Biology A (Salters-Nuffield) A-Level

Biology A (Salters-Nuffield) A-Level glossary

501 key terms for Biology A (Salters-Nuffield) A-Level, A to Z, each linked to the topic pages that use it.

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A

Abiotic factors: The non-living physical and chemical conditions of a habitat, such as light intensity, temperature, water availability, pH, salinity, oxygen concentration and…Pearson Edexcel
Abundance: How many individuals occur there; population size reflects how well conditions and resources within the niche support survival and reproduction.Pearson Edexcel
Acrosome: A lysosome-like vesicle at the tip of the sperm head that releases digestive enzymes to penetrate the egg's protective layers.Pearson Edexcel
Acrosome reaction: Binding to the zona pellucida triggers the release of hydrolytic enzymes from the acrosome that digest a path through the zona pellucida.Pearson Edexcel
Action potential propagation: The action potential travels along the axon as adjacent regions reach threshold.Pearson Edexcel
Activators: Transcription factors that stimulate transcription by helping RNA polymerase bind.Pearson Edexcel
Active immunity: Protection where the person's own lymphocytes produce antibodies and memory cells, developing slowly but lasting a long time.Pearson Edexcel
Active mRNA: The processed transcript, produced by transcription of a gene, that is able to be translated at the ribosome.Pearson Edexcel
Active transport: Movement of solutes against a concentration gradient using carrier proteins and energy from ATP hydrolysis to ADP and Pi.Pearson Edexcel
Actual risk: The measured probability of an outcome, such as the increased chance of heart disease from a high saturated fat diet or smoking.Pearson Edexcel
Adaptation: An inherited feature that improves survival and reproduction in a particular niche.Pearson Edexcel
Adaptation and speciation: Adaptation occurs through natural selection acting on inherited variation over generations, and new species can evolve if populations become reproductively…Pearson Edexcel
Aerobic exercise: Regular activity that lowers blood pressure, improves the HDL:LDL balance and helps control body mass, reducing CHD risk.Pearson Edexcel
Aerobic respiration: The splitting of the respiratory substrate to release carbon dioxide as a waste product and the reuniting of hydrogen with atmospheric oxygen with the release…Pearson Edexcel
Agarose gel: Acts as a molecular sieve so that shorter DNA fragments travel further and faster than longer fragments in a given time.Pearson Edexcel
Allele frequency: How common an allele is in a population, which can change progressively across generations in response to environmental conditions.Pearson Edexcel
Allele frequency change over time: Over many generations the frequency of advantageous alleles increases as individuals pass them to offspring, so the population becomes better adapted.Pearson Edexcel
Alleles: Versions of the same gene that occupy the same locus on homologous chromosomes.Pearson Edexcel
Allopatric speciation: Speciation that requires a physical barrier, such as a river or mountain range, separating populations, after which independent mutation, selection and…Pearson Edexcel
Alternative polyadenylation: A post-transcriptional change that alters the 3′ end of mRNA.Pearson Edexcel
Alternative splicing: Removes introns and joins exons in different combinations, producing different mature mRNA molecules from the same pre-mRNA.Pearson Edexcel
Amniocentesis: Amniotic fluid containing fetal cells is sampled with a needle, usually at about 15–20 weeks of pregnancy, and the cells are cultured and karyotyped or…Pearson Edexcel
Amyloplast: An oval body similar in shape to a chloroplast but lacking thylakoids and containing smooth, electron-lucent starch grains.Pearson Edexcel
Amyloplasts: Colourless leucoplasts that store starch grains in plant cells.Pearson Edexcel
Anaerobic decomposition: Waterlogged or acidic conditions slow decomposition and can produce methane instead of carbon dioxide.Pearson Edexcel
Anaerobic metabolism: Pyruvate is reduced to lactate, regenerating the oxidised coenzyme so glycolysis can continue when oxygen is absent.Pearson Edexcel
Animal experiments: Allow controlled manipulation of environment, such as enriched or deprived conditions, to test nurture effects on brain development.Pearson Edexcel
Animal models in neuroscience research: These allow invasive recording and controlled sensory experience that is not possible in humans, supporting human research rather than replacing ethical human…Pearson Edexcel
Annealing: A stage near 55 °C that allows primers to bind by complementary base pairing to sequences flanking the target.Pearson Edexcel
Antagonistic muscle action: The circular and radial iris muscles have opposing contractions: in bright light circular muscle contracts and radial relaxes, and in dim light radial…Pearson Edexcel
Antagonistic muscle pairs: Muscles that work opposite each other, so that as one contracts the other relaxes, enabling controlled movement.Pearson Edexcel
Anthropogenic causes of climate change: Human activities that increase atmospheric greenhouse gases, especially carbon dioxide and methane, such as burning fossil fuels, agriculture, landfill and…Pearson Edexcel
Antibiotic stewardship: Codes of practice for antibiotic prescription, such as prescribing only when needed, using narrow spectrum antibiotics and completing the course, which reduce…Pearson Edexcel
Antibody: A Y-shaped glycoprotein produced by plasma cells, with variable regions that bind a specific antigen epitope to form an antigen–antibody complex.Pearson Edexcel
Anticoagulants: Reduce blood clotting, so they lower the risk of thrombosis but increase bleeding risk.Pearson Edexcel
Anticodon: Part of a transfer RNA molecule that is complementary to the mRNA codons, while the molecule carries a specific amino acid.Pearson Edexcel
Antigen: A molecule, often a protein or glycoprotein on a pathogen's surface, recognised as non-self and triggering an immune response.Pearson Edexcel
Antigen-presenting cell: A cell such as a macrophage that displays processed antigen fragments on its surface and activates T helper cells.Pearson Edexcel
Antigenic drift: The gradual accumulation of mutations in surface antigen genes.Pearson Edexcel
Antigenic shift: A major reassortment of genes, producing a new subtype.Pearson Edexcel
Antigenic variation: A change in pathogen surface antigens so that existing antibodies and memory cells no longer bind effectively.Pearson Edexcel
Antihypertensives: Lower blood pressure, reducing the mechanical stress that damages artery walls and promotes atheroma formation, but can cause hypotension or dizziness.Pearson Edexcel
Application of molecular techniques to plants and animals: Profiles reveal genetic relationships in plants and animals, for example confirming pedigree or identifying endangered species.Pearson Edexcel
Artificial passive immunity: Immunity following injection of antibodies produced in another organism, for example antivenom, which neutralise a toxin immediately and form no memory cells.Pearson Edexcel
Aseptic technique: Disinfect the bench, flame the loop and bottle neck, lift the lid minimally, work near a Bunsen flame, and never open a dish containing a grown culture.Pearson Edexcel
Atheroma and thrombosis mechanisms: Atheroma builds over decades, and high blood pressure damages the endothelium, promoting plaque formation and thrombosis.Pearson Edexcel
ATP: The immediate energy source because its hydrolysis releases energy directly for conformational changes in the carrier protein.Pearson Edexcel
ATP as universal energy currency: Hydrolysis of its terminal phosphoanhydride bond releases energy in a single rapid step, providing an immediate supply for biological processes.Pearson Edexcel
ATP synthase: The enzyme through which protons diffuse back, using the energy of this flow to phosphorylate ADP to ATP.Pearson Edexcel
ATP synthesis: Net ATP is produced by substrate-level phosphorylation, after phosphate from ATP is used to phosphorylate the hexose.Pearson Edexcel
ATPase: The enzyme that catalyses the hydrolysis of ATP to ADP and inorganic phosphate, re-energising the myosin head for another cycle.Pearson Edexcel
Atrioventricular node (AVN): Introduces a slight delay to allow the ventricles to fill before the impulse travels onwards.Pearson Edexcel
Autonomic control of the pupil: In bright light parasympathetic motor neurones constrict the pupil, while in dim light sympathetic motor neurones dilate it.Pearson Edexcel

B

B effector cells: These differentiate into plasma cells that secrete antibodies, which bind specific antigens and can neutralise, agglutinate or opsonise pathogens.Pearson Edexcel
B memory cells: Long-lived cells that respond rapidly to re-infection by dividing into plasma cells and more memory cells, giving a faster and greater secondary response.Pearson Edexcel
Bactericidal: Kills bacteria directly, so the number of viable cells falls over time.Pearson Edexcel
Bacteriostatic: Stops bacteria growing and dividing, so numbers stay roughly constant while the drug is present.Pearson Edexcel
Barriers to infection: The skin, stomach acid, and gut and skin flora protect the body by preventing pathogen entry or creating conditions hostile to pathogens.Pearson Edexcel
Behavioural adaptation: An action that improves survival and reproduction, such as staying in a burrow during the day to avoid desert heat.Pearson Edexcel
Benedict's test: A laboratory test that gives a positive result with reducing sugars and can detect the products of hydrolysis.Pearson Edexcel
Benefits: These include producing medicines such as human insulin, improving crop yield or nutritional content, and reducing pesticide use.Pearson Edexcel
Binary fission: Bacteria reproduce asexually by this process, so growth is measured as an increase in population number rather than cell size.Pearson Edexcel
Biocompatibility: A property for which materials such as titanium are chosen, alongside strength and low density, when prostheses replace missing limbs or joint surfaces.Pearson Edexcel
Biodegradability: Plant fibres and starch-based bioplastics can break down, cutting plastic pollution, though some need industrial composting.Pearson Edexcel
Biodiversity: The variety of life, including species richness, genetic diversity, and ecosystem diversity.Pearson Edexcel
Biofuels: Fuels derived from recently living biological material, such as crops or waste, which can replace fossil fuels.Pearson Edexcel
Biomass units: Dry mass per unit area per unit time, for example g m⁻² year⁻¹, used when calculating the efficiency of biomass transfer.Pearson Edexcel
Biotic factors: Factors involving other living organisms, including competition for food, water, space, light and mates, predation, disease and grazing.Pearson Edexcel
Block to polyspermy: The released contents harden the zona pellucida and destroy sperm-binding receptors so no second sperm can enter.Pearson Edexcel
Body mass index: Mass in kg divided by height in m squared (kg m⁻²), used as an indicator of obesity.Pearson Edexcel
Buffer: A solution used to maintain a constant pH and conduct electricity during electrophoresis.Pearson Edexcel

C

Calcium and magnesium in plant structure and function: Calcium ions (Ca²⁺) are components of calcium pectate in the middle lamella, affect membrane permeability and act in enzyme regulation, while magnesium ions…Pearson Edexcel
Carbon capture: A method that stores carbon dioxide underground, removing it from the atmosphere or preventing its release.Pearson Edexcel
Carbon cycle: The movement of carbon between the atmosphere, oceans, land and organisms, which these methods alter by changing the balance between fixation and release.Pearson Edexcel
Carbon dioxide and methane sources: Burning fossil fuels releases CO₂, while agriculture, landfill and fossil-fuel extraction release CH₄, for example a coal-fired power station adds CO₂ and…Pearson Edexcel
Carbon dioxide production: Released by decarboxylation in the link reaction and also produced as the Krebs cycle completes the oxidation of glucose.Pearson Edexcel
Carbon dioxide records: Ice cores and direct atmospheric measurements show past and present CO₂ concentrations.Pearson Edexcel
Carbon fixation in the Calvin cycle: Carbon dioxide joins RuBP, a 5-carbon sugar phosphate, in a reaction catalysed by RUBISCO, forming an unstable 6-carbon product that splits into two GP…Pearson Edexcel
Carbon fixation into glucose: Hydrogen is stored in a fuel by combining it with carbon dioxide to make glucose.Pearson Edexcel
Carbon sinks: These absorb carbon dioxide, and planting trees increases them as a method to reduce atmospheric levels of the gas.Pearson Edexcel
Cardiac cycle: A repeating sequence of atrial systole, ventricular systole and cardiac diastole.Pearson Edexcel
Cardiac output: The volume of blood pumped by one ventricle per minute, calculated as heart rate multiplied by stroke volume.Pearson Edexcel
Cardiovascular control centre: A centre in the medulla oblongata that adjusts heart rate via autonomic nerves acting on the sinoatrial node.Pearson Edexcel
Carrier identification: Testing an adult's DNA to show whether they carry one copy of a recessive allele, so the probability of an affected child can be estimated.Pearson Edexcel
Carrier proteins: Integral membrane proteins that bind specific solutes and change shape to transport them.Pearson Edexcel
Cation channels: Structures kept open by rhodopsin in darkness so Na⁺ enters, and closed by light so Na⁺ entry falls.Pearson Edexcel
CD4 receptors: Receptors on helper T cells that HIV binds to before fusing with the cell.Pearson Edexcel
Cell cycle and interphase: The sequence of events from one cell division to the next, in which interphase is the longest phase and includes G₁, S and G₂.Pearson Edexcel
Cell signalling: A neurone meets another cell at a junction, and neurotransmitter released from the presynaptic knob carries the signal across to the postsynaptic membrane.Pearson Edexcel
Cell specialisation and tissue function: Cells specialise, and cells with the same function and similar structure group together to form a tissue.Pearson Edexcel
Cellular energy demand and supply: Cells maintain a high ATP/ADP ratio so that ATP can be regenerated continuously and energy is available without delay.Pearson Edexcel
Cellular respiration pathways: Glycolysis is the first stage of both aerobic and anaerobic respiration, occurring in the cytoplasm with or without oxygen.Pearson Edexcel
Cellulose: A polymer of β-glucose joined by β-1,4 glycosidic bonds, with adjacent units rotated 180° so the chain is straight and unbranched.Pearson Edexcel
Cellulose microfibrils: Structures in plant cell walls arranged in a matrix that resist tensile forces, with their angle affecting the direction and strength of the wall.Pearson Edexcel
Cerebellum: A region at the back, beneath the cerebral hemispheres, that coordinates balance, posture and fine motor skills.Pearson Edexcel
Cerebral hemispheres: The largest, outermost, folded region of the brain, divided into left and right halves and joined by the corpus callosum.Pearson Edexcel
CFTR chloride channel: A channel that is faulty because of a mutation in the CFTR gene, so chloride ions are not transported normally.Pearson Edexcel
CFTR gene: The gene that codes for a chloride ion channel protein in cell membranes, in which many different mutations can cause cystic fibrosis.Pearson Edexcel
Chain of infection: The sequence of transmission that hospital practice codes break, linking each code to a specific contributory cause of hospital acquired infections.Pearson Edexcel
Changing rainfall patterns: Some regions become wetter and others drier, so water availability for photosynthesis, germination and breeding shifts.Pearson Edexcel
Chemiosmosis: The process in which protons flow back through ATP synthase, and the energy of this flow drives the phosphorylation of ADP to ATP.Pearson Edexcel
Chloroplast: A large oval body with a double membrane, stacked thylakoids forming grana and a granular stroma.Pearson Edexcel
Cholesterol: Present in animal cell membranes and modulates membrane fluidity and stability.Pearson Edexcel
Circular DNA: A single circular DNA molecule lying free in the cytoplasm, not enclosed in a nuclear envelope.Pearson Edexcel
Classification: A means of organising the variety of life based on relationships between organisms using differences and similarities in phenotypes and in genotypes.Pearson Edexcel
Climate change: Alters rainfall patterns and shifts seasonal cycles, and both changes affect plants and animals.Pearson Edexcel
Climate modelling: Combining measured data with scientific understanding of processes to project future climate change, such as projected temperature rise or sea-level change.Pearson Edexcel
Climax community: The final, stable, self-sustaining stage in equilibrium with the prevailing climate.Pearson Edexcel
Closed culture: A system in which numbers rise through lag, log, stationary and death phases as nutrients fall and toxic waste accumulates.Pearson Edexcel
Co-ordination: Requires communication between receptors, processing centres and effectors so that the whole organism responds appropriately to a stimulus.Pearson Edexcel
Codon: A sequence on messenger RNA that the ribosome reads during translation, with anticodons on transfer RNA complementary to it.Pearson Edexcel
Coenzymes in respiration: Reduced NAD and reduced FAD carry hydrogen to the electron transport chain, where oxidative phosphorylation generates most ATP.Pearson Edexcel
Collagen: A long, insoluble fibrous protein of three polypeptide chains wound into a triple helix, cross-linked to provide tensile strength.Pearson Edexcel
Community: All the populations of different species living and interacting in the same habitat.Pearson Edexcel
Complementary base pairing: Occurs between adenine and thymine, or adenine and uracil in RNA, and between cytosine and guanine.Pearson Edexcel
Concentration difference: A steep concentration difference between the two sides of the exchange surface increases the net movement of molecules by diffusion.Pearson Edexcel
Condensation: A reaction in which two monosaccharides join with the loss of one water molecule per glycosidic bond formed.Pearson Edexcel
Conservation: Protecting and preserving the natural environment, which can conflict with human needs for resources.Pearson Edexcel
Conservation biology: Conservation efforts aim to mitigate the threats that reduce populations, cause extinctions and lower genetic diversity.Pearson Edexcel
Continuous variation and normal distribution: Phenotype values form a smooth range, often with a normal distribution when frequency is plotted against the measured value.Pearson Edexcel
Control of variables: Keeping other variables constant, such as light intensity or temperature, to obtain valid results when investigating a factor.Pearson Edexcel
Control variables: Include pH, substrate concentration, enzyme concentration, volumes and time intervals, so that temperature is the only factor affecting rate.Pearson Edexcel
Controlled variables: Named variables such as light intensity, water volume, salinity, pH, humidity and handling time kept constant so that temperature is the only independent…Pearson Edexcel
Coordination centre: Part of a feedback loop that processes the deviation detected by the receptor, alongside the receptor and effector.Pearson Edexcel
Correlation: An association between variables, such as cholesterol levels and CVD, which alone does not prove causation because confounding factors such as diet, smoking…Pearson Edexcel
Correlation and causation: A correlation shows two variables changing together, while causation requires a plausible mechanism and supporting evidence.Pearson Edexcel
Cortical reaction: Triggered by sperm-oocyte membrane fusion, cortical granules fuse with the oocyte membrane and release their contents into the zona pellucida.Pearson Edexcel
Cristae: Folded structures within a double membrane that indicate a mitochondrion in an electron micrograph.Pearson Edexcel
Critical evaluation: The process by which the scientific community evaluates new data through peer review, replication and debate, which can overturn older groupings.Pearson Edexcel
Critical period: A restricted early developmental window of high plasticity in the visual cortex, during which experience of clear images is needed for normal wiring.Pearson Edexcel
Critical periods and experience-dependent plasticity: Early deprivation shifts the balance of ocular dominance, informing ideas about critical periods in human brain development.Pearson Edexcel
Cross-cultural studies: Compare development across different cultural settings to identify universal aspects suggesting nature versus culturally variable aspects suggesting nurture.Pearson Edexcel
Cross-sectional area: Calculated using A = πr² after halving the measured diameter to obtain the radius.Pearson Edexcel
Crossing over: The exchange of corresponding segments of alleles between non-sister chromatids of a homologous pair at chiasmata.Pearson Edexcel
CVS: Chorionic villi are sampled, usually at about 11–14 weeks, giving earlier results than amniocentesis but with a slightly higher risk of miscarriage.Pearson Edexcel
Cytoplasm: The material inside the cell where metabolism occurs.Pearson Edexcel

D

DCPIP: An electron acceptor that becomes reduced and changes colour when illuminated chloroplasts carry out the light-dependent reactions.Pearson Edexcel
Decomposition: Autolysis by the body's own enzymes, then putrefaction by micro-organisms; its extent indicates elapsed time and is faster in warm, moist, oxygen-rich…Pearson Edexcel
Degenerate: Most amino acids are encoded by more than one codon, although each codon specifies only one amino acid.Pearson Edexcel
Denaturation: Above an optimum, hydrogen and ionic bonds break, the active site loses its shape and substrate no longer fits, so rate falls sharply.Pearson Edexcel
Dendrochronology: Uses tree-ring width and density; wide rings often indicate favourable growth conditions, narrow rings less favourable conditions.Pearson Edexcel
Differential gene expression: All cells contain the same genes, but only some are transcribed and translated in each cell type, so cells become specialised.Pearson Edexcel
Diffusion: A passive and relatively slow process that only supplies an organism if every cell lies close to the exchange surface.Pearson Edexcel
digitalis soup: A variable herbal mixture prepared by William Withering from foxglove, whose observations suggested benefit but lacked controlled comparison.Pearson Edexcel
Disaccharides: Two monosaccharides joined by a glycosidic bond, forming soluble transport forms such as sucrose.Pearson Edexcel
Distinguishing learning from fatigue and adaptation: Habituation is a learned change in the nervous system's response, not a change in receptor sensitivity, and a habituated animal still responds normally to…Pearson Edexcel
Distribution: Where a species is found; a species occurs only where all its niche requirements are satisfied, so absence indicates a limiting factor.Pearson Edexcel
DNA: The genetic material that stores the instructions for proteins.Pearson Edexcel
DNA methylation: Adding methyl groups to cytosine bases, typically in CpG-rich promoter regions, usually reducing transcription by making the promoter less accessible to…Pearson Edexcel
DNA polymerase: Catalyses the formation of phosphodiester bonds between adjacent nucleotides, joining the 3′-OH of one nucleotide to the 5′-phosphate of the next.Pearson Edexcel
DNA profiling: A technique that compares variable repeat sequences between individuals, producing a distinctive band pattern used for identification and determining genetic…Pearson Edexcel
DNA replication and sister chromatids: During S phase DNA replication produces two identical copies of each chromosome, each consisting of two sister chromatids held at a centromere.Pearson Edexcel
DNA-specific stain: A dye such as acetic orcein or Feulgen reagent that makes chromosomes visible by binding to DNA.Pearson Edexcel
Dopamine: A naturally occurring brain chemical, a neurotransmitter involved in motor control, whose deficiency due to loss of dopaminergic neurons in the substantia…Pearson Edexcel
double blind trials: Trials in which neither participants nor assessors know who receives the treatment, reducing bias.Pearson Edexcel
Double membrane envelope: The envelope that encloses the organelle and controls the movement of substances into and out of the chloroplast.Pearson Edexcel
Dynamic equilibrium: Conditions fluctuate within narrow limits rather than remaining absolutely constant.Pearson Edexcel

E

Ecological succession: The directional change in a community over time, from colonisation by pioneer species through seral stages to a climax community.Pearson Edexcel
Ecosystem: The community plus the non-living surroundings it interacts with, so energy flow and nutrient cycling are included.Pearson Edexcel
Efficiency: The useful biomass or energy passed to the next trophic level compared with the amount available at the previous level, expressed as a percentage.Pearson Edexcel
Electric field: When applied, it causes negatively charged DNA fragments to migrate towards the positive electrode because of their phosphate groups.Pearson Edexcel
Electrocardiograms (ECGs): Record the electrical activity of the heart, and abnormal traces help diagnose cardiovascular diseases like arrhythmias or myocardial infarctions.Pearson Edexcel
Electron transport and chemiosmosis: Excited electrons pass along an electron transport chain, releasing energy that pumps protons and drives ATP synthesis by chemiosmosis.Pearson Edexcel
Electron transport chain: A chain of carriers on the inner mitochondrial membrane to which reduced NAD and reduced FAD donate electrons.Pearson Edexcel
Endemism: Describes species unique to a specific location, restricted to a particular geographic area.Pearson Edexcel
Endergonic and exergonic reactions: Phosphorylation of ADP is endergonic and requires an input of energy, whereas hydrolysis of ATP is exergonic and releases energy.Pearson Edexcel
Endothelial dysfunction: Damage to the endothelium lining an artery makes it more permeable and less able to regulate what enters the wall, and is the initiating event.Pearson Edexcel
Energy balance: Calculated as energy intake minus energy expenditure, where a positive value means weight gain and a negative value means weight loss.Pearson Edexcel
Energy conservation and attention: Saving energy and allowing attention to be directed to new or significant stimuli.Pearson Edexcel
Energy expenditure: The energy used in metabolism and activity.Pearson Edexcel
Energy intake: The chemical energy from food, measured in kJ or kcal.Pearson Edexcel
Energy losses: Energy lost in respiration, movement, excretion, egestion and as heat, and energy in biomass not eaten or digested, which makes transfer inefficient.Pearson Edexcel
Energy units: Units such as kJ m⁻² year⁻¹, used when calculating the efficiency of energy transfer between trophic levels.Pearson Edexcel
Enhanced greenhouse effect: Occurs when extra greenhouse gases trap more heat, raising global mean temperature, as distinct from the natural greenhouse effect that keeps Earth warm…Pearson Edexcel
Enzyme activity: Rises with temperature because particles gain kinetic energy and collide more often, forming more enzyme–substrate complexes.Pearson Edexcel
Enzyme-controlled termination of signals: Acetylcholinesterase hydrolyses acetylcholine, clearing the cleft so the signal ends and the synapse resets.Pearson Edexcel
Epigenetic changes: Changes that modify gene activation without altering the DNA base sequence, affecting whether and how strongly a gene is transcribed, and are reversible.Pearson Edexcel
Ester bond: Formed between the glycerol hydroxyl group and the fatty acid carboxyl group, with three formed in total per triglyceride.Pearson Edexcel
Ethical concerns: These include animal welfare and whether it is right to alter organisms.Pearson Edexcel
Ethics and translation of animal findings to humans: Invasive procedures on sentient animals need justification, and findings must be translated to humans cautiously because brains differ.Pearson Edexcel
Ethics of genetic information: Social, moral and ethical issues include privacy and consent for genomic data, discrimination by insurers or employers, unequal access to testing and…Pearson Edexcel
Evolution: A change in the allele frequency within a population over generations, not simply a change in an individual organism.Pearson Edexcel
Evolution of biodiversity over time: Over time the variety of life has become extensive through evolutionary processes such as natural selection and speciation.Pearson Edexcel
Evolutionary race: Reciprocal selection in which host immune defences select pathogen variants that evade them, and those variants then select host variants better able to…Pearson Edexcel
Extension: A stage near 72 °C where DNA polymerase adds free nucleotides to the primer, synthesising a complementary strand.Pearson Edexcel
Extensors: Muscles that increase joint angle, such as the triceps brachii at the elbow.Pearson Edexcel
Extracellular digestion: Micro-organisms secrete enzymes that break down proteins, carbohydrates and lipids into small soluble molecules before absorption.Pearson Edexcel
Extracellular enzymes: Enzymes produced by cells and secreted to catalyse reactions outside of cells, often in the gut lumen.Pearson Edexcel
Extrapolation: Extending a trend or relationship beyond the range of available data to estimate future values.Pearson Edexcel
Eyepiece graticule: Used with a stage micrometer to measure fibre diameter, being calibrated against the stage micrometer before use.Pearson Edexcel

F

F508del: The common three-nucleotide deletion that removes phenylalanine at position 508, leading to misfolding of the CFTR protein, which is degraded before reaching…Pearson Edexcel
Facultative anaerobes: Organisms that can switch their oxygen requirement, unlike obligate aerobes and obligate anaerobes.Pearson Edexcel
Fast twitch fibres: Fibres with fewer mitochondria and less myoglobin but more glycogen and glycolytic enzymes, so they contract powerfully for short bursts.Pearson Edexcel
Fibrin: Insoluble protein forming a mesh that traps red blood cells and platelets to produce a clot.Pearson Edexcel
Fick's Law: The rate of diffusion is proportional to (surface area × concentration difference) ÷ thickness, so changing any of these factors alters the rate.Pearson Edexcel
Flagellum: A helical structure that rotates to move the cell.Pearson Edexcel
Fluid mosaic model: Describes the membrane as a fluid bilayer with a mosaic of proteins and other components that can move laterally within the bilayer.Pearson Edexcel
fMRI: Detects changes in blood oxygenation as a proxy for neural activity, mapping active brain regions during tasks to investigate function.Pearson Edexcel
Foam cells: Macrophages that have engulfed oxidised low-density lipoprotein (LDL) until they form these cells, following the entry of monocytes into the intima.Pearson Edexcel
Forensic entomology: Use of blowfly life cycle stages (eggs, larvae, pupae, adults) and accumulated temperature (degree days) to refine the estimate of time since death.Pearson Edexcel
Forensic identification: Comparing a crime-scene profile with a suspect's profile, where a match supports identification while a mismatch excludes.Pearson Edexcel
Frameshift: A shift caused by insertion or deletion of a number of bases not divisible by three, altering every subsequent codon and usually producing a truncated or…Pearson Edexcel
Free water molecules: Water molecules that are not bound to solutes, so adding solute reduces their concentration.Pearson Edexcel
Fuel molecules and stored energy: Glucose is a fuel that stores energy in its bonds for respiration and growth.Pearson Edexcel
Fundamental niche: The niche set by abiotic factors such as light, water, pH and temperature.Pearson Edexcel
Future climate scenarios: Model projections are scenarios rather than certainties because they depend on assumptions about human activity and multiple models may produce a range of…Pearson Edexcel

G

Gas exchange with the atmosphere: Oxygen produced when water is split is released into the atmosphere.Pearson Edexcel
Gaseous exchange: Reduced when mucus obstructs airways and bronchioles, which also increases infection risk.Pearson Edexcel
Gel electrophoresis: A technique that separates DNA fragments of different length by moving them through an agarose gel matrix under an applied electric field.Pearson Edexcel
Gene: A sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain.Pearson Edexcel
Gene flow: The movement of alleles between populations through migration and successful interbreeding; isolation reduces or prevents this movement.Pearson Edexcel
Gene mutation: A change in the DNA base sequence that produces a new version of a gene and is the source of new alleles.Pearson Edexcel
Genetic code: The definite order of bases along one strand of DNA, which carries the coded information.Pearson Edexcel
Genetic modification: Introduces a gene into a plant, animal or microorganism so that it makes a useful drug.Pearson Edexcel
Genetic variation and drug response: Variation can affect drug metabolism, efficacy and risk of side effects, so variants in genes coding for drug-metabolising enzymes can show whether a standard…Pearson Edexcel
Genetic variation and mutation: Variation within a population arises from mutation and sexual reproduction, producing a range of phenotypes.Pearson Edexcel
Genetically identical daughter cells: Mitosis separates sister chromatids so each daughter nucleus receives an identical set of chromosomes, giving cells with the same chromosome number as the…Pearson Edexcel
Genome sequencing: Projects produce data about the base sequences of individuals and populations, revealing an individual's DNA base sequence and genetic variants.Pearson Edexcel
Genomics: The large-scale study of genomes and DNA sequences, providing evidence for evolution by comparing DNA sequences and genomes.Pearson Edexcel
Genotype-environment interaction: Phenotype is the observable characteristic produced by the interaction of genotype with the environment, so environmental factors can alter gene expression…Pearson Edexcel
Gluconeogenesis: The conversion of pyruvate into glucose.Pearson Edexcel
Glycogen: A highly branched polysaccharide that allows rapid hydrolysis to glucose for respiration.Pearson Edexcel
Glycoproteins and glycolipids: Involved in cell recognition, adhesion, and as receptors.Pearson Edexcel
Glycosidic bond: The bond formed between two monosaccharide units, which is broken by hydrolysis.Pearson Edexcel
Golgi apparatus: A structure that modifies, packages and sorts proteins and lipids into vesicles.Pearson Edexcel
Grana: Stacks of thylakoids that increase the surface area for light absorption and hold photosystems, electron carriers and ATP synthase.Pearson Edexcel
Greenhouse gases and infrared absorption: Carbon dioxide and methane absorb outgoing infrared radiation emitted by the Earth's surface, then re-emit it in all directions, warming the lower atmosphere.Pearson Edexcel
Gross primary productivity: The total energy fixed by photosynthesis.Pearson Edexcel
Growth and asexual reproduction: Mitosis allows growth by increasing cell number and enables asexual reproduction, for example in strawberry runners, where offspring are clones of the parent.Pearson Edexcel

H

Habitat: The place where an organism lives, defined by its physical features, such as a rocky shore or a hedgerow.Pearson Edexcel
Habituation: A simple form of learning in which an animal gradually stops responding to a repeated stimulus that proves harmless or unimportant.Pearson Edexcel
Haemoglobin: A compact, soluble globular protein with a quaternary structure of four polypeptide chains, each containing a haem group that binds oxygen.Pearson Edexcel
Haploid: Describes gametes, so that fusion of one sperm nucleus with one egg nucleus restores the diploid chromosome number.Pearson Edexcel
Haploid gametes: Gametes with half the diploid chromosome number, produced by meiosis, which combine at fertilisation to add further variation.Pearson Edexcel
Hardy-Weinberg equation: Models allele and genotype frequencies in an ideal population, using p + q = 1 for two alleles and p² + 2pq + q² = 1 for genotypes.Pearson Edexcel
HDL: Lipoprotein particles that return excess cholesterol to the liver, where it is excreted, so higher levels are generally protective and associated with lower…Pearson Edexcel
Helper T cells: Cells destroyed by budding HIV virions, so that falling numbers impair immune responses and opportunistic infections cause symptoms and eventually death.Pearson Edexcel
Heterozygosity index (H): A measure of genetic diversity within a species, calculated as the number of heterozygotes divided by the number of individuals in the population.Pearson Edexcel
Hill reaction: The process in which isolated chloroplasts, when illuminated, transfer electrons to an electron acceptor such as DCPIP, showing they carry out the…Pearson Edexcel
Histone modification: Acetylation or methylation of histone tails changes chromatin packing: acetylation loosens chromatin and increases transcription, while some methylation…Pearson Edexcel
Homeostasis: The maintenance of a stable internal environment around a set point despite external changes.Pearson Edexcel
Hormonal control: Endocrine glands secrete hormones into the blood, which carry them to target cells bearing complementary receptors and alter cell activity.Pearson Edexcel
Hospital acquired infections: Infections arising from contributory causes such as pathogen transmission via hands, equipment and air, vulnerable patients, invasive procedures and…Pearson Edexcel
Hubel and Wiesel and visual cortex development: They used kittens and monkeys with monocular deprivation, and their recordings revealed ocular dominance columns and experience-dependent shifts in cortical…Pearson Edexcel
Human impact on ecosystems: Human activities such as habitat destruction, pollution, overexploitation, invasive species and climate change threaten the variety of life.Pearson Edexcel
Hydrogen bonding: Bonds form between complementary bases on opposite DNA strands, holding the strands together as they twist into a double helix.Pearson Edexcel
Hydrolysis: A process that uses water to split a glycosidic bond, regenerating monosaccharides, and can be catalysed by dilute acid or by specific enzymes.Pearson Edexcel
Hyperpolarisation: The change in a rod cell when light closes cation channels and Na⁺ entry decreases.Pearson Edexcel
Hypothalamus: A region lying below the thalamus and above the pituitary gland that maintains homeostasis, regulating body temperature, blood water potential, hunger and…Pearson Edexcel

I

IAA: Auxin, a hormone transported by polar transport from shoot tips that redistributes laterally in shoots and roots to produce directional growth responses.Pearson Edexcel
Independent assortment: Homologous pairs align independently at the equator, so maternal and paternal chromosomes of different pairs are distributed randomly into gametes.Pearson Edexcel
Index of diversity (D): A formula used to compare habitats, calculated as N(N - 1) divided by the sum of n(n - 1), where N is total individuals and n is individuals per species.Pearson Edexcel
Infection prevention and control: Hospital practice codes including hand hygiene, isolation, sterilisation, PPE, environmental cleaning and surveillance, which interrupt transmission and limit…Pearson Edexcel
Inflammation: Histamine released by mast cells dilates arterioles and increases capillary permeability, so more blood, phagocytes and plasma proteins reach the site,…Pearson Edexcel
Inheritance and genetic relationships: A child inherits bands from both parents, so a candidate parent must share bands with the child, and profiles reveal relationships in plants and animals.Pearson Edexcel
Inhibition zone: A clear zone around a disc on the bacterial lawn, whose diameter in millimetres gives a quantitative comparison of antimicrobial activity.Pearson Edexcel
Initial rate: Measured from the gradient at the start of the reaction, when substrate concentration is highest and the rate is steepest.Pearson Edexcel
Interdependence of organisms and environment: Human activity affects all organisms, including plants, fungi and microorganisms, not only large animals.Pearson Edexcel
Interferon: Released by virus-infected cells, it binds receptors on uninfected cells and switches on antiviral proteins that inhibit viral replication.Pearson Edexcel
Intracellular enzymes: Enzymes that catalyse reactions inside cells, for example in the cytosol or mitochondria.Pearson Edexcel
Ion movement and permeability: The membrane is more permeable to K⁺ than to Na⁺, and during the action potential voltage-gated Na⁺ channels open so Na⁺ enters, then K⁺ channels open so K⁺…Pearson Edexcel
Ion-dipole forces: Electrostatic attractions formed between water and charged ions, such as sodium and chloride.Pearson Edexcel
Isolation of chloroplasts: Carried out from leaf tissue using a cold, isotonic, buffered solution, keeping the preparation cold to maintain enzyme activity.Pearson Edexcel

K

Keyhole surgery: A minimally invasive technique using small incisions and an endoscope with a camera, so surgeons repair joints with less tissue damage, less pain and faster…Pearson Edexcel

L

L-Dopa: A precursor of dopamine that can cross the blood–brain barrier, unlike dopamine itself, and is converted to dopamine by enzymes in the brain.Pearson Edexcel
lac operon: A named example of gene control in prokaryotes in which lactose binds the repressor protein, freeing the operator so RNA polymerase transcribes the structural…Pearson Edexcel
Lactate: Accumulates in muscle tissue and diffuses into the blood, then is transported to the liver where it is oxidised back to pyruvate.Pearson Edexcel
LDL: Lipoprotein particles that deliver cholesterol to cells and artery walls, so higher levels are generally harmful and associated with increased CVD risk.Pearson Edexcel
Levels of organisation: The hierarchy runs cell, tissue, organ, system, organism, with increasing complexity at each level.Pearson Edexcel
Levodopa: A drug developed to increase dopamine levels in Parkinson's disease, which may become less effective over time.Pearson Edexcel
Life-cycle considerations: Sustainability requires considering life-cycle impacts, including energy, fertiliser and transport, as well as land and water use and competition with food…Pearson Edexcel
Lifestyle versus inherited risk: Inherited susceptibility, such as faulty LDL receptors in familial hypercholesterolaemia, combines with lifestyle factors such as diet, smoking and inactivity…Pearson Edexcel
Ligaments: Structures that connect bone to bone and stabilise joints.Pearson Edexcel
Lignin: A substance in secondary walls that makes xylem vessels rigid and waterproof so they withstand negative pressure.Pearson Edexcel
Linkage: Genes close together on the same chromosome tend to be inherited together, reducing independent assortment and producing fewer recombinant gametes.Pearson Edexcel
Lipid-soluble hormones: Hormones such as oestrogen or cortisol that can cross the cell membrane, bind an intracellular receptor, and the hormone-receptor complex acts as a…Pearson Edexcel
Locus: The location of genes on a chromosome; its plural is loci.Pearson Edexcel
Lysis: Bursting of the cell caused by disruption of the cell wall or cell membrane.Pearson Edexcel
Lysozyme: An enzyme in tears, saliva and nasal secretions that hydrolyses peptidoglycan in bacterial cell walls, causing lysis.Pearson Edexcel

M

Macerate: To soften and separate tissue, as hydrochloric acid does to root tips so the squash produces a single layer.Pearson Edexcel
Many-stepped process: A sequence in which each step is controlled and catalysed by a specific intracellular enzyme, allowing energy to be released in small, usable amounts.Pearson Edexcel
Marker gene: Used to select transformed cells after the vector has been transferred into the host cell.Pearson Edexcel
MDMA: A drug taken up by transporter proteins that causes serotonin to be released into the synaptic cleft, prolonging and intensifying stimulation.Pearson Edexcel
Medical therapies: Uses of stem cells to replace damaged or lost cells, for example bone marrow transplant for leukaemia.Pearson Edexcel
Medulla oblongata: The lowest brainstem region, continuous with the spinal cord, that controls autonomic functions such as heart rate, breathing rate and blood pressure.Pearson Edexcel
Membrane potentials: At rest the axon membrane is polarised, with the inside negative relative to the outside, and the potential rises during depolarisation before the resting…Pearson Edexcel
Membrane transport and ion movement: Depolarisation opens voltage-gated Ca²⁺ channels so Ca²⁺ enters, and receptor binding opens cation channels so Na⁺ enters and depolarises the cell.Pearson Edexcel
Meristematic cells: Actively dividing cells found in root tips that provide cells in all stages of mitosis.Pearson Edexcel
Meselson and Stahl's experiment: Grew E.Pearson Edexcel
Mesosomes: Inward folds of the plasma membrane that increase membrane surface area and are associated with processes such as respiration and DNA segregation.Pearson Edexcel
Metabolic demand: The requirement of a large, active organism that diffusion is too slow over long distances to meet.Pearson Edexcel
Middle lamella: A layer rich in calcium pectate that cements adjacent plant cell walls together.Pearson Edexcel
Mitochondrial structure and function: The site housing the enzymes for the link reaction and Krebs cycle, unlike glycolysis which occurs in the cytoplasm.Pearson Edexcel
Modification, sorting and packaging by the Golgi apparatus: The Golgi modifies the protein, for example trimming or adding carbohydrate, then packages it into vesicles.Pearson Edexcel
Molecular phylogeny: Comparing conserved molecules, which led to the three-domain system of Bacteria, Archaea and Eukaryota.Pearson Edexcel
Monocular deprivation: Covering one eye during the critical period, which leaves cortical cells that should respond to that eye instead driven by the open eye.Pearson Edexcel
Mononucleotide: Consists of a pentose sugar, which is deoxyribose in DNA or ribose in RNA, a phosphate group and a nitrogenous base.Pearson Edexcel
Monosaccharides: Single sugar units, such as glucose, that are soluble and easily respired to release energy.Pearson Edexcel
Motor neurone: Carries impulses from the CNS to an effector such as a muscle or gland, with a large cell body in the CNS, many dendrites and a long axon.Pearson Edexcel
MRI: Uses strong magnetic fields and radio waves to align hydrogen nuclei, and emitted signals build high-resolution images of soft tissue without ionising…Pearson Edexcel
Mucus: Becomes abnormally thick and sticky because chloride ions are not transported normally and water movement changes.Pearson Edexcel
Multifactorial causation of disease: CVD risk arises from several interacting factors, and no single factor acts alone.Pearson Edexcel
Multiple loci and allele effects: Each contributing allele adds a small effect, so the combined effect of many loci produces a graded range of phenotypes rather than discrete categories.Pearson Edexcel
Muscle fibre: A multinucleate cell bounded by a sarcolemma, containing sarcoplasm, myofibrils, T-tubules and sarcoplasmic reticulum.Pearson Edexcel
Mycobacterium tuberculosis: A bacterium spread in airborne droplets from coughing or sneezing and inhaled into the alveoli, where it is engulfed by phagocytes but resists digestion.Pearson Edexcel
Myelination and conduction speed: The myelin sheath insulates the membrane so depolarisation occurs only at nodes of Ranvier, and the impulse jumps from node to node in saltatory conduction,…Pearson Edexcel
Myogenic: Describes cardiac muscle that initiates its own contractions without external nervous stimulation.Pearson Edexcel

N

Narrow limits: The range within which negative feedback maintains systems by opposing the initial stimulus and correcting fluctuations.Pearson Edexcel
Natural active immunity: Immunity following infection, where antigens stimulate the person's own B and T lymphocytes to produce plasma cells, antibodies and memory cells.Pearson Edexcel
Natural selection: The process by which individuals whose phenotype gives them a greater chance of surviving and reproducing pass on more copies of an allele, so its frequency…Pearson Edexcel
Natural variation: Fibres vary, so several samples of the same type are tested and a mean calculated to reduce its effect.Pearson Edexcel
Negative feedback: A mechanism that reverses a deviation from the set point to restore the original level, opposing the initial stimulus.Pearson Edexcel
Nervous control: Receptors, sensory neurones, relay neurones, motor neurones and effectors transmit information rapidly using action potentials and neurotransmitters.Pearson Edexcel
Net primary productivity: The energy or biomass remaining after plants use some of the energy they fix for respiration.Pearson Edexcel
Neurotransmitter action and specificity: Acetylcholine diffuses across the cleft and binds complementary receptors on the postsynaptic membrane, and threshold must be reached for an action potential…Pearson Edexcel
Niche: The role a species plays in its ecosystem: where it lives, what it consumes, when it is active, and how it interacts with other species.Pearson Edexcel
Nitrate and nitrogen metabolism: Nitrate ions (NO₃⁻) provide nitrogen for amino acids, proteins, nucleic acids and chlorophyll; deficiency causes stunted growth and chlorosis.Pearson Edexcel
Non-overlapping: Each base is read only once and belongs to only one codon, with the reading frame set by the start codon.Pearson Edexcel
Non-specific defence: Barriers form the first line of defence and are non-specific, preventing pathogen entry or creating conditions hostile to pathogens.Pearson Edexcel
Normal flora: Gut flora and skin flora compete with pathogens for nutrients and space and can produce antimicrobial substances.Pearson Edexcel
Nucleolus: A region inside the nucleus where ribosomal RNA is made and ribosomes are assembled.Pearson Edexcel

O

Obesity: Associated with increased risk of type 2 diabetes and cardiovascular disease, and can result from sustained positive energy balance.Pearson Edexcel
Obligate anaerobes: Organisms that are poisoned by oxygen, in contrast with obligate aerobes which need it for aerobic respiration.Pearson Edexcel
Ocular dominance: The cortical wiring in which cells respond to one eye; monocular deprivation in the window shifts it towards the non-deprived eye.Pearson Edexcel
One-way flow: Maintained by valves, so that blood moves through the heart in a single direction.Pearson Edexcel
Optimism bias: A factor shaping perception in which a person may accept the statistics yet feel the risk applies to others.Pearson Edexcel
Optimum: The temperature identified from the graph of development against temperature at which development is greatest, with very high temperatures denaturing enzymes…Pearson Edexcel
Optimum temperature: The temperature at which an organism's enzymes work best, for example near 37 °C in humans and above 70 °C in thermophilic microorganisms.Pearson Edexcel
Organ and system integration: Different tissues combine to form an organ, and organs cooperate as an organ system to carry out a major function of the body.Pearson Edexcel
Organism metabolism: Plants and animals in cold environments may have slower metabolism, while those in hot environments risk denaturation.Pearson Edexcel
Osmosis: The net movement of free water molecules through a partially permeable membrane from a region of higher water concentration to a region of lower water…Pearson Edexcel
Oxidative phosphorylation: The synthesis of ATP associated with the electron transport chain in mitochondria, including the role of chemiosmosis and ATP synthase.Pearson Edexcel
Oxygen debt: The extra oxygen consumed after exercise, which acts as the final electron acceptor in the electron transport chain.Pearson Edexcel

P

Pancreatic ducts: When blocked, digestive enzymes cannot reach the small intestine, so food is poorly digested and nutrients are absorbed less efficiently.Pearson Edexcel
Parkinson’s disease: A condition involving degeneration of dopamine-secreting neurones, so dopamine levels fall and movement becomes difficult to control.Pearson Edexcel
Partially permeable membrane: A membrane that allows small water molecules through but restricts larger solute particles, so water moves down its concentration gradient.Pearson Edexcel
Passive: Osmosis is passive and does not require metabolic energy, continuing until concentrations equalise or the membrane is opposed by external pressure.Pearson Edexcel
Passive immunity: Protection where ready-made antibodies are supplied, acting immediately but fading as donated antibodies are broken down, with no memory formed.Pearson Edexcel
Passive transport: Movement of substances down a concentration gradient that does not require ATP hydrolysis.Pearson Edexcel
Pathogen entry: Pathogens may enter the body via the respiratory tract, gastrointestinal tract, skin breaks, genitourinary tract and conjunctiva of the eye.Pearson Edexcel
Peer review: Independent experts evaluate the design, methods, data and conclusions of a study before publication, and may recommend rejection or revision.Pearson Edexcel
Peptide bond: A bond formed by condensation between the carboxyl group of one amino acid and the amino group of another, releasing water.Pearson Edexcel
Perceived risk: What a person believes the chance of an outcome to be, a subjective judgement that often diverges from the actual risk.Pearson Edexcel
Percentage cover: An estimate of abundance made in quadrats, and processed with means and simple graphs to link distribution and abundance to abiotic factors.Pearson Edexcel
Personalised medicine: A patient's genome can be analysed to predict disease risk, guide drug choice and adjust dosage, because genetic variation affects how a person responds to…Pearson Edexcel
PET: Uses a radioactive tracer that emits positrons; annihilation produces gamma rays detected to show metabolic activity, useful for tumours and functional mapping.Pearson Edexcel
PGD: Embryos produced by IVF are screened before implantation; a single cell is removed and tested, and embryos without the condition are selected for transfer.Pearson Edexcel
Phagocytosis: A phagocyte engulfs a pathogen in a vesicle; lysosomes fuse and release enzymes that digest it.Pearson Edexcel
Phenotype: The observable characteristic, shaped by genotype and environment.Pearson Edexcel
phloem: Living sieve tubes with sieve plates and companion cells, occupying the outer vascular bundle and translocating organic solutes such as sucrose in either…Pearson Edexcel
Phloem sieve tubes: Tubes with sieve plates and companion cells, located in the outer part of each vascular bundle.Pearson Edexcel
Phospholipid bilayer: Hydrophilic phosphate heads face aqueous environments and hydrophobic hydrocarbon tails face each other, forming a barrier to water-soluble molecules.Pearson Edexcel
Photolysis of water: Light energy is used to split apart the strong bonds in water molecules, releasing hydrogen and oxygen.Pearson Edexcel
Photolysis of water and oxygen production: Water is split by photolysis to replace electrons lost from chlorophyll, producing oxygen as a by-product, along with protons and electrons.Pearson Edexcel
Photophosphorylation and NADP reduction: Electrons are used to reduce NADP to NADPH in photophosphorylation, the light-driven formation of ATP.Pearson Edexcel
Physiological adaptation: An internal process that improves survival and reproduction, such as producing concentrated urine to conserve water.Pearson Edexcel
Phytochrome: A photoreceptor protein with two interconvertible forms, Pr and Pfr, which absorb red light at about 660 nm and far-red light at about 730 nm.Pearson Edexcel
Pioneer species: The first colonisers of bare, hostile ground, able to tolerate extremes and low nutrient availability.Pearson Edexcel
placebo: An inactive treatment given to a control group, allowing comparison of real effects against expectation.Pearson Edexcel
Plant respiration: The process that releases energy, which is subtracted from gross primary productivity to give net primary productivity.Pearson Edexcel
Plaque formation: The build-up of an atheroma containing foam cells, cholesterol, calcium salts and fibrous tissue, which narrows the lumen and stiffens the artery wall.Pearson Edexcel
Plasma cell: A specialised B effector cell that secretes large quantities of antibody.Pearson Edexcel
Plasma membrane: A phospholipid bilayer that bounds every cell and controls what enters and leaves.Pearson Edexcel
Plasmids: Small circular DNA molecules separate from the main chromosome that can carry extra genes.Pearson Edexcel
Plasmodesmata: Cytoplasmic channels that bridge neighbouring cytoplasms through pits, allowing transport and communication between adjacent plant cells.Pearson Edexcel
Plasticity: The capacity for change in the visual cortex, which is high during the critical period and falls as the period ends.Pearson Edexcel
Platelet inhibitors: Make platelets less sticky, reducing clot formation in narrowed arteries, but also increase bleeding risk.Pearson Edexcel
Pluripotency: The ability of a cell to differentiate into cells of all three germ layers but not extra-embryonic tissues.Pearson Edexcel
Pollen in peat bogs: Preserved plant pollen; because different species tolerate different climates, changes in pollen types indicate past vegetation and climate shifts.Pearson Edexcel
Polygenic inheritance: Inheritance in which multiple alleles for the same gene at many loci contribute to a single phenotype, rather than a single gene determining it.Pearson Edexcel
Polynucleotide: Formed when mononucleotides are linked by condensation reactions that create phosphodiester bonds and release water.Pearson Edexcel
Polypeptide chain: A chain formed when amino acids join by peptide bonds, folding to give a protein.Pearson Edexcel
Polyspermy: Prevented when the zona pellucida hardens after fertilisation, blocking further sperm.Pearson Edexcel
Population: All the organisms of one species in a habitat at one time.Pearson Edexcel
Positive control: A disc soaked in a substance such as a disinfectant, included to confirm the method can detect inhibition.Pearson Edexcel
Positive feedback: A mechanism that amplifies a deviation, moving the system further from the set point.Pearson Edexcel
Post-transcriptional changes: Changes that occur after transcription but before translation, modifying the mRNA transcript.Pearson Edexcel
Pr and Pfr: The two interconvertible forms of phytochrome; Pfr accumulates in red light and reverts to Pr in darkness or far-red light.Pearson Edexcel
Practical investigation: Heart structure and function can be investigated by dissection, by observing a dissected or model heart, and by interpreting ECG traces and pressure or volume…Pearson Edexcel
Pressure and flow: Arteries carry blood at high pressure while veins return blood at low pressure, and elastic recoil smooths pulsatile flow.Pearson Edexcel
Primary structure: The sequence of amino acids, which determines the three-dimensional structure and properties of a protein.Pearson Edexcel
Promoter: A suitable sequence required so that the host's transcription and translation machinery expresses the inserted gene.Pearson Edexcel
Prosthesis: A device that replaces a missing limb or joint surface, such as a below-knee running blade or a hip or knee replacement, restoring movement for sporting…Pearson Edexcel
Protein denaturation: The process at very high temperatures that denatures enzymes and so reduces development, used to explain the biological basis of the results.Pearson Edexcel
Protein synthesis and folding in the rER lumen: Proteins made on ribosomes on the rER are threaded into the rER lumen, where they fold and may gain sugar groups.Pearson Edexcel
Proteomics: The large-scale study of proteins, providing evidence for evolution by comparing proteins across species.Pearson Edexcel

Q

Quadrat: A sampling method used with transects to study distribution, with percentage cover or frequency as measures of abundance.Pearson Edexcel
Quadrats: Placed randomly, for example with a random number generator and grid coordinates, then used to count individuals or estimate percentage cover.Pearson Edexcel

R

Raised blood pressure: Results from increased resistance to flow through the narrowed, stiffened artery, and in turn causes further endothelial damage, making the process…Pearson Edexcel
Recombinant chromatids: Chromatids carrying new combinations of alleles, produced when crossing over links alleles from one homologue to alleles from the other.Pearson Edexcel
Redox reactions and coenzymes: Oxidation removes hydrogen, forming reduced coenzyme, commonly reduced NAD, which acts as a hydrogen carrier.Pearson Edexcel
Reduced NAD: Formed when oxidised NAD is converted during the oxidation of lactate back to pyruvate in the liver.Pearson Edexcel
Reduced reliance on fossil fuels: Plant-based products can replace oil-based plastics, lowering demand for finite crude oil and reducing carbon emissions if grown and processed sustainably.Pearson Edexcel
Reduction of GP to GALP using ATP and reduced NADP: GP, a 3-carbon compound, is reduced to GALP using ATP and reduced NADP made in the light-dependent reactions.Pearson Edexcel
Reflex arcs and coordination: The sensory neurone synapses with a relay neurone in the spinal cord, which synapses with a motor neurone, so the response is rapid and involuntary because…Pearson Edexcel
Reforestation: Replanting trees in areas where forests have been removed, restoring habitats and increasing carbon dioxide absorption.Pearson Edexcel
Regeneration of RuBP and the role of RUBISCO: Most GALP regenerates RuBP using further ATP, while RUBISCO catalyses carbon fixation, the joining of carbon dioxide with RuBP.Pearson Edexcel
Relay neurone: Lies entirely within the CNS, connecting sensory and motor pathways, with many short dendrites and a short axon.Pearson Edexcel
Renewable energy: Using sources such as solar or wind avoids combustion of fossil fuels, lowering carbon dioxide emissions.Pearson Edexcel
Renewable resources: Plant fibres and starch come from crops, so using them can reduce reliance on finite crude oil.Pearson Edexcel
Replicates: Several organisms used at each temperature so that a mean can be calculated, reducing the effect of individual variation.Pearson Edexcel
Repressors: Transcription factors that inhibit transcription by blocking RNA polymerase or recruiting co-repressors.Pearson Edexcel
Reproductive isolation: A barrier to interbreeding, such as geographical separation, different mating seasons or incompatible courtship behaviour, that prevents interbreeding between…Pearson Edexcel
Respiratory substrate: A molecule such as glucose that is split during aerobic respiration to release carbon dioxide as a waste product.Pearson Edexcel
Restoration of diploidy: The sperm nucleus fuses with the oocyte nucleus, restoring the diploid chromosome number and forming the zygote.Pearson Edexcel
Retinal: Combines with opsin to form rhodopsin, and is isomerised by light, altering rhodopsin.Pearson Edexcel
Reverse transcriptase: An enzyme of HIV that makes DNA from viral RNA, which then integrates into the host genome.Pearson Edexcel
Rhodopsin: Formed from opsin and retinal, it is sensitive to light intensity and keeps cation channels open in darkness.Pearson Edexcel
Ribosomes: Structures present in every cell that assemble polypeptides.Pearson Edexcel
Rigor mortis: Muscle contraction that develops as ATP is lost and actin and myosin lock, then resolves as enzymes break the complex down, so its degree indicates time since…Pearson Edexcel
Risk assessment: This considers the likelihood and severity of harm, and controls such as containment, buffer zones and monitoring reduce risk.Pearson Edexcel
Risk communication: The use of absolute risks, clear comparisons and trusted sources to convey risk, because health messages alone may not change behaviour.Pearson Edexcel
Risk factor interaction and compounding: Risk factors interact and compound, so several moderate factors can matter more than one severe factor.Pearson Edexcel
Risks: These include genes escaping into wild populations, effects on non-target species, reduced biodiversity, and possible allergic or toxic effects of the new…Pearson Edexcel
RNA editing: A post-transcriptional change that changes the nucleotide sequence of mRNA, which can alter codons and amino acids.Pearson Edexcel
Root tip squash: A preparation in which a stained root tip is squashed under a coverslip to spread cells into a single layer for clear observation.Pearson Edexcel
Rough endoplasmic reticulum: Stacked flattened sacs studded with ribosomes, distinguished from smooth endoplasmic reticulum by the presence of bound ribosomes on the membrane surface.Pearson Edexcel
Running blade: A prosthesis that stores elastic strain energy during impact and returns it during push-off, improving running economy for an athlete with a lower-limb…Pearson Edexcel

S

Saprobionts: Mainly bacteria and fungi that carry out external digestion, with fungi extending hyphae into dead material.Pearson Edexcel
Sarcomere: The repeating unit of actin and myosin filaments within a myofibril, giving the striated appearance.Pearson Edexcel
Saturated fatty acid: Has only single carbon–carbon bonds in its hydrocarbon chain, so it is straight and packs closely together.Pearson Edexcel
Scale bar: A feature used to establish scale and magnification, allowing the size of structures to be judged before they are identified.Pearson Edexcel
Schwann cells: Wrap around axons to form the myelin sheath, and play a crucial role in facilitating the regeneration and repair of damaged peripheral axons.Pearson Edexcel
Scientific conferences: Meetings that allow researchers to present findings, receive questions and criticism, and compare results with other scientists.Pearson Edexcel
Sclerenchyma fibres: Dead at maturity with thick lignified walls and small lumens, they sit as caps outside the phloem and provide support.Pearson Edexcel
Seasonal cycles: Cycles such as earlier springs or later autumns change when resources appear.Pearson Edexcel
Secondary succession: Succession that starts where soil already exists, for instance after a fire, so it proceeds faster.Pearson Edexcel
Secondary thickening: The addition of extra cellulose and lignin to the primary wall, increasing rigidity and waterproofing.Pearson Edexcel
Secretion of extracellular enzymes by exocytosis: Secretory vesicles fuse with the plasma membrane, releasing the protein outside the cell by exocytosis.Pearson Edexcel
Selection pressure and differential survival: In a given environment, some individuals have alleles giving a selective advantage, so they are more likely to survive and reproduce.Pearson Edexcel
Semi-conservative replication: Each new DNA molecule contains one original parental strand and one newly synthesised strand.Pearson Edexcel
Sensory neurone: Carries impulses from receptors to the central nervous system, with a cell body in a dorsal root ganglion, a long dendron and a short axon.Pearson Edexcel
Serotonin: A neurotransmitter whose concentration in the synapse is increased by MDMA, prolonging and intensifying postsynaptic stimulation.Pearson Edexcel
Sex linkage: Occurs when a gene is located on a sex chromosome, commonly the X chromosome in mammals.Pearson Edexcel
Sinoatrial node (SAN): Acts as a pacemaker by generating waves of depolarisation across the atria, causing atrial systole.Pearson Edexcel
Sliding filament theory: Actin filaments are pulled past myosin filaments, causing the sarcomere to shorten, while the filaments themselves do not shorten.Pearson Edexcel
Slow twitch fibres: Fibres with many mitochondria, high myoglobin and dense capillaries, so they resist fatigue for endurance.Pearson Edexcel
Smooth endoplasmic reticulum: A structure that lacks ribosomes and synthesises lipids.Pearson Edexcel
Solvent control: A disc soaked only in the solvent used to make the extract, included to show the solvent itself is not causing inhibition.Pearson Edexcel
Speciation: Continued divergence of isolated populations may eventually prevent the production of fertile offspring, forming new species.Pearson Edexcel
Species concept: The idea that members of a species interbreed to produce fertile offspring, around which classification is built.Pearson Edexcel
Species distribution: Species may survive only where conditions suit them, so their distribution moves towards the poles or to higher altitude, or they become locally extinct.Pearson Edexcel
Specific intracellular enzyme: A catalyst that controls and catalyses one particular step of respiration inside the cell.Pearson Edexcel
Specificity: Only substrates with a complementary shape and chemistry can bind to the active site, forming a stable enzyme–substrate complex.Pearson Edexcel
SSRIs: Drugs developed to increase serotonin levels in depression by blocking serotonin reuptake, which may cause nausea or increased anxiety initially.Pearson Edexcel
Starch: A polymer of α-glucose joined by α-1,4 glycosidic bonds, with α-1,6 branches in amylopectin, used for storage in plants.Pearson Edexcel
Statins: Reduce cholesterol synthesis in the liver, lowering LDL cholesterol and slowing plaque build-up, but may cause muscle pain.Pearson Edexcel
Stem cell: An unspecialised cell that can divide by mitosis to self-renew and differentiate into specialised cells.Pearson Edexcel
Stimulus–response pathways: Receptors detect a stimulus and generate an action potential in a sensory neurone, which carries impulses to the CNS, where relay neurones pass signals to…Pearson Edexcel
Stop codon: A signal that terminates translation, does not code for an amino acid and releases the completed polypeptide.Pearson Edexcel
Stopping smoking: Removes carbon monoxide, which reduces oxygen carriage, and nicotine, which raises heart rate and blood pressure, so CHD risk falls.Pearson Edexcel
Stroke volume: The volume of blood ejected by one ventricle per beat, so cardiac output depends on both rate and volume.Pearson Edexcel
Stroma: The fluid-filled interior of the chloroplast containing enzymes such as Rubisco that catalyse the light-independent reactions, including carbon fixation.Pearson Edexcel
Structure and function: The wall structure of each blood vessel matches its job as an exchange or transport tube.Pearson Edexcel
Structure related to function: Structure at each level is related to function, so damage at one level can affect the whole system.Pearson Edexcel
Studies of damaged brain areas: Link specific brain regions to functions and show plasticity after injury, informing nature through localisation and nurture through reorganisation.Pearson Edexcel
Substitution: A change where one base is replaced by another during DNA replication, which may alter a single codon and hence one amino acid.Pearson Edexcel
Succession: A predictable sequence of colonising organisms on a body, where the stage reached indicates time since death.Pearson Edexcel
Surface area to volume ratio: A large surface area to volume ratio increases the number of collisions between diffusing molecules and the surface, so more molecules cross per unit time.Pearson Edexcel
Sustainability: Using resources without depleting them, balancing human demand with conservation, for example by sustainable forestry or using agricultural waste.Pearson Edexcel
Sympatric speciation: Speciation occurring without a physical barrier, often via polyploidy in plants or via behavioural, temporal or mechanical reproductive isolation within one…Pearson Edexcel

T

T helper cells: These release cytokines that stimulate B cells, T killer cells and macrophages, coordinating the immune response.Pearson Edexcel
T killer cells: These recognise and destroy infected or abnormal cells, often by inducing apoptosis, and do not secrete antibodies.Pearson Edexcel
Temperature: The independent variable, controlled at set values, for example using water baths, with enzyme and substrate equilibrated before mixing.Pearson Edexcel
Tendons: Structures that attach muscle to bone and transmit force.Pearson Edexcel
Tensile strength: The maximum force a fibre withstands per unit cross-sectional area before breaking, calculated as breaking force divided by cross-sectional area.Pearson Edexcel
Thermostable DNA polymerase: An enzyme such as Taq polymerase that survives the repeated high temperatures which would denature a normal enzyme.Pearson Edexcel
Thin diffusion pathway: A short distance reduces the distance over which molecules must diffuse, increasing the rate of diffusion.Pearson Edexcel
three-phased testing: Testing that progresses from safety and dosage in small groups, to efficacy and side effects in larger patient groups, then to larger-scale comparison with…Pearson Edexcel
Thrombin: An active enzyme that catalyses the conversion of soluble fibrinogen into insoluble fibrin.Pearson Edexcel
Thromboplastin: Released from damaged tissue and platelets at the site of vessel injury, it triggers the conversion of prothrombin to thrombin.Pearson Edexcel
Thrombus: A clot that may form on an atheroma, narrowing or blocking an artery in cardiovascular disease.Pearson Edexcel
Thylakoid membranes: Membranes that create a small internal compartment allowing proton accumulation and a proton gradient for chemiosmosis and ATP synthesis.Pearson Edexcel
TIR1: A receptor that IAA binds to, leading to changes in transcription factors that switch genes for cell elongation, division and differentiation on or off.Pearson Edexcel
Tonoplast: The membrane that bounds the large permanent vacuole, controlling entry and exit of solutes to and from the vacuole.Pearson Edexcel
Totipotency: The ability of a cell to differentiate into all cell types, including extra-embryonic tissues such as placenta.Pearson Edexcel
Transcription: RNA polymerase binds to the DNA template (antisense) strand and synthesises mRNA using complementary base pairing.Pearson Edexcel
Transcription factors: Proteins that bind to specific DNA base sequences, often near the promoter of a target gene, to help or block RNA polymerase.Pearson Edexcel
Transects: Used to study distribution along a gradient, with a line transect recording organisms touching the line and a belt transect placing quadrats at intervals.Pearson Edexcel
translocation: The movement of organic solutes such as sucrose through phloem sieve tubes, which can occur in either direction.Pearson Edexcel
Transport and exchange: Blood vessels act as transport and exchange tubes, with arteries and veins transporting blood and capillaries aiding exchange.Pearson Edexcel
Transport medium: A solution that carries dissolved oxygen, glucose, amino acids and mineral ions in blood plasma or in xylem and phloem sap.Pearson Edexcel
Triglyceride: Formed when one glycerol molecule condenses with three fatty acid molecules, which may be identical or different.Pearson Edexcel
Triplet: A set of three bases, also called a codon, that specifies one amino acid.Pearson Edexcel
Triplet code: Every three bases, called a codon, on mRNA specifies one amino acid.Pearson Edexcel
Tropomyosin: At rest it blocks the myosin-binding sites on actin, and it is held in position by troponin.Pearson Edexcel
Troponin: At rest it holds tropomyosin in position; when Ca²⁺ binds to it, tropomyosin moves and exposes the myosin-binding sites on actin.Pearson Edexcel
Twin studies: Compare concordance rates between monozygotic and dizygotic twins to estimate heritability of brain traits, though identical twins also share environment.Pearson Edexcel

U

Ultrastructure: The internal detail of a cell, such as shape, internal membranes and relative size, used to recognise organelles in electron microscope images rather than…Pearson Edexcel
Uncertainty and limitations: Models have limitations including uncertainty in future greenhouse gas emissions, incomplete understanding of feedbacks, natural variability, sparse or…Pearson Edexcel
Units and measurement: Units must match, so if values differ in time unit, convert one before subtracting.Pearson Edexcel
Unsaturated fatty acid: Contains one or more carbon–carbon double bonds, which introduce kinks in the chain and prevent close packing.Pearson Edexcel

V

Vascular bundle arrangement: Sclerenchyma outside, phloem middle and xylem inside, with xylem towards the stem interior and phloem towards the exterior.Pearson Edexcel
Vector: A carrier such as a plasmid into which the drug-producing gene is inserted before transfer into a host cell.Pearson Edexcel
Ventilation centre: A centre in the medulla oblongata that controls the rate and depth of breathing by stimulating the diaphragm and intercostal muscles.Pearson Edexcel
Vesicular transport between membrane-bound organelles: Vesicles bud from the rER and carry the protein to the Golgi apparatus, a stack of flattened, membrane-bound cisternae.Pearson Edexcel
Viable cells: Cells that are alive; bactericidal action directly lowers their counts, whereas bacteriostatic action leaves numbers approximately constant.Pearson Edexcel

W

Waist-to-hip ratio: Waist circumference divided by hip circumference; a higher ratio indicates more abdominal fat and greater CHD risk.Pearson Edexcel
Water as a solvent and transport medium: A solvent for ions, enzymes and metabolites, allowing reactions in solution, and a transport medium in xylem and phloem moving minerals and assimilates.Pearson Edexcel

X

xylem vessels: Wide, lignified, hollow tubes formed from dead cells with no end walls, lying towards the stem interior and transporting water and mineral ions upward while…Pearson Edexcel

Z

Zona pellucida: A glycoprotein coat surrounding the egg that sperm must bind to and digest, and which hardens after fertilisation to prevent polyspermy.Pearson Edexcel

Where these terms come from

Each definition is one we publish on a Biology A (Salters-Nuffield) A-Level topic page or study guide. Open a term to see how each exam board words it and which topic it belongs to. For the full course, go to Biology A (Salters-Nuffield) revision or the glossary for every subject.

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Biology A (Salters-Nuffield) topics

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