Biology

    AQA
    A-Level

    Specification: 7402

    The AQA A-Level Biology specification covers 53 topics with 1142 learning objectives (7402). Use the topic browser below to explore subtopics, exam tips, common mistakes, and key terminology for each area of the course.

    Starting AQA A-Level Biology this term? The whole specification is below, unit by unit.

    This subject will help you develop key knowledge and skills required for exam success.

    53

    Topics

    1142

    Objectives

    963

    Exam Tips

    1592

    Pitfalls

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    Official Specification

    Download the AQA A-Level Biology specification PDF

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    Key Features

    • Master key concepts
    • Develop exam technique
    • Apply knowledge effectively

    About AQA A-Level Biology

    AQA A-Level Biology offers a comprehensive and rigorous journey through the science of life, from the chemistry of biological molecules to the complex interactions within ecosystems. The specification is structured around eight core topics that build logically: starting with Biological molecules and Cells, moving through how organisms exchange substances with their environment and the role of genetic information, then exploring variation, relationships between organisms, and the intricacies of energy transfers. This progression ensures that you develop deep, integrated understanding of both the unity and diversity of living systems.

    A standout feature of AQA Biology is its clear narrative that connects small-scale processes to whole-organism and ecological dynamics. You'll investigate fundamental concepts like enzyme action, DNA replication, and cell division, then see how these underpin larger themes such as homeostasis, nervous coordination, and gene expression. Throughout, the specification sharpens your practical skills via 12 required practicals that are woven into the theory, preparing you not just for the written exams but also for future scientific study and a mindset of evidence-based enquiry.

    The course also places a strong emphasis on synoptic and analytical skills, particularly in Paper 3, where you are challenged to synthesise knowledge across all topics and interpret experimental data. Whether you are aiming for a career in medicine, biotechnology, ecology, or another bioscience field, AQA A-Level Biology equips you with a thorough grounding in both the key principles and the scientific method, all supported by a wealth of high-quality resources developed by an experienced exam board.

    Assessment Structure

    The AQA A-Level Biology qualification is assessed entirely through three written examinations at the end of the two-year course, with no coursework component contributing to the final grade. Paper 1 covers Topics 1–4 (Biological molecules, Cells, Organisms exchange substances, Genetic information), is 2 hours long, worth 91 marks, and accounts for 35% of the A-level. Paper 2 covers Topics 5–8 (Energy transfers, Organisms respond to changes, Genetics and populations, Control of gene expression), also 2 hours, 91 marks, and 35%. Paper 3 is synoptic, drawing on any content from Topics 1–8, includes a 25-mark essay, lasts 2 hours, is worth 78 marks, and makes up the remaining 30%. Additionally, students must complete a Practical Endorsement (pass/fail) by carrying out twelve required practicals and keeping a lab book; this is reported separately on the certificate and is essential for many university science applications.

    Why Choose AQA?

    • AQA's specification is renowned for its logical and coherent structure, making it easier to grasp how concepts interconnect. The journey from molecule to ecosystem is clearly signposted, and the regular cycling of themes helps consolidate learning.
    • The board provides exceptionally strong support for practical work, with detailed teacher guidance, student-friendly handbooks, and accessible equipment lists for the 12 required practicals. This makes skill development manageable and directly relevant to the exams, where practical-based questions can account for at least 15% of the marks.
    • As the most widely taken Biology A-Level in England, AQA boasts a vast library of online resources, past papers, mark schemes, and examiner reports. This abundance means you can practise effectively and learn from real examiner feedback, giving you a clear advantage in understanding what top answers look like and where common mistakes occur.

    Frequently Asked Questions

    Assessment Objectives

    AO1
    30%-35%

    Demonstrate knowledge and understanding of scientific ideas, processes, techniques and procedures

    AO2
    40%-45%

    Apply knowledge and understanding of scientific ideas, processes, techniques and procedures in a theoretical context, in a practical context, when handling qualitative data and when handling quantitative data

    AO3
    25%-30%

    Analyse, interpret and evaluate scientific information, ideas and evidence, including in relation to issues, to make judgements and reach conclusions and to develop and refine practical design and procedures

    Exam Structure

    AQA A-Level (7402)

    Paper 1: Topics 1–4

    2h

    Duration

    91

    Marks

    35%

    Weighting

    Paper 2: Topics 5–8

    2h

    Duration

    91

    Marks

    35%

    Weighting

    Paper 3: Synoptic topics 1–8

    2h

    Duration

    78

    Marks

    30%

    Weighting

    Common Exam Mistakes

    Pitfalls to avoid in your exams

    • •Assuming all carbon-containing compounds are organic; correct by remembering that simple carbon compounds like carbon dioxide (CO2) and carbonates are inorganic because they lack carbon-hydrogen bonds.
    • •Misinterpreting a 'similar biochemical basis' to mean organisms have identical molecules; correct by understanding that while the classes of molecules (e.g., proteins) and monomers (e.g., amino acids) are shared, the specific sequences (e.g., amino acid sequences) differ between species.
    • •Stating species are related solely because of similar physical appearances when asked for biochemical evidence; correct by specifically referencing shared molecules like DNA, RNA, or proteins.
    • •Defining a monomer as just a small molecule, with no reference to the larger molecule it builds; a full definition needs both halves
    • •Calling a disaccharide such as maltose a monomer because it is smaller than starch; it is a dimer, not a monomer
    • •Naming glycerol or a fatty acid as a monomer of a triglyceride, when lipids are not polymers
    • •Saying the monomers of DNA are bases rather than nucleotides; the monomer is the nucleotide, which contains a base, sugar and phosphate
    • •Answering with the elements present, carbon, hydrogen and oxygen, when asked to identify a repeating unit

    Top Examiner Tips

    Expert advice for exam success

    • •When evaluating evolutionary relationships, always specify the exact biochemical evidence being compared, such as the 'DNA base sequence' or 'amino acid sequence', rather than just writing 'DNA'.
    • •The concept of a shared biochemical basis is an excellent linking theme for synoptic essays, connecting biological molecules to classification, evolution, and the genetic code.
    • •Read the monomer off the diagram you are given - a question about an unfamiliar polymer like chitin is testing whether you can spot the repeating unit, not whether you have memorised it.
    • •Pair every monomer with its polymer and bond when you revise: glucose-polysaccharide-glycosidic, amino acid-polypeptide-peptide, nucleotide-polynucleotide-phosphodiester.
    • •In a definition question write both halves of the idea in one sentence: the small repeating unit and the larger molecule it builds.
    • •When a question says 'use Figure 1', every point you make must be visible in the figure - the flipped monomer and the straight chain are there to be seen.
    • •Check the branching before you mention 1,6 bonds; if the drawn chain is straight, a 1,6 bond is a marked error.
    • •Structure means monomer, bond, chain shape and chain interactions - not the elements the molecule contains.

    Specification Topics

    53 topics

    Monomers and polymers

    Life on Earth is incredibly diverse, yet the underlying biochemical basis is fundamentally similar across all organisms. Every living thing is constructed from four main groups of carbon-based organic molecules: carbohydrates, lipids, proteins, and nucleic acids. Organic molecules contain carbon bonded to hydrogen. Carbon's ability to form four strong covalent bonds allows it to build complex chains and rings. The same monomers—monosaccharides, amino acids, and nucleotides—are used universally. Furthermore, the same fundamental reactions, condensation and hydrolysis, assemble and dismantle these polymers. This shared biochemistry, including the universal use of DNA, RNA, and ATP, provides strong evidence for evolution and a common ancestor.

    The variety of life, both past and present, is extensive, but the biochemical basis of life is similar for all living things., Monomers are the smaller units from which larger molecules are made., Polymers are molecules made from a large number of monomers joined together. +3 more

    18 objectives17 tips28 pitfalls6 subtopics

    Carbohydrates

    Monosaccharides are single sugar units that act as the monomers from which larger carbohydrates are made. Glucose, galactose and fructose are the common monosaccharides named in this specification. Monosaccharides can join in condensation reactions to form disaccharides and polysaccharides, while hydrolysis separates larger carbohydrates into smaller sugar units. This monomer relationship is the key distinction: a monosaccharide is one building block, whereas a larger carbohydrate contains sugar units joined by bonds.

    Monosaccharides are the monomers from which larger carbohydrates are made. Glucose, galactose and fructose are common monosaccharides., A condensation reaction between two monosaccharides forms a glycosidic bond., Disaccharides are formed by the condensation of two monosaccharides: maltose is a disaccharide formed by condensation of two glucose molecules sucrose is a disaccharide formed by condensation of a glucose molecule and a fructose molecule lactose is a disaccharide formed by condensation of a glucose molecule and a galactose molecule. +5 more

    24 objectives21 tips36 pitfalls8 subtopics

    Lipids

    Lipids contain carbon, hydrogen and oxygen, but with proportionally less oxygen than carbohydrates. They are insoluble in water but soluble in organic solvents like ethanol. The specification names two groups: triglycerides and phospholipids. Triglycerides consist of one glycerol joined to three fatty acids. They are non-polar, hydrophobic molecules, making them ideal energy storage lipids in adipose tissue. Phospholipids consist of one glycerol joined to two fatty acids and a phosphate-containing group. This structure makes them amphipathic: they have a hydrophilic phosphate head and hydrophobic fatty acid tails. This property is essential for forming the bilayer of cell membranes.

    Triglycerides and phospholipids are two groups of lipid., Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid., A condensation reaction between glycerol and a fatty acid (RCOOH) forms an ester bond. +5 more

    24 objectives23 tips38 pitfalls8 subtopics

    General properties of proteins

    Amino acids are the monomers from which proteins are made. Each amino acid has a central carbon atom bonded to four groups: an amine group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain called the R group. The amine and carboxyl groups must be drawn on the same central carbon. The twenty amino acids common to all organisms differ only in their R group. For example, glycine has a hydrogen atom as its R group, whereas cysteine has a side chain containing sulfur. The sequence of R groups influences a protein's properties because R groups vary in size, charge and polarity, so they determine the bonds that form when the protein folds.

    Amino acids are the monomers from which proteins are made. The general structure of an amino acid as: where NH represents an amine group, COOH represents a carboxyl group and R represents a side chain. The twenty amino acids that are common in all organisms differ only in their side group., A condensation reaction between two amino acids forms a peptide bond., Dipeptides are formed by the condensation of two amino acids. Polypeptides are formed by the condensation of many amino acids. +6 more

    27 objectives24 tips40 pitfalls9 subtopics

    Many proteins are enzymes

    Reactions require an initial energy input, the activation energy, to break bonds and reach the transition state. Enzymes provide an alternative pathway with a lower activation energy. When a substrate binds to the active site, an enzyme-substrate complex forms. Interactions with specific R groups (charged, polar, or hydrophobic) lining the active site can strain bonds in the substrate. Alternatively, the active site holds multiple substrates close together in the correct orientation. Consequently, less energy is needed. At a given temperature, a greater proportion of molecules possesses kinetic energy greater than or equal to this lowered activation energy, increasing the frequency of successful collisions and raising the reaction rate without needing extra heat.

    Each enzyme lowers the activation energy of the reaction it catalyses., The induced-fit model of enzyme action., The properties of an enzyme relate to the tertiary structure of its active site and its ability to combine with complementary substrate(s) to form an enzyme-substrate complex. +3 more

    18 objectives17 tips26 pitfalls6 subtopics

    Structure of DNA and RNA

    DNA and RNA are nucleic acids, polymers of nucleotides, and the information they carry lies in the sequence of bases along the strand. In DNA a sequence of three bases, a triplet, codes for one amino acid, so the base sequence of a gene determines the sequence of amino acids in a polypeptide, which determines its tertiary structure and therefore its function. Because the genetic code is universal, the same triplets code for the same amino acids in almost all organisms, which is why a human gene inserted into a bacterium still yields a human protein. DNA is the long-term store: it is stable, the bases are protected on the inside of the double helix by the sugar-phosphate backbone, and it can be copied exactly.

    Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are important information-carrying molecules., In all living cells, DNA holds genetic information and RNA transfers genetic information from DNA to the ribosomes., Ribosomes are formed from RNA and proteins. +4 more

    21 objectives19 tips31 pitfalls7 subtopics

    DNA replication

    Semi-conservative replication ensures genetic continuity between generations of cells. During the S phase of interphase, before nuclear division, the DNA double helix unwinds. Each original strand acts as a template. Free nucleotides bind via complementary base pairing, and DNA polymerase joins them to form a new strand. Consequently, each new DNA molecule consists of one original parental strand and one newly synthesised strand. Because base pairing dictates the sequence, the genetic code is copied exactly. This provides genetic continuity: daughter cells produced by mitosis inherit an identical copy of the parent cell's genome, preserving essential genetic information across cell generations, barring rare mutations. Note that replication itself produces two identical DNA molecules (sister chromatids), not two daughter cells; mitosis then separates them into daughter cells.

    The semi-conservative replication of DNA ensures genetic continuity between generations of cells., The process of semi-conservative replication of DNA in terms of: unwinding of the double helix breakage of hydrogen bonds between complementary bases in the polynucleotide strands the role of DNA helicase in unwinding DNA and breaking its hydrogen bonds attraction of new DNA nucleotides to exposed bases on template strands and base pairing the role of DNA polymerase in the condensation reaction that joins adjacent nucleotides., Students should be able to evaluate the work of scientists in validating the Watson–Crick model of DNA replication.

    9 objectives9 tips14 pitfalls3 subtopics

    ATP

    ATP is a phosphorylated nucleotide derivative comprising the organic base adenine, the pentose sugar ribose, and a chain of three phosphate groups. It serves as the immediate energy source for biological processes. The bonds between the phosphate groups are unstable. While breaking the terminal phosphate bond requires a small input of activation energy, the overall hydrolysis reaction to form ADP and inorganic phosphate is highly exergonic, releasing approximately 30.5 kJ mol⁻¹ of usable energy. This provides a small, manageable packet of energy, preventing thermal damage to the cell. ATP hydrolase catalyses this breakdown, while ATP synthase re-forms ATP from ADP and Pi during respiration and photosynthesis, ensuring continuous recycling.

    A single molecule of adenosine triphosphate (ATP) is a nucleotide derivative and is formed from a molecule of ribose, a molecule of adenine and three phosphate groups., The hydrolysis of ATP can be coupled to energy-requiring reactions within cells., The inorganic phosphate released during the hydrolysis of ATP can be used to phosphorylate other compounds, often making them more reactive.

    9 objectives8 tips13 pitfalls3 subtopics

    Water

    Water is polar, forming hydrogen bonds between molecules. This gives it five key biological properties. 1) It is a metabolite in condensation and hydrolysis reactions. 2) It is an important solvent for metabolic reactions, as polar molecules dissolve easily. 3) It has a high specific heat capacity, buffering temperature changes because energy breaks hydrogen bonds rather than raising kinetic energy, protecting enzymes from denaturation. 4) It has a large latent heat of vaporisation, providing a cooling effect (e.g. sweating) with little water loss. 5) Strong cohesion between molecules supports continuous water columns in plant xylem under tension, and produces surface tension where water meets air, supporting small organisms.

    Water is a major component of cells. It has several properties that are important in biology. In particular, water: is a metabolite in many metabolic reactions, including condensation and hydrolysis reactions is an important solvent in which metabolic reactions occur has a relatively high heat capacity, buffering changes in temperature has a relatively large latent heat of vaporisation, providing a cooling effect with little loss of water through evaporation has strong cohesion between water molecules; this supports columns of water in the tube-like transport cells of plants and produces surface tension where water meets air.

    3 objectives3 tips4 pitfalls

    Inorganic ions

    Inorganic ions are charged particles that contain no carbon-hydrogen bonds. They dissolve because water is polar and surrounds each ion, so they occur in solution in cytoplasm, blood plasma and tissue fluid. Some are present in large amounts: sodium, potassium and chloride in plasma and cytoplasm, and hydrogencarbonate in plasma. Others act at minute concentrations: iron in haemoglobin, and copper and zinc as enzyme cofactors. Concentration is no measure of importance, since a plant starved of magnesium cannot make chlorophyll and becomes chlorotic. Note that while iodine is needed for thyroxine synthesis, it is incorporated as atoms, not free ions.

    Inorganic ions occur in solution in the cytoplasm and body fluids of organisms, some in high concentrations and others in very low concentrations., Each type of ion has a specific role, depending on its properties., Students should be able to recognise the role of ions in the following topics: hydrogen ions and pH; iron ions as a component of haemoglobin; sodium ions in the co-transport of glucose and amino acids; and phosphate ions as components of DNA and of ATP.

    9 objectives6 tips9 pitfalls3 subtopics

    Structure of eukaryotic cells

    Eukaryotic cells contain membrane-bound organelles. The cell-surface membrane is a phospholipid bilayer controlling exchange. The nucleus has a double-membrane envelope with pores, containing linear DNA bound to histones, and nucleoli. Mitochondria have a folded inner membrane (cristae) and matrix, releasing ATP. Chloroplasts, found in plants and algae, contain thylakoids stacked into grana and stroma for photosynthesis. The Golgi apparatus modifies, packages and transports proteins and lipids, forming vesicles and lysosomes (containing hydrolytic enzymes). Ribosomes synthesise proteins. Rough ER has ribosomes and transports proteins; smooth ER synthesises lipids. Plant, algal and fungal cell walls provide support. Plant cell vacuoles maintain turgor.

    The structure of eukaryotic cells, restricted to the structure and function of: cell-surface membrane nucleus (containing chromosomes, consisting of protein- bound, linear DNA, and one or more nucleoli) mitochondria chloroplasts (in plants and algae) Golgi apparatus and Golgi vesicles lysosomes (a membrane-bound organelle that releases hydrolytic enzymes) ribosomes rough endoplasmic reticulum and smooth endoplasmic reticulum cell wall (in plants, algae and fungi) cell vacuole (in plants)., In complex multicellular organisms, eukaryotic cells become specialised for specific functions., Specialised cells are organised into tissues, tissues into organs and organs into systems. +1 more

    13 objectives11 tips19 pitfalls4 subtopics

    Structure of prokaryotic cells and of viruses

    Prokaryotes are typically 0.5-5 micrometres across against 10-100 micrometres for a eukaryotic cell, and the differences are not just of scale. Prokaryotic cytoplasm contains no membrane-bound organelles at all: no nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus or chloroplasts, so respiratory enzymes are located on the cell-surface membrane instead. Its ribosomes are smaller, 70S rather than the 80S of eukaryotes. Its genetic material is one circular DNA molecule lying free in the cytoplasm as a nucleoid, not wound around histone proteins and not enclosed by a nuclear envelope, whereas a eukaryote has several linear chromosomes associated with histones inside a nucleus.

    Prokaryotic cells are much smaller than eukaryotic cells. They also differ from eukaryotic cells in having: cytoplasm that lacks membrane-bound organelles smaller ribosomes no nucleus; instead they have a single circular DNA molecule that is free in the cytoplasm and is not associated with proteins a cell wall that contains murein, a glycoprotein., In addition, many prokaryotic cells have: one or more plasmids a capsule surrounding the cell one or more flagella., Viruses are acellular and non-living. The structure of virus particles to include genetic material, capsid and attachment protein.

    12 objectives8 tips14 pitfalls3 subtopics

    Methods of studying cells

    An optical microscope passes visible light through a specimen using glass lenses. It is limited by the relatively long wavelength of light, giving a maximum resolution of about 0.2 micrometres. It cannot resolve small organelles but can view living, coloured specimens. Electron microscopes use electron beams with much shorter wavelengths, providing significantly higher resolution. A transmission electron microscope (TEM) transmits electrons through an ultra-thin section to reveal 2D internal ultrastructure. A scanning electron microscope (SEM) sweeps a beam across a surface to produce a 3D image. Electron microscopy requires a vacuum, meaning specimens must be dead, and involves complex preparation that can create artefacts.

    The principles and limitations of optical microscopes, transmission electron microscopes and scanning electron microscopes., Measuring the size of an object viewed with an optical microscope., The difference between magnification and resolution. +1 more

    16 objectives10 tips18 pitfalls4 subtopics

    All cells arise from other cells

    Differentiation makes cells specialised, and many differentiated cells no longer divide. A mature mammalian red blood cell loses its nucleus, so it has no nuclear DNA to replicate and cannot divide. A phloem sieve tube element also loses its nucleus at maturity; it retains mitochondrial and plastid DNA but lacks nuclear DNA, so it cannot divide. Neurones and mature skeletal muscle fibres are terminally differentiated: they leave the cell cycle and enter a non-dividing state called G0. Xylem vessel elements are dead at maturity. Some differentiated cells, such as hepatocytes, can re-enter the cell cycle to repair tissue, but division is mostly restricted to specific populations: stem cells in bone marrow, intestinal crypts and the basal layer of the skin, and plant meristems at root and shoot tips.

    Within multicellular organisms, not all cells retain the ability to divide., Eukaryotic cells that do retain the ability to divide show a cell cycle., DNA replication occurs during the interphase of the cell cycle. +11 more

    56 objectives36 tips65 pitfalls14 subtopics

    Transport across cell membranes

    Every membrane in a cell is built to the same basic plan: a phospholipid bilayer about 7 nanometres thick with proteins embedded in it. The reason lies in the phospholipid molecule itself, which has a hydrophilic phosphate head and two hydrophobic fatty acid tails. In an aqueous environment the molecules arrange themselves automatically so that the heads face the water on each side of the membrane while the tails point inwards away from it, producing a stable double layer. Because that arrangement follows from the molecule, it is the same whether you are looking at the cell-surface membrane, the double membrane of the nuclear envelope, mitochondrion or chloroplast, the endoplasmic reticulum, the Golgi apparatus, a lysosome or a vesicle. Membranes differ in the proteins and lipids they contain, which gives each membrane its particular functions.

    The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the same., The arrangement and any movement of phospholipids, proteins, glycoproteins and glycolipids in the fluid-mosaic model of membrane structure., Cholesterol may also be present in cell membranes where it restricts the movement of other molecules making up the membrane. +5 more

    32 objectives20 tips35 pitfalls8 subtopics

    Cell recognition and the immune system

    Cells carry specific molecules on their cell-surface membrane, mostly proteins or glycoproteins. A protein's highly specific tertiary structure gives it a unique 3D shape, acting as an identity label. The immune system uses these to identify four main targets. First, pathogens (like bacteria) carry foreign antigens. Second, cells from other organisms of the same species carry different antigens, which explains why transplanted tissue or mismatched blood transfusions are attacked. Third, abnormal body cells, such as cancer or virus-infected cells, display altered antigens. Finally, toxins are non-self molecules (not cells) that also stimulate an immune response.

    Each type of cell has specific molecules on its surface that identify it. These molecules include proteins and enable the immune system to identify: pathogens cells from other organisms of the same species abnormal body cells toxins., Definition of antigen. The effect of antigen variability on disease and disease prevention., Phagocytosis of pathogens. The subsequent destruction of ingested pathogens by lysozymes. +15 more

    54 objectives46 tips80 pitfalls18 subtopics

    Surface area to volume ratio

    Surface area increases with the square of a linear dimension while volume increases with its cube, so as an organism or structure gets larger its surface area to volume ratio falls. A 1 mm cube has 6 mm² of surface and 1 mm³ of volume, a ratio of 6:1; a 2 mm cube has 24 mm² over 8 mm³, a ratio of 3:1. The biological consequence is that exchange happens across surfaces while demand is generated throughout the volume. A large organism therefore has proportionally less surface through which to absorb oxygen and nutrients and to lose heat and waste, and the distance from the surface to the innermost cells becomes too great for diffusion to supply them in time. Small organisms can rely on diffusion across the body surface alone.

    The relationship between the size of an organism or structure and its surface area to volume ratio., Changes to body shape and the development of systems in larger organisms as adaptations that facilitate exchange as this ratio reduces., Students should be able to appreciate the relationship between surface area to volume ratio and metabolic rate.

    9 objectives9 tips13 pitfalls3 subtopics

    Gas exchange

    Gas exchange surfaces share key features: large surface area, short diffusion pathway, and a maintained concentration gradient. Single-celled organisms have a large surface area to volume ratio, allowing direct diffusion across the cell-surface membrane. Insects draw air through spiracles into tracheae, then highly branched tracheoles with thin walls, providing a short pathway directly to tissues. Fish gills use filaments covered in lamellae to increase surface area. Blood and water flow in opposite directions (counter-current principle), maintaining a diffusion gradient along the entire lamella. Dicotyledonous leaves use spongy mesophyll for a large internal surface area, with stomata pores allowing gases to diffuse in and out.

    Adaptations of gas exchange surfaces, shown by gas exchange: across the body surface of a single-celled organism in the tracheal system of an insect (tracheae, tracheoles and spiracles) across the gills of fish (gill lamellae and filaments including the counter-current principle) by the leaves of dicotyledonous plants (mesophyll and stomata)., Structural and functional compromises between the opposing needs for efficient gas exchange and the limitation of water loss shown by terrestrial insects and xerophytic plants., The gross structure of the human gas exchange system limited to the alveoli, bronchioles, bronchi, trachea and lungs. +4 more

    21 objectives17 tips30 pitfalls7 subtopics

    Digestion and absorption

    Digestion is chemical hydrolysis: a water molecule is used to break a bond, the reverse of the condensation reaction that built the molecule. Starch, triglycerides and proteins are too large, and largely too insoluble, to cross the phospholipid bilayer of the epithelial cells lining the gut, so each is broken into smaller products first. Hydrolysis of glycosidic bonds in carbohydrates gives monosaccharides; hydrolysis of ester bonds in triglycerides gives a monoglyceride and fatty acids; hydrolysis of peptide bonds in proteins gives amino acids. Each bond type has its own class of enzyme, and each enzyme is specific because its active site is complementary to only one substrate. Digestion occurs in the lumen of the gut, and the small soluble products are then absorbed across the cell-surface membrane of the epithelial cells lining the ileum, passing into the blood or the lacteal.

    During digestion, large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes., Digestion in mammals of: carbohydrates by amylases and membrane-bound disaccharidases lipids by lipase, including the action of bile salts proteins by endopeptidases, exopeptidases and membrane- bound dipeptidases., Mechanisms for the absorption of the products of digestion by cells lining the ileum of mammals, to include: co-transport mechanisms for the absorption of amino acids and of monosaccharides the role of micelles in the absorption of lipids.

    9 objectives9 tips13 pitfalls3 subtopics

    Mass transport in animals

    Haemoglobin is a globular protein with quaternary structure: four polypeptide chains, two alpha and two beta in adult humans, each folded into its own tertiary structure and each holding one haem group containing an iron ion. Quaternary structure means more than one polypeptide chain, held together by interactions between the chains such as hydrogen bonds, ionic bonds and hydrophobic interactions. Because each haem group binds one oxygen molecule, one haemoglobin molecule carries four oxygen molecules, and the product is called oxyhaemoglobin. The plural in the haemoglobins matters: the same basic architecture appears across many organisms and at different life stages, but the primary structure of the globin chains differs. A different amino acid sequence gives a different tertiary and quaternary structure, and so a different affinity for oxygen.

    The haemoglobins are a group of chemically similar molecules found in many different organisms. Haemoglobin is a protein with a quaternary structure., The role of haemoglobin and red blood cells in the transport of oxygen., The loading, transport and unloading of oxygen in relation to the oxyhaemoglobin dissociation curve. +11 more

    43 objectives39 tips65 pitfalls14 subtopics

    Mass transport in plants

    Xylem vessels are dead, hollow cells stacked end to end with their end walls broken down, and with walls thickened by lignin, so they form a continuous open tube from root to leaf. Cohesion-tension theory explains how water rises through them without any cell doing work. Water evaporates from the surfaces of mesophyll cells and diffuses out through the stomata, which is transpiration. This lowers the water potential of those cells, so water is drawn out of the xylem in the leaf, and that creates tension, a negative pressure, at the top of the column. Water molecules are polar and form hydrogen bonds with one another, so they stick together; this cohesion means the entire column is pulled up as a single unit. Water also adheres to the lignified walls. The column must be unbroken, which is why lignin holding the vessel open matters and why an air bubble stops the flow entirely.

    Xylem as the tissue that transports water in the stem and leaves of plants. The cohesion-tension theory of water transport in the xylem., Phloem as the tissue that transports organic substances in plants., The mass flow hypothesis for the mechanism of translocation in plants. The use of tracers and ringing experiments to investigate transport in plants. +1 more

    12 objectives12 tips18 pitfalls4 subtopics

    DNA, genes and chromosomes

    A prokaryotic cell has no nucleus. Its DNA lies free in the cytoplasm as a single short, circular molecule, in a region sometimes called the nucleoid. This DNA is not wound around histone proteins, so it is often described as naked. Many prokaryotes also carry plasmids, which are small, separate circles of DNA holding extra genes such as those for antibiotic resistance, but a plasmid is not the cell's main DNA molecule. When the cell divides by binary fission, the circular DNA replicates and a copy passes to each daughter cell. The key features to remember are that the main DNA is short, circular, and not associated with proteins.

    In prokaryotic cells, DNA molecules are short, circular and not associated with proteins., In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones., Together a DNA molecule and its associated proteins form a chromosome. +8 more

    33 objectives28 tips46 pitfalls11 subtopics

    DNA and protein synthesis

    The genome is the complete set of genes in a cell. It belongs to a specific cell or organism, not a species or population. Note that the genome is not simply 'all the DNA', as eukaryotic DNA includes significant non-coding regions outside of genes, such as multiple repeats. The proteome is the full range of proteins that a cell is able to produce. The two are linked but distinct. Every body cell of an organism carries the same genome, yet a liver cell and a muscle cell contain different proteins because different genes are expressed. In eukaryotes, the proteome is larger than the gene count because pre-mRNA from one gene can be alternatively spliced, and proteins can be modified after translation.

    The concept of the genome as the complete set of genes in a cell and of the proteome as the full range of proteins that a cell is able to produce., The structure of molecules of messenger RNA (mRNA) and of transfer RNA (tRNA)., Transcription as the production of mRNA from DNA. The role of RNA polymerase in joining mRNA nucleotides. +4 more

    22 objectives18 tips32 pitfalls7 subtopics

    Genetic diversity can arise as a result of mutation or during

    A gene mutation is a change to the base sequence of the DNA in a chromosome, producing a new allele of that gene. Because the base sequence determines the sequence of codons on mRNA, a change can alter which amino acid is inserted at that point in the polypeptide. A different amino acid sequence changes the primary structure. Since the R groups of the amino acids determine where ionic bonds, hydrogen bonds and disulfide bridges form, the tertiary structure may also change. For an enzyme, this altered 3D shape means the active site may no longer be complementary to its substrate, preventing enzyme-substrate complexes from forming. Some mutations are silent: because the genetic code is degenerate, a changed triplet may still code for the same amino acid, so the primary structure is unchanged.

    Gene mutations involve a change in the base sequence of chromosomes., They can arise spontaneously during DNA replication and include base deletion and base substitution., Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids. +5 more

    25 objectives21 tips33 pitfalls8 subtopics

    Genetic diversity and adaptation

    Genetic diversity is defined strictly as the number of different alleles of genes in a population. It is not the number of genes, nor the number of species. All members of a species carry the same genes; what differs is which version, or allele, of each gene they possess. A population is a group of individuals of the same species living in the same area that can interbreed. The greater the number of different alleles present in this population, the greater the genetic diversity. High genetic diversity is crucial because it provides a wider range of phenotypes, meaning there is more variation for natural selection to act upon if environmental conditions change.

    Genetic diversity as the number of different alleles of genes in a population., Genetic diversity is a factor enabling natural selection to occur., The principles of natural selection in the evolution of populations. +8 more

    33 objectives30 tips49 pitfalls11 subtopics

    Species and taxonomy

    The biological species concept defines a species by reproductive compatibility, not by appearance. Two organisms belong to the same species only if they can interbreed and the offspring are themselves fertile, so that alleles keep flowing between them and they share a single gene pool. A horse and a donkey interbreed and produce a mule, but the mule is sterile because its odd chromosome number prevents homologous chromosomes pairing in meiosis; horse and donkey are therefore separate species. The word fertile is what carries the definition, because offspring that cannot themselves reproduce are an evolutionary dead end and the two parent populations continue to evolve apart.

    Two organisms belong to the same species if they are able to produce fertile offspring., Courtship behaviour as a necessary precursor to successful mating., The role of courtship in species recognition. +6 more

    28 objectives27 tips44 pitfalls9 subtopics

    Biodiversity within a community

    Biodiversity is the variety of living organisms in an area, and the area can be any size: a single pond, a hedgerow, a stretch of river downstream of a power station, a rainforest, or the whole planet. The scale has to be stated, because a measurement made in one place does not describe another. That is the basis of a standard exam criticism: data collected at one site, or in one month, describe that community at that time and cannot be generalised to all communities or to the whole year. Biodiversity is also measured at more than one level, including the number of species in a community, the genetic variety within a species, and the range of habitats present, and questions often turn on which level a given figure actually measures.

    Biodiversity can relate to a range of habitats, from a small local habitat to the Earth., Species richness is a measure of the number of different species in a community., An index of diversity describes the relationship between the number of species in a community and the number of individuals in each species. +1 more

    12 objectives10 tips19 pitfalls4 subtopics

    Investigating diversity

    Genetic diversity is compared using four methods. Originally, scientists measured the frequency of observable characteristics. However, most traits are polygenic and modified by the environment, making phenotypes an indirect guide to genotypes. Modern methods compare molecules directly. Comparing DNA base sequences is the most direct; fewer differences indicate closer relatedness. Comparing mRNA base sequences works similarly, as mRNA is complementary to the DNA template strand. Finally, comparing the amino acid sequences of proteins encoded by DNA and mRNA also reveals diversity. Because the genetic code is degenerate, amino acid sequences may underestimate DNA differences, but they still provide clear evidence of evolutionary relationships.

    Genetic diversity within, or between species, can be made by comparing: the frequency of measurable or observable characteristics the base sequence of DNA the base sequence of mRNA the amino acid sequence of the proteins encoded by DNA and mRNA., Students should be able to: interpret data relating to similarities and differences in the base sequences of DNA and in the amino acid sequences of proteins to suggest relationships between different organisms within a species and between species appreciate that gene technology has caused a change in the methods of investigating genetic diversity; inferring DNA differences from measurable or observable characteristics has been replaced by direct investigation of DNA sequences. Knowledge of gene technologies will not be tested., Quantitative investigations of variation within a species involve: collecting data from random samples calculating a mean value of the collected data and the standard deviation of that mean interpreting mean values and their standard deviations.

    9 objectives7 tips14 pitfalls3 subtopics

    Photosynthesis (A-level only)

    The light-dependent reaction takes place on the thylakoid membranes of the chloroplast. Chlorophyll absorbs light, and the energy raises an electron to a higher energy level so that it leaves the molecule, which is photoionisation. The electrons pass along a chain of electron carriers embedded in the membrane, losing energy at each transfer, and that energy actively moves protons from the stroma into the thylakoid space. A proton gradient builds across the membrane, and protons pass back into the stroma through ATP synthase; their movement drives the synthesis of ATP from ADP and inorganic phosphate, which is chemiosmosis, called photophosphorylation here. At the end of the chain, electrons and protons combine with NADP to form reduced NADP. The electrons lost from chlorophyll are replaced by photolysis of water, which splits into protons, electrons and oxygen.

    The light-dependent reaction in such detail as to show that: chlorophyll absorbs light, leading to photoionisation of chlorophyll some of the energy from electrons released during photoionisation is conserved in the production of ATP and reduced NADP the production of ATP involves electron transfer associated with the transfer of electrons down the electron transfer chain and passage of protons across chloroplast membranes and is catalysed by ATP synthase embedded in these membranes (chemiosomotic theory) photolysis of water produces protons, electrons and oxygen., The light-independent reaction uses reduced NADP from the light- dependent reaction to form a simple sugar., The hydrolysis of ATP, also from the light-dependent reaction, provides the additional energy for this reaction. +4 more

    23 objectives20 tips36 pitfalls7 subtopics

    Respiration (A-level only)

    Respiration transfers energy from organic molecules to ATP. ATP is synthesised by substrate-level phosphorylation in glycolysis and the Krebs cycle, and by oxidative phosphorylation. In oxidative phosphorylation, electrons from reduced coenzymes pass along the electron transfer chain, releasing energy to actively transport protons. The resulting proton gradient drives ATP synthase to phosphorylate ADP. ATP is the immediate energy source for cells. It is hydrolysed by ATP hydrolase to ADP and Pi, releasing manageable energy for processes such as active transport, muscle contraction and protein synthesis. Glycolysis initially requires ATP to phosphorylate glucose, so respiration both consumes and produces ATP. Phosphorylation makes glucose more reactive and less stable, allowing it to be split more readily; this is accepted as lowering the activation energy of the reaction.

    Respiration produces ATP., Glycolysis is the first stage of anaerobic and aerobic respiration. It occurs in the cytoplasm and is an anaerobic process., Glycolysis involves the following stages: phosphorylation of glucose to glucose phosphate, using ATP production of triose phosphate oxidation of triose phosphate to pyruvate with a net gain of ATP and reduced NAD. +5 more

    25 objectives19 tips33 pitfalls8 subtopics

    Energy and ecosystems (A-level only)

    Producers form the entry point of carbon and chemical energy into an ecosystem. In terrestrial ecosystems, plants synthesise organic compounds using carbon dioxide from the atmosphere. In aquatic ecosystems, producers utilise dissolved carbon dioxide and hydrogencarbonate ions. Through photosynthesis, this inorganic carbon is converted into organic compounds, primarily sugars. While some sugars are respired, the remainder are used to synthesise other biological molecules, forming the plant's biomass. This biomass represents the chemical energy available to consumers and decomposers. Therefore, the rate at which producers synthesise these organic compounds directly determines the energy available to the rest of the food web.

    In any ecosystem, plants synthesise organic compounds from atmospheric, or aquatic, carbon dioxide., Most of the sugars synthesised by plants are used by the plant as respiratory substrates., The rest are used to make other groups of biological molecules. +8 more

    41 objectives26 tips41 pitfalls11 subtopics

    Nutrient cycles (A-level only)

    Nutrients are recycled within ecosystems, whereas energy flows through and is lost. Nitrogen and phosphorus follow this pattern: producers absorb inorganic ions from soil or water, building them into organic molecules. Elements pass along food chains, and microorganisms return them to the inorganic pool when organisms die. Nitrogen enters mainly as nitrate, absorbed by active transport into roots, and is used to make amino acids, proteins, ATP and DNA. It has a vast atmospheric reservoir of nitrogen gas, fixed mainly by nitrogen-fixing bacteria and archaea, though lightning also contributes. Phosphorus has no significant gaseous stage; its reservoir is rock, which weathers to release phosphate ions into soil water. Phosphate is needed for phospholipids, ATP, DNA and RNA.

    Nutrients are recycled within natural ecosystems, exemplified by the nitrogen cycle and the phosphorus cycle., Microorganisms play a vital role in recycling chemical elements such as phosphorus and nitrogen., The role of saprobionts in decomposition. +4 more

    28 objectives20 tips33 pitfalls7 subtopics

    Survival and response (A-level only)

    A stimulus is a detectable change in the internal or external environment; a response is the change in behaviour or physiology that follows. Responding raises the chance of surviving long enough to reproduce. The general pathway involves a stimulus, receptor, coordinator, effector and response. Receptors are specific to one form of energy, such as a Pacinian corpuscle detecting pressure. Coordinators include the nervous system, hormonal systems, or plant growth factors. Effectors vary: animals use muscles or glands, whereas plants use elongating cells. Responses range from reflex withdrawals to taxes and kineses that keep invertebrates in favourable conditions.

    Organisms increase their chance of survival by responding to changes in their environment., In flowering plants, specific growth factors move from growing regions to other tissues, where they regulate growth in response to directional stimuli., The effect of different concentrations of indoleacetic acid (IAA) on cell elongation in the roots and shoots of flowering plants as an explanation of gravitropism and phototropism in flowering plants. +3 more

    24 objectives15 tips25 pitfalls6 subtopics

    Receptors (A-level only)

    Receptors are transducers: they convert the energy of a stimulus into the electrical energy of a nerve impulse. Each type is specific, because only one form of energy can open its particular channel proteins. A Pacinian corpuscle, found deep in the skin and around joints and tendons, contains stretch-mediated sodium ion channels that open only when the membrane is physically deformed, so it responds to pressure and vibration and is completely insensitive to light, sound or chemicals. When pressure is applied, sodium ions enter and the membrane depolarises, and that local depolarisation is the generator potential. A generator potential is graded: a greater or faster pressure change deforms more channels, admits more sodium ions and produces a larger generator potential.

    The Pacinian corpuscle should be used as an example of a receptor to illustrate that: receptors respond only to specific stimuli stimulation of a receptor leads to the establishment of a generator potential., The basic structure of a Pacinian corpuscle., Deformation of stretch-mediated sodium ion channels in a Pacinian corpuscle leads to the establishment of a generator potential. +1 more

    16 objectives12 tips21 pitfalls4 subtopics

    Control of heart rate (A-level only)

    Cardiac muscle is myogenic: it contracts rhythmically without nerve impulses. The intrinsic rhythm originates in the sinoatrial node (SAN) in the right atrium, which depolarises spontaneously to set the basic heart rate. A wave of depolarisation spreads across both atria, causing atrial systole. Cardiac muscle cells are electrically coupled via gap junctions, allowing ions to pass directly. A band of non-conducting connective tissue between the atria and ventricles prevents the wave passing straight down. Consequently, ventricular contraction only begins after the wave is routed through the atrioventricular node (AVN).

    Myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity., The roles of the sinoatrial node (SAN), atrioventricular node (AVN) and Purkyne tissue in the bundle of His., The roles and locations of chemoreceptors and pressure receptors and the roles of the autonomic nervous system and effectors in controlling heart rate.

    9 objectives7 tips11 pitfalls3 subtopics

    Nerve impulses (A-level only)

    A motor neurone carries impulses from the central nervous system to an effector. Its cell body, containing the nucleus and much rough endoplasmic reticulum for making proteins such as neurotransmitter enzymes, lies in the spinal cord, and many short dendrites carry impulses towards it. One long axon carries impulses away, ending in synaptic knobs at the effector. Schwann cells wrap repeatedly around the axon, and the many layers of their plasma membrane form the myelin sheath, which is rich in lipid and acts as an electrical insulator. Between adjacent Schwann cells are short gaps, the nodes of Ranvier, where the axon membrane is exposed to tissue fluid and voltage-gated sodium ion channels are concentrated. Depolarisation can occur only at these nodes, so the action potential jumps from node to node by saltatory conduction, which is faster than conduction along an unmyelinated axon.

    The structure of a myelinated motor neurone., The establishment of a resting potential in terms of differential membrane permeability, electrochemical gradients and the movement of sodium ions and potassium ions., Changes in membrane permeability lead to depolarisation and the generation of an action potential. The all-or-nothing principle. +3 more

    18 objectives17 tips28 pitfalls6 subtopics

    Synaptic transmission (A-level only)

    A synapse is the junction between the synaptic knob at the end of one neurone and the membrane of the next cell. The presynaptic knob contains many mitochondria, supplying ATP for making and packaging neurotransmitter, a large amount of smooth endoplasmic reticulum, and vesicles filled with acetylcholine. Its membrane holds voltage-gated calcium ion channels. The synaptic cleft is a gap of about 20 nm. The postsynaptic membrane carries specific receptor proteins whose binding sites are complementary in shape to acetylcholine and which form part of sodium ion channels, and acetylcholinesterase in the cleft hydrolyses acetylcholine into choline and ethanoic acid. A neuromuscular junction has the same basic components but its postsynaptic membrane is the folded sarcolemma of a muscle fibre, forming the motor end plate, and it is the muscle fibre that is excited to contract.

    The detailed structure of a synapse and of a neuromuscular junction., The sequence of events involved in transmission across a cholinergic synapse in sufficient detail to explain: unidirectionality temporal and spatial summation inhibition by inhibitory synapses., A comparison of transmission across a cholinergic synapse and across a neuromuscular junction. +1 more

    13 objectives11 tips19 pitfalls4 subtopics

    Skeletal muscles are stimulated to contract by nerves and act as

    Muscles can only pull. A muscle shortens when it contracts and generates force, but it cannot push itself back to its original length, so something else must stretch it, and that something is another muscle pulling the same bone the other way. That is an antagonistic pair: as one muscle, the agonist or prime mover, contracts, its partner, the antagonist, relaxes and is stretched. At the elbow the biceps contracts to flex the joint while the triceps relaxes; to extend it, the triceps contracts and the biceps relaxes. The bones between them are incompressible, so the pull is transmitted rather than absorbed, and the joint acts as a pivot so the bone works as a lever. Tendons, made of collagen and almost inelastic, attach muscle to bone so that contraction moves the bone rather than stretching the attachment. Without a rigid skeleton the force would simply deform the body.

    Muscles act in antagonistic pairs against an incompressible skeleton., Gross and microscopic structure of skeletal muscle. The ultrastructure of a myofibril., The roles of actin, myosin, calcium ions and ATP in myofibril contraction. +3 more

    18 objectives16 tips27 pitfalls6 subtopics

    Principles of homeostasis and negative feedback (A-level only)

    Homeostasis is the maintenance of a relatively constant internal environment despite changes outside the body and in the body's own activity. The internal environment means the blood and the tissue fluid that bathe the cells: their temperature, pH, water potential and the concentrations of glucose and ions. Every control system has the same parts: a set point, receptors that detect a deviation from it, a coordinator, usually a centre in the brain or an endocrine gland, that receives that information, effectors that carry out a correction, and feedback of the new value to the receptors. Notice the word restricted rather than constant. Levels fluctuate around the set point rather than being held exactly, because a correction can only begin once a deviation has been detected.

    Homeostasis in mammals involves physiological control systems that maintain the internal environment within restricted limits., The importance of maintaining a stable core temperature and stable blood pH in relation to enzyme activity., The importance of maintaining a stable blood glucose concentration in terms of availability of respiratory substrate and of the water potential of blood. +3 more

    18 objectives18 tips29 pitfalls6 subtopics

    Control of blood glucose concentration (A-level only)

    Blood glucose concentration is influenced by diet, exercise, hormones and body mass. Glucose rises after a carbohydrate meal as gut enzymes hydrolyse starch and disaccharides and the products are absorbed from the small intestine. It falls when cells respire glucose, especially during exercise, and when the liver and muscle cells convert glucose to glycogen (glycogenesis) under the influence of insulin. During fasting, the liver breaks down glycogen to glucose (glycogenolysis) and can synthesise glucose from non-carbohydrate sources such as glycerol and amino acids (gluconeogenesis). Glucagon stimulates both glycogenolysis and gluconeogenesis, while adrenaline stimulates glycogenolysis. Body mass is a risk factor for type 2 diabetes rather than an acute control factor.

    The factors that influence blood glucose concentration., The role of the liver in glycogenesis, glycogenolysis and gluconeogenesis., The action of insulin by: attaching to receptors on the surfaces of target cells controlling the uptake of glucose by regulating the inclusion of channel proteins in the surface membranes of target cells activating enzymes involved in the conversion of glucose to glycogen. +6 more

    27 objectives26 tips40 pitfalls9 subtopics

    Control of blood water potential (A-level only)

    Osmoregulation is the homeostatic control of the water potential of blood plasma, which matters because body cells are bathed in tissue fluid derived from plasma, and water moves between these compartments by osmosis. If blood becomes too concentrated, water leaves cells by osmosis and they shrink; if too dilute, water enters and animal cells may burst. The controlled variable is water potential, measured in kPa; pure water is 0 kPa and any solution is negative, so more concentrated plasma has a lower, more negative water potential. Osmoreceptors in the hypothalamus monitor this. Sweating, a salty meal or not drinking makes blood water potential fall; water leaves osmoreceptor cells by osmosis and they shrink, triggering ADH release, so the kidney reabsorbs more water and a small volume of concentrated urine is produced.

    Osmoregulation as control of the water potential of the blood., The roles of the hypothalamus, posterior pituitary and antidiuretic hormone (ADH) in osmoregulation., The structure of the nephron and its role in: the formation of glomerular filtrate reabsorption of glucose and water by the proximal convoluted tubule maintaining a gradient of sodium ions in the medulla by the loop of Henle reabsorption of water by the distal convoluted tubule and collecting ducts.

    9 objectives8 tips13 pitfalls3 subtopics

    Inheritance (A-level only)

    A genotype is the genetic constitution of an organism, representing the specific alleles it carries. It can be written for a single gene (e.g., Ff), two genes in a dihybrid cross (e.g., FfGg), or a sex-linked gene (e.g., $X^R X^r$). Alleles are different versions of a gene at a specific locus. While diploid organisms typically have two alleles for each autosomal gene, XY males possess only one allele for X-linked genes (e.g., $X^R Y$). A genotype belongs to an individual, whereas a gene pool describes a population, and a genome is the complete set of genes. The genotype is fixed at fertilisation; cellular differences arise from gene expression, not different alleles.

    The genotype is the genetic constitution of an organism., The phenotype is the expression of this genetic constitution and its interaction with the environment., There may be many alleles of a single gene. +4 more

    21 objectives18 tips32 pitfalls7 subtopics

    Populations (A-level only)

    A species is a group of organisms able to interbreed and produce fertile offspring, but its members are rarely spread evenly across the landscape. They usually occupy separate patches of suitable habitat, and each patch holds a population. Lemurs of one species living on opposite sides of a river, or monkeyflowers growing at different altitudes on the same mountain, are one species divided into several populations. What keeps them a single species is gene flow: if individuals move between the populations, or gametes do, as when a bee carries pollen from one hillside to another, alleles are shared and the two gene pools stay similar.

    Species exist as one or more populations., A population as a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed., The concepts of gene pool and allele frequency. +2 more

    15 objectives15 tips24 pitfalls5 subtopics

    Evolution may lead to speciation (A-level only)

    No two members of a population are identical. Some characteristics vary in discrete categories, such as blood group or flower colour in a species with two pigment alleles, and this discontinuous variation is usually controlled by one or a few genes with little environmental influence. Others, such as height, body mass or the concentration of a chemical a plant produces, vary continuously across a range, because many genes each make a small contribution and the environment adds more. Plotting a continuous characteristic for a large sample gives a normal distribution, with most individuals near the mean and few at the extremes, and the standard deviation measures how widely they are spread around that mean. This spread is the raw material of natural selection, because selection can only favour phenotypes that already exist in the population.

    Individuals within a population of a species may show a wide range of variation in phenotype., This is due to genetic and environmental factors., The primary source of genetic variation is mutation. +11 more

    42 objectives40 tips63 pitfalls14 subtopics

    Populations in ecosystems (A-level only)

    A population is all the organisms of one species living in the same place at the same time that can interbreed. A community is all the populations of all the different species living in that same place at the same time, and the point of the word is that those populations interact: they compete for the same resources, eat one another, pollinate one another, parasitise one another. Community is a purely biotic term, so soil, temperature and water are not part of it. Communities are described by sampling them. Species richness counts how many different species are present; an index of diversity goes further and uses the number of individuals of each species as well, so it distinguishes a community with one dominant species from one where individuals are spread evenly.

    Populations of different species form a community., A community and the non-living components of its environment together form an ecosystem., Ecosystems can range in size from the very small to the very large. +11 more

    46 objectives40 tips67 pitfalls14 subtopics

    Alteration of the sequence of bases in DNA can alter the

    A gene mutation is a change in the base sequence of DNA. These might arise spontaneously during DNA replication, for example if DNA polymerase inserts an incorrect nucleotide, producing a substitution. Other types arise through different errors: deletion removes a base and addition inserts one; unless the number changed is a multiple of three, this causes a frameshift, altering every subsequent triplet. Duplication repeats a section of bases. Inversion reverses a sequence of bases so its triplets are read backwards. Translocation moves a sequence of bases to a different position within the DNA sequence. These changes can alter the mRNA codons, potentially changing the amino acid sequence and the protein's tertiary structure.

    Gene mutations might arise during DNA replication. They include addition, deletion, substitution, inversion, duplication and translocation of bases., Gene mutations occur spontaneously., The mutation rate is increased by mutagenic agents. +5 more

    24 objectives23 tips35 pitfalls8 subtopics

    Most of a cell’s DNA is not translated (A-level only)

    A stem cell is an undifferentiated cell that can keep dividing by mitosis and can differentiate into other cell types. Totipotent cells are the most versatile of all: they can divide and produce every type of body cell and, in mammals, the extra-embryonic tissues such as the placenta, so a single totipotent cell can give rise to a whole organism. The fertilised egg is totipotent, and so are the cells produced by the first few divisions after fertilisation. Every one of these cells contains exactly the same DNA as every other cell in the body; what makes a specialised cell different is not which genes it has but which genes it expresses. As development proceeds, a totipotent cell transcribes only part of its DNA and translates only the mRNA produced from those genes, and the proteins produced determine the structure and function it takes on.

    Totipotent cells can divide and produce any type of body cell., During development, totipotent cells translate only part of their DNA, resulting in cell specialisation., Totipotent cells occur only for a limited time in early mammalian embryos. +5 more

    24 objectives20 tips33 pitfalls8 subtopics

    Regulation of transcription and translation (A-level only)

    A transcriptional factor is a protein that controls whether a particular gene is transcribed. Many are held inactive in the cytoplasm until a signal arrives; once activated they pass through a nuclear pore into the nucleus and bind to a specific base sequence in the promoter region, the stretch of DNA lying just before the gene. The binding site of the factor is complementary in shape to that sequence, so each factor only affects its own target genes. A stimulatory factor helps RNA polymerase attach to the promoter, so transcription begins, mRNA is produced and the polypeptide is made. An inhibitory factor blocks the promoter, so RNA polymerase cannot bind and no mRNA is transcribed. The gene itself is unchanged in every case; only the amount of mRNA changes.

    In eukaryotes, transcription of target genes can be stimulated or inhibited when specific transcriptional factors move from the cytoplasm into the nucleus., The role of the steroid hormone, oestrogen, in initiating transcription., Epigenetic control of gene expression in eukaryotes. +4 more

    21 objectives19 tips33 pitfalls7 subtopics

    Gene expression and cancer (A-level only)

    A tumour is a mass of cells produced by uncontrolled mitosis. Benign and malignant tumours differ in ways you can be asked to compare. Benign tumours grow slowly and remain compact, usually surrounded by a capsule of fibrous tissue. Their cells are differentiated, resembling normal cells with normal-sized nuclei. Because they do not invade, surgical removal is usually a permanent cure. They can cause harm by pressing on an organ, causing blockages, or releasing excess hormones. Malignant tumours grow rapidly, are irregular in shape and lack a capsule, so they invade surrounding tissue. Their cells are undifferentiated, with large, dark nuclei, and can spread (metastasise).

    The main characteristics of benign and malignant tumours., The role of the following in the development of tumours: tumour suppressor genes and oncogenes abnormal methylation of tumour suppressor genes and oncogenes increased oestrogen concentrations in the development of some breast cancers.

    6 objectives6 tips8 pitfalls2 subtopics

    Using genome projects (A-level only)

    The genome is the complete set of genes in a cell, or all the DNA in a cell, including non-coding DNA. Sequencing determines the order of bases along that DNA. The Human Genome Project mapped the vast majority of the human genome between 1990 and 2003. Genomes of bacteria, viruses, and model organisms like the fruit fly have also been sequenced, many concurrently or earlier. Comparing sequences between species measures how closely related organisms are; more similar sequences imply a more recent common ancestor. This evidence has led to species originally grouped by appearance being reclassified and renamed based on their evolutionary relationships.

    Sequencing projects have read the genomes of a wide range of organisms, including humans., Determining the genome of simpler organisms allows the sequences of the proteins that derive from the genetic code (the proteome) of the organism to be determined., This may have many applications, including the identification of potential antigens for use in vaccine production. +2 more

    15 objectives14 tips22 pitfalls5 subtopics

    Recombinant DNA technology (A-level only)

    Recombinant DNA is DNA made by joining together DNA from two different organisms, usually from different species. The sequence of operations is fixed and worth learning as a whole. The required gene is isolated - cut out with restriction endonucleases, made from mRNA using reverse transcriptase, or built in a gene machine. Promoter and terminator regions are added so that the host's RNA polymerase knows where to start and stop. The fragment is inserted into a vector, normally a plasmid or a virus, using the same restriction enzyme to create complementary sticky ends and DNA ligase to seal the sugar-phosphate backbones. The vector carries the DNA into a host cell; a cell that takes it up is transformed, and a whole organism containing DNA from another species is transgenic.

    Recombinant DNA technology involves the transfer of fragments of DNA from one organism, or species, to another., Since the genetic code is universal, as are transcription and translation mechanisms, the transferred DNA can be translated within cells of the recipient (transgenic) organism., Fragments of DNA can be produced by several methods, including: conversion of mRNA to complementary DNA (cDNA), using reverse transcriptase using restriction enzymes to cut a fragment containing the desired gene from DNA creating the gene in a ‘gene machine’. +7 more

    30 objectives28 tips45 pitfalls10 subtopics

    Differences in DNA between individuals of the same species can be exploited

    A DNA probe is a short, single-stranded DNA fragment with a base sequence complementary to the target allele. It carries a label, such as a radioactive phosphorus-32 tag detected on X-ray film, or a fluorescent tag visible under UV light. To locate an allele, the sample DNA is first amplified by PCR. It is then heated to break hydrogen bonds, making it single-stranded. The labelled probe is added and the mixture cooled. If the target allele is present, the probe binds to its complementary sequence via hydrogen bonding—this is DNA hybridisation. Finally, unbound probes are washed away, and the label's presence is detected to locate the specific allele.

    The use of labelled DNA probes and DNA hybridisation to locate specific alleles of genes., The use of labelled DNA probes that can be used to screen patients for heritable conditions, drug responses or health risks., The use of this information in genetic counselling and personalised medicine. +1 more

    13 objectives10 tips13 pitfalls4 subtopics

    Genetic fingerprinting (A-level only)

    Variable number tandem repeats (VNTRs) are loci where a short base sequence is repeated end to end. The repeat sequence is identical between individuals, but the number of times it is copied varies, creating different alleles. Many VNTRs occur in non-coding DNA; because these often do not alter polypeptides, changes in repeat number are usually not selected against, allowing many alleles to accumulate. A person inherits one allele at each locus from each parent. While unrelated people might share a repeat number at a single locus, testing multiple independent loci multiplies the separate probabilities. This makes a match at every locus vanishingly improbable, except in monozygotic twins who share identical DNA.

    An organism’s genome contains many variable number tandem repeats (VNTRs). The probability of two individuals having the same VNTRs is very low., The technique of genetic fingerprinting in analysing DNA fragments that have been cloned by PCR, and its use in determining genetic relationships and in determining the genetic variability within a population., The use of genetic fingerprinting in the fields of forensic science, medical diagnosis, animal and plant breeding. +1 more

    15 objectives10 tips15 pitfalls4 subtopics

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