Biology A-Level

Biology A-Level glossary

1892 key terms for Biology A-Level, A to Z, each linked to the topic pages that use it.

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A

abiotic components: Non-living, physical and chemical conditions such as light intensity, temperature, water availability, pH, oxygen and carbon dioxide concentration, soil type…AQA
abiotic conditions: Tolerated conditions such as temperature, pH, light intensity and water availability.AQA
abiotic factors: Non-living influences such as light intensity, temperature, water availability and soil pH that affect which organisms survive and how the community is…AQA, OCR
abnormal body cells: Cells such as cancer or virus-infected cells that display altered antigens.AQA
abnormal methylation: Hypermethylation of a tumour suppressor gene's promoter prevents its transcription, while reduced methylation of an oncogene increases it.AQA
ABO blood groups: A gene with three alleles, I^A, I^B and I^O, giving six possible genotypes and four phenotypes.AQA
Abscisic acid: Acts as a stress hormone, promoting stomatal closure and maintaining seed dormancy.OCR
absence of a nucleus: A feature of mature mammalian red blood cells, with no other organelles, leaving more room for haemoglobin.AQA
Absence of introns: Prokaryotic genes are continuous because their DNA contains no introns, so there is no pre-mRNA stage.AQA
absolute refractory period: The time immediately after an action potential when the membrane cannot produce another one because the voltage-gated sodium ion channels are inactivated.AQA
abundance and percentage cover: A valid measure of abundance, such as percentage cover or frequency, with repeats taken to calculate a mean.AQA
acceptance of scientific evidence: A claim is accepted only after independent repetition and scrutiny by other scientists, which explains the delay in accepting structures as real.AQA
Accessory pigments: Chlorophyll b and carotenoids absorb wavelengths that chlorophyll a absorbs poorly and transfer the energy to chlorophyll a.OCR
Accumulation of nitrogenous waste: The build-up of urea and other nitrogenous waste in the blood rather than being excreted, linked to reduced filtration.OCR
Accuracy of dates, pages and authorship: Details such as date, page and author must be accurate rather than approximate, so the citation matches the source actually used.OCR
Accuracy of observation transferred to paper: Representing the specimen's shape and relative proportions faithfully, with structures large enough to show the detail actually observed.OCR
Accurate arithmetic and significant figures: Carrying out arithmetic accurately, including correct handling of decimals, with significant figures reflecting the precision of the measurements.OCR
Accurate measurement technique: Operating equipment correctly, such as reading a meniscus at eye level or zeroing a balance before weighing.OCR
Accurate observation and recording: Distinguishing genuine structures from artefacts and recording observations accurately at the stated magnification.OCR
Accurate reading of scales and menisci: Reading the instrument correctly, including the bottom of the meniscus at eye level and the smallest scale division before recording.OCR
acellular structure: Lacking a cell-surface membrane, cytoplasm and organelles, so a virus particle is not a cell.AQA
Acetyl coenzyme A: Formed when the remaining 2-carbon acetyl group combines with coenzyme A, delivering the acetyl group into the Krebs cycle.OCR
acetylcholinesterase inhibition: A drug that inhibits acetylcholinesterase leaves acetylcholine in the cleft for longer, so receptors are repeatedly bound and more action potentials follow.AQA
acid hydrolysis and squashing: Warming in 1 mol dm⁻³ hydrochloric acid at 60°C hydrolyses the middle lamella to separate cells, and pressing firmly without twisting squashes cells into a…AQA
Acid hydrolysis of non-reducing sugars: Boiling a fresh sample with dilute hydrochloric acid to hydrolyse the glycosidic bonds, then neutralising with sodium hydrogencarbonate before repeating the…AQA
actinomyosin bridges: Structures formed when myosin heads attach to the myosin binding sites on actin.AQA
Actinomyosin cross-bridge formation: Myosin heads attach to the exposed binding sites on actin, forming cross-bridges to initiate contraction.AQA
Action potential ion movements: Voltage-gated Na⁺ channels open and Na⁺ rushes in, then Na⁺ channels close and K⁺ channels open so K⁺ leaves the axon.OCR
activation energy: The initial energy input needed to break bonds and reach the transition state, which each enzyme lowers for the reaction it catalyses.AQA
Activation energy and enzyme–substrate complexes: Enzymes lower the activation energy of a reaction and bind substrate at a complementary active site to form an enzyme–substrate complex.OCR
activation of helper T cells: After binding to a presented antigen, the helper T cell is activated and divides by mitosis to form a clone.AQA
active immunity: The recipient's own immune system is stimulated to produce antibodies and memory cells, so protection develops slowly but lasts a long time.AQA, OCR
Active immunity and memory cells: Vaccination introduces a harmless form of an antigen so the adaptive immune system makes a primary response and memory B and T cells.OCR
Active loading: The process at the source in which sucrose is actively loaded into sieve tube elements, lowering water potential.OCR
active site: A small depression formed when the polypeptide folds, with a specific tertiary structure determined by the sequence of amino acids.AQA
active sites and binding sites: Sites whose tertiary structure must stay complementary to the substrate, or whose variable regions form specific antigen-binding sites complementary to a…AQA
active transport: The process by which pyruvate crosses the inner mitochondrial membrane through a specific carrier protein, requiring energy.AQA, Pearson Edexcel
active transport and ATP: The sodium-potassium pumps actively transport three sodium ions out for every two potassium ions taken in, using ATP.AQA
Active transport and maintenance of gradients: Energy from respiration drives active transport, including ion pumps that maintain concentration gradients across membranes.OCR
active transport of calcium ions: ATP is used to actively transport calcium ions back into the sarcoplasmic reticulum during relaxation.AQA
Adaptations of exchange surfaces: Gill lamellae have a large surface area, thin epithelium and a rich blood supply; tracheoles have thin walls and fluid-filled tips that deliver oxygen…OCR
addition and deletion: Adding or deleting a base, or any number of bases not a multiple of three, shifts the reading frame.AQA
adenine and ribose: The organic base and the pentose sugar that, together with three phosphate groups, form a molecule of ATP.AQA
Adenylate cyclase and cyclic AMP: The membrane-bound enzyme adenylate cyclase catalyses the conversion of ATP into cyclic AMP.AQA
ADH: Released by the posterior pituitary in response to stimulation by the hypothalamus; increases the permeability of the collecting duct to water by inserting…OCR
adhesion to the wall: Water also adheres to the lignified walls of the xylem vessels.AQA
ADP recycling: The ADP and phosphate produced return to the thylakoid membranes, where photophosphorylation converts them back to ATP.AQA
Adrenal cortex and medulla: The adrenal gland has an outer cortex and an inner medulla, each with distinct hormones and functions.OCR
Adrenaline and fight-or-flight: The adrenal medulla secretes adrenaline and noradrenaline, which increase heart rate and blood glucose as part of the fight-or-flight response.OCR
advantage depends on environment: Whether an allele benefits its possessor is entirely dependent on the specific environment, so it may be disadvantageous elsewhere.AQA
Advantages and limitations of immobilisation: A product can be made continuously, the enzyme can be recovered and reused, and product contamination is reduced, but each method trades enzyme leakage…OCR
Advantages of cloning plants artificially: Rapid, large-scale production of uniform plants, including rare or endangered species, and meristem tissue gives virus-free plantlets.OCR
Aerenchyma: Air spaces in hydrophytes that provide buoyancy and a reservoir of gases for submerged tissues.OCR
Aesthetic reasons for maintaining biodiversity: Reasons concerning cultural, recreational and intrinsic value, such as landscapes people value, wildlife watching, and the ethical view that species have…OCR
affinity: Indicated by the position of the curve: a shift to the left means higher affinity, while a shift to the right means lower affinity.AQA
affinity for oxygen: Determined by the environment around the haem groups, which differs because primary structure determines tertiary and quaternary structure.AQA
agglutination: Antibodies bind to antigens on several pathogens at once, clumping them together so large complexes form and phagocytosis is more efficient.AQA, OCR
agonist and antagonist: The agonist or prime mover contracts, while its partner relaxes and is stretched.AQA
agonists and antagonists: A drug that mimics the neurotransmitter triggers depolarisation and more action potentials, while one with a shape complementary to the receptor but no…AQA
Airway structure and support: Cartilage rings in the trachea and bronchi keep the airways open, while smooth muscle in bronchioles changes airway diameter to control airflow.OCR
algal bloom: A surface bloom formed when the extra nitrate and phosphate remove the limiting factor on algal growth, so algae multiply rapidly.AQA
All-or-nothing law and impulse frequency: A stronger stimulus gives more frequent impulses, not bigger ones, so stimulus strength is coded by impulse frequency rather than amplitude.OCR
all-or-nothing principle: Below threshold no action potential is produced, and above threshold every action potential is the same size.AQA
allele: A different version, or different base sequence, of a gene at the same locus.AQA, Pearson Edexcel
Allele frequencies: Proportions of the gene pool, where for two alleles p + q = 1, with p the dominant and q the recessive allele frequency.OCR
allele frequency: The proportion of a gene pool made up by one particular allele, written as a decimal or a percentage.AQA
allele frequency change: The shift in allele proportions because breeders are not a random sample, with the advantageous allele increasing over successive generations.AQA
Allele frequency changes: The frequency of the advantageous allele increases in the population over subsequent generations.AQA
allele frequency over generations: The proportion or percentage of the allele in the population rises over many generations, not the number of alleles.AQA
allele passed to offspring: Survivors must reproduce and pass the advantageous allele on, whether via gametes in sexual reproduction or binary fission in asexual organisms.AQA
alleles: Different versions of the same gene, that is different base sequences at the same locus.AQA
Alleles, genotypes and phenotypes: Alleles are assigned as dominant or recessive, genotypes are labelled with allele symbols, and phenotypes are labelled separately in the diagram.OCR
Allopatric (geographical) isolation: A physical barrier, such as a river or mountain range, separates populations and prevents interbreeding.OCR
allopatric speciation: Speciation that begins with a geographical barrier, like a river or mountain range, physically dividing a population, after which the separated groups diverge.AQA
alpha cells of the islets of Langerhans: The cells that secrete glucagon when blood glucose concentration falls.AQA
alpha-glucose polymers: Glycogen and starch are formed by the condensation of alpha-glucose.AQA
altered tertiary structure: Random changes to existing proteins often change this negatively, for example an enzyme's active site becomes non-complementary.AQA
alternate monomers rotated 180 degrees: A feature of cellulose and chitin chains, where alternate beta monomers are flipped through 180 degrees so their hydroxyl groups can line up and hydrogen…AQA
alternative respiratory substrates: Other respiratory substrates include the breakdown products of lipids and amino acids, which enter the Krebs cycle.AQA
alternative splicing: The process by which pre-mRNA from one gene can be spliced in different ways, making the proteome larger than the gene count in eukaryotes.AQA
Alveolar and capillary adaptations for diffusion: Alveolar walls are thin and moist, giving a short diffusion distance and a large surface area, and capillaries have thin walls and a steep concentration…OCR
Alveoli: Clusters of tiny air sacs at the ends of bronchioles that provide a massive surface area for gas exchange.AQA
Amine and carboxyl groups: The —NH₂ and —COOH groups that are common to all amino acids.OCR
Amino acid and protein synthesis: Triose phosphate is built into amino acids, and therefore proteins, when nitrogen is available.OCR
amino acid binding site: The site at the opposite end of tRNA from the anticodon loop, which attaches a specific amino acid.AQA
amino acid sequence: A mutation changing the base sequence can alter the amino acid added, so the encoded polypeptide has a different amino acid sequence.AQA
amino acid sequence comparison: Compares the amino acid sequences of the same protein in the same way, since the base sequence determines the amino acid sequence.AQA
amino acid sequences: Comparing the amino acid sequence of the proteins encoded by DNA and mRNA also reveals diversity.AQA
amino acids: Examples of monomers which join by peptide bonds to make dipeptides and polypeptides.AQA
ammonification: The nitrogen in amino acids is surplus to the saprobiont's needs, so it is deaminated and released as ammonia, which dissolves to give ammonium ions.AQA
amphipathic and bilayer formation: A molecule with both hydrophilic and hydrophobic properties is amphipathic, so in water phospholipids arrange themselves automatically into a bilayer.AQA
Amphipathic properties and membranes: Having a hydrophilic phosphate head and hydrophobic fatty acid tails lets phospholipids form bilayers in membranes.OCR
Amplification by PCR and cDNA from reverse transcriptase: Polymerase chain reaction can amplify the gene, and reverse transcriptase can make complementary DNA from mRNA.OCR
amplification of a change: An accelerating departure from the starting value, usually ending only when something else intervenes, which is how positive feedback appears.AQA
amylase and maltose: Amylase hydrolyses glycosidic bonds in starch to produce maltose, which a membrane-bound disaccharidase such as maltase then hydrolyses to glucose.AQA
amylose and amylopectin: Starch is a mixture of two polysaccharides, not a single molecule: amylose, a long unbranched chain of 1,4 glycosidic bonds that coils into a helix, and…AQA, OCR
Anabolic reactions and biosynthesis: Respiration supports anabolic reactions such as protein synthesis, DNA replication and the synthesis of other cell molecules.OCR
anaerobic ATP supply: Glycolysis is the cell's only anaerobic source of ATP, and it stops if the pool is exhausted.AQA
Anaerobic conditions: Created by sealing the vessel or layering oil over the culture, so that oxygen is excluded.OCR
anaerobic regeneration of ATP by phosphocreatine: The immediate reserve that donates its phosphate group to ADP to form ATP rapidly and anaerobically, for example during a short sprint, and does not produce…AQA
Anaerobic respiration effects: Anaerobic respiration can distort results because CO₂ is produced without O₂ uptake, raising the apparent RQ.OCR
anaerobic stage: Glycolysis is described as this because it does not require oxygen, so a cell can keep making ATP when oxygen runs short.AQA
Analogous structures: Structures that arise from convergent evolution and do not indicate close relatedness.OCR
anaphase: Two separate groups of V shaped chromatids moving towards opposite poles with a clear gap between them.AQA, OCR
anatomical adaptation: A structural feature of an organism, such as a thick insulating layer of blubber or a streamlined body shape.AQA
Anatomical adaptations: Physical features of body form or structure, such as a streamlined shape, thick fur or a waxy cuticle.OCR
Animal and recombinant sources: Some medicines have been obtained from animals, for example insulin historically extracted from animal pancreas, and human insulin is now produced by modified…OCR
Animal communicable diseases: In animals the key groups are bacteria, viruses, fungi and protoctists, for example tuberculosis, HIV, ringworm and malaria.OCR
animal use in production and testing: Production and testing of vaccines usually involve animals, and mice are used to produce the cells for monoclonal antibodies.AQA
Annotation as recorded evidence: Recording observed features beyond a name, for example colour, texture, relative size or a measured dimension, with a title identifying the specimen.OCR
Anomalies versus random variation: Small scatter is expected, whereas a single value far outside the pattern is not.OCR
Anomaly: A measurement that does not fit the pattern shown by the other readings.OCR
antagonistic intercostal muscles: During inspiration the external intercostal muscles contract while the internal intercostal muscles relax, an antagonistic pair.AQA
Antagonistic muscle action: External intercostals contract to raise the ribs in inspiration, while internal intercostals lower them in forced expiration.OCR
antagonistic pairs: Muscles that work against each other, so that as one contracts the other relaxes and is stretched.AQA
Antibiotic resistance: Antibiotic use acts as a selection pressure: resistant bacteria survive, reproduce and pass on resistance alleles, so resistant strains become more common.OCR
antibodies: Have variable regions complementary to a specific antigen.AQA
antibody: A protein or immunoglobulin with a quaternary structure, secreted by plasma cells derived from B lymphocytes in response to a specific antigen.AQA
Antibody concentration: In the primary response it rises gradually and may take days, whereas the secondary response produces a higher and longer-lasting antibody concentration.OCR
Antibody function in immune defence: Binding to antigen to neutralise pathogens or toxins, marking them for destruction, and forming antigen–antibody complexes destroyed by phagocytosis.OCR
Antibody specificity: Each antibody binds specifically to its antigen, and the difference between antibodies lies in the consequence of binding.OCR
Antibody structure: A Y-shaped molecule of four polypeptide chains: two identical heavy chains and two identical light chains, held by disulfide bridges.OCR
anticodon: Three exposed bases at one end of transfer RNA.AQA
Antidiuretic hormone (ADH): Produced in the hypothalamus and secreted into the blood by the posterior pituitary gland, travelling to the kidneys.AQA
antigen: A molecule, usually a protein or glycoprotein and usually found on a cell surface, that is recognised as foreign and stimulates an immune response.AQA
antigen-antibody complex: Formed when an antibody's binding site, with a tertiary structure complementary to a specific antigen, binds to that antigen.AQA
antigen-presenting cell: A cell displaying antigen on its cell-surface membrane, where the displayed antigen may be foreign or an abnormal self-antigen.AQA
Antigen-presenting cells: Macrophages display pathogen antigens from the digested pathogen on their cell-surface membrane, helping to activate other immune responses.OCR
antigens in vaccines: A vaccine introduces antigens into the body without causing the disease, and they may come from a dead pathogen, an attenuated living pathogen, an isolated…AQA
Antimicrobial chemical defences: Compounds such as tannins, alkaloids and terpenoids, plus enzymes and the oxidative burst, that act against pathogens.OCR
Apical dominance: The suppression of lateral bud growth by the shoot apex.OCR
apoptosis: Programmed cell death, triggered when damage cannot be mended.AQA
Apparatus selection: Selecting appropriate apparatus, for example a protractor or clinometer for plant growth, or a ruler-drop or timing device for animal reaction time.OCR
Apparatus selection by range and resolution: Choosing equipment whose range and resolution suit the quantity being measured, rather than choosing by habit.OCR
Apparatus selection matched to purpose: Choosing equipment whose precision and function match the measurement required by the task, rather than choosing by convenience.OCR
Applications in analysing nucleic acids and proteins: Uses include checking PCR products, restriction digests and plasmid digests, separating DNA fragments for sequencing or blotting, and separating proteins for…OCR
Applications in DNA analysis and quantification: PCR provides enough DNA for profiling, sequencing or cloning from tiny or degraded samples, and can be made quantitative.OCR
Applications of predicted amino acid sequences: Comparing predicted sequences between organisms reveals conserved regions and possible mutations.OCR
Appropriate apparatus and resolution: Choosing an instrument and scale whose resolution and scale suit the quantity being measured, with readings taken at the stated intervals.OCR
Aquaporins: Inserted by ADH to increase the permeability of the collecting duct to water, so more water is reabsorbed by osmosis.AQA, OCR
aquatic ecosystems: Systems in which producers utilise dissolved carbon dioxide and hydrogencarbonate ions as their carbon source.AQA
artefact: A structure visible in a prepared specimen that was not present in the living cell, such as air bubbles, folds in a section, scratches, debris or material…AQA
Artificial embryo twinning: An early embryo is split into separate cells or groups before cells specialise, each group is cultured briefly and implanted into a surrogate so several…OCR
artificial fertilisers: Inorganic salts of known composition that dissolve quickly, so the ions are immediately available and the dose can be matched precisely to the crop.AQA
Artificial immunity: Results from a medical intervention, such as vaccination or injection of antibodies such as antivenom.OCR
aseptic technique: A method that keeps unwanted microorganisms out when growing a bacterial lawn on sterile nutrient agar to test antimicrobial substances.AQA, OCR
Asexual reproduction: Reproduction such as budding or fragmentation that produces offspring genetically identical to the parent.OCR
Asexual reproduction and cloning: Mitosis enables asexual reproduction, producing genetically identical offspring or clones, and in plants underlies vegetative propagation.OCR
Association and dissociation of oxygen: At high pO₂ in the lungs oxygen associates with haemoglobin to form oxyhaemoglobin, while at lower pO₂ in respiring tissues oxygen dissociates and is released.OCR
Asthma: Constricts bronchioles, lowering forced expiratory volume in one second.AQA
ATP: The immediate energy source for cells, hydrolysed to ADP and Pi to release manageable energy for processes.AQA
ATP and ADP as energy carriers: Adding the terminal phosphate to ADP (phosphorylation) stores energy, while removing it (dephosphorylation) releases energy.OCR
ATP as the universal energy currency: ATP is the immediate, universal energy currency, transferring energy in small, usable amounts to reactions such as active transport, protein synthesis, DNA…OCR
ATP hydrolase: The enzyme that catalyses ATP hydrolysis to ADP and Pi, releasing energy.AQA
ATP hydrolysis: ATP is hydrolysed to ADP and inorganic phosphate by ATP hydrolase, releasing energy in a small, immediately usable amount.AQA
ATP hydrolysis on the myosin head: ATP hydrolase hydrolyses ATP to ADP and Pi, releasing energy to recock the head so it can bind further along.AQA
ATP in the cross-bridge cycle: ATP binding detaches myosin heads from actin and ATP hydrolysis re-cocks them for the next cycle.OCR
ATP production: If blood glucose falls too low, cells cannot produce enough, so active transport and nerve function fail.AQA
ATP requirement: Shortening the fibres requires ATP, supplied by mitochondria clustered near the spindle.AQA
ATP synthase: A membrane protein that acts as a channel and enzyme, through which protons diffuse back down their gradient.OCR
atrial systole: The atria contract, atrial pressure rises above ventricular pressure, the atrioventricular valves are already open and atrial contraction completes…AQA
atrioventricular delay: The AVN delays the wave for approximately 0.1 seconds, allowing atria to empty and ventricles to fill.AQA
Atrioventricular valves: These separate atria and ventricles and are anchored by tendinous cords that prevent inversion during ventricular contraction.AQA
attachment protein and receptor: The virus carries attachment proteins that bind to complementary receptor molecules on the host cell-surface membrane, so a virus infects only cells with the…AQA
attachment proteins and host receptors: Proteins projecting from the surface that bind to complementary receptor molecules on specific host cells, determining which cells the virus can infect.AQA
Autoimmune disease: A condition in which the immune system attacks the body's own tissues because self is not distinguished from non-self.OCR
automated sequencing: Sequencing carried out by machines rather than by hand, with methods that are continuously updated.AQA
Autonomic control of heart rate: The autonomic nervous system modulates the intrinsic rhythm, with sympathetic nerves speeding the SAN and parasympathetic nerves slowing it.OCR
Autonomic nervous system: The part of the peripheral nervous system controlling involuntary effectors such as smooth muscle, cardiac muscle and glands.OCR
Autosomal linkage: Genes linked on the same autosome tend to be inherited together, so the ratio departs from 9:3:3:1 with an excess of the two parental phenotypes and fewer…OCR
Auxin: Produced mainly in shoot tips, it moves to regions of elongation and causes cell elongation, and an unequal distribution produces the differential growth seen…OCR
Auxin (IAA): A substance produced by the apex that inhibits bud outgrowth; applied in a paste to the cut stump it restores suppression so the buds stay dormant.OCR
Auxins: Used as selective weedkillers, affecting broad-leaved plants more than narrow-leaved cereals, and as rooting powders for cuttings.OCR
AVN: Receives the impulse and introduces a delay, allowing the atria to finish emptying before the ventricles contract.OCR
Axis labelling and linear scales: Each axis carries units and the scale is linear and labelled, with the independent variable on the x-axis.OCR
Axis labelling with quantity and unit: Each axis is labelled with the quantity and its unit, using a solidus or brackets so the unit is unambiguous.OCR
axon and dendrites: The long axon carries impulses away from the cell body towards the effector, while many short dendrites carry impulses towards the cell body.AQA
axon diameter and resistance: A wider axon offers less resistance to the flow of local currents in the cytoplasm, so depolarisation spreads faster.AQA

B

B lymphocyte structure and maturation: B lymphocytes mature in bone marrow and carry antibody-like surface receptors, each with a receptor of one shape.OCR
bacterial lawn: A lawn grown on sterile nutrient agar, spread evenly, on which discs soaked in different antimicrobials are placed well apart.AQA
Balance of cell gain and loss: The balance between mitosis and apoptosis determines final cell number and arrangement, so loss of control of either mechanism alters body form.OCR
balance of division and cell death: In healthy tissue the rate of cell division is balanced by the rate of cell death, so tissue size stays constant.AQA
base analogues: Chemicals like 5-bromouracil that resemble normal bases and are incorporated during replication, but pair incorrectly to cause substitutions.AQA
base deletion: A base is removed so that all subsequent triplets are shifted, causing a frameshift.AQA
base sequence: The order of bases in DNA, which carries information and which a gene is a length of.AQA
base substitution: One base is replaced by a different base, affecting one triplet only.AQA
Batch versus continuous fermentation: In batch fermentation the culture is closed, with nutrients added at the start and product harvested at the end, whereas continuous fermentation adds…OCR
behavioural adaptation: Something the organism does, such as migration, courtship displays or basking to raise body temperature.AQA
Behavioural adaptations: Actions or activity patterns, such as migration, nocturnal foraging, huddling or basking.OCR
belt transect: A tape along an environmental gradient with quadrats at regular intervals, used because random placement would hide the pattern.AQA
Benedict's test for reducing and non-reducing sugars: Add Benedict's reagent and heat in a water bath; a brick-red precipitate shows a reducing sugar, and a positive result only after acid hydrolysis and…OCR
benign and malignant tumours: Benign tumours grow slowly and remain compact, usually surrounded by a capsule of fibrous tissue, whereas malignant tumours grow rapidly, are irregular in…AQA
Beta cell destruction and autoimmune cause: The beta cells of the islets of Langerhans are destroyed, usually by an autoimmune response, so little or no insulin is produced.AQA
beta cells of the islets of Langerhans: Cells in the pancreas that secrete insulin when they detect a rise in blood glucose concentration.AQA
beta-glucose polymer: Cellulose is formed by the condensation of beta-glucose, so alternate residues are flipped through 180 degrees and the chain is straight and unbranched.AQA
beta-glucose rotation: Alternate beta-glucose molecules must be rotated through 180 degrees before their hydroxyl groups align and the bond can form, producing the straight…AQA
betalain pigment leakage: The red pigment contained in the vacuoles stays inside while the tonoplast and cell-surface membrane are intact and leaks out once they are damaged.AQA
biconcave disc: The shape of a mature mammalian red blood cell, giving a large surface area to volume ratio so oxygen diffuses in and out rapidly.AQA
bile salts and emulsification: Made in the liver, they do not hydrolyse anything but emulsify lipid droplets into many tiny droplets, greatly increasing the surface area on which lipase can…AQA
binomial: A universal two-word name for each species, made of the name of its genus and species, as in Homo sapiens.AQA
Binomial nomenclature: A two-part Latin name given to every species: a genus name capitalised followed by a species name in lower case, both italicised when printed.OCR
binucleate cells: If cytokinesis is prevented, mitosis can occur without it, and the result is one cell containing two or more nuclei.AQA
Biochemical tests for carbohydrates: Tests using Benedict's solution for reducing sugars and non-reducing sugars, and iodine/potassium iodide for starch.AQA
Biodiversity and ecosystem function: The variety of life and processes such as soil stability, breeding populations and nutrient cycling that management maintains.OCR
Biological drawing conventions: Feature continuous, unbroken lines with no shading or sketchy strokes, draw structures in proportion, include a title, and use straight, ruled label lines…AQA
biological molecules: Groups of molecules made from sugars synthesised during photosynthesis that are not respired, including carbohydrates, lipids, proteins and nucleic acids.AQA
biological species concept: Defines a species by reproductive compatibility, not by appearance, so that organisms belong together only if they can interbreed and the offspring are…AQA
biomass: The plant material formed from synthesised organic compounds, representing the chemical energy available to consumers and decomposers.AQA, OCR
biotic conditions: Relationships such as food, predators and competitors.AQA
Biotic factors: Living influences such as predation, competition and disease that affect which organisms survive and how the community is structured.OCR
Births, deaths, immigration and emigration: Births and immigration add individuals to a population, while deaths and emigration remove them, so change in population = (births + immigration) − (deaths +…OCR
biuret test: Detects peptide bonds, so it is a test for peptides and proteins rather than for individual amino acids.AQA
Biuret test for proteins: Add an equal volume of biuret solution to the sample; a purple colour indicates peptide bonds, so protein is present, while blue is negative.OCR
Bivalents and chiasmata: In prophase I homologous chromosomes pair as bivalents and crossing over occurs at chiasmata.OCR
blastocyst: The developmental stage by which totipotency is lost, with an outer trophoblast and an inner cell mass.AQA
Blood glucose and salt balance: Aldosterone increases sodium reabsorption and helps regulate blood pressure, while cortisol raises blood glucose and supports the response to long-term stress.OCR
Blood glucose monitoring: Monitoring blood glucose allows doses to be adjusted and reduces the risk of hypoglycaemia.OCR
Blood markers of kidney function: Urea and creatinine concentrations in blood indicate how well the kidneys are filtering, since poor filtration allows these waste products to accumulate.OCR
Blood transport of hormones: Hormones are carried in the plasma to reach cells throughout the body, and their blood concentration depends on the balance between secretion and breakdown.OCR
blue to purple: A positive result is a colour change from blue to purple, lilac or violet, while a negative result stays blue.AQA
Body mass: A risk factor for type 2 diabetes rather than an acute control factor influencing blood glucose concentration.AQA
Bohr effect: The effects of carbon dioxide concentration on the dissociation of oxyhaemoglobin, where hydrogen ions lower pH and reduce the affinity for oxygen.AQA
bomb calorimeter: A sealed vessel in which a weighed, dried sample is ignited in pure oxygen so that combustion is complete.AQA
Bonding and R-group interactions: Peptide bonds hold the primary sequence, hydrogen bonds between backbone C=O and N–H groups hold secondary structure, and hydrogen bonds, ionic bonds,…OCR
brain dependence on glucose: Brain cells cannot respire fatty acids and depend on glucose almost entirely.AQA
branching and 1,6 bonds: Glycogen is alpha-glucose with 1,4 and many 1,6 bonds, so it has shorter chains and more branches than amylopectin.AQA
branching and rapid hydrolysis: Branch points create many terminal glucose units that hydrolytic enzymes can act on simultaneously, so glucose is released rapidly.AQA
branching diagrams: A tree in which every branch point is a common ancestor and the branches show the order in which lineages separated.AQA
Breakdown of self-tolerance: Failure of self-tolerance, so self antigens are treated as foreign and provoke an immune response.OCR
Bronchioles: A network of narrower tubes inside the lungs that bronchi divide repeatedly into, and which terminate in clusters of tiny air sacs.AQA
Buffering: Acting on solutions because the same carbon carries both an acidic carboxyl group and a basic amine group.OCR
bundle of His: The AVN passes the wave into this structure, which divides into Purkyne tissue and conducts the wave down the septum to the apex of the heart.AQA
Bunsen flame: A lit flame worked close to because its upward convection current reduces the chance of airborne microorganisms landing in the vessel.OCR

C

calcium channel blockers: A drug that blocks calcium ion channels, so fewer calcium ions enter the knob and fewer vesicles fuse.AQA
calcium ion release: Depolarisation travels down the transverse tubules and causes the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.AQA
Calcium ion release and active reuptake: An action potential reaching the sarcoplasmic reticulum releases calcium ions into the sarcoplasm; when stimulation ceases, calcium ions are actively…AQA
Calcium-triggered exocytosis: An arriving action potential opens voltage-gated Ca²⁺ channels, and Ca²⁺ entry into the presynaptic knob triggers vesicle fusion and release of…OCR
Calculating magnification and actual cell size: Actual size is found using magnification = image size ÷ actual size, allowing real cell dimensions to be determined.OCR
calibration: Lining the two scales up, counting how many eyepiece divisions cover a known length on the micrometer, and dividing to find the real value of one eyepiece…AQA
Calibration and correct set-up: Setting up and calibrating equipment before taking readings, for example using a blank with a colorimeter.OCR
Calibration and measurement: Specimen size is measured using an eyepiece graticule calibrated with a stage micrometer.OCR
calibration curve: A plot of absorbance, or transmission, against known glucose concentration, from which the concentration of the unknown sample is read after treating it in…AQA
Calibration of the eyepiece graticule: The scale in the eyepiece must be calibrated for each objective using a stage micrometer of known scale, giving a calibration factor that converts graticule…OCR
Callose deposition and cell-wall thickening: Production of callose by some tissues in response to infection to thicken walls and block spread.OCR
calorimetry: A technique used to estimate the chemical energy store in dry biomass by burning it completely and capturing the heat released.AQA
Calvin cycle in the stroma: The light-independent stage happens in the stroma, where the enzymes for carbon fixation are located.OCR
Cambium: Tissue between xylem and phloem that allows secondary thickening in dicotyledonous plants.OCR
Capillary action: The process by which the solvent is allowed to travel up the paper or TLC plate.OCR
Capillary structure and permeability: The capillary wall is thin and permeable, allowing exchange between plasma and tissue fluid surrounding cells.OCR
capsid and genetic material: Genetic material (DNA or RNA) that codes for viral proteins, enclosed within a protein coat.AQA
capsule: A slime layer of polysaccharide outside the cell wall that helps the cell stick to surfaces, retains moisture and makes the bacterium harder for a phagocyte…AQA
Carbohydrate synthesis and storage: Triose phosphate is converted to hexose sugars such as glucose, which can be used in respiration or stored as starch, polymerised into cellulose for cell…OCR
carbon 1 inversion: The positions of the hydrogen atom and the hydroxyl group on carbon 1 are inverted between the two isomers.AQA
carbon dioxide concentration: A factor whose low level leaves RuBP unreacted, so less GP is formed.AQA
Carbon dioxide transport forms: Most is transported as hydrogencarbonate ions (HCO₃⁻) formed in red blood cells by carbonic anhydrase, some is bound to haemoglobin as carbaminohaemoglobin…OCR
carbon fixation: Carbon dioxide combines with ribulose bisphosphate, a reaction catalysed by rubisco.AQA
Carbon fixation by rubisco: Carbon dioxide joins the five-carbon acceptor ribulose bisphosphate (RuBP), a reaction catalysed by the enzyme rubisco.OCR
Carbon, hydrogen and oxygen: The elements present in carbohydrates, lipids, proteins and nucleic acids, forming the bulk of every one of these classes.OCR
carboxyl and amine groups: The backbone groups that are always joined, never the R groups, leaving a free amine group at one end and a free carboxyl group at the other.AQA
cardiomyocyte replacement: Stem cells differentiate into cardiomyocytes, replacing cells lost as scar tissue after a heart attack.AQA
cardiomyocytes: Heart muscle cells, which are the specific specification example of cells formed from unipotent cells.AQA
carrier proteins: A specific protein in the inner mitochondrial membrane through which pyruvate is transported by active transport.AQA
carrier testing: Screening that can reveal carrier status for a heritable condition such as cystic fibrosis.AQA
carrying capacity: The maximum population size that an ecosystem can support over a prolonged period.AQA
Carrying capacity and resource limits: The maximum population an ecosystem can sustain; when numbers exceed it, mortality rises and the population falls back.OCR
Carrying units through to the answer: Carrying units through and stating them, so the final answer carries the correct unit.OCR
Causation: A causal link is supported by strong epidemiological data combined with a biological mechanism, such as high blood pressure damaging endothelium.AQA
causes of change: Natural selection is the most important cause of that change but not the only one, since mutation, migration and genetic drift also alter frequencies.AQA
cDNA and reverse transcriptase: Reverse transcriptase with free DNA nucleotides builds a single-stranded DNA copy from mature mRNA, and DNA polymerase then adds the second strand.AQA
cell cycle checkpoints: Points at which the cell cycle is regulated, such as at the end of G1, at the end of G2 and during metaphase.AQA
cell plate and Golgi vesicles: In a plant cell, vesicles from the Golgi apparatus line up along the equator and fuse, forming this, which grows outwards until it meets the existing wall.AQA
Cell shape and support: Maintaining cell shape and providing mechanical support, one of the linked roles of the cytoskeleton.OCR
Cell signalling and receptors: Membrane proteins act as receptors for cell signalling, and the cell-surface membrane carries receptor molecules for cell signalling.OCR
cell specialisation: Cells are specialised for different functions, so coordination between distant tissues is required.AQA, OCR
Cell structure: Prokaryotic cells have no nucleus, with DNA as a circular loop in the cytoplasm plus small circular plasmids, and have a cell wall (not cellulose).OCR
Cell structure and wall composition: Prokaryotae have no nucleus or membrane-bound organelles and a peptidoglycan wall, Fungi a chitin wall, Plantae a cellulose wall, and Animalia no cell wall.OCR
cell-surface antigens: Specific molecules, including proteins, on the surface of each type of cell that identify it.AQA
Cell-surface receptor specificity: The hydrophilic hormone binds to a specific receptor on the cell-surface membrane of a liver cell, changing the receptor's shape.AQA
cell-surface receptors: Adrenaline attaches to these on the cell-surface membranes of target cells, rather than entering the cell to act.AQA
cellular differentiation: The process by which a stem cell develops into a specific type of cell, with unipotent cells limited to a single lineage.AQA
Cellular metabolism: Enzymes catalyse reactions such as those of respiration and photosynthesis, controlling the metabolic pathways inside cells.OCR
Cellulose and microfibrils: Built from β-glucose with adjacent units inverted, so each chain is straight and neighbouring chains are held by hydrogen bonds into strong microfibrils.OCR
Central (alpha) carbon: The atom at the centre of an amino acid to which the four groups are bonded.OCR
Central nervous system (CNS): The brain and spinal cord, which the peripheral nervous system links to the rest of the body.OCR
centromere attachment: Spindle fibres attach at the centromere, one fibre from each pole, so the chromosome is held under tension from both sides.AQA
Cerebellum: The region below and behind the cerebrum that coordinates muscular activity, balance and posture.OCR
Cerebrum: The largest part, divided into two hemispheres, whose outer cerebral cortex handles conscious thought, memory, reasoning and voluntary movement.OCR
Chain-termination mechanism: A method relying on a template strand, a primer, DNA polymerase and a mixture of normal deoxynucleotides plus modified dideoxynucleotides that terminate chain…OCR
Change in allele frequency over generations: Over time the frequency of advantageous alleles rises and disadvantageous alleles fall, shifting the population's characteristics.OCR
change over generations: Evolution has occurred whenever the proportions change over generations, whether or not a new species appears.AQA
changing species composition: The loss of earlier species is linked to the change in species composition seen at the climax community.AQA
channel and carrier proteins: Proteins through which charged ions cross membranes rather than through the bilayer.AQA
channel proteins and facilitated diffusion: Vesicles carrying glucose transporter channel proteins fuse with the cell-surface membrane, so more channel proteins are present and glucose enters the cell…AQA
Charge-based migration in an electric field: Nucleic acids carry negative phosphate groups, so they migrate towards the positive electrode; proteins are given a uniform negative charge by SDS, so they…OCR
Checkpoint control mechanisms: Control points such as G1, G2 and the spindle assembly checkpoint monitor conditions such as DNA damage, completion of replication and chromosome attachment…OCR
Checkpoints and control of division: These regulate progression through the cycle, ensuring DNA is replicated and undamaged before division.OCR
chemical energy store: What plant biomass represents, being subsequently available to higher trophic levels in the ecosystem.AQA
chemical mutagens: Agents that work in other ways, such as base analogues incorporated in place of normal bases, alkylating agents that add chemical groups, and intercalating…AQA
chemiosmosis: Protons pass back into the stroma through ATP synthase, and their movement drives the synthesis of ATP from ADP and inorganic phosphate.AQA
chemoreceptors: Receptors in the aortic and carotid bodies that detect a rise in blood carbon dioxide, which lowers blood pH by raising hydrogen ion concentration.AQA
Chi-squared (χ²) test: A test that compares observed counts with expected counts to judge whether a difference is due to chance, using raw counts rather than percentages or ratios.AQA, OCR
chloroplast extraction: Carried out by grinding leaf tissue in ice-cold, buffered, isotonic solution, then filtering and centrifuging.AQA
Chloroplast ultrastructure: A lens-shaped organelle bounded by a double membrane called the envelope, with a fluid interior called the stroma and a third membrane system of thylakoids…OCR
choice chamber: Offers contrasting conditions, like damp and dry, where animals are introduced and allowed to distribute.AQA
Choice of widely separated points: Two points on the line of best fit that are far apart, used because a larger separation reduces reading error.OCR
Cholesterol: A small, rigid, amphipathic steroid whose hydrophobic region sits among fatty acid tails while its hydroxyl group faces the aqueous surface, restricting…OCR
Cholesterol and fatty-acid saturation: Cholesterol restricts phospholipid movement at moderate temperatures and prevents rigid packing at low temperatures, while more unsaturated fatty acids…OCR
cholesterol between phospholipids: A lipid that sits between phospholipid molecules in the cell membrane bilayer, binding to their fatty acid tails.AQA
chromatid separation: The centromeres divide and the spindle fibres shorten, pulling the sister chromatids apart to opposite poles centromere first.AQA
chromatid separation in anaphase: The centromere divides and the microtubules shorten, pulling the two sister chromatids apart towards opposite poles.AQA
chromatin: The complex of DNA and histone proteins, which condenses further before nuclear division to form the chromosomes visible under a light microscope.AQA
Chromosome behaviour during division: Chromosomes condense and become visible in prophase, line up along the equator in metaphase, sister chromatids are pulled apart to opposite poles in anaphase,…OCR
chromosome condensation: Chromosomes condense by supercoiling and become visible as two chromatids joined at a centromere.AQA
chromosome content diagrams: Diagrams showing the chromosome content of cells after the first and second meiotic division, when given the chromosome content of the parent cell.AQA
chromosome number maintained: Each daughter cell receives one chromatid from every chromosome, so both end up with the same number of chromosomes as the parent cell.AQA
circular DNA: The main DNA molecule of a prokaryotic cell is circular and lies free in the cytoplasm, not enclosed within a nucleus.AQA
circular DNA and nucleoid: A single circular DNA molecule that is free in the cytoplasm and is not associated with proteins, lying as a nucleoid.AQA
circular DNA replication: The single circular DNA molecule is replicated, and the two circular DNA copies attach to the cell-surface membrane at opposite ends as the cell elongates.AQA
circular organelle DNA: The DNA of mitochondria and chloroplasts is short, circular and not associated with protein, like the DNA of prokaryotes.AQA
CITES: An international agreement that regulates trade in endangered species so that a species is not depleted by legal trade.OCR
Clarity and convention in biological drawing: Drawing with a sharp pencil using continuous, unbroken lines and no shading or artistic sketching, so structural outlines are unambiguous.OCR
Classification: The practice of arranging organisms into groups, which a sound system should link to evolutionary history.OCR
Classification and relationships: Names reflect evolutionary relationships through the genus grouping, which groups together closely related species.OCR
classification tables: In such a table each column is one organism and each row one level, so a missing entry is found by reading across the row and down the column.AQA
Classifying conservation scenarios: Deciding whether a described method is in situ or ex situ before selecting an advantage or limitation of it.OCR
Classifying management examples: Deciding whether a management example is conservation or preservation and justifying the choice by whether sustainable use is intended.OCR
cleavage furrow: In an animal cell, a ring of protein filaments just beneath the cell-surface membrane contracts, pinching the membrane inwards to form this, which deepens…AQA
Climate change: Altered temperature and rainfall shift species ranges and disrupt life-cycle timing, so some species decline and others move in.OCR
climax community: The final stable community, determined by the climate.AQA
Clinical benefits of controlling bacterial infection: Curing or controlling infections, reducing complications and transmission, and enabling surgery and immunosuppression.OCR
clonal expansion: The activated T cell divides by mitosis to form a clone.AQA
clonal expansion by mitosis: The activated B cell divides repeatedly by mitosis, producing a clone of genetically identical cells.AQA
clonal selection: The antigen selects the B cell whose membrane-bound antibody fits it when a complementary antigen binds.AQA
Clone: A genetically identical organism or group of cells derived from a single parent or original cell.OCR
Cloning technique: A method in which the new plant is genetically identical to its parent, produced without meiosis or fertilisation.OCR
Closed (batch) culture: A culture containing a fixed volume of nutrient medium with no additions or removals, so nutrients fall and waste accumulates.OCR
clover-leaf shape: The form tRNA takes when hydrogen bonds between complementary bases within the same strand pull it into a folded structure.AQA
co-transport: Sodium ions re-enter the cell down their concentration gradient through a co-transporter protein, carrying glucose or an amino acid with them.AQA
codominance: Both alleles are expressed in the heterozygote, producing a third phenotype, and both proteins are made so both characteristics appear rather than an average.AQA
Codon: A sequence of three adjacent nucleotides on an mRNA molecule that codes for a specific amino acid.Pearson Edexcel
Codon–anticodon pairing and peptide bond formation: Anticodons pair with complementary mRNA codons, and peptide bonds form between adjacent amino acids, producing a polypeptide whose sequence is set by the…OCR
codons and anticodons: DNA triplets are transcribed into mRNA codons, and most codons are read by a complementary tRNA anticodon carrying a specific amino acid.AQA
Coenzyme: An organic cofactor, often derived from a vitamin, that binds temporarily and transfers chemical groups or electrons between reactions.OCR
Coenzyme A: Carries acetyl groups into the Krebs cycle and is distinct from NAD and FAD.OCR
Coenzymes: Small organic molecules that assist enzymes by carrying chemical groups or electrons between reactions, and are not proteins and not catalysts themselves.OCR
Cofactor: A non-protein substance required for an enzyme to catalyse a reaction, which may be an inorganic ion bound loosely or tightly to the enzyme.OCR
cohesion: Water molecules are polar and form hydrogen bonds with one another, so they stick together and the entire column is pulled up as a single unit.AQA
Cohesion and adhesion: Cohesion between water molecules and adhesion to xylem walls, combined with tension from evaporation, pull water up the xylem.OCR
Cohesion and surface tension: Attraction between water molecules and surface tension allow continuous columns in xylem and support organisms on the water surface.OCR
Cold conditions protecting DNA and aiding precipitation: Chilling the ethanol and sample reduces DNA solubility and slows enzyme activity, encouraging precipitation and protecting DNA.OCR
Collagen: Three polypeptide chains wound into a triple helix, cross-linked in fibrils, giving high tensile strength in tendons, skin and blood-vessel walls.OCR
Collapse of the specific response: Fewer helper T cells means reduced activation of B cells and cytotoxic T cells, resulting in decreased antibody production.AQA
Colonisation of bare ground: The arrival of organisms on bare, previously uncolonised ground where no soil or seed bank exists, for example after a volcanic eruption or glacial retreat.OCR
colorimetry: Used to calculate rate from colorimeter readings at set intervals, naming the filter and the reading taken at each time interval.AQA
colorimetry and absorbance: The colour of the surrounding solution is measured in a colorimeter using a filter of the complementary colour and calibrating with distilled water as the…AQA
Colorimetry and calibration: A colorimeter records the absorbance or transmission of light through a coloured solution and is calibrated with a blank or distilled water before use, with a…OCR
Colorimetry and calibration curves: React the sample to produce a coloured product, measure absorbance with a colorimeter, and plot absorbance against concentration for standard solutions to…OCR
colorimetry and filter choice: Reading the remaining blue solution in a colorimeter using a red filter, because a blue solution absorbs red light.AQA
colour change and quantification: The enzyme converts an added substrate to produce a colour change, and the intensity of the colour is proportional to the quantity bound.AQA
Colour changes observed: Iodine solution stains starch blue-black, and methylene blue stains nuclei and other acidic components blue.OCR
colour vision: Three cone types with different optical pigments allow colour perception, depending on the relative stimulation of the three cone types.AQA
Combined effects on variation: Genotype sets a potential and the environment determines where within that potential the phenotype falls.OCR
common ancestry: The idea supported by shared biochemistry, including the universal use of DNA, RNA and ATP, that living things descend from a common ancestor.AQA
common first stage: Glycolysis is the first stage of both anaerobic and aerobic respiration, the one stage common to both.AQA
common names: Names that cause confusion because one may be used for several species and one species may have several in different regions.AQA
Common stem cell origin: Erythrocytes and neutrophils both derive from a common multipotent stem cell in bone marrow.OCR
community: All the populations of all the different species living in that same place at the same time, whose populations interact.AQA
compact helix for storage: Coiling into a compact helix means a great deal of glucose is stored in a small volume.AQA
companion cells: Cells adjacent to sieve tube elements, packed with mitochondria, that keep the elements alive and use ATP to pump hydrogen ions out.AQA
Comparing cell structure across eukaryotic organisms: Plant, animal and fungal cells show shared features such as a nucleus and membrane-bound organelles, and differences such as cell walls, chloroplasts and…OCR
comparing species: Comparing sequences between species measures how closely related organisms are; more similar sequences imply a more recent common ancestor.AQA
Comparing values with repeats and trends: Looking at the spread of the repeat readings and the trend across the range tested to identify a candidate anomaly.OCR
Comparison of muscle tissue types: Distinguishing skeletal muscle from cardiac muscle, which is branched with centrally placed nuclei and intercalated discs, and from smooth muscle, which is…OCR
Comparison of scale, speed and cost: These methods generate very large volumes of short reads rapidly and cheaply per base, and the reads are assembled or aligned by computer.OCR
Comparison with a known size ladder: Fragment size is estimated by comparison with a standard ladder of fragments of known sizes run alongside the sample.OCR
Compartmentalisation of photosynthesis: The two main stages occur in different compartments: the light-dependent stage on the thylakoid membranes and the light-independent stage in the stroma.OCR
Compartmentalisation within cells: Membranes surround organelles such as the nucleus, mitochondria and chloroplasts, keeping different reactions separate so incompatible processes occur in…OCR
competition for resources: When food, water, light, mineral ions, territory or mates are in short supply, the individuals best able to obtain them survive and breed.AQA
competitive exclusion: Light-demanding pioneers lose the interspecific competition for light, so their population falls and they are excluded from the area.AQA
competitive inhibition: An inhibitor with a similar shape to the substrate binds to the active site, but its effect is reduced by increasing substrate concentration.AQA
Competitive inhibitors: These resemble the substrate and bind the active site, so they compete with substrate molecules, and their effect is reduced by raising substrate concentration.OCR
complementary base pairing: Free RNA nucleotides align opposite exposed bases, with RNA uracil pairing with DNA adenine, adenine with thymine, cytosine with guanine, and guanine with…AQA, OCR
Complementary base pairing of free nucleotides: Free nucleotides align opposite the template by complementary base pairing: adenine with thymine and cytosine with guanine.OCR
Complementary base pairing with uracil: In RNA, adenine pairs with uracil, so the mRNA sequence is complementary to the DNA template.OCR
complementary binding: An antibody's binding site has a tertiary structure complementary to a specific antigen, so the two bind.AQA
complementary binding site: A binding site with a tertiary structure complementary to one antigen, so binding is highly specific and forms an antigen-antibody complex.AQA
complementary shape: The property of a substrate that allows it to bind to the active site, along with complementary charges.AQA
Complete citation elements for different source types: Giving enough detail for a reader to find the exact source you used, with the required details recorded for a website, a textbook or a journal article.OCR
Computational alignment and analysis: Needed to make comparisons of large sequence datasets meaningful and to interpret the data.OCR
Computer modelling: Building a simulation of a biological system, then changing variables to predict outcomes that would be slow or unsafe to test directly.OCR
Concentration gradient: Varied by changing the acid concentration; a steeper gradient increases the rate of diffusion.OCR
Concentration gradients: Gradients that specialised exchange surfaces, with transport systems, maintain steep so that diffusion continues at a useful rate.OCR
condensation: A reaction in which the hydroxyl group of glycerol reacts with the carboxyl group of a fatty acid, a water molecule is released, and an ester bond links the…AQA, OCR
Condensation and ester bonds: A triglyceride forms when one glycerol molecule condenses with three fatty acid molecules, releasing three water molecules and forming three ester bonds.OCR
Condensation and hydrolysis: Condensation joins monosaccharides with the loss of water, and hydrolysis reverses this.OCR
condensation and water loss: A reaction between two monosaccharides in which a hydroxyl group on each reacts and one molecule of water is eliminated as the bond forms.AQA
condensation arithmetic: n amino acids give n-1 peptide bonds and n-1 water molecules, so a polypeptide of 100 residues required 99 condensation reactions.AQA
condensation of many glucose units: Many glucose units join by condensation, with one glycosidic bond formed and one water molecule released per join, to make a polysaccharide.AQA
Condensation polymerisation: Nucleotides join in a condensation reaction that forms phosphodiester bonds between adjacent nucleotides and releases a water molecule each time.OCR
condensation reaction: A reaction in which a hydroxyl group leaves the carboxyl group and a hydrogen atom leaves the amine group, so one molecule of water is eliminated.AQA
Condensation reactions: A reaction in which the hydroxyl group of one monosaccharide and the hydrogen of another combine, water is released, and a glycosidic bond forms.AQA, OCR
Conflicting evidence: Evaluated by comparing study designs, sample sizes, and control variables, and by identifying limitations such as small sample sizes, lack of control groups,…AQA
conflicting human needs: Human needs such as food, timber and housing that often conflict with conservation, so managing the conflict is required.AQA
conformational change: A change in the tertiary structure of the enzyme that puts strain on bonds in the substrate and lowers the activation energy.AQA
Confounding factors: Variables that must be held constant so they do not distort the result, identified by asking what else could affect the dependent variable besides the…OCR
Conjugated proteins and prosthetic groups: A conjugated protein contains a non-protein prosthetic group permanently associated with the polypeptide, such as the haem group containing an iron ion (Fe²⁺)…OCR
Consequences of loss of cell-cycle control: Failure of regulation, such as mutation of proto-oncogenes or tumour suppressor genes, can cause uncontrolled cell division and tumour formation.OCR
Conservation: The same homeobox sequence is highly conserved, so homologous Hox clusters are found across animals.AQA, OCR
conservation measures: Can balance biodiversity with agricultural production, such as maintaining hedgerows, leaving field margins uncultivated, crop rotation, intercropping, and…AQA
Conservation of one original strand per daughter molecule: Each resulting DNA molecule contains one original (conserved) strand and one newly synthesised strand, which is what semi-conservative means.OCR
Consistency of citation style: Using one recognised style throughout a piece of work, applied to every source.OCR
Consistent precision in recorded data: Tables should have consistent decimal places across a data column.OCR
constant region: The rest of the molecule, the same in all antibodies of a given class, which binds to receptors on phagocytes, marking the antigen-antibody complex for…AQA
Constant, readable scale choice: A simple step such as 1, 2, 5 or 10 units per major square, kept constant along the whole axis, makes intermediate values easy to read.OCR
continuation of glycolysis: Oxidised NAD returns to the cytoplasm and accepts hydrogen from the next triose phosphate, so the net two ATP per glucose keep being made.AQA
continuous variation: A smooth range of phenotypes, such as human height or mass, usually polygenic with many genes each having a small additive effect, plus environmental…AQA, OCR
Continuous versus discontinuous variation: Continuous variation varies smoothly across a range and is usually polygenic, whereas discontinuous variation places individuals into distinct categories and…OCR
Contrast versus magnification: Stains increase contrast, while magnification is provided by the lenses.OCR
Control of blood distribution by arterioles: Smooth muscle in the wall changes lumen diameter, so vasoconstriction and vasodilation direct blood to different capillary beds.OCR
Control of cell division: Mitosis is controlled by checkpoints and regulated rates, so proliferation occurs only where and when needed.OCR
Control of environmental variables: Light intensity, temperature, humidity and air movement all affect stomatal behaviour, so only one should be varied while others are held constant.OCR
Control of variables: Controlling confounding variables such as temperature, light intensity or handling so the response can be attributed to the stimulus.OCR
Control variables: Include time in the solution, volume of solution, cube dimensions and temperature when not the variable being tested.OCR
Controlled incubation: Incubating at a controlled, appropriate temperature so that growth is effective and the culture is not exposed to unnecessary contamination.OCR
Controlled stepwise release of energy: Energy is released from organic molecules such as glucose in a series of controlled enzyme-catalysed reactions rather than a single explosive step.OCR
Controlled variables: Include temperature, glucose concentration, yeast mass and pH, with a water bath keeping temperature constant because respiration is enzyme-controlled.AQA, OCR
Controlling variables with appropriate equipment: Applying correct technique to control a variable, such as maintaining temperature with a water bath.OCR
controls and control variables: A sterile water control disc is included, and a named variable such as disc size, volume of antimicrobial, incubation temperature and time, or the same…AQA
Convergent evolution: Organisms evolving similar features from different ancestors, so grouping by appearance alone can misrepresent descent.OCR
convergent similarity: Similar features that can evolve independently in unrelated organisms exposed to similar conditions, so they do not show close relatedness.AQA
cooperative binding: Binding the first oxygen molecule changes the haemoglobin's shape, making further binding easier, which gives the curve its sigmoid shape.AQA, OCR
Coordination of the cardiac cycle: Shown by an ECG trace, in which a normal trace has a P wave, a QRS complex and a T wave.OCR
coronary arteries: Vessels that branch from the base of the aorta and supply the cardiac muscle itself with oxygenated blood.AQA
Correct microscope set-up and focusing sequence: Begin on low power to locate and focus the specimen using the coarse focus, then switch to the high-power objective and refocus with the fine focus only.OCR
Correct reagent and conditions for each test: Each test needs the correct sample volume, reagent excess and, for Benedict's, a water bath at around 70–80 °C.OCR
Correct selection and use of dissection instruments: Set out the correct scalpel, scissors, forceps and mounted needle, use forceps to steady tissue rather than fingers, and pass instruments handle-first.OCR
Correct tabulation with units: Writing each value immediately into a pre-prepared table whose headings state the quantity and unit, with consistent decimal places or significant figures.OCR
Correct unit conversion direction: Performing unit conversions in the correct direction, for example multiplying by 1000 to convert mm to µm.OCR
Correct units and significant figures: Recording the value with its unit and rounding to an appropriate number of significant figures consistent with the resolution of the apparatus.OCR
Correct use of the light microscope: Placing the slide on the stage and securing it with clips, selecting the lowest-power objective first, adjusting the coarse focus to find the image, then…OCR
corrective response: A response produced by effectors in the opposite direction to the original change, returning the variable towards the set point.AQA
Correlation: Variables changing together, which must be distinguished from causation when analysing data on the incidence of cardiovascular disease.AQA
correlation against causation: A correlation between diet and type II diabetes does not by itself establish causation, because other variables may be involved.AQA
correlation not causation: A valid limitation is that a correlation between an environmental factor and expression does not show causation.AQA
counter-current flow: Water and blood flowing in opposite directions across the gill lamellae, maintaining a concentration gradient along the whole length of the lamella.AQA
Counter-current gas exchange in gills: A flow of water and blood across the lamellae in opposite directions that maintains a concentration gradient along the whole length, maximising oxygen…OCR
coupled reactions: The linking of ATP hydrolysis to energy-requiring reactions within cells, so energy is used directly rather than lost as heat.AQA
Coupling of electron transport to ATP synthesis: The flow of protons down their gradient provides the energy to phosphorylate ADP, forming ATP, which is chemiosmosis.OCR
cristae: Folds of the inner mitochondrial membrane that increase surface area so that more electron carriers and ATP synthase can be accommodated.AQA
Cristae and surface area for the electron transport chain: Folds of the inner membrane that increase the surface area available for the electron transport chain and ATP synthase.OCR
critical value: The value read from the row for your degrees of freedom and the p = 0.05 column, compared with the calculated value to determine significance.AQA, OCR
crossing over: In prophase I, homologous chromosomes form bivalents and sections of chromatid are exchanged at the chiasmata, producing chromosomes that carry new…AQA
Crossing over and independent assortment: Crossing over between non-sister chromatids of homologous chromosomes in prophase I exchanges alleles, while independent assortment of homologous chromosomes…OCR
Cyclins and cyclin-dependent kinases: Proteins whose concentration varies cyclically bind to and activate cyclin-dependent kinases, and the active complexes phosphorylate target proteins that move…OCR
Cytokinins: Can be used to delay leaf senescence, extending the shelf life of cut foliage and produce.OCR
cytoplasm: The location where glycolysis occurs, outside the mitochondrion, and where the process needs no oxygen.AQA
cytotoxic T cells: Cells that kill infected cells by releasing perforin, making the cell-surface membrane freely permeable.AQA

D

Data interpretation and evaluation: Quoting data points with units to describe trends before explaining them, and evaluating limitations such as small sample sizes or lack of statistical tests.AQA
Data logger: Uses sensors to record readings automatically at set time intervals, giving continuous data and reducing human error.OCR
Data presentation formats: A scatter graph is used for continuous variables, a bar chart with gaps between bars for discrete categories, and a ruled table for raw readings.OCR
Data presentation, rate calculation and evaluation of error: Recording readings at intervals, calculating a rate as change divided by time, repeating to reduce random error, and evaluating a source of error with an…OCR
Data recording and analysis: Taking replicates and recording data in a suitable table to allow a trend to be identified.OCR
DCPIP: A blue dye acting as an artificial electron acceptor in place of NADP, which turns from blue to colourless when reduced.AQA
Deamination and urea: The breakdown of excess amino acids in the liver produces urea, and removing it helps control the solute concentration of body fluids.OCR
Decapitation: Removing the shoot tip, which releases lateral buds so that they then grow.OCR
Decarboxylation: The loss of a carbon as carbon dioxide when pyruvate is converted, removing one carbon per pyruvate.OCR
decomposers: Bacteria and fungi that break down dead organisms and waste, releasing inorganic nutrients.AQA, OCR
decomposers and saprobionts: The chemical energy remaining in dead plants, faeces and urine passes to these, which secrete enzymes, digest material extracellularly and absorb the products.AQA
decreased histone acetylation: Acetyl groups are removed from the histones by histone deacetylase, the histones become more positively charged, bind the negatively charged DNA more…AQA
deep in the skin: Where the oval, onion-like corpuscle is found, along with joints, ligaments and tendons.AQA
Defining and measuring a response: Requires you to define the plant or animal response being measured and choose a measurable variable, such as growth angle, direction of growth or reaction time.OCR
definition of epigenetics: Heritable changes in gene function, without changes to the base sequence of DNA.AQA
deformation: The change caused by pressure applied to the corpuscle, which deforms the lamellae and stretches the membrane of the neurone ending.AQA
degeneracy: Because the code is degenerate, the new triplet may specify the same amino acid, in which case nothing changes at all.AQA
Degeneracy and unambiguity: Most amino acids are encoded by more than one codon, yet each codon specifies only one amino acid.OCR
Degeneracy of the code: Several codons can specify the same amino acid, so the amino acid sequence is often more reliably predicted than the exact base sequence is from the protein.OCR
Degeneracy of the genetic code: Because the genetic code is degenerate, a substitution may be silent and code for the same amino acid.OCR
degenerate code: More than one triplet (codon) codes for the same amino acid, so most amino acids are specified by more than one codon.AQA, Pearson Edexcel
degree of control: Greater when separate mechanisms actively pull the value back from either side, so corrections are faster and the variable stays within a narrower range.AQA
degrees of freedom: The number of phenotypic categories minus one, used to select the appropriate critical value.AQA, OCR
dehydrogenase activity: Measured by the rate of colour loss, where the faster the colour loss the greater the activity and the faster the light-dependent reaction.AQA
Dehydrogenation: The removal of hydrogen from pyruvate, with the removed hydrogen reducing NAD to reduced NAD.OCR
denaturation: At high temperatures or extreme pH, hydrogen and ionic bonds break, altering the tertiary structure so the active site is no longer complementary.AQA
Denaturation by temperature and pH change: Increased vibration or altered hydrogen ion concentration breaks hydrogen and ionic bonds in the tertiary structure, the active site changes shape, substrate…AQA
Dendrites: Structures of a neurone that receive impulses, while the axon transmits impulses away.OCR
denitrification: In anaerobic, waterlogged soils, denitrifying bacteria reduce soil nitrates into nitrogen gas, returning it to the atmosphere and reducing soil fertility.AQA
density gradient centrifugation: A method in which DNA is centrifuged in a density gradient, settling at a level matching its density, with heavier DNA settling lower in the tube.AQA
Dependence on the light-dependent stage: The stage occurs in the stroma and does not require light directly, but stops without the products of the light-dependent reactions.OCR
Dependent variable: The factor measured, such as oxygen bubbles per minute or volume of gas collected, from which rate is calculated.OCR
depolarisation: The membrane becoming depolarised to roughly plus 40 mV, so the inside becomes positive, once the threshold is reached.AQA
Destruction of helper T cells: HIV replication destroys these cells, leading to a significant fall in their numbers.AQA
Detritivores: Organisms that fragment dead material to increase surface area for microbial action.OCR
Development of body form: Body form arises because cells divide and die in controlled patterns, with mitosis supplying new cells and apoptosis sculpting structures.OCR
deviation from set point: A change in a variable that receptors detect, setting in motion a response that reverses that change.AQA
Diagnosis of AIDS: Requires a low helper T cell (CD4) count or an AIDS-defining illness, not simply the presence of antibodies.AQA
Dialysis versus transplant as treatments: Dialysis removes waste by diffusion down a concentration gradient across a partially permeable membrane, while a transplant replaces filtration and…OCR
diaphragm contraction and flattening: The diaphragm muscles contract, so the diaphragm flattens from its domed shape.AQA
diastole: All chambers relax, ventricular pressure falls below atrial pressure so the atrioventricular valves open and the ventricles fill passively, and ventricular…AQA
Diet, weight control and exercise: Controlled first by manipulating the diet, losing weight and exercising, with drugs and sometimes insulin added later.AQA
different alleles and phenotypes: Many different alleles produce many different phenotypes, so phenotypic variation results in differences in survival and reproductive success.AQA
different selection pressures: Each population faces different selection pressures, favouring different alleles.AQA
differential membrane permeability: Potassium ion channels are open at rest while the voltage-gated sodium ion channels are shut, so the membrane is far more permeable to potassium ions than to…AQA
Differential partitioning and relative affinity: Separation depends on differential partitioning: molecules with greater affinity for the stationary phase move more slowly than those with greater affinity…OCR
differential reproduction: Survivors must breed and pass their favourable alleles on, so differential reproductive success is the mechanism.AQA
Differential survival and reproduction: Individuals with characteristics better suited to the environment are more likely to survive and reproduce, passing on the advantageous alleles.OCR
differentiation: The process in which a cell transcribes only part of its DNA and translates only the mRNA produced from those genes, and the proteins produced determine the…AQA, OCR
diffusion at the alveolus: Oxygen diffusing from the alveolus into the blood and carbon dioxide diffusing the other way, each across the alveolar epithelium and the capillary endothelium.AQA
Diffusion distance: The distance from the outside to the centre of an organism, which increases as the organism becomes larger.OCR
Diffusion distance and cell position: Many plant cells are deep inside the plant, far from the surface, so the diffusion distance is too great.OCR
Digestion and absorption of products: Extracellular digestion breaks large food molecules into smaller soluble molecules that can be absorbed, while the enzyme remains in the gut lumen.OCR
dilution series: A series produced by calculating volumes of stock solution and distilled water so every tube has the same final volume.AQA
Dimensional consistency of units: A unit must match the measured quantity, such as a cubed unit for volume rather than an area unit.OCR
Dipeptide: A molecule containing two amino acids joined by one peptide bond.OCR
dipeptides: Formed by the condensation of two amino acids, with one peptide bond and one molecule of water released.AQA
Direct and indirect transmission routes: Direct routes include touching, body fluids and sexual contact, while indirect routes include droplets, airborne aerosols, faecal–oral contamination, vectors…OCR
direct DNA investigation: Gene technology has caused a change in the methods of investigating genetic diversity, with direct investigation of DNA sequences replacing inference from…AQA
Direct mRNA production: In prokaryotes, transcription results directly in the production of mRNA from DNA, without the need for splicing.AQA
directional selection: Individuals at one extreme of the range have the advantage, so the modal value shifts in that direction over generations and the whole distribution curve…AQA
disaccharide formation: Two monosaccharides join by condensation, forming a glycosidic bond and releasing water.AQA
Disaccharides and polysaccharides: Two monosaccharides joined by a glycosidic bond give a disaccharide; many monosaccharides joined this way give a polysaccharide.OCR
discontinuous variation: Distinct categories, such as ABO blood group or flower colour, usually controlled by one gene or a few genes with distinct alleles, with little environmental…AQA, OCR
discrete impulses: Each action potential is a separate event that cannot merge with or overlap the one before.AQA
disease: Removes the individuals whose immune systems cannot cope, so alleles giving resistance are favoured.AQA
disruptive selection: Favours both extremes and selects against the intermediates, so a single curve becomes two peaks.AQA
Dissection technique and safety: Use a scalpel or scissors on a dissecting board, cutting away from your body, and disinfect surfaces and wash hands afterwards to minimise infection risk.AQA
Distinguishing reasons from conservation methods: A reason explains why biodiversity matters, whereas a method explains how it is conserved, such as a nature reserve or a seed bank.OCR
Distribution versus abundance: Distribution describes where organisms are found across a habitat, while abundance describes how many there are.OCR
disulfide bridges: Covalent bonds between the sulfur-containing R groups of two cysteine residues, far stronger than the other two, so proteins rich in them resist heat and pH…AQA
Divergence through mutation and natural selection: Separated populations accumulate different mutations and experience different selection pressures, so allele frequencies diverge.OCR
division of the cytoplasm: Usually occurs after telophase, producing two new cells, and is not part of nuclear division itself.AQA
DNA base sequencing: The determination of the order of bases along the DNA.AQA
DNA double helix: Two antiparallel strands run in opposite directions and wind into a double helix, with the sugar–phosphate backbone on the outside and the paired bases on the…OCR
DNA helicase: Unwinds DNA and breaks the hydrogen bonds between complementary bases in the polynucleotide strands.AQA
DNA hybridisation: The binding of labelled DNA probes to complementary sequences after the DNA has been made single-stranded.AQA
DNA ligase: Catalyses the formation of phosphodiester bonds between adjacent nucleotides, sealing the backbones to produce a recombinant plasmid.AQA
DNA mass versus chromosome number: During the S phase the mass of DNA in the cell doubles, but the chromosome number remains the same.AQA
DNA methylation: Increased methylation of cytosine bases in the promoter inhibits transcription by preventing protein binding and condensing chromatin.AQA
DNA polymerase: Catalyses the condensation reaction that joins adjacent nucleotides by phosphodiester bonds, and its active site is complementary to only one end of the…AQA
DNA probes: A short, single-stranded DNA fragment with a base sequence complementary to the target allele, carrying a label.AQA
DNA replication errors: Mutations that might arise spontaneously during DNA replication, for example if DNA polymerase inserts an incorrect nucleotide, producing a substitution.AQA
DNA replication in S phase: During the synthesis phase of interphase, DNA is replicated so each chromosome becomes two sister chromatids before mitosis.OCR
DNA stain: A stain such as toluidine blue added to the sample so that chromosomes become visible for identifying stages of mitosis.AQA
DNA versus RNA nucleotides: In DNA the sugar is deoxyribose and bases are A, T, C, G; in RNA the sugar is ribose and uracil replaces thymine.OCR
DNA without histones: Prokaryotic DNA is not wound around histone proteins, so it is often described as naked.AQA
Domain: A higher, broader taxonomic rank than a kingdom, so the three-domain system sits above the kingdom level.OCR
domain to species: The order runs domain, kingdom, phylum, class, order, family, genus, species, and the eight names must be learned in sequence.AQA
dominant: An allele that is expressed whenever it is present, so the homozygous dominant and the heterozygote share a phenotype.AQA
double circulation: The pathway in which blood is traced through the right side to the lungs and the left side to the body.AQA, OCR
double helix: A DNA molecule is a double helix with two polynucleotide chains wound together.AQA
Double membrane and compartmentalisation: A smooth outer membrane and an inner membrane surround the mitochondrion, compartmentalising the reactions of respiration.OCR
downstream codons: Every codon after the mutation is regrouped from different bases and specifies a completely different sequence of amino acids.AQA
dry mass per unit area: The other measure of biomass, found by heating a sample to constant mass at a low temperature to remove variable water content.AQA
duration of protection: Active protection lasts for years, whereas passive protection is gone once the introduced antibodies are broken down, usually within weeks.AQA
Dwarf mutants and hormone-replacement experiments: Naturally occurring loss-of-function mutants combined with replacement by applied hormone provide causal evidence, separating the observation from the causal…OCR
Dynamic: Populations, species composition and physical conditions change over time, for example through succession or seasonal change.OCR
dynamic equilibrium: A state in which populations fluctuate around their carrying capacities rather than remaining fixed, held within limits by negative feedback.AQA

E

Ecological reasons for maintaining biodiversity: Reasons concerning the functioning of ecosystems, where diverse communities tend to be more stable and resilient and provide services such as pollination,…OCR
Economic reasons for maintaining biodiversity: Reasons concerning direct and indirect value to people, such as medicines derived from wild species, food crops and their wild relatives, timber, fisheries,…OCR
ecosystem: A community of organisms interacting with each other and with their non-living environment.AQA, OCR
ecosystem boundaries: What sets the boundary is the question being investigated, so the investigator chooses it rather than it being fixed in nature.AQA
Ecosystem management techniques: Specific methods such as coppicing, rotational grazing, quotas, mesh-size limits and replanting that limit extraction and protect regenerative capacity.OCR
Effect of missing cofactors on reaction rate: Without the cofactor the active site may be the wrong shape or the substrate cannot be chemically transformed, so the rate falls.OCR
Effect of mutations on protein function and production: A change in amino acid sequence alters tertiary structure and therefore protein function, while mutations in non-coding regions can affect protein production…OCR
Effect of substrate concentration on inhibition: Raising substrate concentration reduces the effect of a competitive inhibitor because more substrate molecules occupy the active sites, but adding more…OCR
Effect of temperature on enzyme-controlled respiration: Varying temperature using a water bath and allowing equilibration, recognising that enzyme denaturation causes a fall above an optimum.OCR
effectors: Animals use muscles or glands, whereas plants use elongating cells.AQA
Effects of human activities on ecosystems: Activities such as deforestation, overfishing, pollution, urbanisation and intensive agriculture can reduce biodiversity, degrade habitats and deplete…OCR
eight taxonomic levels: The hierarchy comprises domain, kingdom, phylum, class, order, family, genus and species, from the largest and most inclusive group down to the smallest.AQA
elastic recoil: The elastic tissue in an artery wall stretches as blood is forced in during ventricular systole and recoils during diastole, smoothing flow into a continuous…AQA
Elastic recoil and ventilation: Elastic fibres allow airways and alveoli to stretch during inhalation and recoil during exhalation.OCR
Elastin: Extensible and recoils, so it functions in tissues that must stretch and return, such as lung and artery walls.OCR
Electrical activity of the heart: An ECG records this as a trace of voltage against time, showing the rhythm and coordination of the cardiac cycle.OCR
Electrochemical gradient: The gradient created across the inner mitochondrial membrane stores potential energy.OCR
electron transfer chain: Electrons pass along a chain of electron carriers embedded in the membrane, losing energy at each transfer.AQA
Electron transport and chemiosmosis: Electron transport pumps protons into the thylakoid space, and their flow back through ATP synthase produces ATP by chemiosmosis.OCR
Electron transport chain: A series of electron carrier proteins embedded in the membrane to which NADH and FADH₂ deliver electrons.OCR
Electrophoretic separation by charge and size: Charged molecules such as amino acids or proteins migrate through a gel under an applied voltage, the direction depending on charge and the rate on charge and…OCR
ELISA: A test that uses the specificity of antibodies to detect and measure a molecule, performed in wells of a microtitre plate.AQA
Emphysema: Destroys alveolar walls, reducing surface area for diffusion.AQA
empty magnification: Magnifying beyond the resolving power, which produces a larger but blurred image revealing no new detail.AQA
emulsion test: A test for lipids that uses the fact that lipids dissolve in ethanol but not in water.AQA
Emulsion test for lipids: Add ethanol and shake, then pour into water; a cloudy white emulsion indicates lipid.OCR
Encoding stimulus intensity: A larger stimulus produces a larger generator potential and a higher frequency of impulses, encoding intensity.OCR
Endocrine signalling: Chemical signalling that is generally slower and longer-lasting than nervous signalling, which uses neurones and neurotransmitters.OCR
endocytosis and the phagosome: The phagocyte membrane extends around the pathogen, engulfing it into a vesicle called a phagosome.AQA
endopeptidases: Enzymes named for their site of action, acting on internal peptide bonds.AQA
endopeptidases and exopeptidases: The former hydrolyse peptide bonds inside the chain, creating shorter peptides with more ends, while the latter remove terminal amino acids.AQA
endosymbiotic theory: The idea that these organelles evolved from free-living prokaryotes engulfed by ancestral eukaryotic cells, supported by their shared features.AQA
energy flow: Light is captured by producers, passes along food chains and is lost as heat at every transfer, so the system needs a constant input.AQA
energy flows and matter cycles: Nutrients are recycled within ecosystems, whereas energy flows through and is lost.AQA
energy for reduction: The additional energy needed for the reduction of glycerate 3-phosphate comes from the hydrolysis of ATP made in the light-dependent reaction.AQA
energy per gram: The high proportion of carbon-hydrogen bonds means about twice as much energy per gram is released as from carbohydrate when oxidised.AQA
energy per gram of dry mass: Calculated by dividing the energy released by the dry mass of the sample, usually given in kJ g-1 and scalable to kJ m-2 for an area.AQA
Energy value of respiratory substrates: The energy released when a substrate is oxidised, compared per gram or per unit mass, since equal masses do not yield equal energy.OCR
Enucleation: The removal of an egg cell's own nucleus so that the somatic nucleus can be transferred in.OCR
environment and gene expression: Environmental factors alter epigenetic tags, and these changes can be maintained during DNA replication and passed to daughter cells.AQA
Environmental and social factors increasing transmission: Overcrowding, poor sanitation and climate raise the risk of transmission.OCR
Environmental causes of variation: Variation arising from differences in conditions such as diet, temperature or exposure, and it is not heritable.OCR
environmental factors: Conditions during development and later life, including light intensity, temperature, humidity, the supply of mineral ions, diet and disease.AQA
Environmental influences: Diet, temperature, light and other conditions that influence how genes are expressed or how an organism develops.OCR
environmental trigger: A change in the environment that causes the altered methylation or acetylation.AQA
enzyme activation: Binding of adrenaline causes the activation of enzymes involved in the conversion of glycogen to glucose.AQA
Enzyme activity and temperature: Temperature affects enzyme-controlled reactions such as carbon fixation, so rate rises to an optimum then falls as enzymes denature.OCR
Enzyme catalysis: Polymerases catalyse synthesis and nucleases catalyse hydrolysis.OCR
Enzyme concentration and rate: Increasing enzyme concentration raises the rate proportionally, provided substrate is not limiting.OCR
enzyme specificity: Determined by the tertiary structure of the active site being complementary to the substrate.AQA
Enzyme specificity and active-site complementarity: The active site is specific because its shape and chemistry, including charged and hydrophobic residues, only fit particular substrates.OCR
Enzyme-catalysed formation of new strands: Enzymes catalyse the joining of nucleotides into a new strand along each template.OCR
enzyme-substrate complex: Formed when a substrate with a complementary shape and complementary charges binds to the active site.AQA
Enzyme–substrate complex formation: Both types lower activation energy by forming an enzyme–substrate complex at the active site, and the products are released.OCR
enzymes: Catalysts with active sites complementary to a substrate, such as amylase.AQA
Enzymes as biological catalysts: Biological catalysts that lower the activation energy of a reaction, allowing metabolic reactions to proceed at a rate sufficient for life.OCR
epigenetic silencing: The process by which parts of the genome are switched off, so progressively fewer genes remain available for expression and potency becomes restricted.AQA
epigenetics: Control that determines which eukaryotic genes are transcribed, without altering the DNA base sequence.AQA
epistasis: Occurs when one gene masks or modifies another, signalled by a 9:3:4, 12:3:1 or 9:7 ratio rather than independent assortment.AQA, OCR
equator alignment: Chromosomes line up along the equator, attached by their centromeres to spindle fibres running to both poles.AQA
Erythrocyte structure and function: The cell loses its nucleus and organelles, becomes packed with haemoglobin and adopts a biconcave shape, giving a large surface area for oxygen transport.OCR
Ester bond: The —COO— link formed between the hydroxyl group of glycerol and the carboxyl group of a fatty acid.OCR
Estimating real cell dimensions: Working in reverse from image size and magnification to find actual size, then sanity-checking that the answer is plausible for a cell or organelle.OCR
Estimating species richness and abundance: Sampling records which species are present and how many individuals of each there are in a limited number of small areas, rather than counting the whole…OCR
ethanol and carbon dioxide: In plants and yeast, pyruvate is decarboxylated to ethanal, releasing carbon dioxide, and ethanal then accepts hydrogen from reduced NAD, forming ethanol.AQA
Ethanol fermentation in yeast and some plants: Pyruvate is decarboxylated to ethanal, which is then reduced to ethanol, releasing carbon dioxide.OCR
Ethene: Used commercially to ripen fruit such as bananas and tomatoes during transport and storage.OCR
Ethical issues: The use of stem cells raises ethical issues, which is why the statement refers to potential uses.OCR
Ethical justification for using organisms: Treating organisms with care and justifying their use by using the minimum number, the least sentient suitable species and avoiding unnecessary pain or stress.OCR
ethical objections: Concerns such as using embryonic stem cells destroying an embryo, which some regard as a potential life.AQA
Eukaryotic cell diversity: Recognising cellular components across a range of eukaryotic cells, not just one example, and comparing animal, plant and fungal cells.OCR
eutrophication: A fixed sequence following the arrival of leached nitrate in rivers and lakes, where the extra nitrate and phosphate fertilise the water.AQA
evaluating both sides: Weighing points in favour and points against, then reaching a judgement.AQA
evaluating evidence: Weighing evidence and reaching a justified conclusion, not simply stating an opinion, using any data supplied and stating what further evidence would help…AQA
evidence and acceptance: Accumulating experimental evidence demonstrated that DNA, rather than protein, was responsible for bacterial transformation and viral infection, proving it…AQA
Evidence for classification: Shared characteristics, biochemical evidence such as DNA and protein sequences, and evolutionary relationships are used to place organisms in the correct taxon.OCR
Evolution: A change in allele frequency in a population over generations, not an individual organism changing during its lifetime.OCR
Evolution and biodiversity: Adaptation and selection are major factors in evolution and contribute to the diversity of living organisms.AQA
Evolutionary descent: The basis of new classification systems, which clarify relationships rather than relying only on observable or superficial similarity.OCR
Evolutionary relationships between species: Inferred from the overall degree of similarity across the genome, where greater similarity indicates a closer relationship and more recent common ancestry.OCR
Ex situ conservation methods: Methods that protect organisms away from their natural habitat, such as zoos, botanic gardens, seed banks and captive breeding programmes.OCR
Ex vivo: A delivery strategy in which cells are removed, modified, checked and returned.OCR
Exchange at capillaries and return of blood in veins: A single endothelial cell layer and narrow lumen give a short diffusion distance and large surface area for exchange, while valves prevent backflow of…OCR
Excitatory and inhibitory postsynaptic effects: Depolarisation of the postsynaptic membrane gives an excitatory effect (EPSP), while hyperpolarisation gives an inhibitory effect (IPSP).OCR
excitatory only at the neuromuscular junction: A neuromuscular junction is always excitatory, with no inhibitory version, whereas a cholinergic synapse between neurones may be excitatory or inhibitory.AQA
Excluding anomalous values from means: Anomalies are normally excluded from a mean, but you should state why.OCR
Excretion: The removal of metabolic waste products from the body, which keeps the internal environment within limits so metabolism and enzyme activity continue.OCR
Exocrine versus endocrine organisation of the pancreas: Most of the gland is exocrine tissue arranged as acini of enzyme-secreting cells draining into ducts, while small clusters of endocrine cells, the islets of…OCR
Exocytosis and ATP supply: Secretory vesicles fuse with the cell surface membrane, releasing the protein by exocytosis, with mitochondria supplying the ATP for these transport and…OCR
exons: Sections of a gene whose base sequence codes for amino acids, and they are expressed.AQA
exopeptidases: Enzymes named for their site of action, acting on terminal peptide bonds.AQA
Experimental versus established treatments: Many medical applications remain experimental, so the word potential matters.OCR
Exponential (log) phase: A phase in which, provided nutrients are plentiful and conditions suitable, the population doubles at a constant rate, giving a straight line on a logarithmic…OCR
Expulsive reflexes: Active processes such as coughing, sneezing, vomiting and diarrhoea that forcibly remove pathogens from the respiratory or digestive tract.OCR
extinction risk: Without genetic diversity, a population may contain no alleles conferring resistance or adaptation, increasing the risk of extinction.AQA
extracellular digestion: Digestion of dead organisms and waste outside the cell, carried out by hydrolytic enzymes secreted by saprobionts, whose products are then absorbed.AQA
Extracellular enzyme action: Enzymes are secreted from cells and act outside them, for example amylase and protease in the digestive tract.OCR
eyepiece graticule: A scale etched into the eyepiece whose divisions have no fixed value until they are calibrated.AQA

F

facilitated diffusion: Passive movement of substances down a gradient without ATP, using channel proteins or carrier proteins.AQA
FAD: Accepts hydrogen to become reduced FAD in the Krebs cycle, delivering electrons to the electron transport chain.OCR
faeces and excretion: Not everything eaten is digested, so a large fraction leaves the herbivore in faeces, and surplus amino acids are deaminated, losing energy in urine.AQA
faeces and urine F: The chemical energy lost to the environment in faeces and urine.AQA
falling surface area to volume ratio: As organisms grow larger, this ratio falls, driving adaptations that facilitate exchange.AQA
fast-twitch anaerobic fibres: Fibres that contract rapidly and forcefully but fatigue quickly, because they rely on anaerobic glycolysis and have few mitochondria and little myoglobin.AQA
favourable environment: The optimal environment in which a mobile organism is maintained by taxes and kineses, reducing predation or water loss.AQA
fertile offspring: Offspring that can themselves reproduce, so that alleles keep flowing between the parent populations and they share a single gene pool.AQA
fetal haemoglobin: Must take oxygen from maternal blood across the placenta, so it has a higher affinity than maternal haemoglobin.AQA
Fibrosis: Thickens alveolar walls, lengthening the diffusion pathway and reducing elasticity, so forced vital capacity falls.AQA
Fibrous proteins: Long, strand-like molecules whose amino acid chains run parallel along one axis and are held together by cross-links, so they are insoluble in water and…OCR
Fight-or-flight coordination: Adrenaline released from the adrenal glands under nervous stimulation raises heart rate and blood glucose during the fight-or-flight response.OCR
Filtration and reabsorption in the nephron: The processes by which the nephrons filter blood and reabsorb useful solutes effectively, both of which are reduced in kidney failure.OCR
First and second messengers: Adrenaline and glucagon are first messengers, while cyclic AMP is the second messenger that carries the signal inside the cell.AQA
Five-kingdom classification: Sorting organisms into Prokaryotae, Protoctista, Fungi, Plantae and Animalia using observable and cellular features.OCR
flagellum: A whip-like structure that rotates to propel the bacterium, allowing it to move towards nutrients.AQA
flattened and folded body forms: Body shape changes that keep cells close to the surface or increase surface area without significantly raising volume.AQA
Flow of organic molecules and ATP: The glucose made in photosynthesis is the respiratory substrate, and the ATP and intermediate compounds produced feed biosynthetic reactions.OCR
Fluid mosaic model: Describes membranes as a phospholipid bilayer in which proteins are scattered like tiles in a mosaic and can move laterally, giving fluidity.AQA, OCR, Pearson Edexcel
Fluorescent detection and size separation: Each terminating base carries a distinguishable label, so fragments of different lengths are separated by electrophoresis and read in order of size, giving…OCR
forensic comparison: Comparing VNTR band patterns from a crime scene with a suspect to establish a high probability of presence.AQA
Forensic, paternity and conservation applications: Uses include forensic identification from crime-scene samples, paternity and kinship testing, identifying tissue or blood samples, and monitoring genetic…OCR
Fossil record: Shows change over time and transitional forms linking ancestral and descendant groups, though the record is incomplete.OCR
founder effect: A situation in which a few individuals colonise a new area so that their allele frequencies become the starting point.AQA
four bases and twenty amino acids: DNA contains only four different bases, whereas proteins are built from twenty different amino acids, allowing a practically limitless variety of structures.AQA
four chambers: A mammal has two atria and two ventricles.AQA
four polypeptide chains: Each folded into its own tertiary structure and each holding one haem group containing an iron ion.AQA
Four-carbon intermediate: The compound that a two-carbon acetyl group joins to form a six-carbon compound, and which is regenerated so the cycle can repeat.OCR
fragment assembly: Computer software finds the overlaps between reads and assembles them back into the complete sequence.AQA
frame shift: A change in the nature of all base triplets downstream from the mutation.AQA
frameshift: Caused when the number of bases changed is not a multiple of three, altering every subsequent triplet.AQA
Frameshift and its consequences: Insertion or deletion of bases can cause a frameshift, shifting every subsequent codon and usually producing a very different, often non-functional protein.OCR
fructose: One of the common monosaccharides named in the specification.AQA
functional protein: A protein that may contain one or more polypeptides, and may also include non-protein prosthetic groups.AQA
functional RNA: RNA molecules that are never translated and whose job is done by the RNA itself, including ribosomal RNA and transfer RNAs.AQA
Furanose ring: A five-membered ring formed by ribose, using four carbons plus one oxygen.OCR

G

Gamete formation and segregation: Gametes are shown each carrying a single allele, reflecting segregation during meiosis, and are combined systematically in a Punnett square or cross diagram.OCR
gametes: In sexual diploid organisms, offspring receive one allele per gene from each parent via these.AQA
Gametes versus spores: In animals meiosis produces gametes directly, whereas in plants and many algae meiosis produces spores that develop into a haploid gametophyte.OCR
Gas exchange between photosynthesis and respiration: The oxygen released in photosynthesis is used in respiration, and the carbon dioxide released in respiration is used in photosynthesis.OCR
gel electrophoresis: A process that separates DNA fragments, which are negatively charged due to phosphate groups and so migrate through the gel towards the anode, with the gel…AQA
Gel electrophoresis and fragment size separation: Fragments are separated by gel electrophoresis; shorter fragments travel further, so band positions reflect fragment lengths.OCR
Gene: A sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain.Pearson Edexcel
gene and environment interaction: The way environmental conditions affect how genes are expressed and how the body develops.AQA
gene control of the cell cycle: Genes control the cycle, some coding for proteins that stimulate the cycle to continue and others for proteins that halt it.AQA
gene flow: Individuals or gametes moving between populations, so that alleles are shared and the two gene pools stay similar.AQA, OCR
gene mutation: A change in the base sequence of the DNA in a chromosome, producing a new allele of that gene.AQA
gene pool: A single shared set of alleles, which organisms share when they can interbreed and their offspring are fertile.AQA
general structure of an amino acid: A central carbon atom bonded to an amine group, a carboxyl group, a hydrogen atom and a variable R group.AQA
generator potential: The local depolarisation produced, which is graded: the greater the pressure, the more channels are deformed, the more sodium ions enter and the larger it is.AQA, OCR
Genetic biodiversity: The variety of alleles and genes within a species or population.OCR
genetic bottleneck: A situation in which a population crashes and the survivors carry only part of the original variety.AQA
Genetic causes of variation: Variation arising from mutation, meiosis (crossing over and independent assortment) and random fertilisation, and it is heritable.OCR
genetic continuity: The inheritance of identical genetic information from parent to daughter cells during mitosis, preserving essential genetic information across cell generations.AQA
Genetic continuity and stability: Because chromosomes are copied and divided equally, the genotype is maintained, giving continuity and stability of the genome.OCR
genetic counselling: Provides advice following screening, taking a family history, drawing a pedigree and explaining the results to enable an informed decision.AQA
genetic diversity: The number of different alleles of each gene in a population.AQA, OCR
genetic drift: A change in allele frequency caused by chance rather than by selection, where only a sample of each generation's alleles is passed on.AQA, OCR
Genetic identity of clones: Both routes give clones because the nuclear genetic material is identical, and SCNT can clone an existing adult whereas twinning clones an embryo that already…OCR
Genetic modification of microorganisms: Bacteria and fungi can be genetically modified to produce human proteins such as insulin, allowing microbes to make products they would not naturally produce.OCR
genetic relationships: Judged from the number of shared bands, as closely related individuals share more bands.AQA
genetic screening: Using labelled DNA probes to screen patients for heritable conditions, drug responses or health risks.AQA
Genetic sources of variation: Different alleles and gene mutations, which are inherited and set the range of possible phenotypes.OCR
Genetic uniformity and its consequences: Identical, uniform crops with no genetic variation, so a single pest or disease can devastate a whole clone, and pathogens accumulate over generations.OCR
genetic variation: Crossing over in prophase I and independent assortment in metaphase I generate genetic variation among the products, giving offspring a new allele combination.AQA, OCR
Genetic variation as raw material for natural selection: Members of a species differ genetically, providing the variation on which natural selection can act.OCR
Genetic variation within populations: Populations show variation in characteristics, arising from mutation and, in sexual reproduction, from meiosis and random fertilisation.OCR
genetically identical daughter cells: Cells produced by mitosis that have the same chromosome number as the parent cell and are genetically identical to it, barring mutation.AQA, OCR
genome: The complete set of genes in a cell or organism, not simply all the DNA, as eukaryotic DNA includes non-coding regions outside of genes.AQA
genome sequencing: Reads the base sequence of DNA directly, and sequences can be compared base by base.AQA
Genotype: The genetic constitution of an organism; the specific alleles it possesses for a trait.Pearson Edexcel
Genotype frequencies: p² for homozygous dominant, 2pq for heterozygous and q² for homozygous recessive, summing to 1.OCR
genotype versus environment: Monozygotic twins share their alleles, so phenotypic differences between them are largely attributed to environmental factors, though somatic mutations and…AQA
genus: The first word of the binomial, which begins with a capital letter and carries classification information.AQA
genus and species: The genus groups closely related species, while the species epithet identifies one species within it; the genus comes first and the species second.AQA, OCR
Germ-line therapy: Would alter gametes or embryos and is not used clinically in humans.OCR
Gibberellin: Promotes stem elongation and can break seed dormancy, triggering germination.OCR
Gibberellin and seed germination: Gibberellin-deficient seeds fail to germinate, and adding gibberellin triggers germination, including in some light-requiring seeds where it substitutes for…OCR
Gibberellin and stem elongation: Dwarf short-stemmed mutants show reduced stem elongation, and applying gibberellin to dwarf plants restores elongation, showing the hormone causes the growth.OCR
Gibberellins: Applied to promote stem elongation and to break seed dormancy, for example in malting barley or to increase grape size.OCR
Glucagon and α cells: Cells that detect a fall in blood glucose and secrete glucagon, which acts mainly on liver cells to stimulate glycogenolysis and gluconeogenesis, raising…OCR
Gluconeogenesis: The production of glucose from non-carbohydrate sources, mainly glycerol and amino acids, stimulated by glucagon and increasing as glycogen stores fall during…AQA
glucose: One of the common monosaccharides named in the specification.AQA
glycerate 3-phosphate: Two molecules of this three-carbon compound are produced when carbon dioxide reacts with ribulose bisphosphate.AQA
glycerol and three fatty acids: A triglyceride is made from one molecule of glycerol and three molecules of fatty acid.AQA
Glycogen structure: A polymer of α-glucose with 1,4 links and more frequent 1,6 branches, giving a compact store.OCR
Glycogenesis: The conversion of glucose into glycogen for storage, occurring in liver and muscle cells when blood glucose is high and insulin is present.AQA
Glycogenesis and glycogenolysis: Processes by which the liver controls blood glucose.OCR
glycogenolysis: The breakdown of glycogen to glucose in the liver, stimulated by both glucagon and adrenaline, which raises blood glucose concentration.AQA
Glycolysis and its net ATP yield: Occurs in the cytoplasm, where glucose is converted to pyruvate with a net gain of 2 ATP and 2 reduced NAD.OCR
glycoproteins and glycolipids: On the outer surface, act in cell recognition, receptors and as antigens.AQA, OCR
glycosidic bond: A bond formed by a condensation reaction between two monosaccharides, in which a single oxygen atom bridges the two sugar rings.AQA
glycosidic bonds: The bond formed between monosaccharides when water is released, and broken when water is added.AQA, OCR
glycosidic, ester and peptide bonds: Bonds hydrolysed in carbohydrates, triglycerides and proteins respectively, each with its own class of enzyme.AQA
Golgi apparatus: Modifies, packages and transports proteins and lipids, forming vesicles and lysosomes.AQA, OCR
gradient: A steeper concentration or water potential gradient increases the rate of net movement.AQA
Gradient from a line of best fit: The change in y divided by the change in x, taken from the line of best fit, giving units of y-unit per x-unit.OCR
Graph interpretation: Reading a rate-versus-factor graph to identify the plateau and state which other factor is then limiting.OCR
Growth and cell replacement: Roles of mitosis in increasing cell number in multicellular organisms and in repairing and replacing damaged or worn-out cells and tissues.OCR
growth and reproduction: What net primary production is available for, being incorporated into organic molecules like cellulose, proteins and lipids.AQA
growth, repair and asexual reproduction: The biological roles of mitosis, including replacement of worn-out cells.AQA

H

habitat: Where a species lives, rather than its role or position.AQA
Habitat destruction, fragmentation and pollution: These reduce the variety of species and habitats able to survive in an area.OCR
Habitat diversity: The range of different habitats present in an area.OCR
haem group and iron: Each haem group binds one oxygen molecule, so one haemoglobin molecule carries four oxygen molecules.AQA
Haemoglobin affinity: How readily oxygen binds at a given pO₂; fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin.OCR
haemoglobin and antibodies: Proteins with a quaternary structure of four polypeptides, each with a haem group in haemoglobin, or with variable regions forming specific antigen-binding…AQA
Halving of chromosome number: Meiosis produces haploid cells from a diploid parent cell, so the chromosome number is halved.OCR
Hardy–Weinberg principle: Models allele and genotype frequencies in an idealised population that is large, randomly mating, with no mutation, no migration and no selection.OCR
Hazard identification and control: Identifying biological, chemical and physical hazards when using living organisms and stating a control for each, such as gloves, eye protection, disinfection…OCR
health risk alleles: Alleles associated with increased probability of a condition, such as BRCA1 mutations.AQA
Heart anatomy: The external and internal structure of the mammalian heart, learned as a labelled three-dimensional organ rather than a flat diagram.OCR
Heart rate and rhythm interpretation: Reading the trace by counting QRS complexes in a time interval to find heart rate and measuring the distance between successive R waves to judge rhythm…OCR
heat loss from respiration: Much of the assimilated energy is lost as heat to the surroundings during respiration.AQA
heavy isotope labelling: Growing bacteria in a medium with nitrogen-15, so all DNA bases contained it, before transferring cells to a medium with ordinary nitrogen-14.AQA
helper T cells: Activated cells that release cytokines to stimulate phagocytes and B cell division.AQA
Hepatic portal vein: Vessel that brings nutrient-rich blood from the gut to the liver.OCR
Hepatocytes: Cells arranged around a central vein, which perform deamination and glycogen storage.OCR
herd immunity: If a large enough proportion of people are immune, an infected person is unlikely to meet a susceptible one, so transmission is interrupted and the pathogen…AQA
Herd immunity and epidemic prevention: High uptake reduces the number of susceptible hosts so the pathogen cannot spread easily, which can prevent epidemics and protect people who cannot be…OCR
heritable characteristics: Characteristics coded for by alleles, which is why they can be selected for.AQA
Heritable variation: Variation in the trait that must already exist in the population before selection can act.OCR
Heterozygosity: The proportion of individuals heterozygous at a locus, calculated as heterozygous individuals divided by total individuals sampled at a locus.OCR
heterozygous: Describes an organism with two different alleles at the same locus on homologous chromosomes.AQA
heterozygous carrier: A heterozygote for a recessive allele, so two of them having children allows a recessive characteristic to reappear.AQA
Hexose: A sugar with six carbon atoms and the molecular formula C₆H₁₂O₆, of which glucose is an example.OCR
hierarchy: A set of groups nested inside larger groups, like boxes within boxes, with no overlap between groups.AQA
Hierarchy of protein structure: The levels in which the shape of a protein arises, from the primary sequence through secondary and tertiary folding to quaternary association of chains.OCR
high and low pressure vessels: Arteries carry blood at high pressure away from the heart, while veins carry blood at low pressure back to the heart.AQA
High latent heat of vaporisation: Much energy is needed to evaporate water, so sweating or transpiration cools organisms effectively.OCR
High specific heat capacity: Large energy changes cause only small temperature changes, so water buffers temperature in organisms and habitats.OCR
high-throughput methods: Modern automated methods in which very large numbers of small DNA fragments are sequenced in parallel by machines.AQA
Histology of exchange surfaces: The tissue layers of the exchange surface shown on the slide, such as epithelium, capillaries or air spaces.OCR
histone acetylation: Adding acetyl groups to histones reduces their positive charge, weakening their attraction to DNA's phosphate groups and loosening chromatin to stimulate…AQA
histone proteins: Basic, positively charged proteins that the negatively charged phosphate groups of the DNA backbone are attracted to, so the DNA coils tightly around groups…AQA
Homeobox (Hox) genes: Genes that encode transcription factors which bind DNA and switch target genes on or off, setting up regional identity along the anterior–posterior axis.OCR
Homeostasis: Coordinated communication allows the maintenance of a stable internal environment despite external change.OCR
Homeostasis and the internal environment: Keeping the internal environment within limits so enzymes and cells function, helped by removing wastes that disturb pH or solute balance.OCR
Homeostatic control of core temperature and blood pH: Core temperature and blood pH are homeostatic variables kept within narrow limits by negative feedback.AQA
homologous chromosomes: The two chromosomes of a pair that carry the same genes in the same order at the same loci, even though the alleles at those loci may differ.AQA
homologous pairs: Pairs carrying the same genes at the same loci, one from each parent, though alleles may differ and sex chromosomes may differ in size.AQA
Homologous structures: Structures that indicate shared ancestry because they share a common basic structure despite different functions.OCR
homozygous: Describes an organism with two identical alleles at a specific locus on homologous chromosomes.AQA
Honest treatment of anomalous results: Recording anomalous readings and flagging them rather than silently discarding them, so they can be evaluated during analysis.OCR
Hormone detection in urine: Hormones and their breakdown products, such as hCG in pregnancy testing, can be detected in urine to diagnose a condition.OCR
Hormone secretion into the blood: Islet cells pass their hormones straight into the blood, so they lie close to capillaries rather than to ducts.OCR
Hormone sensitivity versus hormone concentration: Dwarfism can result from reduced sensitivity to gibberellin as well as low hormone, so level alone is not the whole story.OCR
Hormones: Chemical messengers secreted by endocrine cells directly into the blood plasma, not into ducts.OCR
Hormones and target-cell receptors: Hormones travel in the blood to target cells with complementary receptors, so they can affect many cells throughout the body, whereas nervous impulses act on…OCR
Host cell destruction: New particles assemble and bud from the cell, acquiring their lipid envelope and destroying the helper T cell.AQA
host machinery used: The host's machinery replicates the nucleic acid, and the host's ribosomes translate viral mRNA into viral proteins.AQA
Human Genome Project: Mapped the vast majority of the human genome between 1990 and 2003.AQA
Human management of evolutionary change: Human practices such as antibiotic prescribing or pesticide rotation can be managed to slow the evolution of resistance.OCR
Human population growth: Rising numbers increase demand for land, food, water and energy, so habitats are destroyed or degraded and biodiversity falls.OCR
Human-directed choice of breeding parents: Humans identify individuals with the desired phenotype and use them as parents, rather than the environment doing so.OCR
Humane handling and housing: Keeping organisms in suitable conditions during measurement so they are handled humanely.OCR
Humid conditions: Warm and humid surroundings, often under a plastic bag or propagator, that reduce water loss by transpiration while roots develop.OCR
humoral response: The response of B lymphocytes to a foreign antigen, clonal selection and the release of monoclonal antibodies.AQA
humus: Material formed when dead remains are decomposed by saprobionts, which holds water and releases mineral ions.AQA
hydrogen acceptors: Pyruvate is the hydrogen acceptor in animals, while ethanal is the hydrogen acceptor in plants and yeast.AQA
Hydrogen bonding: The δ⁺ hydrogen of one water molecule is attracted to the δ⁻ oxygen of a neighbouring molecule, and each water molecule can bond with up to four others.OCR
hydrogen bonds: Form between the slightly positive and slightly negative parts of the backbone in secondary structure, and between polar R groups in tertiary structure; each…AQA
hydrogen bonds between chains: Many hydrogen bonds form between parallel cellulose chains, forming microfibrils that give tensile strength to the cell wall.AQA
Hydrogen content and reduced coenzymes: Lipids contain proportionally more hydrogen and less oxygen, so more hydrogen is removed and passed to NAD and FAD, producing more reduced coenzymes.OCR
hydrogen ions and pH: Hydrogen ion concentration determines pH, and higher concentrations lower pH, altering ionic and hydrogen bonds in an enzyme's tertiary structure so the…AQA
hydrolysis: The reverse of condensation, in which water is added across each ester bond, splitting the triglyceride back into glycerol and three fatty acids.AQA, OCR
hydrolysis reaction: Breaks a chemical bond between two molecules and involves the use of a water molecule.AQA
Hydrolysis reactions: The reverse process in which water is added and the glycosidic bond breaks, releasing the monosaccharide units.OCR
hydrolytic enzymes: Secreted onto the substrate and include proteases, lipases, carbohydrases and nucleases.AQA
hydrophilic head: The phosphate head is charged and polar, so it interacts with water.AQA
hydrophilic heads and hydrophobic tails: The phosphate head faces the water on each side of the membrane while the two fatty acid tails point inwards away from it.AQA
Hydrophobic and amphipathic properties: Triglycerides are non-polar and hydrophobic, while phospholipids are amphipathic with a hydrophilic phosphate head and hydrophobic fatty acid tails.AQA
hydrophobic core as a barrier: The tails point inwards away from the water, creating a hydrophobic core that prevents water-soluble, polar or charged substances from crossing the membrane.AQA
hydrophobic tails: The two remaining fatty acid tails are non-polar, so they are repelled by water.AQA
hydrophobic versus amphipathic: A triglyceride is non-polar and hydrophobic, whereas a phospholipid has a hydrophilic phosphate-containing head and hydrophobic tails, so it is amphipathic…AQA
Hydrophyte: A plant living in or on water, gaining buoyancy and rapid gas exchange through air spaces in stems and leaves, stomata on upper leaf surfaces and poor root…OCR
hydrostatic pressure: At the arteriole end it is high enough to force water and small solutes out through the gaps, forming tissue fluid.AQA
Hydrostatic pressure gradients: Loading sucrose at the source lowers water potential so water enters by osmosis, raising pressure, while unloading at the sink causes water to leave and…AQA
Hydrostatic versus osmotic forces: At the arterial end, hydrostatic pressure from the heart's pumping is higher than the opposing osmotic pull of plasma proteins, giving net filtration outward.OCR
hypermethylation of tumour suppressor genes: The promoter region of a tumour suppressor gene becomes hypermethylated, so it is not transcribed and the protein that would slow the cell cycle or trigger…AQA
Hypersensitive response and localised cell death: Localised cell death that isolates the pathogen after entry.OCR
hypothalamus: The region that regulates body temperature, water balance and hunger, and links the nervous and endocrine systems via the pituitary gland.AQA, OCR
Hypothalamus and posterior pituitary: The hypothalamus detects water potential changes and produces ADH, while the posterior pituitary secretes it into the blood.AQA
Hypothalamus–pituitary link: The hypothalamus links the nervous and endocrine systems by releasing hormones that control the pituitary gland.OCR

I

IAA: Indoleacetic acid, the growth factor the specification names, produced in the meristems at shoot tips and root tips.AQA
Identical twins: Clones of each other that form when a single fertilised egg divides and the two embryos develop separately.OCR
Identification using standards and standardised conditions: Because Rf depends on the solvent and conditions, comparisons must use the same solvent and temperature, so known standards are run alongside samples.OCR
Identifying respiratory substrates: Using the RQ value to identify which respiratory substrate is being respired, comparing it with reference values of about 1.0 for carbohydrate, 0.7 for lipid…OCR
identifying transformed cells: Showing that the plasmid has been taken up, since only a small fraction of host cells take up a recombinant plasmid.AQA
immediate energy source: The role ATP serves for biological processes, releasing energy in small, manageable amounts.AQA
Immobilised enzymes as reusable catalysts: Catalysts fixed to a solid support or trapped in a matrix so they stay in place while substrate flows past, allowing the enzyme to be recovered and reused.OCR
immune rejection and antigens: Donated cells carry foreign antigens and may be rejected, whereas the patient's own adult or iPS cells carry the patient's own antigens, avoiding rejection.AQA
immunological evidence: Antibodies bind to similar proteins from another species, and the more precipitate formed, the closer the relationship.AQA
In situ conservation methods: Methods that protect organisms in their natural habitat, such as nature reserves, national parks, marine protected areas and habitat restoration.OCR
in vitro and in vivo: In vitro means outside a living organism, while in vivo means inside a living organism.AQA
In vitro exponential amplification: A method that copies a target DNA sequence outside a living organism, doubling the target each cycle so that n cycles give 2ⁿ copies.OCR
In vivo: A delivery strategy in which the vector is introduced directly, for example an adenovirus carrying a functional allele to lung cells.OCR
incompressible skeleton: The bones are incompressible, so the pull is transmitted rather than absorbed and the force would otherwise simply deform the body.AQA
increase in dry mass: The rate of biomass accumulation that net primary production appears as, measured per unit area per unit time.AQA
increased DNA methylation: Methyl groups attach to cytosine bases in the promoter region, so transcription factors can no longer bind there and the DNA coils more tightly, so the gene…AQA
increased mutation rate: Mutagenic agents raise the rate at which gene mutation occurs, rather than causing a particular mutation.AQA
increased reproductive success: Individuals carrying an advantageous allele are more likely to survive and reproduce, producing more surviving offspring than those without it.AQA
increased unloading: The harder a tissue respires the more carbon dioxide it produces and the more oxygen haemoglobin releases to it, so the effect is self-regulating.AQA
independent and dependent variables: The environmental condition is the independent variable; the number of animals per compartment, or time taken in a maze, is the dependent variable.AQA
Independent assortment: In metaphase I bivalents line up on the equator and independent assortment of homologues occurs.OCR
independent loci: Loci that are inherited independently, so the separate probabilities at each locus multiply.AQA
independent plasmid replication: Plasmids replicate separately and independently, so their copy number is controlled by plasmid-encoded mechanisms rather than by the host cycle.AQA
independent segregation: Homologous chromosome pairs align randomly on the equator at metaphase I, and which chromosome goes to which pole is independent of other pairs.AQA
Independent variable: The factor that is changed, such as light intensity, carbon dioxide concentration or temperature, while the others are controlled.OCR
Independent, dependent and controlled variables: Varying one factor at a time while holding others constant, and stating which variable is independent, which is dependent and which are controlled.OCR
index of diversity: Describes the relationship between the number of species in a community and the number of individuals in each species.AQA
indoleacetic acid: A substance whose different concentrations affect cell elongation in the roots and shoots of flowering plants, explaining gravitropism and phototropism.AQA
induced fit: A model in which the active site moulds itself around the substrate as it enters, becoming fully complementary.AQA
induced pluripotency: The reversal of the pattern of gene expression that made a cell specialised, reactivating genes associated with pluripotency so the cell can divide…AQA
Induced-fit model of enzyme action: The active site is not a rigid shape but moulds around the substrate as it binds, so the substrate and active site become complementary.OCR
Ineffectiveness against viruses: Antibiotics do not affect viruses, so misuse for viral illness is ineffective and wastes the drug.OCR
inference from phenotype: Investigations used to infer DNA differences indirectly from measurable or observable characteristics, which is unreliable because such characteristics are…AQA
Inflammation: Part of wound repair in which damaged cells release histamine, causing vasodilation and increased permeability of capillaries so more phagocytes reach the site.OCR
informed consent: Human trials expose volunteers to risk, and seriously ill patients may find it hard to give genuinely informed consent.AQA
informed decision: A choice enabled by counselling that provides information about the risk of passing on a condition rather than dictating choices.AQA
ingested energy I: The chemical energy store in ingested food.AQA
inheritance of alleles: Offspring receive one allele per gene from each parent, and heterozygous parents pass the allele to half their offspring on average.AQA
inherited and non-inherited variation: Only genetic differences are passed on to offspring, whereas environmentally caused characteristics cannot be inherited.AQA
inhibiting DNA replication: Some drugs prevent DNA replication during S phase by resembling or damaging nucleotides, stopping DNA polymerase from building a complementary strand.AQA
Inhibitory synapses and hyperpolarisation: Inhibitory synapses release neurotransmitters that open Cl⁻ and K⁺ channels, causing hyperpolarisation, preventing threshold being reached.AQA
initiating transcription: Begins when RNA polymerase, and in eukaryotes the necessary transcription factors, bind to the promoter region.AQA
inorganic ions: Charged particles that contain no carbon-hydrogen bonds and occur in solution in the cytoplasm and body fluids of organisms.AQA
insertional inactivation: Disrupting a second marker gene, which distinguishes recombinant plasmids from empty plasmids because the desired gene disrupts the second marker.AQA
insoluble and osmotically inactive: Being large and insoluble means there is no osmotic effect, so water potential is not lowered and water does not enter by osmosis.AQA
insulin and glucagon: Secreted by different cells of the islets of Langerhans, they have opposite effects, with one lowering blood glucose and the other raising it.AQA
Insulin and β cells: Cells of the islets of Langerhans that detect a rise in blood glucose and secrete insulin, which increases glucose uptake by cells, stimulates glycogenesis…OCR
Insulin injection versus oral administration: Insulin must be injected rather than swallowed because it is a protein and would be hydrolysed by proteases in the gut.AQA
Insulin resistance: Body cells become less responsive to insulin even though insulin is still produced, often linked to obesity and lifestyle.OCR
Insulin resistance and receptor sensitivity: Insulin is still produced and released, but target cells respond poorly because their receptors are fewer, faulty or less sensitive.AQA
Insulin therapy and route of administration: Treatment for Type 1 diabetes in which insulin is commonly injected because insulin is a protein digested in the gut.OCR
interactions between polypeptides: Hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between R groups, not peptide bonds.AQA
interbreeding: Members of the same species can do this to produce fertile offspring, keeping alleles flowing between them and giving them a shared gene pool.AQA
Intercepts and axis origin: The value where the line of best fit crosses the chosen axis, read at the true zero position if the axis does not start at zero.OCR
Intermembrane space and proton accumulation: The space lying between the outer and inner membranes, where protons accumulate during chemiosmosis.OCR
internal environment: The blood and the tissue fluid that bathe the cells, including their temperature, pH, water potential and the concentrations of glucose and ions.AQA
International conservation agreements: Formal commitments between countries that coordinate action across national borders to protect species and habitats.OCR
interphase: The stage in which the nucleus is intact with a visible nucleolus and no distinct chromosomes, and which is an active period of growth and DNA replication.AQA, OCR
Interphase and S phase: The stage of the cell cycle during which DNA replication occurs, specifically in the S (synthesis) phase, strictly before mitosis begins.AQA
Interpretation of stained sections and photomicrographs: Recognising tissue features from images, where the stain makes structures visible, and using observed features such as nuclear position and fibre arrangement…OCR
Interpreting D values: A value close to 1 indicates high biodiversity; a value close to 0 indicates low biodiversity.OCR
interpreting data: Reading a trace, flow diagram or passage about an unfamiliar system and working out which kind of feedback it shows rather than recalling it.AQA
Interpreting data physiologically: Explaining what measurements or changes show about the function being carried out, giving the biological reason rather than only reporting data.OCR
interpreting expression data: Describing the pattern using figures quoted from the graph or table, including units, then explaining it using a named mechanism of gene expression.AQA
Interpreting pressure–time graphs of the cardiac cycle: Identifying systole and diastole and the points at which the atrioventricular and semilunar valves open and close.OCR
Interpreting rate of reaction graphs: Compare initial rate and maximum rate: competitive inhibition raises the substrate concentration needed to reach a given rate but can still reach the same…OCR
interspecific competition: An interaction in which two species needing the same resource both grow more slowly, and the better-adapted species may exclude the other through competitive…AQA, OCR
intracellular and extracellular enzymes: Enzymes catalyse a wide range of reactions inside and outside cells that determine structures and functions from cellular to whole-organism level.AQA
Intracellular and whole-cell movement: Movement of materials inside the cell, such as vesicles along microtubules, and movement of the whole cell or cell extensions such as cilia.OCR
Intracellular enzyme action: Enzymes catalyse reactions inside cells, working within the cytoplasm or organelles, for example catalase in peroxisomes or respiratory enzymes in mitochondria.OCR
intraspecific competition: Competition between members of the same species for the same resource, becoming more intense as the population rises.AQA
Intraspecific versus interspecific variation: Intraspecific variation is within a species, whereas interspecific variation is between species.OCR
Intrinsic (integral) proteins: Span the bilayer, often acting as channel or carrier proteins for transport.OCR
intrinsic and extrinsic proteins: Proteins scattered through the bilayer, with some spanning it and some sitting on one surface.AQA
introns: Non-coding sequences within a gene, lying between the exons.AQA
introns and exons: Non-coding sequences within genes are called introns, and they separate the coding sequences called exons.AQA
Iodine test for starch: Add iodine in potassium iodide solution; blue-black indicates starch, while orange-brown is negative.OCR
ion availability: Whether the ions a fertiliser contains can be taken up at once, which is slow and sustained for organic matter but immediate for inorganic salts.AQA
Ion charge and biological function: The charge and size of an ion can be linked to a named biological role, for example hydrogen ions control pH and enzyme activity.OCR
ion properties: The charge, size and chemistry of an ion, which determine its specific role.AQA
ionic bonds: Form between oppositely charged R groups, and a change in pH adds or removes hydrogen ions, alters those charges and breaks the bonds.AQA
ionising radiation: High-energy radiation, such as X-rays and gamma rays, which breaks DNA strands and disrupts the base sequence.AQA
ionising radiation and UV: Agents that damage DNA directly; ultraviolet light makes adjacent thymine bases bond together, distorting the helix so that replication mis-copies it.AQA
Ions as cofactors and structural components: Magnesium ions are the central ion in chlorophyll, and phosphate ions are components of ATP, DNA, RNA and phospholipids.OCR
Ions in pH regulation and buffering: Hydrogen ions determine pH, hydrogencarbonate ions act as a buffer in blood, and hydroxide ions contribute to alkaline conditions.OCR
iron ions in haemoglobin: Each of the four haem groups in a haemoglobin molecule holds one iron ion, binding one oxygen molecule, so one haemoglobin carries four oxygen molecules.AQA
Islets of Langerhans and their cell types: Small clusters of endocrine cells scattered among the acini, in which alpha cells secrete glucagon and beta cells secrete insulin.OCR
isomers of glucose: Alpha-glucose and beta-glucose have the same molecular formula but a different arrangement of atoms.AQA

J

justified conclusions: A conclusion that follows from the evidence discussed, for example that diet is likely to be one risk factor among several rather than the sole cause.AQA
justifying with figures: Quoting values, times and the direction of change from the data to support a conclusion, since the name on its own is not the mark point.AQA

K

Keratin: Contains many cysteine residues forming disulfide bridges, giving hardness and waterproofing in hair, nails and skin.OCR
kinesis: A non-directional response to a non-directional stimulus, involving changing the rate of movement or turning.AQA
Kinetic energy and collision frequency: Raising temperature increases the kinetic energy of enzyme and substrate molecules, so more successful collisions form enzyme-substrate complexes and the rate…AQA
kinks and packing: Each double bond puts a kink in the chain, so kinked unsaturated tails cannot pack as closely as straight saturated tails.AQA
Known standards: Substances run on the same chromatogram under identical conditions, with which the sample's Rf values are compared to identify it.OCR
Krebs cycle: Acetylcoenzyme A reacts with a four-carbon molecule, releasing coenzyme A and producing a six-carbon molecule that enters a series of oxidation-reduction…AQA

L

labelled antibodies: In medical diagnosis the antibody carries a label rather than a drug, such as an enzyme, fluorescent dye or radioactive marker, allowing the specific antigen…AQA
labelled DNA probes: Short, single-stranded DNA sequences complementary to a specific allele, each carrying a fluorescent or radioactive tag.AQA
labelling: The probe carries a radioactive phosphorus-32 tag detected on X-ray film, or a fluorescent tag visible under UV light.AQA
Lack of splicing: Bacteria cannot splice pre-mRNA, so they fail to produce functional proteins if given unmodified eukaryotic genes containing introns.AQA
lactate fermentation: In animals, pyruvate acts as the hydrogen acceptor from reduced NAD, forming lactate.AQA
Lactate fermentation in mammals: Pyruvate is reduced to lactate by lactate dehydrogenase, using the hydrogen from reduced NAD, and yields no extra ATP.OCR
Lag phase: A period in which cells are metabolically active but not yet dividing, synthesising enzymes and ribosomes.OCR
lamellae: Concentric rings of connective tissue separated by a viscous gel, forming the mechanical part of the receptor.AQA
large population: A condition that removes genetic drift, the chance sampling error that shifts frequencies when only a few individuals reproduce.AQA
large surface area: A feature of gas exchange surfaces that provides more space for diffusion, shown by branching tracheoles, folded mesophyll or numerous gill lamellae.AQA, OCR
large total surface area: The large area for diffusion provided by the large number of alveoli.AQA
Large-scale culture in fermenters: Microorganisms reproduce rapidly and can be cultured on a large scale in fermenters, with aseptic technique and controlled conditions such as temperature, pH…OCR
lateral movement of membrane components: Phospholipids, proteins, glycoproteins and glycolipids able to move laterally, giving the membrane flexibility to bend and fuse.AQA
leaching: The washing of soluble ions down through the soil by rain or irrigation water, mattering most for nitrate because every nitrate salt is soluble and nothing in…AQA
left and right shifts: A curve shifted to the left shows a higher affinity for oxygen, while a curve shifted to the right shows a low affinity.AQA
levels of biodiversity: Biodiversity is measured at more than one level, including the number of species in a community, the genetic variety within a species, and the range of…AQA
levels of classification: The largest group is the domain, and each level below it divides its members into smaller, more closely related groups until the smallest group, the species,…AQA
Levels of organisation: The hierarchy runs from cell to tissue to organ to organ system to organism.OCR
Levels of potency: The range of cell types a stem cell can become: totipotent, pluripotent, multipotent and unipotent.OCR
Lifestyle and drug management of Type 2 diabetes: Treatment that begins with lifestyle measures such as diet and exercise, then drugs such as metformin, which reduce blood glucose, with insulin possibly added…OCR
Lifestyle and obesity: Factors often linked to Type 2 diabetes, which is managed with diet, exercise and medication.OCR
Ligation into a vector: DNA ligase joins the gene into the vector to form a recombinant DNA molecule.OCR
Light absorption and electron excitation: Light absorbed by photosystems excites electrons in chlorophyll, which are passed along an electron transport chain.OCR
light intensity: A factor whose low level slows photoionisation, so less ATP and reduced NADP are made and less GP is reduced.AQA
Light versus electron microscopy: Light microscopes use visible light and give lower magnification and resolution but allow living specimens and colour, while electron microscopes use a beam…OCR
Light-dependent and light-independent stages: Light drives the light-dependent stage, producing ATP and reduced NADP, while carbon dioxide supply feeds the light-independent stage via rubisco-catalysed…OCR
Light-dependent stage on thylakoid membranes: This stage happens on the thylakoid membranes, where photosystems, electron carriers and ATP synthase are embedded.OCR
limited NAD pool: A cell holds only a small pool of this coenzyme, so unless reduced NAD is unloaded again the pool is quickly exhausted and glycolysis stops.AQA
limiting factors: Whichever factor is in shortest supply at any moment, setting the rate of photosynthesis: light intensity, carbon dioxide concentration or temperature.AQA, OCR
limits of a sample: Do not overclaim from one gene or a small sample, since one gene may give a different answer from the rest of the genome.AQA
lineage splitting: Speciation is always this rather than a species appearing from nothing, because every species alive today descends from an earlier species.AQA
linear DNA: Very long DNA molecules in the nucleus of eukaryotic cells that have two ends rather than forming a loop.AQA
Lines and curves of best fit: An appropriate line or curve of best fit shows the trend, recognising scatter, rather than joining every plotted point dot-to-dot.OCR
Link between resource use and biodiversity: Human activities can reduce biodiversity and deplete resources, and management aims to limit these effects.OCR
link reaction: Pyruvate is oxidised to acetate, producing reduced NAD, and acetate combines with coenzyme A to produce acetylcoenzyme A.AQA
linkage: Genes at different loci on the same chromosome tending to be inherited together rather than assorting independently, with closer loci giving stronger linkage.AQA
Linking a measured substance to organ function: Interpreting a diagnostic result requires comparing the measured concentration with a normal range and linking any deviation to a specific organ or process.OCR
Lipase: The enzyme that catalyses the hydrolysis of triglycerides during digestion.OCR
Lipid synthesis: Triose phosphate is converted to glycerol and fatty acids, which combine to form lipids.OCR
Liver as a glucose buffer: The liver acts as a glucose buffer through glycogenesis, glycogenolysis and gluconeogenesis, with glucose released by the liver entering the blood and…AQA
Liver as effector organ: The key organ on which insulin and glucagon act, where glucose is converted to glycogen and glycogen is broken down to glucose.OCR
liver as target organ: The main target of glucagon, because hepatocytes carry many glucagon receptors.AQA
Local conservation agreements: Agreements made at a smaller scale, such as a wildlife trust managing a reserve with a landowner or a community protecting a hedgerow or pond.OCR
local currents: Sodium ions spreading sideways inside the axon from the depolarised point, depolarising the adjacent patch of membrane to threshold.AQA
Local versus long-distance signalling: Signalling can be local, for example a neurotransmitter diffusing across a synapse, or long-distance, a hormone carried in the bloodstream.OCR
locating alleles: The sample DNA is amplified by PCR, heated to break hydrogen bonds, the labelled probe added and cooled, unbound probes washed away, and the label's presence…AQA
Location of glycolysis in the cytoplasm: Glycolysis happens in the cytoplasm, not inside a mitochondrion, so it needs no organelle membranes.OCR
lock and key: An older model in which the active site is a rigid shape that exactly matches the substrate.AQA
locus: The fixed position that a gene occupies on a particular DNA molecule, and therefore on a particular chromosome.AQA
Longitudinal section: A cut made parallel to the long axis of the stem, exposing elongated vessel elements running up the stem.OCR
Loop of Henle: Establishes a solute gradient in the medulla, enabling osmotic water reabsorption.OCR
loss of potency: The narrowing of potential as development continues, explained by progressively more genes being switched off and no longer transcribed.AQA
Losses between trophic levels: Energy and matter lost through respiration, egestion, excretion and parts not consumed, with much energy lost as heat.OCR
low oxygen environments: An organism that struggles to load oxygen because the partial pressure of oxygen is low needs haemoglobin with a high affinity and a curve shifted to the left.AQA
Lowering activation energy: Globular protein catalysts lower the activation energy of a reaction, speeding it up.OCR
lumen diameter: A narrow lumen relative to wall thickness keeps pressure high, whereas a vein has a wide lumen.AQA
Lungs: The paired organs situated within the thoracic cavity that house the branching network of bronchi, bronchioles and alveoli.AQA
lymphatic return: Excess tissue fluid drains into the lymphatic system and returns to the blood.AQA
Lysis and crenation in animal cells: Lacking a cell wall, animal cells in a higher water potential solution swell and may burst (lysis), and in a lower water potential solution they shrink and…OCR
lysosomes: Organelles that fuse with the phagosome and release hydrolytic enzymes (lysozymes) that digest the pathogen.AQA, OCR
lysosomes and lysozymes: The lysosome is the organelle (vesicle); lysozyme is the hydrolytic enzyme it releases, destroying the ingested pathogen by hydrolysis.AQA

M

magnification: How many times larger the image is than the real object, calculated as size of image divided by size of real object; it is a ratio with no units.AQA, OCR
Magnification and actual size: Using the stated magnification or scale bar to estimate the actual size of a named organelle.OCR
Magnification and scale: Include a magnification or scale bar where appropriate to indicate size.AQA
Magnification calculation: Total magnification is calculated by multiplying eyepiece magnification by objective magnification.OCR
Magnification formula: The relationship magnification = image size ÷ actual size, which must be used and rearranged to find whichever quantity is required.OCR
maintaining a concentration gradient: Keeping the oxygen concentration in the alveolus higher than in the blood arriving in the pulmonary capillaries, and the carbon dioxide concentration lower.AQA
Maintaining concentration gradients: A steep concentration gradient is maintained by ventilation, by blood flow, or by active transport, so substances keep moving across.OCR
Maintaining pure cultures: Aseptic technique keeps cultures pure when working with microorganisms in both agar plates and broth.OCR
maltose, sucrose, lactose: Maltose is two glucose molecules, sucrose is glucose and fructose, and lactose is glucose and galactose.AQA
managed succession: Deliberately arresting or deflecting succession so that the species adapted to the open stages are not outcompeted and lost.AQA
Management strategies such as quotas and protected areas: Strategies such as quotas, protected areas, rotation of extraction or regulation of abstraction that aim to make use sustainable.OCR
many mitochondria: More of these organelles means more ATP released in aerobic respiration, for active transport or for movement.AQA
many ribosomes: More of these, or extensive rough endoplasmic reticulum, means rapid synthesis of protein, often for export.AQA
Margin of error: The range within which the true value is expected to lie.OCR
mark-release-recapture: Capturing a first sample, counting and marking the individuals harmlessly, releasing them, allowing time for mixing, then capturing a second sample.AQA
mark-release-recapture calculation: Using given data in the formula (number in first sample × number in second sample) divided by number marked in second sample to estimate population size.AQA
Mark-release-recapture for mobile animals: An estimate of population size using the Lincoln index: estimated population = (number marked in first sample × total in second sample) ÷ number of marked…OCR
marker genes: Genes inserted into the vector and used to detect genetically modified cells or organisms.AQA
Mass flow: The accepted mechanism of translocation, driven by a pressure gradient, moving sap from source to sink.OCR
Mass flow hypothesis: Explains translocation as bulk flow driven by a hydrostatic pressure gradient, with sucrose actively loaded at the source and unloaded at the sink.AQA
mass of carbon: One way biomass can be measured, per given area, useful as carbon forms the backbone of organic molecules and removes variation from differing mineral contents.AQA
mass transport systems: Developed by larger organisms to carry substances between the exchange surface and the cells.AQA
Matching citations to sources actually used: A claim taken from a page of a textbook is cited to that page, not to a different book.OCR
Matching graph type to data type: Continuous data suit a line graph or scattergraph with a continuous scale, while categoric or discrete data suit a bar chart.OCR
Matching instrument to measurement: Choosing appropriate instrumentation means matching the instrument to the quantity being measured and the precision required.OCR
Matching method to ecosystem and consistent recording: Quadrats suit plants and slow-moving organisms, sweep nets and pitfall traps suit invertebrates and kick-sampling suits streams, while consistent recording…OCR
Matching technique to agar plates versus broth: Agar plates provide a solid surface for colonies while broth is liquid for suspension growth, so the correct procedure differs in detail.OCR
Matching units to apparatus and scale: Choosing a prefix and magnitude appropriate to the apparatus, for example mm for a leaf length measured with a ruler.OCR
maternal inheritance: Mitochondrial DNA is typically inherited maternally via the egg's cytoplasm, so mitochondrial conditions often pass from mothers to offspring.AQA
Matrix containing Krebs cycle enzymes, DNA and ribosomes: The gel-like interior enclosed by the inner membrane, containing enzymes of the Krebs cycle, mitochondrial DNA and ribosomes.OCR
Maximising product yield: Manipulating growing conditions keeps the microorganism growing efficiently rather than being limited by a depleted nutrient or a harmful condition,…OCR
maximum impulse frequency: The ceiling on frequency set by the refractory period, so a shorter refractory period allows a higher maximum frequency of impulses.AQA
Measurement of specimen size: Measurements are taken by counting graticule divisions across the specimen and multiplying by the calibration factor, giving a real size in micrometres.OCR
Measuring cylinder: A piece of laboratory glassware that suits approximate volumes.OCR
Measuring respiration rate via gas exchange: Following respiration rate by the rate of oxygen uptake or by the rate of carbon dioxide release.OCR
mechanical stability: Support provided to membranes, especially in animal cells lacking a cell wall, which must withstand being squeezed through capillaries.AQA
Mechanism of breathing: Muscles alter the volume of the thorax, creating pressure changes that move air in and out of the lungs.OCR
Medical and therapeutic applications: Stem cells may be used to replace damaged or lost tissues, for example bone marrow transplants to restore blood cells, or potential therapies for repairing…OCR
medulla oblongata: The region at the base of the brain that controls involuntary homeostatic processes such as heart rate, breathing rate and blood pressure.AQA, OCR
Meiosis: A source of genetic variation through independent assortment of chromosomes and crossing over between homologous chromosomes, creating new allele combinations.OCR
Meiosis and gamete formation: Produces haploid gametes, halving the chromosome number so gametes carry one allele of each gene.OCR
membrane fluidity: A property regulated by cholesterol, which prevents the membrane from becoming too fluid at high temperatures and from becoming too rigid at low temperatures.AQA
Membrane structure and the phospholipid bilayer: The phospholipid bilayer and membrane proteins are disrupted by higher temperature, ethanol concentration or detergent concentration, which increases…OCR
membrane-bound disaccharidases: These sit in the cell-surface membrane of the epithelial cells on the microvilli of the ileum, completing digestion to monosaccharides at the site of…AQA
memory B and T cells: Cells that secrete no antibody at all but simply persist, circulating in the blood for years.AQA
memory cell formation: The antigens trigger a primary response: clonal selection, division by mitosis and differentiation into plasma cells and, crucially, memory cells.AQA
memory cells: Formed when a B cell with a complementary antibody is selected and clones after the antigen triggers a primary immune response, so a later infection meets a…AQA, OCR
meristems: The growing regions at shoot tips and root tips where cells are actively dividing and growth factors are produced.AQA
Meselson and Stahl: Scientists who tested DNA replication models by growing bacteria in a medium with the heavy isotope nitrogen-15, then transferring cells to a medium with…AQA
mesophyll and stomata: Spongy mesophyll gives a large internal surface area, while stomata are pores allowing gases to diffuse in and out of the leaf.AQA
messenger RNA: A linear copy of one gene, made in the nucleus by transcription, long enough to carry the codons for a whole polypeptide but tiny beside a chromosome.AQA
Metabolic demand: The need of active tissues, especially in warm-blooded animals with high respiration rates, for oxygen supply and carbon dioxide removal.OCR
Metabolic waste products: Wastes produced by reactions inside cells, for example carbon dioxide from respiration and urea from the breakdown of excess amino acids in the liver.OCR
metaphase: The stage in which chromosomes or chromatids align at the equator, also called the metaphase plate.AQA, OCR
metastasis: Malignant cells can break off and spread in the blood or lymph to form secondary tumours.AQA
Method choice: Depends on the question: random for an overall estimate across an area, non-random for change along a gradient or patchy species.OCR
methyltransferase and HDAC inhibitors: Drugs that inhibit DNA methyltransferase reduce methylation of tumour suppressor genes so they are expressed again, while histone deacetylase inhibitors…AQA
micelles: Formed when bile salts associate with monoglycerides and fatty acids, and transport these products to the epithelial membrane.AQA
microfibrils: Straight beta-glucose chains of cellulose hydrogen bond to each other in parallel to form microfibrils and then fibres.AQA
Microorganisms as antibiotic sources: Microorganisms such as the fungus Penicillium and bacteria of the genus Streptomyces are sources of antibiotics.OCR
Microorganisms in fermentation: Bacteria and fungi, such as yeast, are used in processes such as cheese and yoghurt production, where a named process is linked to the microorganism…OCR
Micropropagation: Tissue culture producing many genetically identical plantlets from a small explant by mitosis.OCR
microscopy limitations: Electron microscopy requires a vacuum, meaning specimens must be dead, and involves complex preparation that can create artefacts.AQA
microvilli: Finger-like projections of the cell-surface membrane that increase the surface area of cells specialised for absorption, such as the epithelial cells lining…AQA
microvilli and villi: Provide a vast surface area for absorption in the ileum.AQA
milky-white emulsion: Millions of tiny droplets suspended through the liquid that scatter light, so the mixture turns cloudy.AQA
mineral ions: Ions required by plants to synthesise proteins and nucleic acids, with nitrate ions absorbed by roots combined with organic acids to form amino acids.AQA
Mitochondria: Have a double membrane with folded cristae and carry out aerobic respiration, releasing ATP.OCR
Mitochondrial matrix: The site in the cell where the link reaction takes place, after pyruvate made in the cytoplasm is moved into the mitochondrion.AQA, OCR
mitosis: The part of the cell cycle in which a eukaryotic cell divides to produce two daughter cells, each with identical copies of DNA produced during DNA replication.AQA
Mitosis and cytokinesis: Mitosis separates the replicated chromosomes into two nuclei, and cytokinesis divides the cytoplasm to form two separate cells.OCR
mitosis versus meiosis: Mitosis gives two genetically identical diploid cells and meiosis four genetically different haploid cells.AQA
mitotic index: Calculated as the number of cells in mitosis divided by the total number of cells counted, multiplied by 100.AQA
mitotic phase: The much shorter part of the cycle, consisting of nuclear division by mitosis followed immediately by cytokinesis.AQA
Mixtures of substrates: A value between the reference ranges indicates a mixture of substrates being respired.OCR
Model cells: Cubes of agar jelly containing a pH indicator such as phenolphthalein, set with dilute alkali so the jelly is pink, used to measure diffusion without living…OCR
Modification in the Golgi: The Golgi apparatus modifies the protein, for example by adding carbohydrate, and packages it into secretory vesicles.OCR
moist permeable surface: A surface permeable to oxygen and carbon dioxide that is kept moist so that gases dissolve before diffusing.AQA
Moist surfaces and blood supply: In animals, an efficient moist surface and a good blood supply help gas exchange, with gases dissolving before diffusing.OCR
Molecular evidence: Comparisons of ribosomal RNA and other conserved gene sequences revealed that prokaryotes fall into two very distinct groups.OCR
molecular evidence for classification: Comparing DNA base sequences, mRNA base sequences or amino acid sequences to determine evolutionary relationships between diverse species.AQA
monoculture: Replaces mixed communities with a single species, reducing available niches.AQA
Monohybrid inheritance: A cross shown in a genetic diagram where parental genotypes are stated and gametes are combined to give offspring genotypes and phenotypes.OCR
monomer: A smaller unit from which larger molecules are made, named together with the larger molecule it builds.AQA
Monomer structure: A nucleotide has three components: a pentose sugar, a phosphate group and a nitrogenous base.OCR
monosaccharides: Examples of monomers, such as glucose, which join by glycosidic bonds to make disaccharides and polysaccharides.AQA
Monosynaptic: Describes a reflex such as the knee-jerk in which the sensory neurone synapses directly onto the motor neurone, so no relay neurone is involved.OCR
monozygotic twins: The exception to a vanishingly improbable match at every locus, because their DNA is identical.AQA
motor end plate: The folded sarcolemma of a muscle fibre, which carries far more receptors so a single action potential is normally enough to make the muscle fibre contract.AQA
mRNA hydrolysis: Once the complex is attached, the enzyme in it cuts the mRNA into fragments, which are then hydrolysed in the cytoplasm.AQA
mRNA sequences: Comparing the base sequence of mRNA works similarly to DNA, as mRNA is complementary to the DNA template strand.AQA
Mucous membranes: Membranes that line body openings and secrete sticky mucus that traps pathogens, which ciliated cells then move away from the lungs or other surfaces.OCR
Mucus, cilia and airway defence: Ciliated epithelium and goblet cells trap particles in mucus and move it away from the lungs, and the nasal cavity warms, moistens and filters incoming air.OCR
multinucleate fibre: One very long cell formed when many cells fused, with a shared cytoplasm called sarcoplasm containing many mitochondria and an extensive sarcoplasmic reticulum.AQA
multiple repeats: Between genes there are non-coding multiple repeats of base sequences.AQA
multipotent: Cells found in mature mammals that can divide to form a limited number of different cell types.AQA
murein cell wall: A cell wall that contains murein, a glycoprotein.AQA
muscles pull only: A muscle shortens when it contracts and generates force, but it cannot push itself back to its original length.AQA
mutagens: Mutagenic agents that increase the mutation rate above the natural spontaneous rate.AQA
Mutation: A change to the base sequence, usually arising during DNA replication, occurring at random and independently of whether the change would be useful.AQA, OCR, Pearson Edexcel
mutation comes first: Variation is already present, arising from mutation, and the pathogen is the selection pressure, not the source of the allele.AQA
mutation in the pathogen: A change in the base sequence of the pathogen's DNA or RNA, which changes the sequence of amino acids in the antigen.AQA
mutualism: A relationship in which the fungus gains organic compounds, principally sugars from photosynthesis, and the plant gains increased uptake of water and…AQA, OCR
mycorrhizae: Mutualistic associations between fungal hyphae and plant roots, whose fine hyphae grow far out into the soil and add an enormous absorbing surface area to the…AQA
myelin sheath: The many layers of the Schwann cell's plasma membrane around the axon, rich in lipid and acting as an electrical insulator.AQA
myelination and nodes of Ranvier: Provides electrical insulation, so that in myelinated neurones depolarisation only occurs at the nodes of Ranvier.AQA
myogenic contraction: Cardiac muscle contracts rhythmically without stimulation by nerve impulses.AQA
myoglobin and mitochondria: Myoglobin stores oxygen and makes slow fibres look red, while many mitochondria support aerobic respiration and resistance to fatigue.AQA
myosin head cocking: Hydrolysis of ATP to ADP and Pi releases energy that recocks the myosin head before the next power stroke.AQA

N

NAD: Accepts hydrogen and electrons to become reduced NAD during glycolysis and the Krebs cycle, and is oxidised at the electron transport chain.OCR
NAD regeneration: Both pathways regenerate oxidised NAD, allowing glycolysis to continue.AQA
named bond formed: The chemical bond formed depends on the molecules involved, such as a glycosidic, peptide, ester or phosphodiester bond.AQA
Named stains and their targets: Iodine solution stains starch granules and is used to make plant cell contents visible, while methylene blue stains nuclei and other acidic components in…OCR
Nastic responses: Non-directional movements, often involving reversible turgor changes, for example leaf folding in Mimosa.OCR
Natural clones in plants: Asexual reproduction producing genetically identical offspring from one parent, each new plantlet arising by mitosis so it carries the same alleles.OCR
Natural cloning in animals: Cloning that occurs naturally, distinguished from sexual reproduction by whether genetic material from two parents is combined.OCR
natural fertilisers: Organic materials such as manure, slurry, compost and bone meal whose ions are not immediately available because saprobionts must first digest the organic…AQA
Natural immunity: Results from exposure that is not a medical intervention, such as infection or antibodies transferred from mother to fetus or infant.OCR
Natural selection: Changes allele frequencies when heritable variation affects survival and reproduction, as individuals with advantageous alleles reproduce more and pass those…OCR
Natural selection and allele frequency change: Evolution by natural selection changes allele frequencies in a population over generations.OCR
Natural selection mechanisms: Random mutation creates variation, a specific selection pressure acts on the population, individuals with advantageous alleles experience differential…AQA
negative feedback: A self-correcting system in which the change in glucose concentration triggers the corrective hormone response, and the restored level reduces further hormone…AQA, OCR
Negative feedback pathway: A receptor detects a stimulus, a coordination centre compares it with the set point, and an effector produces a response that reverses the change.OCR
Nervous and endocrine coordination: The nervous system provides rapid electrical coordination, while the endocrine system provides slower hormonal coordination.OCR
Nervous coordination by the hypothalamus: Detecting blood temperature and coordinating nervous and hormonal responses.OCR
Nervous versus endocrine transmission: The nervous system transmits electrical impulses along neurones and uses neurotransmitters at synapses, giving fast, short-lived, targeted responses, while…OCR
nested ecosystems: Small ecosystems sit nested inside larger ones, such as a rock pool within a rocky shore within a coastal ecosystem.AQA
Net ATP and pyruvate yield: Because two triose phosphates are processed, the net yield per glucose is 2 ATP (4 made minus 2 used), 2 reduced NAD and 2 pyruvate.OCR
net ATP gain: Two ATP are used initially and four are produced, giving a net gain of two ATP per glucose molecule.AQA
Net exchange and the compensation point: Net gas exchange depends on which process dominates, and at the compensation point the rates of photosynthesis and respiration are equal.OCR
net primary production: The chemical energy store in plant biomass after respiratory losses are subtracted from gross primary production.AQA
Neutrophil structure and function: Production involves retaining a lobed nucleus and forming many lysosomes and granules, enabling phagocytosis of pathogens.OCR
new combinations: What meiosis and sexual reproduction produce by recombining alleles that already exist, rather than creating alleles.AQA
new combinations of alleles: Because homologues carry different alleles, crossing over creates entirely new combinations of alleles on a single chromosome, increasing genetic diversity.AQA
niche: The role or position of a species within its habitat, governed by adaptation to both abiotic and biotic conditions.AQA
nitrate and phosphate uptake: Nitrate and phosphate enter root cells by active transport, and producers absorb these inorganic ions from soil or water.AQA
nitrification: Nitrifying bacteria oxidise ammonium to nitrite, then to nitrate, a process requiring oxygen that stops in waterlogged soil.AQA
nitrifying bacteria: Organisms that oxidise ammonium to nitrite and nitrite to nitrate, using the released energy, in a process needing well-aerated soil.AQA
Nitrogen: Present in proteins and nucleic acids but not a component of carbohydrates or triglycerides.OCR
Nitrogen cycle: The cycle involving nitrogen fixation, ammonification, nitrification and denitrification.OCR
nitrogen fixation: Nitrogen-fixing bacteria, free-living or mutualistic in legume root nodules, reduce atmospheric nitrogen gas to ammonia.AQA
nitrogen-fixing bacteria: Free-living or mutualistic bacteria in legume root nodules that reduce atmospheric nitrogen to ammonia using nitrogenase and ATP.AQA
Nitrogenous bases: Purine bases are adenine and guanine; pyrimidine bases are cytosine, thymine (DNA) and uracil (RNA).OCR
no membrane-bound organelles: Cytoplasm that lacks membrane-bound organelles, so respiratory enzymes are located on the cell-surface membrane instead.AQA
no migration: A condition that prevents alleles being imported or exported.AQA
no overlap: A species cannot sit in two genera, and a genus cannot be shared between two families, so at every level an organism has exactly one place.AQA
no spindle and no nucleus: No spindle forms and no nuclear envelope breaks down, because prokaryotes have no nucleus; the circular DNA is not wound around histones.AQA
Node: The point on a healthy parent plant just below which a shoot or stem cutting is taken with a clean, sharp blade.OCR
nodes of Ranvier: Short gaps between adjacent Schwann cells where the axon membrane is exposed to tissue fluid and voltage-gated sodium ion channels are concentrated.AQA
non-coding DNA: Much of the nuclear DNA in eukaryotes does not code for polypeptides, and exists between genes and within genes.AQA
non-coding DNA between genes: Multiple repeats of base sequences that lie between genes, so a change in repeat number often has no effect on a polypeptide.AQA
non-competitive inhibition: An inhibitor binds to an allosteric site, changing the active site shape so fewer enzyme-substrate complexes form; this inhibition is typically reversible.AQA
Non-competitive inhibitors: These bind at a site other than the active site, changing the shape of the active site so the substrate can no longer bind, and adding more substrate does not…OCR
Non-conducting tissue: A layer between atria and ventricles that prevents impulses passing directly, so the impulse is relayed via the AVN.OCR
non-disjunction: Failure of homologous chromosomes to separate at anaphase I, or of sister chromatids to separate at anaphase II, so both structures are pulled to the same pole.AQA
non-living and metabolism: Lacking independent metabolism and being unable to replicate without a host cell.AQA
non-overlapping and reading frame: Each base belongs to one triplet only and the bases are read in a fixed reading frame from a fixed starting point.AQA
non-overlapping code: The code is read in triplets from a fixed starting point, so each base belongs to one triplet only.AQA
Non-overlapping reading: Each base is read once, so bases are not shared between codons.OCR
Non-random sampling: Deliberate sampling where a belt or line transect follows a chosen gradient, and stratified or opportunistic placement targets selected zones.OCR
non-specific defence: An immune response that targets any pathogen, unlike the specific lymphocyte response.AQA
normal distribution: The result of plotting a continuous characteristic for a large sample, with most individuals near the mean and few at the extremes.AQA
NPP = GPP - R: The relationship in which respiratory losses are subtracted from gross primary production to give net primary production.AQA
nuclear envelope: The structure enclosing this DNA, containing pores.AQA
nuclear envelope breakdown and reformation: The nuclear envelope breaks down in prophase and reforms around each group of chromatids in telophase.AQA
nuclear pore: The structure through which an activated transcriptional factor passes on its way from the cytoplasm into the nucleus.AQA
nucleic acid entry: Viral nucleic acid enters the cell by injection or engulfment.AQA
nucleic acids: DNA and RNA are important information-carrying molecules, and both are polymers of nucleotides.AQA
nucleoid region: The region of a prokaryotic cell in which the main DNA lies free in the cytoplasm is sometimes called this.AQA
nucleolus: The part of the nucleus where ribosomal RNA is transcribed and combined with imported proteins to form subunits.AQA
nucleotide components: Three components joined together: a pentose sugar, which is deoxyribose in DNA and ribose in RNA, a phosphate group, and a nitrogen-containing organic base.AQA
nucleotide derivative: A single molecule of adenosine triphosphate is one of these, formed from ribose, adenine and three phosphate groups.AQA
nucleotide structure: Each nucleotide is made of a phosphate group, a deoxyribose sugar and a nitrogenous base.AQA, OCR
nucleotides: Examples of monomers which join by phosphodiester bonds to make the polynucleotides DNA and RNA.AQA
Nucleus: Contains chromatin and a nucleolus and controls the cell via transcription of DNA.OCR
null hypothesis: A statement that there is no significant difference between observed and expected results, written before any calculation so the conclusion has a clear basis.AQA, OCR
number of transport proteins: More channel proteins increase the rate of facilitated diffusion, and more carrier proteins increase the rate of facilitated diffusion or active transport.AQA
nutrient cycling: Nutrients such as carbon and nitrogen are cycled repeatedly between the abiotic reservoir, producers, consumers and saprobionts.AQA
nutrient recycling: Nutrients are recycled within natural ecosystems, as shown by the nitrogen cycle and the phosphorus cycle.AQA
Nutrition modes: Nutrition is varied in Protoctista, heterotrophic by absorption after extracellular digestion in Fungi, photosynthesis in Plantae, and heterotrophic by…OCR

O

Obtaining the gene of interest: The gene is obtained, for example by PCR from DNA or by reverse transcription of mRNA to make cDNA.OCR
oestrogen and breast cancer: Increased concentrations can stimulate mitosis in some breast cancers by stimulating transcription of genes that drive mitosis in breast tissue.AQA
oestrogen receptor: Part of a transcriptional factor held inactive in the cytoplasm, with a receptor site to which oestrogen binds.AQA
oncogene expression: Reduced methylation or increased acetylation of histones associated with the oncogene means it is transcribed more, so cell division is stimulated.AQA
oncogenes: A mutation can turn a proto-oncogene into one of these, whose protein is permanently active or overproduced, so the cell divides uncontrollably.AQA
one amino acid changed: Provided the new triplet still codes for an amino acid, the consequence is limited to a single amino acid changed.AQA
one in four risk: The probability per pregnancy that two heterozygous parents produce a homozygous recessive offspring.AQA
open systems: No ecosystem is closed, because organisms migrate across the boundary, energy enters as light and leaves as heat, and nutrients are carried in and out.AQA
Open versus closed systems: In a closed system blood is confined to vessels, whereas in an open system blood leaves vessels and bathes the tissues in a haemocoel.OCR
Opportunistic secondary infections: These cause the symptoms of AIDS rather than the direct action of HIV.AQA
opposing mechanisms: Separate mechanisms involving negative feedback that control departures in different directions from the original state.AQA
Opsonisation: Antibodies or complement proteins bind to the surface of a pathogen, marking it so phagocytes can recognise and engulf it more readily.OCR
optical vs electron microscopes: An optical microscope passes visible light through a specimen using glass lenses and cannot resolve small organelles, whereas electron microscopes use…AQA
Optimal conditions for cell function: Keeping conditions such as temperature, blood glucose and water potential within narrow limits so enzymes and cells function optimally.OCR
order of reagents: Ethanol is added to the sample first and shaken, then the mixture is poured into an equal volume of water.AQA
Order of substrate use in respiration: Proteins are typically used as a respiratory substrate only when carbohydrates and lipids are limited, and must be deaminated first.OCR
organ: Made of several different tissues working together to perform an overall function, such as the stomach or a leaf.AQA, OCR
organ system: A set of organs working together, such as the digestive system or the gas exchange and circulatory systems.AQA, OCR
Organelle cooperation: The coordinated pathway in which the nucleus, ribosome, rough endoplasmic reticulum, Golgi apparatus, secretory vesicles, cell surface membrane and…OCR
organelle membranes: The nuclear envelope, mitochondrion, chloroplast, endoplasmic reticulum, Golgi apparatus, lysosome or vesicle are built on the same basic plan as the…AQA
Organelle roles in plant cells: Photosynthesis occurs in chloroplasts and aerobic respiration occurs in mitochondria, both within the same plant cell.OCR
Organelles and ribosomes: Eukaryotic cells have membrane-bound organelles such as mitochondria and endoplasmic reticulum, which prokaryotic cells lack, and prokaryotic ribosomes are…OCR
organic compounds: Substances such as sugars that are synthesised from inorganic carbon sources and form the plant's biomass.AQA
organic molecules: Carbon-based compounds containing carbon bonded to hydrogen, built from four main groups: carbohydrates, lipids, proteins and nucleic acids.AQA
oscillation around a set point: The pattern of negative feedback in which the variable rises, a correction follows and it falls back around a steady level.AQA
osmoreceptors: Cells in the hypothalamus that monitor water potential; when blood water potential falls, water leaves them by osmosis and they shrink, triggering ADH release.AQA, OCR
Osmoregulation: Controls blood water potential; if it falls, water leaves osmoreceptors in the hypothalamus by osmosis, causing them to shrink.AQA, OCR
Osmoregulation and water potential: Control of water potential and blood volume that fails when water regulation is lost, disturbing the solute concentration of body fluids.OCR
Osmosis: The movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential.AQA, OCR, Pearson Edexcel
osmosis at the venule end: As hydrostatic pressure falls along the capillary, water re-enters down the water potential gradient.AQA
Overexploitation and intensive agriculture: These reduce species numbers by removing organisms or simplifying habitats.OCR
Oxidation and reduction of NAD: Each triose phosphate is oxidised, losing hydrogen that is picked up by NAD to form reduced NAD.OCR
oxidation to pyruvate: Each triose phosphate is oxidised to pyruvate, during which hydrogen is removed and accepted by NAD.AQA
oxidative phosphorylation: Synthesis of ATP associated with the transfer of electrons down the electron transfer chain and passage of protons across inner mitochondrial membranes,…AQA
Oxidative phosphorylation and ATP yield: More hydrogen removal produces more reduced NAD and reduced FAD, so more ATP is generated by oxidative phosphorylation.OCR
oxidised NAD: The form produced when reduced NAD gives up its hydrogen, which can be used in further glycolysis.AQA
Oxygen and nutrient supply in fermentation: Oxygen supply and nutrient concentration are adjusted to support the required growth rate and maximise formation of the desired product.OCR
Oxygen dissociation curve: A curve plotting percentage oxygen saturation of haemoglobin against partial pressure of oxygen (pO₂), with a sigmoid (S) shape.OCR
Oxygen dissociation curve and the Bohr shift: Affinity changes with pO₂ to give the S-shaped oxygen dissociation curve, and a high pCO₂ lowers affinity further, increasing oxygen release where respiration…OCR
Oxygen uptake: The volume of oxygen absorbed by the body per unit time, reflecting aerobic respiration demand.OCR
oxyhaemoglobin: Formed when haemoglobin associates with, or loads, oxygen in the lungs, and it dissociates and unloads its oxygen in respiring tissues.AQA

P

p squared + 2pq + q squared = 1: An equation for the genotype frequencies, where p squared is the homozygous dominant, 2pq the heterozygous and q squared the homozygous recessive.AQA
pair bond: Formed by courtship and supports the rearing of the young.AQA
Parallel high-throughput sequencing: DNA is fragmented, adaptors are ligated, and millions of clusters are sequenced in parallel, with fluorescent signals detected as each nucleotide is added.OCR
partial pressure of oxygen: High in the lungs so haemoglobin loads oxygen, and low in respiring tissues so oxyhaemoglobin dissociates and unloads its oxygen.AQA
Partition between phases: Pigments partition differently between a stationary phase (a thin silica layer on a plate) and a moving solvent, which is the mobile phase.OCR
Partition between stationary and mobile phases: Compounds in a sample spotted on the stationary phase travel different distances as the solvent moves up by capillary action, according to solubility and…OCR
passive immunity: Ready-made antibodies are received rather than produced, giving immediate protection that is short-lived because no memory cells are generated.AQA, OCR
Passive valve action governed by pressure differences: Valves open and close passively in response to pressure differences across them, not by active muscle contraction of the valve itself.OCR
Passive versus active transport: Facilitated diffusion moves polar molecules and ions down their gradient through carrier or channel proteins without ATP, whereas active transport uses…OCR
paternity testing: Justifying relatedness by matching each of the child's VNTR bands to either the mother or the alleged father.AQA
Pathogen: An organism or agent that causes disease and can be passed between hosts.OCR
pathogen identification: A medical diagnosis use, such as identifying specific strains of pathogens.AQA
pathogens: Organisms such as bacteria that carry foreign antigens.AQA
Pattern formation: Underlying process in which gene regulation, switching target genes on or off, controls where and when other genes are expressed.OCR
PCR amplification: A process in which primers flanking the repeats allow Taq polymerase to create millions of fragment copies whose lengths depend on the repeat number.AQA
peer review: Along with repetition by other scientists, needed before a model is accepted.AQA
Pentose: A sugar with five carbon atoms and the molecular formula C₅H₁₀O₅, of which ribose is an example.OCR
pentose sugar: In DNA it is deoxyribose and in RNA it is ribose.AQA
Pentose sugars: DNA nucleotides contain deoxyribose; RNA nucleotides contain ribose.OCR
peptide bond: The link formed between two amino acid residues, joining the carbon of one carboxyl group to the nitrogen of the next amine group.AQA, OCR
peptide bond formation: Peptide bonds form between adjacent amino acids, catalysed by the ribosome.AQA
peptide bonds detected: A positive result is produced by copper(II) ions forming a coloured complex with the nitrogen atoms of peptide bonds.AQA
percentage change in mass: Calculated for each cylinder because starting masses vary, and plotted against concentration.AQA
percentage cover: What is recorded where individual plants cannot be counted.AQA
percentage efficiency: Transfer between consumer trophic levels is typically around ten per cent, and roughly one per cent from light to producers.AQA
Percentage error calculation: Compares the uncertainty with the measured value: percentage error = (uncertainty ÷ measured value) × 100.OCR
percentage saturation: The value on the y-axis of the oxyhaemoglobin dissociation curve, plotted against partial pressure of oxygen.AQA
perforin: Released by cytotoxic T cells to make the infected cell's membrane freely permeable.AQA
Peripheral nervous system (PNS): The nerves that link the central nervous system to the rest of the body.OCR
Permeability and factors that disrupt membranes: Higher temperature, ethanol concentration and detergent concentration each increase permeability by disrupting the phospholipid bilayer and membrane proteins.OCR
personalised medicine: Uses genetic information to tailor treatments, matching a specific drug or dose to an individual's DNA genotype.AQA
Pesticide resistance in agricultural pests: Pesticide use selects resistant insects, reducing the effectiveness of chemical control in agriculture.OCR
pesticides: Kill non-target insects, disrupting food webs.AQA
pH and active-site ionisation: pH affects the ionisation of amino acids in the active site; each enzyme has an optimum pH, and extremes alter charges and disrupt the active site, reducing…OCR
pH and affinity: Hydrogen ions released when carbon dioxide forms carbonic acid lower the pH inside the red blood cell, and this fall in affinity means more oxygen is unloaded.AQA
Phagocytosis: The process by which phagocytes engulf and digest pathogens, with the cell-surface membrane invaginating to enclose the microorganism in a phagosome.OCR
phagocytosis of clumped bacteria: A phagocyte engulfs the clump, so many bacteria are destroyed at once, and the bacterium is hydrolysed by lysosomes.AQA
Phagosome: A vesicle formed when the cell-surface membrane invaginates to enclose the microorganism; lysosomes fuse with it and release hydrolytic enzymes.OCR
Phases of the cell cycle: An ordered sequence of interphase, consisting of G1, S and G2, followed by mitosis and cytokinesis.OCR
Phenotype: The observable characteristic of an organism, which can vary due to genetic factors, environmental factors, or both acting together.AQA, OCR, Pearson Edexcel
Phloem sieve tube structure and function: Elements that remain living, lose their nucleus and most organelles, and have sieve plates for translocation of assimilates.OCR
Phloem sieve tubes and companion cells: Structures that transport assimilates such as sucrose and amino acids to sinks.OCR
phosphate ions: These form the phosphodiester bonds in the sugar-phosphate backbone of DNA and RNA, and are added to ADP to form ATP.AQA
phosphate uptake: An increase in the uptake of inorganic ions, particularly phosphate, which diffuses very slowly through soil and is quickly depleted in the zone immediately…AQA
phosphodiester bond: Formed by a condensation reaction between two nucleotides.AQA
phosphodiester bonds: The bond links the phosphate group of one nucleotide to the pentose sugar of the next, forming a sugar–phosphate backbone with bases projecting from it.AQA, OCR
Phosphodiester bonds and backbone: Nucleotides join through the sugar–phosphate backbone, with phosphodiester bonds linking the phosphate of one nucleotide to the sugar of the next.OCR
phospholipid: A lipid with two tails and a hydrophilic phosphate-containing head, so it is amphipathic and forms a bilayer in water.AQA
phospholipid as membrane component: A molecule with only two tails and a hydrophilic phosphate-containing head, which in water arranges into a bilayer forming the basis of cell-surface and…AQA
phospholipid bilayer: A stable double layer about 7 nanometres thick with proteins embedded in it, formed because the molecules arrange themselves automatically in an aqueous…AQA, OCR
Phospholipid bilayer and partial permeability: A partially permeable barrier formed from a phospholipid bilayer with proteins, cholesterol and glycoproteins, allowing selective movement of substances.OCR
Phospholipid bilayer permeability: Small non-polar molecules such as oxygen and carbon dioxide diffuse directly through the phospholipid bilayer down a concentration gradient, while ions and…OCR
Phospholipid structure: One glycerol bonded to two fatty acids and one phosphate group, giving a hydrophilic phosphate head and two hydrophobic fatty acid tails.AQA, OCR
Phospholipids: Amphipathic molecules with a hydrophilic phosphate head and two hydrophobic fatty acid tails, so in water they form bilayers with tails inward.OCR
Phosphorus: Present in nucleic acids and in phospholipids, and not characteristic of amino acids.OCR
Phosphorylated nucleotides: Nucleotides in which the sugar carries phosphate groups, as in ADP and ATP, which are built on a nucleotide framework.OCR
Phosphorylation and splitting of glucose: Glucose, a 6-carbon sugar, is phosphorylated by two ATP molecules, then split into two 3-carbon triose phosphate molecules.OCR
phosphorylation of glucose: Glucose is phosphorylated to glucose phosphate using ATP, which makes the molecule more reactive and lowers activation energy.AQA
photoionisation: Chlorophyll absorbs light and loses an electron, which is raised to a higher energy level before leaving the molecule.AQA
photolysis of water: Water splits into protons, electrons and oxygen, and the electrons replace those lost from chlorophyll.AQA, OCR
Photomicrograph: An image taken through a microscope, which you must interpret by recognising cellular components across a range of eukaryotic cells.OCR
photosynthesis: The process during which plants synthesise organic compounds, primarily sugars like glucose, from carbon dioxide.AQA
phototropism and gravitropism: Responses in flowering plants explained by the effect of different concentrations of indoleacetic acid on cell elongation in roots and shoots.AQA
Phylogenetic classification: A modern system that uses shared ancestry as the basis for grouping rather than superficial resemblance.OCR
phylogenetic tree: A record of branching that shows the split related to common ancestry, not a ladder of improvement.AQA
Phylogeny: The study of evolutionary history and the lines of descent connecting organisms.OCR
Physical barriers to pathogen entry: Defences such as the waxy cuticle, cellulose cell walls, bark, and stomatal closure or guard-cell responses that limit entry.OCR
physiological adaptation: An internal process or chemistry of an organism, such as producing very concentrated urine or an enzyme that breaks down an antibiotic.AQA
Physiological adaptations: Functional or biochemical processes within the body, such as concentrating urine, producing antifreeze proteins or regulating salt balance.OCR
Physiological function: The normal role carried out by a cell, tissue, organ or system that maintains life processes.OCR
pioneer species: The first colonisers, adapted to harsh abiotic conditions, such as lichens on bare rock and marram grass on sand.AQA
Pioneer species and weathering: Lichens and mosses that arrive by wind or water dispersal, tolerate harsh conditions and begin weathering rock.OCR
placebo and control group ethics: It is not ethical to withhold an effective treatment, so a control group receives an existing drug rather than a placebo if a treatment already exists.AQA
plagioclimax: What is produced when conservation deliberately arrests or deflects succession, rather than letting it continue to the climax community.AQA
Plant cloning structures: Runners, tubers, bulbs, rhizomes and offsets from which new plantlets arise by mitosis from the parent.OCR
Plant communicable diseases: In plants the key groups are fungi, bacteria, viruses and protoctists, for example black sigatoka, ring rot, tobacco mosaic virus and potato blight.OCR
Plant dispersal of spores by wind, water and soil: Plant pathogens spread through soil contamination, wind and water dispersal of spores, and rain splash.OCR
plant growth factors: Specific factors that move from growing regions to other tissues, where they regulate growth in response to directional stimuli.AQA
Plant-derived medicines: Plants are a source of medicines, for example aspirin from willow and digoxin from foxgloves.OCR
plasma cells: Short-lived antibody factories, surviving only a few days, but each secreting thousands of antibody molecules per second to clear the current infection.AQA
plasma proteins: Too large to leave, so they remain in the capillary and keep its water potential low, that is more negative.AQA
plasmids: Small circular loops of DNA separate from the main circular DNA molecule, carrying genes not essential for survival and replicating independently of it.AQA
Plasmolysed: The state of a plant cell in a solution of lower (more negative) water potential, where it loses water.OCR
pluripotent: Cells found in embryos that divide by mitosis and differentiate into almost any body cell, but not extra-embryonic tissues like the placenta.AQA
Polar auxin transport: The movement of auxin down the stem in a polar direction, where blocking that transport reduces the dominance.OCR
polygenic characteristics: Most traits are polygenic and modified by the environment, making phenotypes an indirect guide to genotypes.AQA
Polygenic inheritance: Control by many genes, each with a small additive effect, producing a range of phenotypes.OCR
Polymer: A large molecule made of many monomers joined together.OCR
polymer synthesis: The reaction that builds polymers, with one water molecule released per bond formed, so n monomers release n-1 water molecules.AQA
polymerase chain reaction: The standard in vitro method, carried out in a thermal cycler with primers, free DNA nucleotides and a heat-stable DNA polymerase.AQA
polypeptide length: A truncated polypeptide can arise from a premature stop codon.AQA
polypeptides: Formed by the condensation of many amino acids; a chain of amino acids, not the same as a folded, functional protein.AQA
polyploidy: Chromosome doubling in plants that produces individuals unable to make fertile offspring with the parent population.AQA
Polysynaptic: Describes reflexes such as the blinking reflex and the withdrawal reflex, which include one or more relay neurones.OCR
population: All the organisms of one species living in the same place at the same time that can interbreed.AQA, OCR
population level: Evolution concerns populations, not individuals, because an individual's alleles are fixed at fertilisation, so an individual cannot evolve.AQA
Positive and negative responses: Shoots show positive phototropism and negative geotropism, while roots show positive geotropism.OCR
Positive feedback: A mechanism that amplifies a change and is rare, occurring in specific events such as certain stages of childbirth.OCR
Post-transcriptional control of mRNA: Control that acts on the mRNA after transcription, for example through splicing or the rate of mRNA degradation.OCR
Post-translational control of proteins: Control that acts on the synthesised protein, for example through activation by phosphorylation or through protein degradation.OCR
post-translational modification: Modifications such as glycosylation, which bacteria cannot perform because they lack organelles like the Golgi apparatus.AQA
postsynaptic receptors: Specific receptor proteins on the postsynaptic membrane whose binding sites are complementary in shape to acetylcholine and which form part of sodium ion…AQA
potential antigens: Proteins found on the surface of the pathogen, identified from the proteome, which the immune system will actually meet and which are candidates for a vaccine.AQA
Potometer: Apparatus used to estimate transpiration rate indirectly by measuring water uptake, with a shoot sealed into a capillary tube containing an air bubble.OCR
Potometry as a proxy measure: A potometer records the rate of water uptake by a shoot as a proxy for transpiration rate, with an air bubble moving along a capillary scale.OCR
power stroke: The bending of the myosin head that pulls the actin filament towards the centre of the sarcomere, driven by the release of ADP and Pi.AQA
Practical applications in agriculture and livestock: Uses include increasing crop yield, improving disease resistance in livestock, producing high-yielding dairy cattle, and developing breeds or varieties with…OCR
Practical technique and precautions: A concentrated extract is spotted on a pencil origin line, the plate is developed in solvent kept below the origin line in a covered container, and the…OCR
pre-messenger RNA: The molecule into which introns are transcribed along with exons, before they are removed by splicing.AQA
pre-mRNA: Transcription copies the whole gene, introns and exons alike, producing this.AQA
Precise biological vocabulary: Using exact terms such as gas exchange, reabsorption or conduction instead of vague terms such as 'works properly'.OCR
Precision and resolution of measurements: The number of significant figures reported should match the resolution of the instrument and the precision of the data, not simply the digits shown on a…OCR
Precision versus accuracy: Precision describes how closely repeated measurements agree with each other, while accuracy describes how close a measurement is to the true value.OCR
predation: An interaction in which a rise in prey numbers supports more predators, whose increased numbers then reduce the prey, so the two populations oscillate out of…AQA, OCR
Predicting offspring ratios: Offspring genotypes and phenotypes are deduced to give the expected ratio, such as 3:1 for a heterozygous monohybrid cross.OCR
Prefix conversion (milli, micro, kilo): Changing between prefixed units by converting the prefix correctly, for example 250 mm equals 0.250 m.OCR
premature stop codon: The new reading frame often throws up a stop codon earlier than normal, so translation halts and a short, truncated polypeptide is released.AQA
Preservation: Protection with minimal human interference, keeping an area in its existing state.OCR
Pressure and flow in the circulatory system: Arteries carry blood away from the heart at high pressure, while veins return blood at low pressure.OCR
Pressure and flow maintenance: A double circulation maintains higher pressure and faster flow to body tissues because blood returns to the heart after the lungs before being pumped to the…OCR
Pressure and volume relationships: Boyle's law links volume and pressure: as volume increases, pressure decreases.OCR
pressure differences across valves: Valves are passive: each opens when the pressure behind it is greater than the pressure in front, and closes when that reverses.AQA
Pressure generation: The thicker muscular wall of the left ventricle generates higher pressure for systemic circulation.OCR
pressure gradient: Air moves in down the pressure gradient, with the converse sequence for expiration.AQA
pressure receptors: Baroreceptors in the aortic arch and carotid sinus that detect blood pressure instead of blood carbon dioxide.AQA
Pressure-driven flow through the heart: Blood moves from higher to lower pressure, so pressure changes drive flow through the heart.OCR
Preventing contamination from air and surfaces: Working close to a Bunsen flame with upward air currents and lifting lids rather than removing them fully reduces contamination.OCR
preventing inbreeding: Comparing band patterns to select the least similar individuals for mating, thereby increasing genetic diversity.AQA
preventing spindle formation: Other drugs, like vinca alkaloids, prevent spindle formation, so chromatids cannot separate during anaphase.AQA
Primary immune response: The response that follows first exposure to an antigen; it is slow because the few lymphocytes with a matching receptor must be selected, then divide and…OCR
Primary non-specific defences: Physical, chemical and mechanical barriers that prevent pathogen entry and act immediately without distinguishing between different pathogens.OCR
Primary pigment: Chlorophyll a is the primary pigment at the reaction centre, where light energy is passed for the light-dependent stage.OCR
primary productivity: The rate at which producers fix energy (gross) and store it as biomass (net).AQA
primary response: The antigen has to find the very few B and T cells whose receptors are complementary to it, which are then activated, divide by mitosis and differentiate.AQA
primary structure: The sequence of amino acids in a polypeptide, which dictates R group positions and therefore determines tertiary folding.AQA
primary structure of a polypeptide: The sequence of amino acids in a polypeptide, which determines how the chain folds and therefore what it can do.AQA
primary succession: Begins on newly exposed land with no soil, like bare rock or blown sand, from colonisation by pioneer species to climax community.AQA
Primary versus secondary immune response: On later exposure to the real pathogen, memory cells give a faster, stronger secondary response that destroys the pathogen before symptoms develop.OCR
Primer design and thermostable polymerase: Two primers complementary to the flanking sequences bind the target, and a thermostable DNA polymerase such as Taq survives the high denaturation temperature.OCR
primers: Short single strands with bases complementary to the sequences at either end of the target region, which anneal to each strand.AQA
probability of a chance match: Made vanishingly small across many loci because the separate probabilities at each locus multiply.AQA
probability of a lymphocyte meeting its antigen: Increased by presentation, because each T cell recognises only one antigen, cells carrying that receptor are rare, and free antigen in lymph is easily missed.AQA
producers: Organisms that form the entry point of carbon and chemical energy into an ecosystem by synthesising organic compounds.AQA
Production of genetically identical daughter cells: Cytokinesis follows mitosis, dividing the cytoplasm to complete the formation of two genetically identical daughter cells.OCR
production versus productivity: Production is the total quantity of chemical energy or new biomass generated in a given area, whereas productivity is the rate of this production.AQA
Products ATP and NADPH: NADP is reduced to NADPH, and the ATP and NADPH produced are used in the light-independent stage.OCR
Programmed cell death: Apoptosis is carried out by controlled enzyme action, removing surplus or damaged cells in a regulated sequence without inflammation.OCR
Prokaryotic vs eukaryotic: A comparison of two cell types that share a cell surface membrane, cytoplasm, ribosomes and DNA but differ in structure and ultrastructure.OCR
promoter region: The site of its target genes to which the oestrogen-receptor complex binds after moving into the nucleus, stimulating RNA polymerase so that transcription…AQA
Proportion of polymorphic loci: Calculated as polymorphic loci divided by total loci sampled, for example 8 ÷ 20 = 0.40, or 40%.OCR
Prosthetic group: A cofactor bound permanently to an enzyme, as in the haem group of catalase.OCR
prosthetic groups: Non-protein groups that a protein may include, such as the haem in haemoglobin or the carbohydrate in a glycoprotein.AQA
Protecting the worker and environment: Aseptic technique protects the worker and environment, and waste is autoclaved to prevent contamination and release.OCR
Protective role of carotenoids: Carotenoids also protect chlorophyll from damage by excess light.OCR
protein denaturation above 45 degrees: Increased permeability above about 45 degrees Celsius is explained by denaturation of membrane proteins, with bonds broken and tertiary structure changed.AQA
Protein fibres: Microfilaments, microtubules and intermediate filaments, which are the systems that make up the cytoskeleton.OCR
Protein kinase and enzyme cascade: Cyclic AMP activates protein kinase, which phosphorylates and activates further enzymes in a cascade ending with glycogen phosphorylase.AQA
Protein synthesis and transport: The ordered pathway in which a gene in the nucleus is transcribed to mRNA, which binds to a ribosome and is translated into a polypeptide.OCR
proteins and lipids give specialisation: Membranes differ in the proteins and lipids they contain, which gives each membrane its particular functions.AQA
proteome: The full range of proteins that a cell is able to produce, which is larger than the gene count in eukaryotes.AQA
Proton gradient: Energy from electron transport pumps protons across the inner mitochondrial membrane from the matrix into the intermembrane space, creating a gradient.OCR
pulmonary and systemic circuits: Blood flows through these circuits in series, with blood passing through the heart twice for every complete circuit of the body.AQA, OCR
pulmonary vein: Vessel that returns oxygenated blood from the lungs to the left atrium.AQA
Purkyne fibres: Carry the impulse to the ventricular apex so the ventricles contract from the apex upwards, aiding efficient ejection of blood.OCR
Purkyne tissue: Carries the wave up the ventricle walls, causing contraction from the apex upwards.AQA
Purpose of staining: Staining increases contrast because many biological specimens are transparent and have little natural colour, so without a stain cell structures are hard to…OCR
Purposeful labelling with leader lines: Placing labels outside the drawing, each with a straight leader line that touches the named structure without crossing other lines or labels.OCR
Pyramid of biomass: A representation of standing crop at each trophic level, which may differ from a pyramid of numbers.OCR
Pyranose ring: A six-membered ring formed when glucose closes, in which five carbons and one oxygen form the ring and carbon 6 sits outside it.OCR
pyruvate not glucose: The molecule that enters the mitochondrion after glycolysis, because glucose is broken down to pyruvate in the cytoplasm.AQA

Q

Qualitative detection of biological molecules: Reagents that give a visible change when a particular biological molecule is present, confirming identity rather than measuring amount.OCR
Qualitative vs quantitative analysis: Benedict's is qualitative but becomes quantitative by filtering and weighing the precipitate, or by using a colorimeter with a calibration curve.AQA
Quantifying results objectively and using replicates: Colour intensity is quantified objectively by colorimeter absorbance or comparison with a colour standard, and replicates are used with a mean calculated so…OCR
Quantitative versus qualitative measurement: Quantitative methods determine the amount or concentration of a substance, not just its presence, whereas a colour change alone is qualitative.OCR
Quantitative versus subjective recording: Both instruments give quantitative data, reducing subjective judgement compared with visual estimation.OCR
quaternary structure: More than one polypeptide chain, held together by interactions between the chains such as hydrogen bonds, ionic bonds and hydrophobic interactions.AQA

R

R group: The variable side chain that differs between amino acids.AQA
R group interactions: Polypeptide tertiary structure is held by these bonds, namely hydrogen, ionic and disulfide bonds, and a different R group alters them.AQA
Ramsar Convention: An international agreement that protects wetlands of international importance as habitats for waterbirds and other species.OCR
random fertilisation: Any genetically unique male gamete can fuse with any genetically unique female gamete, creating novel allele combinations in the zygote.AQA, OCR
random mating: A condition that stops particular genotypes pairing preferentially.AQA
random mutation: A change to the base sequence that occurs at random and independently of whether the change would be useful, and can result in new alleles of a gene.AQA
Random quadrat sampling: A quadrat is placed using random coordinates to avoid bias, and the number of individuals or percentage cover of a species is recorded.OCR
random sampling: Uses coordinates generated independently of the investigator, for example random numbers to position quadrats, so personal choice does not bias placement.AQA, OCR
Random sampling and quadrats: Representative sampling of a habitat, for example with random quadrats, used to identify and count the distinct species present.OCR
Random sampling to estimate abundance: Placing a quadrat randomly, for example using random coordinates, gives percentage frequency, density per m² or percentage cover, and avoids bias.OCR
Random sampling with quadrats: Placing quadrats randomly, for example by generating random coordinates within a grid, to avoid bias in the sample.OCR
Random versus systematic sampling: Random sampling avoids bias when the habitat is uniform, while systematic sampling along a transect follows a change such as a gradient.OCR
rapidly dividing normal tissue: Treatments act on rapidly dividing cells, so normal tissues that divide quickly, like hair follicles and bone marrow, are also damaged.AQA
Rate calculation: Volume transpired is calculated from bubble distance multiplied by the capillary tube's cross-sectional area, giving a rate per unit time.OCR
rate of transport: The speed at which a cell can move substances across a membrane depends on how much membrane there is and on how many transport proteins are set into it.AQA
rate per unit time: Productivity is measured per unit time, such as kJ m-2 year-1, because it is a rate.AQA
Reading frame and codon translation: The reading frame is identified and the codons are translated using a codon table to give the primary sequence of amino acids in the polypeptide.OCR
reading frame preserved: The reading frame is intact because every triplet before and after the altered one is unchanged.AQA
reagent in excess: Using enough Benedict's solution that it does not become limiting, so the high concentrations do not all give the same reading.AQA
Rearranging equations: Identifying which two quantities are given and which is required, then rearranging the formula before substituting values.OCR
Reasons for protection: Maintaining biodiversity, protecting ecosystem services such as flood control and carbon storage, safeguarding genetic resources and meeting ethical or legal…OCR
receptor binding on the cell-surface membrane: Glucagon cannot cross the phospholipid bilayer, so it attaches to receptors on the cell-surface membrane and the signal is relayed inside the cell by a second…AQA
receptor specificity: Only one form of energy can open a receptor's particular channel proteins, so receptors respond only to specific stimuli.AQA
receptor, coordinator, effector: The parts of every control system: receptors detect a deviation from the set point, a coordinator receives that information, and effectors carry out a…AQA
Receptors and medulla integration: Chemoreceptors detect changes in blood CO₂ and pH and baroreceptors detect blood pressure changes, and the medulla oblongata integrates these inputs and…OCR
recessive: An allele expressed only in the homozygote, which is why a recessive characteristic can skip generations and reappear when two heterozygous carriers have…AQA
Recessive phenotype frequency: Equals q², so q is the square root of that frequency and p is then found as 1 − q.OCR
reclassification: New evidence contradicted a grouping based on observable features, so species were renamed or moved.AQA
recognition sites: The specific palindromic base sequence at which a restriction endonuclease cuts the DNA.AQA
recombinant DNA: DNA made by joining together DNA from two different organisms, usually from different species.AQA
redox indicator: Substance such as methylene blue or TTC that changes colour as it is reduced by hydrogen from the respiring cells.AQA
reduced NADP: The reducing agent that donates hydrogen to glycerate 3-phosphate, converting it to triose phosphate, and is oxidised back to NADP.AQA
Reducing versus non-reducing sugars: Benedict's reagent heated with a reducing sugar turns from blue to a brick-red precipitate, while a non-reducing sugar needs prior hydrolysis with dilute…OCR
reduction of GP: The conversion of glycerate 3-phosphate to triose phosphate, for which ATP from the light-dependent reaction supplies energy.AQA
Reduction of GP to TP: Glycerate 3-phosphate (GP) is reduced to triose phosphate (TP) using ATP and reduced NADP supplied by the light-dependent stage.OCR
Reflex action: A rapid, involuntary and stereotyped response to a stimulus, coordinated by a reflex arc so that the brain is not required to make the decision.OCR
Reflex arc: The pathway running from receptor to sensory neurone, into the spinal cord or brain, then to a motor neurone and effector.OCR
Regeneration and replacement rates: The rate at which a population or system regrows; extraction stays below this rate so the resource is not depleted.OCR
Regeneration of NAD in the absence of oxygen: Without oxygen the electron transport chain cannot reoxidise NADH, so NAD must be regenerated for glycolysis to continue.OCR
Regeneration of RuBP: Most triose phosphate is used to regenerate ribulose bisphosphate so the Calvin cycle can continue.OCR
regeneration of the action potential: A fresh action potential produced at the next region of membrane once local currents have depolarised it to threshold.AQA
Regulation without changing DNA sequence: Control at any level changes the amount or activity of the final functional protein, so gene expression is controlled without changing the DNA sequence.OCR
regulatory genes: Genes coding for transcription factors and RNA molecules that determine which genes are transcribed in which cell type and when.AQA
relatedness: The fewer the differences in base or amino acid sequences, the more recently the organisms shared a common ancestor and the more closely related they are.AQA
Relating stained structure to function: Acinar cells appear as groups with dark basal nuclei and granular apical cytoplasm, whereas islet cells form paler, rounded groups with a rich capillary…OCR
Relationship between enlargement and detail: Resolution determines the level of detail visible, while magnification only enlarges the image, so increasing magnification beyond the resolving limit…OCR
Relationship between magnification and calibration: Because the magnification changes between objectives, the graticule must be recalibrated at each power.OCR
Relationship between sequence and shape: A mutation that changes one amino acid alters the primary structure and can change the tertiary shape.OCR
relative refractory period: The period that follows, when the channels have reset but the membrane is hyperpolarised because potassium ion channels close slowly, so only a stronger than…AQA
relative simplicity of DNA: DNA appeared too simple, consisting of only four different nucleotides, which led many scientists to doubt that it carried the genetic code.AQA
relay neurone: The neurone in the central nervous system with which the sensory neurone synapses, and which synapses with a motor neurone.AQA
Relay neurones: Neurones found within the central nervous system that connect sensory neurones to motor neurones, often with many short dendrites and a short axon.OCR
Reliability: Improved by repeats and averaging, with rate expressed per unit time.OCR
Reliability and bias in sampling: Random sampling reduces bias, and using more quadrats or a larger sample improves reliability of the estimates.OCR
Reliability of primary data: Producing primary data in a controlled, repeatable way, taking repeat readings and recording them all rather than discarding any.OCR
Reliability through control and repeats: Control variables such as volumes and conditions, and use repeats, to reduce error and improve reliability.OCR
Reliability through sample number and standardised technique: Increasing the number of samples and using consistent, standardised technique reduces the effects of chance and bias, making conclusions more reliable.OCR
repolarisation: The membrane repolarising when the sodium ion channels close at the peak and voltage-gated potassium ion channels open, so potassium ions diffuse out.AQA
Reporting data honestly and consistently: Round only at the final stage to avoid accumulating error.OCR
Representativeness and reducing bias: Samples are used to represent the whole area, and larger numbers of samples and consistent technique reduce chance and bias.OCR
Reproductive isolating mechanisms and speciation: Mechanisms such as temporal, behavioural or mechanical isolation, or hybrid infertility, maintain separation until populations can no longer produce fertile…OCR
Reproductive isolation: A state in which populations are prevented from interbreeding so that gene flow between them stops.OCR
Reproductive potential and sustainability: Reproductive potential, generation time and the availability of resources shape how quickly a population grows, and population factors relate to…OCR
reproductive separation: A barrier prevents gene flow between populations, and this initial separation can be allopatric or sympatric.AQA
Research applications: Pluripotent and multipotent stem cells provide models for studying development, gene expression and disease, and can be used to test drugs and toxicity.OCR
resistance to fatigue: Slow fibres resist fatigue because they respire aerobically, whereas fast fibres fatigue quickly as the lactate they produce lowers pH.AQA
resolution: The minimum distance two points can be apart and still be distinguished as separate, measured in micrometres or nanometres and determined by the wavelength of…AQA, OCR
Resolution limits of microscopy: Resolution limits what can be seen, so some organelles may not be resolvable at the stated magnification.OCR
Respiration rate: Calculated by dividing the volume of gas evolved or taken up by the time taken.OCR
respiratory loss R: The respiratory losses to the environment.AQA
Respiratory quotient formula: The ratio of carbon dioxide produced to oxygen consumed during respiration, calculated as CO₂ produced ÷ O₂ consumed.OCR
respiratory substrate: Glucose is the main one, oxidised in respiration to release energy for ATP synthesis.AQA
respirometer: Apparatus in which carbon dioxide is absorbed by potassium hydroxide or soda lime, so the liquid movement in the manometer measures oxygen uptake alone.AQA, OCR
response: The change in behaviour or physiology that follows a stimulus.AQA
Resting potential and the sodium–potassium pump: The inside-negative membrane of a resting neurone, kept by a pump moving 3 Na⁺ out for every 2 K⁺ in and by greater K⁺ permeability.OCR
resting potential of minus 70 mV: At rest the inside of the axon is about 70 mV negative relative to the outside.AQA
Restoration at fertilisation: Fusion of two haploid gametes at fertilisation restores the diploid number, keeping chromosome number constant between generations.OCR
restored function: Replaced cells are linked to restored function, such as stronger ventricular contraction and higher ventricular pressure.AQA
restricted limits: The narrow range within which the internal environment is maintained, rather than being kept exactly constant.AQA
restricted phospholipid movement: The limiting of the movement of phospholipids and other molecules making up the membrane, caused by cholesterol holding the fatty acid tails together.AQA
restriction endonucleases: Enzymes whose active site is complementary to a specific palindromic recognition sequence, hydrolysing phosphodiester bonds in the sugar-phosphate backbone.AQA
restriction endonucleases and Southern blotting: DNA is cut with restriction endonucleases, then fragments are transferred to a membrane by Southern blotting and detected with labelled DNA probes, producing…AQA
Restriction enzymes and complementary sticky ends: The gene and a vector such as a plasmid are cut with the same restriction enzyme, producing complementary sticky ends.OCR
Restriction enzymes and sticky ends: Enzymes that recognise specific palindromic sequences and cut DNA, producing sticky ends that allow complementary base pairing.OCR
Retrovirus structure: HIV contains two RNA strands and reverse transcriptase within a protein capsid, surrounded by a lipid envelope derived from the previous host's membrane.AQA
Return of fluid and drainage as lymph: At the venous end, hydrostatic pressure has fallen and the osmotic effect of the retained plasma proteins draws most fluid back into the capillary; the…OCR
return to the original level: The variable coming back to, or towards, its original level once the corrective response has acted.AQA
reverse transcriptase: In retroviruses, this first makes DNA from viral RNA.AQA
reversibility: Because no base has been altered, these changes are in principle reversible, which is what makes them attractive drug targets.AQA
Rf value calculation: Rf = distance travelled by the spot ÷ distance travelled by the solvent front; it is a ratio with no units and is always less than 1.OCR
Rf values and comparison with standards: The distance travelled by the compound divided by the distance travelled by the solvent front, compared with known standards to identify a compound.OCR
Rf values and pigment identification: Rf value = distance moved by pigment ÷ distance moved by solvent front, and is used to identify pigments by comparison with known values.OCR
rhodopsin: Pigment in rods that is broken down by light and is sensitive at low light intensities, enabling dim-light vision.AQA
ribosomal RNA: RNA that, together with protein, makes up a ribosome, which is a non-membrane-bound structure.AQA
ribosome movement: The ribosome moves along the mRNA to the next codon, and the tRNA is released and reused.AQA
Richness and evenness combined: D depends on both the number of species and how evenly individuals are distributed among them.OCR
right shift of the curve: On the dissociation curve this appears as a shift to the right, so at any given partial pressure of oxygen the haemoglobin is less saturated.AQA
Ringing experiments: Removing a ring of phloem-containing bark causes swelling above the ring as sugars accumulate, providing evidence for translocation.AQA
risk against benefit: Rare side effects fall on individuals while the benefit is to the whole population.AQA
Risk assessment and hazard control: Before cutting, confirm the organ's identity and orientation, read the dissection instructions, and set out a clean board, the correct instruments, plus eye…OCR
risk factors: Diet is likely to be one risk factor among several rather than the sole cause, and genetics, ethnicity, age or inactivity also contribute.AQA
risk of tumours: iPS and embryonic cells divide indefinitely, risking tumour formation.AQA
RNA interference: In eukaryotes and some prokaryotes, translation of the mRNA produced from target genes can be inhibited by this process.AQA
RNA polymerase: The enzyme that catalyses phosphodiester bond formation between adjacent RNA nucleotides, joining them into a single-stranded RNA molecule.AQA
RNA polymerase binding: Occurs at the promoter, positioning the enzyme so transcription starts at the correct base and runs in the correct direction.AQA
role of ATP: ATP is used to attach each amino acid to its tRNA and provides energy for translation.AQA
role of blood flow: Removing oxygenated blood and bringing deoxygenated blood, so maintaining the gradient.AQA
Role of buffer in controlling charge: Maintains pH so that the charge on the molecules, and therefore their movement, is controlled.OCR
Role of Ca²⁺ and troponin: Calcium ions released from the sarcoplasmic reticulum bind troponin, moving tropomyosin to expose actin binding sites.OCR
Role of ions in membrane potentials and signalling: Potassium ions maintain the resting potential of neurones, while calcium ions trigger muscle contraction and stimulate exocytosis of neurotransmitters at…OCR
Role of membrane proteins: Carrier or channel proteins allow facilitated diffusion down a gradient, aquaporin channel proteins carry water in osmosis, and carrier proteins use ATP in…OCR
Role of oxygen: Acts as the final electron acceptor, combining with electrons and protons to form water.OCR
Role of plant hormones in differentiation: Auxin and cytokinin concentrations are adjusted to stimulate shoot formation and then root formation.OCR
Role of plasma cells: Differentiated B lymphocytes that secrete antibodies in response to a specific antigen.OCR
Role of reverse transcriptase: It synthesises a DNA copy from viral RNA, which is inserted into the host's DNA.AQA
Role of salt in neutralising phosphate charge: Salt neutralises the negative charges on the phosphate backbone so DNA molecules can aggregate and precipitate.OCR
Role of the spindle apparatus: Spindle fibres form in prophase, attach to centromeres in metaphase, shorten in anaphase to pull sister chromatids apart, and disassemble in telophase.OCR
Roles of plasma, memory, helper and cytotoxic cells: Activated B cells divide and differentiate into plasma cells that secrete antibodies and memory cells that persist, while helper T cells release cytokines and…OCR
root tip meristem: The part of the root containing dividing cells, which is why the last few millimetres of a growing root tip are cut.AQA
Rooting powder: A powder containing auxin that the cut end may be dipped in to encourage root formation.OCR
rough endoplasmic reticulum: The structure to which ribosomes attach to make proteins for secretion or lysosomes.AQA
Rounding conventions in calculations: When multiplying or dividing, the answer takes the smallest number of significant figures in the inputs; when adding or subtracting, it follows decimal places.OCR
rubisco: The enzyme that catalyses the reaction in which carbon dioxide combines with ribulose bisphosphate.AQA
RuBP regeneration: The third ATP molecule used in each turn of the Calvin cycle is used in regenerating ribulose bisphosphate from triose phosphate, a step with an energy cost…AQA
Rules for identifying significant figures: Leading zeros are never significant, trailing zeros after a decimal point are significant, and zeros between non-zero digits are significant.OCR

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Safe cutting technique and hand position: Carry the scalpel with the blade covered and pointing down, cut away from your body and away from your fingers, and never cut towards a hand holding the…OCR
saltatory conduction: Conduction enabled by the myelin sheath, formed by Schwann cells with nodes of Ranvier between them, giving faster transmission.AQA, OCR
Saltatory conduction and refractory period: In myelinated axons the impulse jumps between nodes of Ranvier, which is faster, and the refractory period enforces one-way travel.OCR
same genes in all cells: Every cell contains exactly the same DNA as every other cell in the body, and what makes a specialised cell different is not which genes it has but which…AQA
Sampling individuals and loci: Measuring variation at gene loci, such as allele frequencies or the number of polymorphic loci, with larger random samples giving more reliable estimates.OCR
Sampling reliability: Representative sampling and adequate sample size are needed for a reliable value of D.OCR
SAN: In the right atrial wall, acts as the pacemaker, spontaneously generating impulses that spread across the atria, causing atrial contraction.OCR
SAN as intrinsic pacemaker: The sinoatrial node in the right atrium acts as the pacemaker and fires without nervous input, setting the intrinsic rhythm.OCR
saprobionts: Saprobiotic bacteria and fungi that secrete hydrolytic enzymes onto dead organisms and waste, digest the material extracellularly and absorb the products.AQA, OCR
saprobiotic nutrition: Feeding on dead organic material and on faeces and urine, using extracellular digestion and absorption of the soluble products.AQA
sarcolemma and transverse tubules: The plasma membrane, which folds inwards at intervals as transverse tubules that carry depolarisation deep into the fibre.AQA
sarcomere between Z lines: A repeating unit of a myofibril, running from one Z line to the next, with thin actin filaments anchored at the Z lines and thick myosin filaments lying in…AQA
Sarcomere structure: Repeating units of a myofibril bounded by Z-lines, containing thick myosin filaments and thin actin filaments.OCR
sarcoplasmic reticulum: An extensive structure in the sarcoplasm that stores calcium ions.AQA
saturated: A fatty acid with no carbon-carbon double bonds in the hydrocarbon chain, so the chain is drawn straight.AQA
saturated fatty acids: Fatty acids with no carbon-carbon double bonds, so every carbon carries the maximum possible number of hydrogen atoms and the chain is straight.AQA
saturation: The rate plateaus once all carrier proteins are saturated, because they are occupied and cannot work faster.AQA
Saturation (Vmax): The point at which all active sites are occupied, so increasing substrate concentration further has no effect and the rate plateaus.OCR
Scale and accuracy: An even, clearly numbered scale fills the available space, points are plotted accurately, and readings are recorded to a consistent precision justified by the…OCR
scale of biodiversity: The range of habitats a biodiversity figure relates to, from a small local habitat to the Earth, which has to be stated.AQA
scale of ecosystems: Ecosystems can range in size from the very small to the very large, and scale matters most when you sample.AQA
Schwann cell: A cell that wraps repeatedly around the axon, its many layers of plasma membrane forming the myelin sheath.AQA
scientific consensus: DNA was widely doubted as the genetic material, and this consensus only shifted as accumulating experimental evidence demonstrated that DNA carried the…AQA
secondary productivity: The rate at which consumers store energy as new biomass.AQA
secondary response: On second exposure to the same antigen, memory B cells divide rapidly into plasma cells, producing antibody faster, at a higher concentration and for longer,…AQA, OCR
secondary structure: The folding of the backbone into an alpha helix or beta pleated sheet, held by hydrogen bonds.AQA
Selecting the appropriate operation: Matching the mathematical operation to the question, such as a mean to summarise repeats or a rate to describe change over time.OCR
selection: Changes allele frequencies by changing which individuals reproduce.AQA
selection acts on phenotype: Selection acts on the phenotypes those alleles help to produce, not directly on alleles.AQA
selection pressure: Any factor that makes some phenotypes more likely than others to survive and reproduce.AQA
Selection pressure and antibiotic resistance: Selection pressure favours resistant bacteria, so overuse and incomplete courses increase resistant strains such as MRSA.OCR
selection pressures: When a new selection pressure arises, such as a novel disease, differences in survival and reproductive success mean advantageous alleles are passed on.AQA, OCR
selective advantage: A phenotype that, in a given environment, raises the chance of surviving to reproduce and of producing offspring.AQA
Selective gene expression: The process by which the same genetic information produces distinct outcomes, so differentiation is not a change in the genes present.OCR
Selective gene expression in differentiation: A cell expresses particular genes, switching some on and others off, so it develops the structure and proteins suited to a specialised function.OCR
Selective reabsorption: Process in the proximal convoluted tubule that returns glucose, amino acids, ions and most water to the blood.OCR
Selective retention of large molecules and cells: Large plasma proteins and blood cells are held back because they are too big to pass through the capillary wall.OCR
Selective toxicity of antibiotics against bacterial targets: Antibiotics target bacterial structures or processes, such as cell-wall synthesis or protein synthesis, that human cells lack.OCR
self-renewal: Pluripotent cells divide by mitosis in unlimited numbers.AQA
Self-tolerance: The normal process that prevents the immune system from attacking the body's own tissues, and whose breakdown underlies autoimmunity.OCR
semi-conservative replication: Produces DNA molecules with one original template strand and one newly synthesised strand, ensuring genetic continuity between generations of cells.AQA, Pearson Edexcel
Semilunar valves: These sit at the base of the pulmonary artery and aorta, preventing backflow into the ventricles.AQA
Sensory neurones: Neurones that carry impulses from receptors to the central nervous system, typically with a cell body in a dorsal root ganglion and a long dendron.OCR
separate gene pools: Once separated, the two gene pools change independently of one another.AQA
Separation of homologues then chromatids: Homologous chromosomes separate in anaphase I and sister chromatids separate in anaphase II.OCR
separation of oxygenated and deoxygenated blood: Oxygenated and deoxygenated blood are kept separate by the septum.AQA
Separation of purified DNA from soluble contaminants: The white precipitate can be spooled or collected, separating purified DNA from soluble contaminants left in the supernatant.OCR
Separation of strands and template function: The double helix unwinds and the two strands separate, each acting as a template for a new strand.OCR
Septum: This separates the sides of the heart, preventing oxygenated and deoxygenated blood from mixing.AQA
Sequencing as a source of design information: Gene sequencing identifies the base sequences of genes, promoters and regulatory regions, revealing which DNA elements carry out which functions.OCR
Seral stages and climax community: Each stage is a seral stage, and the community changes until a stable climax community forms, determined by climate.OCR
serial dilution: Mixing equal volumes of stock and distilled water repeatedly so that each tube is half the concentration of the one before.AQA, OCR
set point: The value that a physiological control system works around, with levels fluctuating around it rather than being held exactly.AQA
sex linkage: Revealed when reciprocal crosses give different results and when phenotype frequencies differ between the sexes, because males carry one X and express…AQA, OCR
shading and light intensity: A canopy formed by shrubs and trees cuts light intensity at ground level to a small fraction of full sunlight.AQA
Shape determines function: A change in primary structure can alter the folding and therefore the tertiary structure and function of the protein.OCR
shared monomers: Monosaccharides, amino acids and nucleotides, which are used universally by all life forms.AQA
shared transmission mechanism: An action potential opens calcium ion channels, vesicles release acetylcholine by exocytosis, it diffuses across a cleft and binds to receptors that open…AQA
short diffusion pathway: The extremely short distance between air and blood because the epithelium lies against a capillary endothelium that is also one cell thick.AQA
side effects and dose: A named side effect is explained by damage to a normal rapidly dividing tissue, such as hair loss or reduced white blood cell production.AQA
sieve tube elements and sieve plates: Living cells joined end to end through perforated plates, each element having lost its nucleus and most organelles.AQA
sigmoid curve: The oxyhaemoglobin dissociation curve is S-shaped, plotting percentage saturation of haemoglobin against partial pressure of oxygen.AQA
significance and error bars: Using error bars, standard deviations or statistical tests to state whether a difference is significant, saying what is greater, lower or different.AQA
silent mutation: A substitution that produces a synonymous codon, leaving the amino acid sequence unchanged, so tertiary structure and function are usually unaffected.AQA
simple diffusion: Passive net movement of particles down a concentration gradient, restricted by the bilayer to small non-polar molecules.AQA
simple reflex: A rapid, automatic, involuntary response to a stimulus, which has a protective effect.AQA
Simpson's Index formula: D = 1 − Σ(n/N)², where n is the number of individuals of each species and N is the total number of individuals of all species.OCR
single sensory neurone ending: The naked, unmyelinated axon terminal at the centre of the corpuscle.AQA
Single versus double circulation: A single circulation has blood passing through the heart once per complete circuit, while a double circulation passes through the heart twice.OCR
sinoatrial node: Depolarises spontaneously in the right atrium, setting the heart rate and initiating the wave of depolarisation that causes the atria to contract.AQA
sinoatrial node as pacemaker: The sinoatrial node acts as the pacemaker, originating the wave of electrical activity.AQA
siRNA and microRNA: Double-stranded RNA in the cytoplasm is cut by an enzyme into short lengths of about 21 nucleotides called small interfering RNA; microRNA works in a similar…AQA
sister chromatids: Two identical copies joined at a centromere, which each chromosome comes to consist of after the DNA is replicated in S phase.AQA
Sister chromatids and centromeres: Each chromosome replicates to form two identical sister chromatids joined at a centromere.AQA
Size-dependent sieving through a gel: The gel acts as a molecular sieve, so small fragments travel further and faster than large ones in a set time, producing bands.OCR
Slide preparation technique: Selecting suitable material, placing it on a clean slide, adding a mounting medium such as water, and lowering a coverslip carefully to avoid air bubbles.OCR
Sliding filament model: Actin filaments slide over myosin as myosin heads pivot, so the sarcomere shortens while the filaments themselves do not change length.OCR
slow-twitch aerobic fibres: Fibres that contract slowly and with less force but resist fatigue, because they respire aerobically with many mitochondria, a dense capillary supply and a…AQA
small populations: Groups in which an unrepresentative sample of alleles can easily be passed on, so the effect of chance change in allele frequency is larger.AQA
smooth muscle and vasoconstriction: Arterioles have proportionally more smooth muscle, which contracts to narrow the lumen, controlling blood distribution between organs by vasoconstriction and…AQA
sodium hydroxide and copper sulfate: An equal volume of sodium hydroxide solution makes the sample alkaline, then a few drops of dilute copper(II) sulfate solution are added and mixed gently.AQA
sodium ions and co-transport: Pumped out of an epithelial cell, their concentration gradient drives the co-transport of glucose and amino acids into the cells lining the ileum.AQA
sodium-potassium pump: Pumps in the axon membrane actively transport three sodium ions out for every two potassium ions taken in, using ATP.AQA
Software processing: Entering raw readings into a spreadsheet or statistics package to calculate means, plot graphs and run tests.OCR
soil formation: Pioneers weather rock and, upon death, saprobionts decompose their remains, so dead organic matter mixes with rock particles to form a thin soil.AQA
Soil formation and organic matter: The death and decay of pioneers, together with trapped particles, slowly build a thin soil layer and add organic matter and nutrients.OCR
soil structure: A property improved by the organic matter in natural fertilisers, along with water retention.AQA
Solubility and folding of globular proteins: Globular proteins fold so that hydrophobic residues cluster in the core and hydrophilic residues face the aqueous surroundings, giving a compact, roughly…OCR
Solubility differences between DNA in water and in alcohol: DNA is soluble in water but insoluble in cold ethanol, so adding cold ethanol causes DNA to precipitate out of solution.OCR
solubility in ethanol: Lipids dissolve in ethanol but not in water, so they come out of solution as droplets when the ethanol solution meets water.AQA
Solute gradient: Gradient in the medulla generated by the loop of Henle that allows water reabsorption from the collecting duct.OCR
Solvents dissolving lipids: Organic solvents such as ethanol dissolve membrane lipids and disrupt the bilayer, increasing permeability.OCR
Somatic cell nuclear transfer: A somatic cell's nucleus is kept and transferred into an enucleated egg, and the reconstructed cell is stimulated to divide and develop as an embryo, which is…OCR
somatic cells: Adult body cells, typically skin fibroblasts, from which iPS cells can be produced using appropriate protein transcription factors.AQA
Somatic therapy: Targets body cells such as lung epithelium or bone marrow, and the changes are not inherited.OCR
source to sink: Movement of assimilates from anywhere they are made or released to anywhere they are used or stored.AQA
Source-to-sink transport of assimilates: Sugars and other assimilates are transported from sources to sinks, allowing efficient distribution despite the plant's size.OCR
Sources and sinks: Regions where assimilates are produced or released, and regions where they are used or stored, respectively.OCR
spatial summation: Many rods share a single bipolar neurone, allowing summation of generator potentials, giving high sensitivity in dim light but low acuity.AQA
Specialisation: An adaptation of cell structure, including shape, organelle number and membrane features, that suits a particular function.OCR
Specialised cell structure and function: The structure and proteins suited to a specialised function that a cell develops through selective gene expression.OCR
specialised cells: Eukaryotic cells in complex multicellular organisms that have become adapted for specific functions, with altered shape and organelle proportions.AQA
specialised exchange surfaces: Developed by larger organisms to compensate for a low surface area to volume ratio.AQA
species: A group of organisms able to interbreed and produce fertile offspring.AQA
Species concept: A group of organisms with similar features that can interbreed to produce fertile offspring.OCR
Species evenness: The relative abundance of the different species; a community is even when species have similar numbers of individuals.OCR
species interactions: Populations compete for the same resources, eat one another, pollinate one another or parasitise one another.AQA
species recognition: Allows an animal to recognise a member of its own species, so that gametes are not wasted on a mating which could not produce fertile offspring.AQA
species richness: The number of different species present in a habitat, counted without regard to how many individuals each species has.AQA, OCR
Specific colour changes as evidence: The colour change, not the reagent alone, is the evidence, so the specific observable change must be named for each test.OCR
specific hydrolysing enzymes: Membrane-bound enzymes on the epithelial cells of the ileum hydrolyse each disaccharide back to its monosaccharides.AQA
specific receptors on target cells: Binding sites on the cell-surface membranes of target cells that are complementary in shape to insulin, to which insulin attaches.AQA
Specificity of antigen binding: The variable regions at the tips of the two arms form the antigen-binding sites, giving a shape complementary to one specific antigen.OCR
Specificity of receptor–ligand binding: A ligand binds a specific receptor with a complementary binding site, so only cells with the correct receptor respond.OCR
specimen preparation: Killing, fixing, dehydrating, embedding, sectioning and staining tissue for microscopy, where every step can add something that was never there or destroy…AQA
Specimen preparation and observation of organelles: A specimen is selected and prepared, for example as a thin section or a stained slide, and the correct magnification is chosen to observe cell structure…OCR
spindle microtubules: Protein microtubules forming a framework that runs from organising centres at opposite poles of the cell.AQA
spiracles and valves: Structures that can close via valves and have hairs that trap moist air, reducing the water potential gradient.AQA
splicing: Spliceosomes cut the pre-mRNA at the intron-exon boundaries, remove the introns and join the exons together, forming mature mRNA.AQA
squamous epithelium one cell thick: A single layer of flattened cells forming the alveolar surface, giving a short diffusion pathway.AQA
stabilising selection: Individuals at both extremes are selected against and those near the mean have the advantage, so the mean stays the same while the range narrows and the curve…AQA
Stable internal environment: The maintenance of conditions within narrow limits despite changes in the external environment.OCR
stage micrometer: A slide carrying an accurate scale, usually one millimetre divided into one hundred divisions of ten micrometres.AQA
Stages of mitosis: Nuclear division conventionally described in stages: prophase, metaphase, anaphase and telophase.OCR
Stages of the cell cycle: The sequence of events by which a cell grows and divides to produce two genetically identical daughter cells, consisting of interphase followed by the mitotic…OCR
standard deviation: A measure of how widely individuals are spread around the mean.AQA
Standard deviations: They show the spread of data around a mean and are not a formal significance test.AQA
Standard solutions: A series of known concentrations is prepared, each measured for absorbance, so the unknown can be compared with a curve built from them.OCR
Standardisation: Keeping quadrat size, recording method (percentage cover or frequency) and sample number constant so results are comparable and representative.OCR
Standardised DNA parts and constructs: Sequenced information allows standardised DNA parts to be catalogued and combined into designed gene constructs.OCR
stating the direction of change: Writing more, less, fewer or no at each step, since describing normal synaptic transmission without this caps the answer below full marks.AQA
Stationary phase: A phase in which growth slows as nutrients become limiting and toxic waste builds up, so reproduction balances death.OCR
statistical significance: Whether the difference between observed and expected results is significant, determined by comparing the calculated value with the critical value at p = 0.05.AQA
stem cell: An undifferentiated cell that can keep dividing by mitosis and can differentiate into other cell types.AQA
stem cell potency: The differentiation potential of a stem cell, which for unipotent cells is the most restricted, being limited to a single lineage.AQA
Stem cell self-renewal: Stem cells can divide by mitosis to self-renew, so the stem cell population is maintained while some daughter cells differentiate.OCR
stem cells: Cells found in specific populations such as bone marrow, intestinal crypts and the basal layer of the skin, where division is mostly restricted.AQA
Stem cells in meristems: Undifferentiated cells held in meristems that divide by mitosis and are the source of differentiated tissues.OCR
Sterile technique in culture: The explant is sterilised and placed on sterile nutrient agar to prevent microbial contamination that would ruin the culture.OCR
Sterilisation: Treating glassware, media and loops before use, for example in an autoclave or by flaming, so that contaminants are killed before inoculation.OCR
steroid hormone: A lipid-soluble substance that diffuses straight through the phospholipid bilayer of the cell-surface membrane rather than binding on the outside.AQA
Steroid hormone secretion: The adrenal cortex secretes steroid hormones, including mineralocorticoids such as aldosterone and glucocorticoids such as cortisol.OCR
sticky ends: Short single-stranded runs of exposed bases left by a staggered cut, which can later pair with complementary ends.AQA
stimulus: A detectable change in the internal or external environment.AQA
stimulus response chain: A sequence in which each action by one animal is the stimulus for the next response by its partner, worked through before mating takes place.AQA
Stimulus-specific prefixes: Photo- indicates light, geo- indicates gravity, hydro- indicates water and thigmo- indicates touch.OCR
Stomatal aperture: The opening of stomata, which environmental factors change to affect the transpiration rate.OCR
storage polysaccharides: Starch is the storage carbohydrate of plants and glycogen is the storage carbohydrate of animals and fungi, storing glucose which releases energy when respired.AQA
Storage versus structural polysaccharides: Starch and glycogen are compact, insoluble storage polymers, while cellulose forms hydrogen-bonded microfibrils that give plant cell walls tensile strength.OCR
STR multi-locus comparison: Short tandem repeat numbers at several loci are measured, giving a multi-locus profile that is compared between samples.OCR
straight versus branched chains: A chain described as straight, linear or unbranched where the diagram shows it, or carrying 1,6 branch points where branching is shown.AQA
stretch-mediated sodium ion channels: Channels in the membrane of the neurone ending that open when pressure deforms the membrane, the lamellae or the channels themselves.AQA
stroma: The light-independent reaction occurs in this part of the chloroplast.AQA
structural and functional roles: Enzymes determine structures and functions from cellular to whole-organism level through the reactions they catalyse.AQA
structural polysaccharide: Cellulose is the structural polysaccharide of the plant cell wall, giving the cell wall strength rather than acting as a store.AQA
structural proteins: Proteins such as collagen that provide tensile strength.AQA
structure and function: The link between a cell's structural features and organelle proportions and the specific function it performs.AQA
Structure and treatment differences: Each type differs in structure and replication, so treatment differs: antibiotics act on bacteria but not on viruses.OCR
structure determines polysaccharide: Alpha-glucose allows coiling and branching, giving amylose, amylopectin and glycogen, while beta-glucose forces a straight and unbranched chain, giving…AQA
Structure of haemoglobin and oxygen binding: A quaternary protein with four haem groups, each containing an iron ion (Fe²⁺) that binds one oxygen molecule, so a fully loaded molecule carries four oxygen…OCR
Structure of skeletal muscle tissue: Long, parallel, unbranched cylindrical cells lying side by side in bundles, each multinucleate with flattened nuclei pushed to the periphery just under the…OCR
Structure related to function: Linking a named structural feature directly to the function stated in the question stem.OCR
structure to property reasoning: Making the link explicit between a structural feature of a protein and the property that follows from it, rather than stating them separately.AQA
Structure–function relationship: ADP has two phosphate groups and ATP has three, so ATP has one extra phosphate compared with ADP, attached in a chain.OCR
Structure–function relationship of blood vessels: Each vessel's structure is adapted to its position in the circulatory system, pairing a named structural feature with the function it performs.OCR
Structure–function relationships: Structural differences explain function: branching gives many ends for rapid hydrolysis, helices pack densely for storage, and straight hydrogen-bonded chains…OCR
Structure–function relationships in gas exchange: Relating each observed histological feature, such as a thin epithelium shortening the diffusion distance, to its function in efficient gas exchange.OCR
substitution: A change that replaces one base with another and therefore alters exactly one triplet, with every triplet before and after it unchanged.AQA
substitution and deletion: A substitution is produced when DNA polymerase inserts an incorrect nucleotide, while a deletion removes a base.AQA
substrate-level phosphorylation: A way ATP is synthesised in glycolysis and the Krebs cycle.AQA
sucrose as the transport sugar: The sugar carried by phloem because it is soluble but metabolically fairly inert.AQA
sugar-phosphate backbone: This lies on the outside of the molecule, with the bases pointing inwards.AQA
Sulphur: Occurs only in the amino acids that contain it, so only some proteins contain it.OCR
summation not needed: A single action potential arriving is normally enough to make the muscle fibre contract because the motor end plate carries far more receptors, whereas a…AQA
Sunken stomata: Stomata in pits that trap humid air, reducing the water potential gradient and so transpiration.OCR
sunken stomata and rolled leaves: Adaptations that trap water vapour, lowering the water potential gradient between the leaf air spaces and the atmosphere, limiting water loss but at the cost…AQA
surface area: A greater area of internal or external membranes increases the rate of transport, because more membrane is available for movement.AQA
surface area for absorption: The enormous absorbing area added to the root system by extremely fine hyphae that reach water and ions in soil pores too small for root hairs to enter.AQA
Surface area for reactions: Internal membranes increase surface area for reactions, for example the folded inner membrane (cristae) of mitochondria, and form transport vesicles.OCR
Surface area to volume ratio: As an organism becomes larger, its volume increases faster than its surface area, so this ratio falls.OCR
Surface area to volume ratio and diffusion limits: Multicellular organisms have a small surface area to volume ratio and large distances between cells, so diffusion alone is too slow to coordinate the whole…OCR
Surface area to volume ratio in plants: Multicellular plants are large, so this ratio is small, meaning diffusion across the outer surface would be too slow to supply every cell.OCR
surface proteins: Proteins likely to be displayed on the outside of the cell or virus, picked out from the proteome once the genome is sequenced.AQA
survival to reproductive age: An organism carrying an advantageous allele is more likely to reach this stage and produce offspring.AQA
sustainability: Managing the use of natural resources so that they are maintained, which conservation aims to achieve.AQA
Sustainable resource use: Management of an ecosystem that provides resources at a rate the ecosystem can replace, so the resource and the ecosystem's functions persist for future use.OCR
Sustainable use of environmental resources: Controlling how environmental resources are exploited so that use is sustainable, for example by setting fishing quotas or designating protected areas.OCR
switching off the response: When the variable is back within limits the receptors detect this and the corrective response is stopped, so the system does not overshoot.AQA
sympathetic and parasympathetic control: Neurones that raise heart rate and neurones that lower it.AQA
sympathetic and parasympathetic pathways: Both are autonomic, so involuntary, and both converge on the same effector, the SAN, which is why the two branches act antagonistically.AQA
Sympathetic division: The autonomic division that prepares the body for action, raising heart rate and dilating pupils.OCR
sympatric speciation: Speciation that occurs without a geographical barrier, in populations sharing the same area, with isolation that is behavioural, temporal or mechanical.AQA
synaptic cleft: The gap, about 20 nm wide, that separates the presynaptic and postsynaptic membranes and across which neurotransmitter diffuses.AQA, OCR
synaptic knob: The structure at the end of one neurone, containing many mitochondria, a large amount of smooth endoplasmic reticulum and vesicles filled with acetylcholine.AQA
synchronising mating: Sperm and egg are released at the same time, raising the probability of fertilisation, and the display itself may trigger ovulation or sperm release.AQA
Synthetic and genetically engineered medicines: Many medicines are now produced synthetically or by genetic engineering, such as human insulin from modified bacteria.OCR
Synthetic biology: Designing and constructing new biological parts, devices or systems, or redesigning existing biological systems.OCR
Systematic data recording and apparatus safety: Using apparatus such as a pooter, sweep net or pitfall trap safely and recording data systematically and consistently so results can be analysed.OCR
systematic sampling: A belt transect used where there is an obvious environmental gradient to observe changes along it.AQA
Systole and diastole of atria and ventricles: The coordinated contraction (systole) and relaxation (diastole) of the atria and ventricles in one heartbeat.OCR

T

T lymphocyte structure and maturation: T lymphocytes mature in the thymus, and helper T cells carry receptors that bind antigen presented by phagocytes.OCR
Table headings with quantity and unit: Clear headings state the quantity and its unit, for example 'Time / s' or 'Length / mm', with no units buried in data cells.OCR
Tangents for curved graphs: A tangent drawn at the required point, which allows a gradient to be estimated where the relationship is not linear.OCR
Taq DNA polymerase: An enzyme from a thermophilic bacterium that is not denatured at high temperature and joins free nucleotides to the end of each primer.AQA
Target cell response: Binding triggers a response inside the target, such as opening an ion channel, activating an enzyme cascade or altering gene expression.OCR
Target cells: Cells that respond to a hormone because they carry a complementary receptor.OCR
targeted drug delivery: A therapeutic drug is attached to an antibody whose binding site is complementary to an antigen present in greater numbers on target cells, and the antibody…AQA
taxa: The plural form, used when more than one group is meant, as in 'six taxa of shade-tolerant plants'.AQA
taxis: A directional response in which the organism moves its whole body towards or away from a directional stimulus.AQA
taxon: Each group within the classification hierarchy is called this, at any level, and the word names the group, not the rank.AQA
Taxonomic hierarchy: The system that groups organisms into ranks from domain through kingdom, phylum, class, order, family, genus to species, each level containing groups from the…OCR
taxonomy: The study of how organisms are placed into these groups, with a taxonomist using evidence about ancestry to decide which group an organism belongs to.AQA
telophase: Two dense groups of chromosomes at the poles, beginning to uncoil, with a new nuclear envelope forming and a cleavage furrow or cell plate appearing.AQA, OCR
TEM and SEM principles: A transmission electron microscope transmits electrons through an ultra-thin section to reveal 2D internal ultrastructure, while a scanning electron…AQA
temperature and enzymes: A factor where low temperature slows enzymes such as rubisco, while very high temperature denatures them and closes the stomata, cutting the carbon dioxide…AQA
temperature and kinetic energy: Higher temperatures increase the kinetic energy of ions, leading to a faster rate of passive diffusion of Na⁺ and K⁺ through voltage-gated channels.AQA
Temperature and phospholipid movement: Raising temperature increases phospholipid movement and can open gaps, increasing permeability; very high temperature denatures proteins and disrupts the…OCR
temperature rise of water: Recorded for a known mass of water in which the vessel sits, and used with the specific heat capacity to calculate energy released.AQA
Temperature, kinetic energy and denaturation: Raising temperature increases rate up to an optimum because molecules have more kinetic energy and more successful collisions; above the optimum, bonds…OCR
template strand: Each separated strand acts as this, with free DNA nucleotides attracted to the exposed complementary bases.AQA
Temporal and spatial summation: Summation allows sub-threshold impulses to reach threshold: spatial uses multiple neurones firing simultaneously, while temporal uses one neurone firing…AQA
tension: A negative pressure created at the top of the column when water is drawn out of the xylem in the leaf.AQA
terminally differentiated cells: Cells that leave the cell cycle and enter a non-dividing state called G0, such as neurones and mature skeletal muscle fibres.AQA
Termination of the signal: The signal is transient because the messenger is broken down or removed, so the response stops.OCR
terminator region: A sequence at the far end that causes RNA polymerase to detach, releasing the mRNA and preventing the enzyme running on into neighbouring DNA.AQA
Tertiary and quaternary structure: The precise three-dimensional shape comes from the primary sequence and is held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic…OCR
tertiary structure: The 3D shape of a protein, determined by where ionic bonds, hydrogen bonds and disulfide bridges form between the R groups of the amino acids.AQA
Tertiary structure and active site complementarity: Enzymes are globular proteins whose tertiary structure creates an active site complementary to a specific substrate.AQA
tertiary structure and pH: A change in pH alters the ionic and hydrogen bonds holding a protein's tertiary structure, so the active site is no longer complementary.AQA
Thermal cycling stages: Each cycle has three temperature stages: denaturation near 95 °C separates the strands, annealing around 55–65 °C lets primers bind, and extension near 72 °C…OCR
Thermoregulation in ectotherms: Gaining most heat from the environment and relying heavily on behavioural responses such as basking, seeking shade, burrowing or changing posture, with some…OCR
Thermoregulation in endotherms: Generating heat metabolically and regulating it physiologically through sweating, panting, vasodilation, vasoconstriction, shivering, increased metabolic rate…OCR
Thin diffusion pathway: A thin barrier, often a single layer of flattened cells, gives a short diffusion pathway and speeds diffusion.OCR
Thin sections: Sections mounted in water or a stain such as toluidine blue so that lignified walls are visible under a light microscope.OCR
third base position: The position at which codons for the same amino acid often differ, so a substitution there is the most likely to be silent.AQA
three condensation reactions: Each hydroxyl group reacts with one carboxyl group in a condensation reaction, so three condensations occur for every triglyceride assembled.AQA
Three domains of life: The system placing organisms in Bacteria, Archaea and Eukaryota, separating the two prokaryotic domains.OCR
three phosphate groups: The chain that, with adenine and ribose, makes up a molecule of ATP.AQA
three water molecules: Three molecules of water are released for every triglyceride assembled, one per ester bond.AQA
three-neurone arc: The route from receptor to sensory neurone, relay neurone, motor neurone and effector, in that order.AQA
threshold: The potential of about minus 55 mV that enough sodium ions must lift the inside to, so that positive feedback opens all the remaining voltage-gated sodium ion…AQA, OCR
Tidal volume: The volume of air moved into or out of the lungs in one normal breath at rest, typically about 0.5 dm³.OCR
tissue: A group of similar cells working together to carry out one function, such as squamous epithelium lining the alveoli.AQA, OCR
totipotent cell: A cell that can divide and produce every type of body cell and, in mammals, the extra-embryonic tissues such as the placenta, so a single one can give rise to…AQA
totipotent cells: Can divide and differentiate into any cell type; they contain the full genome but translate only part of their DNA.AQA
Toxin neutralisation: Antibodies bind to toxins produced by pathogens, neutralising them so they cannot bind to host cell receptors and cause harm.OCR
toxins: Non-self molecules, not cells, that also stimulate an immune response.AQA
Tracer experiments: Plants supplied with radioactive 14CO2 incorporate 14C into sugars, which are tracked moving through the phloem using autoradiography.AQA
Trachea: A wide tube supported by C-shaped cartilage rings that prevent collapse, and which bifurcates into two bronchi.AQA
Tracheal system ventilation in insects: Air moves in and out of tracheae through spiracles along the abdomen and thorax by diffusion and by abdominal pumping movements that change pressure.OCR
transcription and splicing: During transcription both introns and exons are transcribed into pre-mRNA, but introns are subsequently removed during splicing to form mature mRNA.AQA
transcription and translation mechanisms: RNA polymerase transcribes mRNA, and ribosomes assemble amino acids as dictated by codons, and these mechanisms are also universal.AQA
Transcription by RNA polymerase: DNA helicase unwinds the double helix and RNA polymerase joins free RNA nucleotides to the template strand by complementary base pairing.OCR
transcription factors: These activate only specific genes, so that eukaryotic cells become specialised for specific functions.AQA, OCR
Transcriptional control of gene expression: Control that determines whether a gene is transcribed into mRNA, for example through transcription factors or chromatin modification affecting RNA polymerase…OCR
transcriptional factor activation: Binding changes the tertiary structure of the receptor, releasing an inhibitor molecule and activating the factor.AQA
transcriptional factors: Proteins that move from the cytoplasm into the nucleus and stimulate or inhibit transcription of target genes.AQA
transduction: The conversion of stimulus energy into a generator potential and then into nerve impulses.AQA, OCR
Transects along environmental gradients: Using a transect to record how distribution changes along an environmental gradient.OCR
Transects along gradients: Distribution along a gradient is measured using a transect, recording organisms at intervals.OCR
Transects to record distribution: A line along which distribution changes is recorded, with quadrats at intervals giving a kite diagram or belt transect data.OCR
Transfer efficiency: Biomass at the higher trophic level divided by biomass at the lower trophic level, multiplied by 100.OCR
transfer RNA: A small folded molecule, held by hydrogen bonds between complementary bases within the same strand, with an anticodon of three exposed bases at one end and an…AQA
transformation: A cell that takes up the vector carrying the DNA is transformed.AQA
Transformation and selection of host cells: The vector is introduced into a host cell by transformation, and marker genes such as antibiotic-resistance genes allow transformed cells to be selected.OCR
transformed host cells: Cells that have taken up a plasmid carrying the fragment, with promoter and terminator regions added, and are then cultured.AQA
transgenic organisms: Organisms whose cells contain transferred DNA that can be translated within them.AQA
Translation at the ribosome: mRNA leaves the nucleus through a nuclear pore and attaches to a ribosome, where tRNA molecules carry specific amino acids.OCR
Translocation: The movement of assimilates, mainly sucrose and amino acids, through the phloem from sources to sinks.OCR
Transpiration: The loss of water vapour from the aerial parts of a plant, mainly through stomata in the leaves.AQA, OCR
Transpiration stream: The upward movement of water through the plant, which also carries dissolved mineral ions.OCR
Transplantation and its risks: Transplanting pancreatic tissue or islet cells can restore insulin secretion but carries risks of rejection and requires immunosuppressants.OCR
transport proteins: Include haemoglobin, which carries oxygen, and the channel and carrier proteins that move ions across membranes.AQA
Transverse section: A cross-section cut at right angles to the stem's long axis, revealing the ring or scattered arrangement of vascular bundles and the position of xylem within…OCR
Treatment and management: Type 1 is treated with insulin injections, while Type 2 is managed with diet, exercise and medication.OCR
triglyceride: A lipid with three hydrophobic tails and no polar group, making it non-polar and insoluble, so it is stored in droplets without affecting water potential.AQA
triglyceride as energy store: A molecule with three hydrocarbon tails and no polar group, so it is non-polar, hydrophobic and insoluble in water, making it an excellent energy store.AQA
Triglyceride structure: One glycerol molecule bonded to three fatty acid molecules by ester bonds, with the fatty acids saturated or unsaturated.AQA, OCR
Triglycerides: Non-polar and insoluble molecules with many carbon–hydrogen bonds, so oxidation releases large amounts of energy per gram, making them good energy stores and…OCR
triose phosphate: The simple sugar produced; two molecules can be joined to give a hexose sugar such as glucose.AQA
triplet: A sequence of three DNA bases that codes for a specific amino acid.AQA
triplet code: In DNA a sequence of three bases codes for one amino acid.AQA
Triplet code and codons: Three bases, a codon, specify one amino acid, and because there are four bases, 4³ = 64 possible codons arise.OCR
Triplet nature of the genetic code: The genetic code is a triplet code read in a fixed reading frame, so each group of three bases specifies one amino acid or a stop signal.OCR
trisomy: A zygote with forty-seven chromosomes in humans, formed when a gamete with n plus one chromosomes is fertilised by a normal gamete, as in Down's syndrome.AQA
Tropisms: Directional growth responses to a unilateral stimulus, such as phototropism to light or geotropism to gravity.OCR
Tropomyosin and troponin function: In relaxed muscle, tropomyosin covers myosin binding sites on actin filaments; calcium ions bind to troponin, altering its tertiary structure and pulling…AQA
tumour suppressor genes: These code for proteins that slow the cell cycle, repair DNA or trigger apoptosis, and a mutation can inactivate them so damaged cells survive and divide.AQA
Turgid: The state of a plant cell in a solution of higher (less negative) water potential, where it takes up water and the cellulose wall prevents lysis.OCR
twenty common amino acids: The amino acids common to all organisms, which differ only in their R group or side chain.AQA
Two divisions after one replication: Meiosis has two divisions after one round of DNA replication, giving four haploid cells.OCR
Type 1 diabetes: An example in which pancreatic beta cells are destroyed and insulin production is reduced.OCR
Types of gene mutation: Gene mutations are changes in the base sequence of DNA, classified as substitution, insertion or deletion, and as silent, missense or nonsense.OCR
Types of vaccine antigen: Vaccines may contain killed pathogens, attenuated live pathogens, toxoids, isolated antigens or subunit components.OCR

U

Ultrafiltration: Pressure-driven filtration at the glomerulus that separates small molecules by size and charge, retaining blood cells and large proteins.OCR
Ultrastructure: The fine structure of prokaryotic and eukaryotic cells, which can be compared to show similarities and differences.OCR
ultraviolet light and thymine dimers: Causes adjacent thymine bases to bond into dimers, distorting the DNA strand and making replication error-prone.AQA
Uncertainty in apparatus: The doubt in a single reading, often taken as half the smallest scale division for an analogue instrument or the stated tolerance for a digital one.OCR
unchanged primary structure: The amino acid sequence stays the same because the substituted base still produces a codon specifying the same amino acid.AQA
Unidirectional transmission and signal termination: Synapses ensure one-way transmission and allow summation of impulses, and acetylcholinesterase breaks down acetylcholine to stop the signal.OCR
Unidirectionality: Transmission is unidirectional as only the presynaptic knob has vesicles and only the postsynaptic membrane has receptors.AQA
unipotent: Cells found in mature mammals that are the most restricted, dividing to produce just one cell type.AQA
unipotent cells: A highly restricted category of stem cell found in mature mammals that can only differentiate into one specific cell type.AQA
Unit conversion between µm, mm and m: Keeping units consistent by converting micrometres to millimetres (1 mm = 1000 µm) or to metres (1 m = 10⁶ µm) before calculating, and stating the unit in the…OCR
Units and labelling: Axes or column headings give both the quantity and its unit, every axis states units, and the graph or table has a title.OCR
Units for rates: A rate combines two quantities, for example cm³ s⁻¹ for gas volume per second, keeping the time element in the denominator.OCR
units of biomass: Units that include area, such as g m⁻² or kg C m⁻², so values can be compared between sites of different sizes.AQA
universal code: The same triplet codes for the same amino acid in almost every organism, so a human gene inserted into a bacterium can yield the human protein.AQA
Universal communication in biology: The same name is used by biologists worldwide, so a species is identified unambiguously and data and research can be shared and compared reliably.OCR
universal genetic code: The same codons specify the same amino acids across organisms, which is why bacteria are used in recombinant DNA technology to produce human proteins.AQA
universal naming: The system in which a species carries the same name in every country and every language, removing the confusion caused by common names.AQA
Universality of the code: The same codons carry the same meaning in almost all organisms.OCR
unsaturated: A fatty acid with one or more carbon-carbon double bonds, so the chain is usually drawn with a kink at the double bond.AQA
unsaturated fatty acids: Fatty acids with one or more carbon-carbon double bonds, each of which puts a kink in the chain.AQA
Urine testing for glucose and protein: Measuring these substances in urine can indicate disease, since glucose suggests diabetes mellitus and protein suggests damage to the glomerulus.OCR
Use of multiple features together: Testing each feature in turn, such as nucleus present, cell wall material, chlorophyll, nutrition mode and number of cells, rather than one alone.OCR
Using microscopical evidence to justify identification: Using observed features from the image, such as nuclear position and fibre arrangement, to justify the identification rather than relying on the label alone.OCR

V

vaccine production: The chosen gene is inserted into a harmless host cell by recombinant DNA technology so the protein can be made in bulk before use.AQA
Validity of measurements: Obtaining measurements by controlling variables, using replicates and recording data accurately.OCR
Valve function: Atrioventricular and semilunar valves prevent backflow, being passive flaps rather than structures that actively pump blood.OCR
variable number tandem repeats: Short base sequences repeated many times between genes, where the number of repeats at a given position varies between individuals and is inherited.AQA
variable plasmid number: Each daughter cell receives a variable number of plasmid copies, because plasmid segregation is not perfectly equal.AQA
Variable R group: The side chain that differs between the twenty or so standard amino acids and gives each its distinct properties.OCR
variable region: The tip of each arm, whose amino acid sequence and tertiary structure differ between antibodies and which forms a binding site complementary to one specific…AQA
Variable repetitive DNA as the basis of profiling: Profiling exploits variation in repetitive, non-coding DNA to produce a pattern characteristic of an individual, so individuals differ in fragment lengths or…OCR
variation as raw material: Natural selection can only act on variation that already exists, and genetic diversity supplies this raw material.AQA
Variation between individuals: Differences revealed by sequencing, such as single nucleotide polymorphisms, which may relate to phenotype, disease or ancestry.OCR
variation for natural selection: A wider range of phenotypes provided by high genetic diversity, giving more variation for natural selection to act upon if environmental conditions change.AQA
Vascular bundles: In stems, these are arranged in a ring near the periphery, with xylem positioned internally and phloem externally.OCR
Vectors and fomites: Vectors are organisms such as mosquitoes and ticks that transmit pathogens, while fomites are indirect objects that carry contamination.OCR
vectors and plasmids: A vector, normally a plasmid or a virus, carries the DNA into a host cell.AQA
Vectors and recombinant DNA formation: A vector such as a plasmid carries the gene into a host cell, and DNA ligase joins gene to vector to form a recombinant plasmid.OCR
Vegetative propagation in horticulture: Horticulturists take cuttings, divide clumps, or separate runners and tubers to propagate desirable varieties cheaply and quickly while preserving the genotype.OCR
vena cava: Vessel that brings deoxygenated blood from the body into the right atrium.AQA
ventilation: The movement of air into and out of the lungs, continually replacing the air in the alveoli.AQA
Verification by re-sequencing: Re-sequencing engineered organisms verifies that the intended construct has been assembled correctly, supporting the design-build-test cycle.OCR
vesicles of acetylcholine: Structures within the presynaptic knob that are filled with acetylcholine.AQA
Vesicular bulk transport: Endocytosis brings material in and exocytosis releases it, both moving large quantities in vesicles and requiring ATP.OCR
Viral replication cycle: Replication starts when gp120 binds to a CD4 receptor on a helper T cell, the envelope fuses with the host membrane, and RNA and enzymes are released into the…AQA
virus-infected body cell: A source of presented antigen that displays viral antigens produced inside it.AQA
visual acuity: High acuity arises because each cone usually connects to its own bipolar neurone, so adjacent cones send separate impulses and are resolved as two points.AQA
Vital capacity: The maximum volume of air a person can forcibly exhale after a maximum inhalation, measured with a spirometer.OCR
VNTR amplification: A modern alternative in which specific VNTR loci are amplified by PCR, producing fragments whose length depends on the number of repeats.AQA
volume and pressure changes: Both changes increase the volume of the thoracic cavity, so the pressure inside falls below atmospheric pressure.AQA
Volumetric flask: A piece of laboratory glassware that makes up to a single calibrated volume.OCR
Volumetric pipette: A piece of laboratory glassware that delivers one accurate fixed volume.OCR

W

washing and controlled variables: Cut cylinders are washed thoroughly to remove pigment released by the cells cut at the surface, and the volume of solution, the exposure time, and the size…AQA
washing and false positives: Washing between every step is essential, because unbound antibody left behind gives a false positive.AQA
Waste disposal, cleaning and instrument storage: Keep the workspace clear, dispose of sharps in the designated container and biological waste as your centre directs, and clean and store instruments afterwards.OCR
water eliminated: One molecule of water is released, eliminated or removed, coming from a hydroxyl group from one molecule and a hydrogen atom from the other.AQA
Water potential: The tendency of water to move, measured in kPa, where pure water at atmospheric pressure is 0 kPa and solutions have negative values.OCR
Water potential gradient: The gradient down which water vapour diffuses out through stomata into the surrounding air, and which drives the transpiration stream.OCR
water potential of blood: The factor being controlled in osmoregulation, measured in kPa, where pure water is 0 kPa and any solution is negative, so more concentrated plasma has a…AQA
wave of depolarisation: A wave of depolarisation spreads across both atria, stimulating them to contract.AQA
Waveform identification (P, QRS, T): The P wave is atrial depolarisation, the QRS complex is ventricular depolarisation and the T wave is ventricular repolarisation.OCR
wavelength and resolution: The relatively long wavelength of light limits an optical microscope to a maximum resolution of about 0.2 micrometres, while electrons have a much shorter…AQA
Wavelength and resolving power of microscopes: Electron microscopes resolve better than light microscopes because electrons have a shorter wavelength than visible light.OCR
wavelength limit: Resolution is determined by the wavelength of the radiation used, so electrons having a much shorter wavelength than light provide higher resolution.AQA
waxy cuticle: A thick physical waterproof barrier that physically reduces evaporation, rather than trapping water vapour.AQA
Whole organism metabolism: The combined activity of enzymes across tissues and organs determines the organism's overall metabolic rate and its ability to function.OCR
Whole-genome data: The complete DNA sequence of an organism, allowing many genes and regions to be compared at once rather than single genes.OCR
Widened wavelength range and efficiency: Having several pigments widens the range of wavelengths usable, increasing the efficiency of light absorption.OCR

X

Xerophyte: A plant facing water shortage that reduces loss while storing or conserving water, using a thick waxy cuticle, sunken stomata in pits, rolled or reduced…OCR
Xylem and phloem functions: The xylem carries water and mineral ions upward from the roots, and the phloem translocates sugars and other assimilates.OCR
Xylem vessel structure and function: Elements that become hollow, lignified and dead, adapted for transporting water and mineral ions.OCR
Xylem vessels: Structures identified by thick lignified walls and a hollow lumen, lying toward the inner side of each bundle and distinguished from phloem in the same bundle.OCR

Z

zero net osmosis point: Where the line crosses zero, the water potential of that solution equals the water potential of the potato cells.AQA
zone of elongation: The tissue just behind the tip where growth factors change the rate at which cells elongate.AQA
zone of inhibition: The clear zone around each disc, measured and used to calculate its area.AQA
zygote: The cell formed at fertilisation, which with the cells of the first few mitotic divisions is totipotent.AQA

Other terms

1,4 and 1,6 linkages: Bonds running from carbon 1 of one residue to carbon 4 of the next, or, at branch points, from carbon 1 to carbon 6.AQA
70S ribosomes: Smaller ribosomes than those of eukaryotes, which are 80S.AQA
80S and 70S: Eukaryotic ribosomes are 80S, larger than the 70S ribosomes of prokaryotes and chloroplasts.AQA
α-glucose versus β-glucose: The two forms differ in the position of the hydroxyl group on carbon 1.OCR
β cell destruction: The β cells of the islets of Langerhans are destroyed by an autoimmune response, so little or no insulin is produced.OCR

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