Edexcel A-Level Biology A (Salters-Nuffield)

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1 Lifestyle, Health and Risk

Transport and the heart

1.1 - Why animals need mass transportA cell needs a continuous supply of materials and a way to remove its wastes. Learn why diffusion can meet these needs over short distances, and how a heart and circulation help many animals supply cells deep inside the body.
1.2 - Water as a transport solventBlood transports nutrients and wastes over distances that diffusion alone cannot cover quickly enough. Water makes this possible for many substances because its molecules interact with their particles and keep them dissolved. This lesson links the charge distribution in a water molecule to its ro...
1.3 - Arteries, veins and capillariesBlood vessels must carry blood over long distances and bring its contents close enough to cells for exchange. Learn why arteries, capillaries and veins have different walls and lumens, and how these differences suit the pressures and movements of blood through them.
1.4 - The heart and cardiac cycleThe heart sends blood through the lungs and then through the body, but its valves do not actively pump. This lesson explains how chambers, vessels and pressure changes produce one-way flow during each heartbeat. You will also see how observation and dissection can test the relationship between he...

Cardiovascular disease and evidence

1.5 - Atherosclerosis and blood pressureAtherosclerosis develops in the wall of an artery before it can obstruct the blood flowing through it. In this lesson, you will follow how altered endothelial function, inflammation and plaque growth change an artery, then explain their effects on blood flow and pressure.
1.6 - Clotting and cardiovascular diseaseBlood clotting seals damaged vessels and limits blood loss while tissue heals. You will follow the conversions that produce a fibrin mesh, then explain how the same response can obstruct an artery and contribute to cardiovascular disease.
1.7, 1.10 - Risk factors and perceived riskIn 1.7, you will connect genetics, diet, age, gender, high blood pressure, smoking and inactivity to cardiovascular disease. In 1.10, you will examine why the chance suggested by evidence can differ from the danger a person feels. Expressing risk as a probability helps you distinguish an increase...
1.8 - Interpreting health-risk dataA larger number of deaths does not always mean a greater risk of dying: the size and composition of the population matter. This lesson shows how to compare illness and mortality data, construct useful displays, and decide what an association can establish. You will also learn how to recognise con...
1.9 - Evaluating health-risk studiesA convincing health-risk claim depends on how the evidence was collected. Learn how study design, participant selection, sample size and measurements affect the conclusions we can draw about cardiovascular disease.

Diet, molecules and prevention

1.11 - Diet and energy budgetsFood supplies energy, while living processes transfer energy out of the body. Learn how to analyse diet and energy-budget data in 1.11(i), then use the balance to explain weight loss, weight gain and the development of obesity in 1.11(ii).
1.12, 1.13 - Carbohydrates and glycosidic bondsGlucose can supply a cell with fuel, but storing thousands of separate glucose molecules would disturb its water balance. In 1.12, you will relate the structures of sugars and storage carbohydrates to their roles in providing and storing energy. In 1.13, you will explain how glycosidic bonds join...
1.14 - Triglycerides and fatty acidsFats and oils contain triglycerides, whose properties depend on the fatty acids attached to them. You will follow how glycerol and three fatty acids join, how that reaction can be reversed, and how carbon–carbon double bonds affect a lipid's structure and physical properties.
1.15 - Cholesterol evidence and lipoproteinsCholesterol is needed by cells, yet some patterns of cholesterol transport increase the risk of cardiovascular disease. In 1.15(i), you will interpret total, LDL and HDL cholesterol data; in 1.15(ii), you will examine why the evidence supports a causal contribution from LDL cholesterol. The key i...
1.16 - Reducing coronary heart disease riskScientific evidence connects diet, physical activity and smoking with changes in coronary heart disease risk. Learn how these changes act, how to calculate two body-measurement indicators, and why neither an indicator nor a study result predicts an individual's future with certainty.
1.17, Core practical 1 - Caffeine, Daphnia and research ethicsA translucent water flea lets you observe a beating heart without dissection. Core practical 1 uses this to investigate caffeine, while 1.17 asks whether the information gained justifies the risks to the invertebrates. You will plan a controlled investigation, process its measurements and judge w...
Core practical 2 - Vitamin C in food and drinkA colour change can turn a comparison between foods into a quantitative investigation. In Core practical 2, you will use a vitamin C standard to calibrate DCPIP, measure food and drink samples, and decide how confidently the results support a claim.
1.18 - Benefits and risks of CVD treatmentsMedicines can reduce cardiovascular disease risk by changing blood pressure, cholesterol metabolism or clot formation. You will distinguish four treatment classes and explain why each useful effect must be considered alongside possible harm.

2 Genes and Health

Exchange surfaces and membranes

2.1 - Gas exchange and Fick's lawOxygen must cross an exchange surface before the blood can deliver it to respiring tissues. This lesson explains how surface area, diffusion distance and concentration difference determine that transfer, uses model-cell geometry and Fick's law to calculate their effects, and connects them to the...
2.2 - Membranes and the fluid mosaic modelA cell membrane keeps the cell's contents separate from its surroundings while allowing selected substances and signals to cross. In 2.2(i), you will connect the arrangement of lipids and proteins to these properties. In 2.2(ii), you will see how measurements and experiments led scientists to dev...
Core practical 3 - Investigating membrane permeabilityBeetroot can colour its surrounding water when cell membranes become more permeable. You will learn how to investigate the effect of temperature or alcohol concentration, measure pigment release with a colorimeter, and decide how strongly the results support a conclusion about membranes.
2.3 - Osmosis and free water moleculesWater can cross a cell membrane even when some dissolved substances cannot. This lesson explains how a difference in free-water concentration produces a net movement of water, why cells may swell or shrink, and how percentage mass change helps us interpret osmosis.
2.4 - Passive and active membrane transportA cell must obtain useful substances, remove products and keep its internal conditions different from its surroundings. Some particles cross a membrane by following a gradient; others need an energy supply or transport inside a vesicle. Understanding the route explains why a membrane protein can...

Genes, proteins and enzymes

2.5 - Nucleotides, DNA and RNADNA and RNA are built from small units called nucleotides. Their sugars, bases and bonds explain both how a strand is held together and how two DNA strands fit together. Learning these structures gives us a foundation for understanding how cells use genetic information.
2.7, 2.8 - Genes and the genetic codeThe order of DNA bases carries information about the order of amino acids in a polypeptide. In 2.8, you will learn what a gene encodes; in 2.7, you will use the triplet, non-overlapping and degenerate genetic code to interpret that information. This explains how a four-letter base alphabet can sp...
2.6 - Transcription and translationA DNA base sequence can determine an amino acid sequence even though DNA and amino acids do not bind directly to one another. This lesson explains the two stages that connect them, and how to follow the information accurately from a DNA template to mRNA and then to a polypeptide.
2.9(i), 2.9(ii) - Amino acids and peptide bondsProteins are built from amino acids, but making a chain requires a particular chemical link. In 2.9(i), you will learn the general amino acid structure; in 2.9(ii), you will follow the condensation reactions that join these building blocks into polypeptides. Tracking the links and water molecules...
2.9(iii), 2.9(iv) - Protein folding, haemoglobin and collagenA polypeptide's amino acid sequence helps determine the shape it can form and the job it can do. In 2.9(iii), you will connect sequence, folding and the interactions that stabilise proteins. In 2.9(iv), haemoglobin and collagen show how different molecular structures suit oxygen transport and mec...
2.10 - Enzymes and activation energyCells need chemical reactions to happen fast enough to sustain life at ordinary biological temperatures. Learn how an enzyme's three-dimensional structure selects its substrates, how it lowers the energy barrier to reaction, and why some enzymes work inside cells while others act outside them.
Core practical 4 - Enzyme concentration and initial ratesAn enzyme can make a reaction faster without increasing the amount of product eventually available from a fixed amount of substrate. Investigate how enzyme concentration and substrate concentration affect the rate at the start, then use graphs and uncertainty to decide what the results support.

Inheritance and genetic health

2.11 - DNA replication and experimental evidenceBefore a cell divides, its DNA must be copied so that each daughter cell can receive the genetic information. In 2.11(i), you will follow how complementary strands are made; in 2.11(ii), you will use Meselson and Stahl’s evidence to distinguish three possible ways of copying DNA.
2.12, 2.14 - Mutations and cystic fibrosisIn 2.12, you will see how a copying error can change DNA and why several different mutations can cause cystic fibrosis. In 2.14, you will follow the consequences from a membrane protein to gas exchange, digestion and reproduction. The same molecular fault can affect several organs because their e...
2.13 - Monohybrid inheritance and pedigreesInheritance can hide an allele for a generation without losing it. You will learn how genes, alleles and dominance connect a parent's genotype to possible offspring, then use crosses and family pedigrees to calculate probabilities and distinguish a certain deduction from an unresolved possibility.
A.1.9 - Chi-squared and inherited ratiosAn inheritance model predicts probabilities, so real offspring counts need not match its ratio exactly. You will learn how to select and calculate a chi-squared test, then use a supplied critical value to decide whether a difference provides evidence against the proposed ratio.
2.15, 2.16 - Genetic screening and ethical choicesGenetic tests can reveal an inherited allele before someone has symptoms, before an embryo is transferred to the uterus, or during pregnancy. In 2.15, you will connect carrier identification, pre-implantation genetic diagnosis and prenatal tests with the information each can provide. In 2.16, you...

3 Voice of the Genome

Cell organisation and microscopy

3.1, 3.2 - Cells and eukaryotic ultrastructureIn 3.1, you will connect the common features of cells to the needs of living organisms. In 3.2, you will learn how the internal structures of eukaryotic cells carry out different jobs, and use those structures to make reasoned predictions about a cell's activity.
3.3 - The secretory pathwayAn enzyme can be made inside a cell yet act outside it. This lesson follows a secreted enzyme from synthesis at the rough endoplasmic reticulum, through the Golgi apparatus, to release at the plasma membrane, and uses that route to explain what happens when transport is interrupted.
3.4 - Prokaryotic cell ultrastructureBacteria carry out the processes of life without a nucleus or mitochondria. Learn how their structures support those processes, how to compare them with eukaryotic cells, and why one historical feature in the specification needs a different interpretation today.
3.5 - Recognising organelles and measuring cellsAn electron micrograph shows a thin slice through a real cell, so its structures look less tidy than a textbook diagram. In 3.5, you will learn to recognise the organelles named in 3.2 from their visible features, record observations carefully and calculate real sizes from calibrated images. You...

Division, development and gene expression

3.6, 3.7 - Gametes and mammalian fertilisationA sperm must reach an egg, cross its protective coat and deliver genetic material without allowing several sperm to fertilise the same egg. In 3.6, you will connect the specialised structures of mammalian gametes to these jobs. In 3.7, you will follow the acrosome reaction, cortical reaction and...
3.8 - Gene loci, linkage and sex linkageA gene's position helps explain how it is inherited. You will locate genes on homologous chromosomes, explain why some alleles tend to travel together, and use sex chromosomes to predict offspring and interpret family pedigrees.
3.9 - Meiosis and genetic variationMeiosis produces cells with one set of chromosomes while reshuffling the genetic information they carry. You will follow how independent assortment and crossing over produce different allele combinations, then calculate the chances of particular gametes without confusing chromosome number with DN...
3.10 - Mitosis and the cell cycleA growing root and a healing skin wound both need new cells with a complete set of genetic instructions. This lesson explains how DNA copying, mitosis and cell division work together to produce daughter cells with the same chromosomes, and why this matters for growth and asexual reproduction.
Core practical 5 - Root-tip squash and mitosisA root grows partly because cells near its tip divide. In Core practical 5, you will learn how to prepare and stain this tissue, recognise mitotic stages under a light microscope, and turn observations into drawings, cell-size measurements and a mitotic index. You will also distinguish what a pre...
3.11 - Stem cells and medical decisionsStem cells can supply new cells, but their developmental potential varies. You will distinguish stem cells, pluripotency and totipotency in 3.11(i), then use scientific evidence and ethical reasoning to discuss medical therapies in 3.11(ii). This explains why a promising idea and a justified trea...
3.12, 3.13 - Differential gene expression and organisationA muscle cell and an insulin-secreting cell can contain the same genes yet carry out very different jobs. In 3.12, you will trace how selective gene expression produces specialised cells, and how the bacterial lac operon regulates protein production. In 3.13, you will connect specialised cells to...

4 Biodiversity and Natural Resources

Biodiversity, adaptation and classification

4.1, 4.2 - Biodiversity, endemism and diversity indicesLife has diversified over a long evolutionary history, yet human activity can remove species and the variation within them. In 4.1 you will connect human pressures to biodiversity loss; in 4.2 you will distinguish biodiversity from endemism and use species counts and genotypes to measure differen...
4.3, 4.4 - Niches, adaptations and natural selectionOrganisms can share a place to live while using it in different ways. In 4.3, you will connect an organism's niche with behavioural, physiological and anatomical adaptations. In 4.4, you will explain how selection acting on inherited differences can change a population over generations, and what...
4.5 - Hardy-Weinberg and reproductive isolationPopulation genetics follows alleles through generations rather than following one family. In 4.5(i), you will use Hardy–Weinberg relationships to calculate frequencies and distinguish a model prediction from evidence of change over time. In 4.5(ii), you will explain how reduced interbreeding can...
4.6 - Classification and molecular phylogenyClassification organises the variety of life into groups that represent relationships between organisms. In 4.6(i), you will use the species concept and compare phenotype and genotype evidence; in 4.6(ii), you will see how molecular evidence and scientific scrutiny can change those groups, includ...

Plants, tissues and natural materials

4.7, 4.8 - Plant cells and electron micrographsA plant cell can make sugars, store starch and remain firm while surrounded by water, but different cell types share these jobs between them. In 4.7, you will connect plant-cell structures with their functions and compare them with animal cells. In 4.8, you will use genuine electron micrographs t...
4.9 - Starch, cellulose and microfibrilsA plant can build both a food reserve and a strong cell wall from glucose. This lesson explains how the form of glucose, the glycosidic bonds and the arrangement of chains make starch suited to storage and cellulose suited to support.
4.10, 4.11 - Xylem, phloem and plant supportA stem must resist bending while moving water and the products of photosynthesis. In 4.10, you will connect cellulose microfibrils and secondary wall thickening to the properties of plant fibres and their human uses. In 4.11, you will compare sclerenchyma, xylem and phloem, including their positi...
Core practical 6 - Identifying tissues in stemsA stained stem section reveals how transport and support tissues fit together. In Core practical 6 you will prepare and examine thin sections, identify sclerenchyma fibres, phloem sieve tubes and xylem vessels, and record their locations. You will also calibrate measurements and make a proportion...
4.12, Core practical 7 - Water, minerals and deficiency investigationsA plant can receive plenty of water and light yet fail to grow normally because one mineral is missing. In 4.12 you will connect water, nitrate, calcium and magnesium to plant function; in Core practical 7 you will investigate those requirements by comparing controlled nutrient solutions and anal...
Core practical 8 - Tensile strength of plant fibresA plant fibre can carry a load because its cell walls resist being pulled apart. Core practical 8 investigates how much pulling force prepared fibres withstand, then accounts for their thickness to estimate tensile strength. You will learn how to make the comparison fair and how measurement uncer...

Natural products and conservation

4.13 - How medicines are testedA patient may improve after taking a substance without the substance being responsible for the improvement. Learn how William Withering investigated foxglove and how contemporary drug testing separates a treatment's effects from alternative explanations while protecting participants.
4.14 - Bacterial growth and logarithmic scalesBacterial populations can change enormously while their surroundings appear unchanged. You will connect the conditions needed for growth to the phases of a culture, calculate population changes and use graphs that keep small and large counts readable.
Core practical 9 - Antimicrobial plant extracts and asepsisA clear patch around a plant extract can show that bacterial growth has been inhibited, but its size depends on the assay as well as the extract. Core practical 9 develops a safe, controlled investigation and shows how to measure, plot and interpret the results without claiming more than they dem...
4.15 - Plant materials and sustainabilityPlants can supply strong fibres and mouldable starch-based materials that replace some oil-based products. Learn why using a renewable raw material can help sustainability, and how to compare the whole life of competing products before deciding which is suitable.
4.16 - Conservation in zoos and seed banksKeeping a species alive involves more than increasing its head count. This lesson evaluates how zoos and seed banks can preserve endangered species and their genetic diversity, and how research, breeding, reintroduction and education contribute to that aim.

5 On the Wild Side

Ecology and field evidence

5.1, 5.2 - Ecosystems and distribution factorsTwo places in the same woodland can support very different organisms. In 5.1, you will distinguish a population, community, habitat and ecosystem. In 5.2, you will explain how living interactions and physical conditions affect how many organisms occur and where they are found, then judge how far...
5.3, Core practical 10 - Niches, quadrats and transectsA plant may grow abundantly beside one part of a path and be absent a few metres away. In 5.3, you will use its niche to explain such patterns; in Core practical 10, you will design a habitat study, identify organisms, sample their distribution and abundance, and measure relevant abiotic conditio...
A.1.9 - Correlation in ecological dataEcologists often measure an organism and an environmental factor in the same place. Learn to plot those paired observations, calculate a rank correlation and judge its statistical support. You will also learn why even a convincing association needs further evidence before it can be called a cause.
5.4 - Succession and climax communitiesA newly exposed surface and an abandoned field can both develop changing communities, but they do not begin with the same resources. In 5.4 you will explain how colonising organisms alter their surroundings, why different species become successful, and what a climax community means in a changing...

Photosynthesis and ecosystem energy

5.5, 5.6 - Photosynthesis and ATPA growing plant needs both materials and energy to make new cells. In 5.5, you will follow these through the overall reaction of photosynthesis; in 5.6, you will see how ATP transfers usable energy to the cell processes that need it.
5.7, 5.9 - Chloroplasts and light-dependent reactionsA chloroplast converts absorbed light energy into chemical forms that can drive the manufacture of organic molecules. In 5.9, you will connect its compartments and membranes with their functions; in 5.7, you will follow electrons from water to reduced NADP and explain how their transfer supports...
5.8 - The Calvin cycle and biosynthesisHow does carbon dioxide become material for a growing organism? You will follow carbon through the Calvin cycle, then connect its sugar products to respiration and the construction of new biological molecules in plants, animals and other organisms.
Core practical 11 - The Hill reaction in isolated chloroplastsA blue dye lets us follow electron transfer in chloroplasts without waiting for a plant to grow. You will learn how to prepare a chloroplast suspension, make controlled measurements and use graphs to judge the evidence. A separate whole-leaf example will show why zero net gas exchange can occur w...
5.10, 5.11 - Productivity and trophic transfersPhotosynthesis supplies organic material to a food web, but only part of the energy fixed by plants becomes new consumer biomass. In 5.10, you will relate gross primary productivity, plant respiration and net primary productivity and calculate missing quantities. In 5.11, you will follow biomass...

Climate, evolution and stewardship

5.12 - Evidence for climate changeClimate evidence comes from instruments and from natural records that preserve clues about the past. In 5.12 you will interpret carbon dioxide and temperature records, pollen in peat bogs and tree rings, then distinguish a correlation from evidence about a cause.
5.13 - Greenhouse gases and human climate changeHuman activities can change climate by altering how energy leaves Earth. This lesson explains how carbon dioxide and methane strengthen the natural greenhouse effect, where the additional gases come from, and why several kinds of evidence support a human cause of modern global warming.
5.14 - Climate models and predictionsClimate evidence describes changes that have already happened; models help us investigate what could happen next. You will use a simple straight line to make and check a prediction, then distinguish that exercise from physical climate models and explain why useful projections still have limits.
5.15, Core practical 13 - Temperature, development and life cyclesClimate change can alter where organisms survive and when they grow, reproduce or migrate. In 5.15, you will connect changing rainfall and seasonal cycles to these responses; in Core practical 13, you will investigate temperature using seedling growth and decide how far a controlled experiment ca...
5.16, Core practical 12 - Temperature, enzyme rate and Q10Temperature changes enzyme activity and therefore the processes that keep plants, animals and microorganisms alive. In 5.16, connect molecular changes to these biological effects; in Core practical 12, measure initial enzyme rates at controlled temperatures and calculate how strongly rate respond...
A.1.9 - Student's t-test in biological investigationsTwo groups of seedlings can have different mean growth even when their growing conditions have no real effect. Student's t-test helps judge a difference between suitable independent sample means in relation to variation and sample size. You will work from individual measurements to a significance...
5.17, 5.19 - Mutation, selection and speciationAn environmental change can favour an inherited variant without producing that variant. In 5.17, you will trace how gene mutation supplies new alleles and natural selection changes their frequencies. In 5.19, you will explain how reduced gene flow can allow populations to diverge through allopatr...
5.18, 5.20 - Evaluating evidence and scientific conclusionsNew evidence gains strength when other scientists can inspect and challenge it. In 5.18 you will examine how genomics, proteomics and the scientific community test evidence for evolution; in 5.20 you will distinguish the evidence behind climate conclusions from the interests and priorities that c...
5.21, 5.22 - The carbon cycle and sustainable managementCarbon atoms move between air, organisms, soil, oceans and rocks, but these transfers occur at very different rates. In 5.21, use those pathways to explain how atmospheric carbon dioxide can be reduced; in 5.22, assess how reforestation and sustainable resources, including biofuels, can meet huma...

6 Immunity, Infection and Forensics

Forensics and molecular methods

6.1 - Estimating time of deathAfter a mammal dies, its temperature, muscles and communities of decomposers change. These changes provide different kinds of evidence about elapsed time. Learn how to interpret them together, use temperature and insect-development graphs, and explain why a defensible estimate usually gives a ran...
6.2 - Decomposition and carbon recyclingA fallen leaf still contains carbon in its organic molecules. In 6.2, follow how microorganisms obtain food from dead material, and how their growth and respiration return that carbon to circulation. This explains the microbial pathway within the carbon cycle you have already studied.
6.3, Core practical 14 - DNA profiling and gel electrophoresisDNA can help identify the source of a biological sample and investigate how organisms are related. In 6.3, you will learn how to interpret profiles from plants and animals; in Core practical 14, you will learn how to separate DNA fragments and judge the quality of the resulting evidence. The prac...
6.4 - Amplifying DNA with PCRA small DNA sample may contain too little of a particular sequence to analyse. The polymerase chain reaction makes many copies of a selected region by repeating DNA synthesis in a tube. You will learn how primers select that region, why the temperature changes, and how to calculate an ideal yield...
6.10 - Alternative splicing and protein diversityA gene does not always lead to just one protein product. You will follow how an initial RNA transcript is processed, how different splicing patterns can change its message, and how to deduce the resulting polypeptides without assuming that the DNA has changed.

Pathogens and immune responses

6.5 - Comparing bacteria and virusesA bacterium is a cell; a virus is a particle that depends on a host cell to produce more viruses. Comparing their structures explains this difference and helps us interpret unfamiliar descriptions without assuming that all bacteria or all viruses look alike.
6.6 - Tuberculosis and HIV infectionTuberculosis and HIV infection can both begin without obvious symptoms, yet their effects on human cells are very different. Following each pathogen from entry into a cell to changes in tissues and immune function explains how serious illness can develop, and why progression is not inevitable.
6.7, 6.11 - Barriers and non-specific defencesA pathogen reaching the body does not automatically establish an infection: it must pass barriers and survive the defences it encounters. In 6.11, we trace major entry routes and explain protection by skin, stomach acid, and gut and skin flora. In 6.7, we follow the rapid non-specific responses t...
6.8, 6.9 - Antibodies, B cells and T cellsAfter a pathogen passes the body's barriers, how can an immune response select the cells that recognise it? In 6.8, you will connect antigens, antibodies, plasma cells and antigen presentation. In 6.9, you will follow the different jobs of B cells and T cells, including the cells retained as memo...
6.12 - Active and passive immunityAn individual can gain protection by making an immune response or by receiving antibodies made elsewhere. You will distinguish these mechanisms from the way they are acquired, then use antibody-response data to explain the importance of immune memory.

Antimicrobials and infection control

6.13 - The evolutionary race between hosts and pathogensHosts have defences, yet pathogens can sometimes avoid or survive them. You will connect these evasion mechanisms to natural selection, explain why selection can act in both populations, and distinguish evidence consistent with an evolutionary race from proof that every stage of one has occurred.
6.14, Core practical 15 - Antibiotics and bacterial inhibitionAn antibiotic can prevent a bacterial population from increasing without killing every cell. In 6.14 you will distinguish bacteriostatic from bactericidal action; in Core practical 15 you will use a controlled school disc comparison to investigate inhibition, measure the results and decide what t...
6.15 - Hospital infections and antibiotic stewardshipHospitals bring together people who are vulnerable to infection, frequent care contacts and intensive antibiotic use. In 6.15, we connect these contributory causes to antibiotic prescribing and hospital practices that prevent infection. Understanding the reason for each practice helps explain why...

7 Run for your Life

Movement and respiration

7.1 - Muscles, bones and antagonistic movementMoving a limb depends on muscles pulling through attachments onto a stable, jointed skeleton. Learn how bones, tendons and ligaments work together, why flexors and extensors act antagonistically, and why a muscle can contract without shortening.
7.10 - Muscle fibres and performanceA skeletal muscle contains specialised cells that generate force, but these cells do not all contract at the same speed or resist fatigue equally well. Learn how a muscle fibre is organised, then connect its contractile proteins, membranes and ATP supply to the demands of sustained activity and r...
7.2 - The sliding filament mechanismA skeletal muscle can pull because many tiny contractile units generate force together. This lesson explains how actin and myosin slide past one another, how calcium controls their interaction, and why ATP is needed for both repeated contraction and relaxation.
7.3 - Aerobic respiration and energy transferA contracting muscle needs a continuing supply of ATP, but its respiratory fuel is not used in one explosive reaction. You will follow the overall material and energy changes in aerobic respiration, then see why they depend on many enzyme-controlled reactions in different parts of the cell.
7.4, 7.7 - Glycolysis, lactate and recoveryGlycolysis can supply ATP both when aerobic respiration keeps pace with demand and when it cannot. In 7.4, follow the investment and production of ATP and the recycling of NAD during lactate formation; in 7.7, discover how lactate can become fuel or be used to rebuild carbohydrate during recovery.
7.5 - The link reaction and Krebs cycleGlycolysis leaves much of glucose's chemical energy in pyruvate. The link reaction and Krebs cycle continue its oxidation, releasing carbon dioxide and transferring energy to ATP and reduced carriers. Following the carbon and the carriers shows why these mitochondrial reactions are essential to a...
7.6 - Oxidative phosphorylationThe link reaction and Krebs cycle transfer much of the energy from respiratory substrates to reduced NAD and reduced FAD. Oxidative phosphorylation uses their electrons to build a proton gradient across the inner mitochondrial membrane, then couples proton return to ATP synthesis. Following elect...
Core practical 16 - Measuring respiration ratesRespiring cells take up oxygen, but how can a tiny gas movement reveal a rate? Learn how to make a controlled respirometer comparison, convert a moving marker into oxygen uptake, and judge how confidently the results answer a biological question.

Exercise regulation and evidence

7.8 - Cardiac electrical control and ECGsThe heart needs to contract in the right sequence as well as at the right rate. In 7.8(i–iii), you will learn how cardiac muscle starts its own electrical activity, how specialised tissues coordinate a heartbeat, and how an ECG provides evidence about that activity. Measuring a trace lets you con...
7.9 - Cardiac output and ventilation controlWorking muscles need oxygen delivered and carbon dioxide removed faster than resting muscles. In 7.9(i), you will calculate the volume of blood pumped each minute; in 7.9(ii), you will explain how the medulla coordinates changes in heart rate and breathing to match changing demand.
Core practical 17 - Exercise and spirometer tracesExercise changes both the depth and frequency of breathing, but moving air is different from consuming oxygen. In Core practical 17, use calibrated spirometer traces to measure tidal volume, breathing rate, respiratory minute ventilation and oxygen consumption, then judge how safely collected dat...
7.11, 7.12 - Feedback, homeostasis and thermoregulationDuring exercise, working muscles release more heat, yet the conditions around body cells must stay suitable for their reactions. In 7.11, you will distinguish negative from positive feedback and see how negative feedback keeps a variable within narrow limits. In 7.12, you will apply this to homeo...
7.13 - Exercise dose and health evidenceExercise changes the demands on the body, but a health claim needs more than a graph with a trend. Learn how too little activity and excessive exercise can carry different risks, and how to judge what exercise-health evidence actually supports.
7.14 - Technology, injury and sportMedical technology can improve movement after an injury or replace some functions of a missing limb. Learn how keyhole surgery and prostheses can support sports participation, and why the outcome also depends on rehabilitation, the person and the activity.
7.15 - Ethics of performance-enhancing substancesAn athlete may value winning, health, freedom of choice and fair competition, yet these values can conflict. This lesson uses the biology of performance enhancement to explain different ethical positions and to build a reasoned view about when substance use could be acceptable.
7.16 - Hormones and transcription factorsA cell can change which proteins it makes without changing the genes it carries. Learn how transcription factors regulate this choice, and how a hormone signal can change transcription and produce a cell response.

8 Grey Matter

Neural signalling and responses

8.1, 8.2 - Neurones, reflexes and pupil responsesHow does a change outside the body make a muscle respond? In 8.1 you will connect the structures of sensory, relay and motor neurones to their jobs, including the role of Schwann cells and myelin. In 8.2(i–ii) you will follow nervous pathways from stimulus to effector, then explain how opposing i...
8.3 - Action potentials and saltatory conductionA nerve impulse can travel along an axon without fading away. This lesson explains how changes in membrane permeability create that impulse, how one region excites the next, and why myelin makes conduction faster.
8.4 - Synapses and acetylcholineAn action potential travels along a neurone, but neighbouring neurones are separate cells. At a chemical synapse, a released molecule carries information across the gap and changes the next cell's membrane potential. This lesson follows acetylcholine from its release to its removal, explaining wh...
8.5 - Rods, light and retinal signallingLight reaching the retina must be converted into an electrical signal before the brain can use it. Learn how a molecular change in a rod cell alters ion movement and transmitter release, and why a rod becoming more negative can lead to more impulses in an optic neurone.
8.6 - Phytochrome, IAA and plant responsesPlants use environmental information to change their growth and development. Learn how phytochrome detects light conditions, how IAA directs unequal growth, and how both signals can change transcription without changing the plant's DNA sequence.
8.7 - Nervous and hormonal coordinationAn animal may need to move one limb quickly while changing the activity of several organs to support that movement. Learn how nervous and hormonal signals reach the right cells, why their effects differ in timing, and how both systems contribute to one coordinated response.

Brain development and learning

8.8, 8.9 - Brain regions and medical imagingA brain scan can show an abnormal structure, a change in metabolism or a response during a task, but these are different kinds of evidence. In 8.8 you will locate four major brain regions and connect them to their functions. In 8.9 you will use those connections to understand what MRI, fMRI, PET...
8.10, 8.11 - Critical periods and visual developmentSeeing depends on developing brain circuits as well as functioning eyes. In 8.10, you will learn how early visual activity refines connections; in 8.11, you will use Hubel and Wiesel's kitten and monkey evidence to judge what animal models reveal about human development.
8.12 - Ethical standpoints in animal researchMedical research can offer future treatments while causing harm to the animals used to investigate them. This lesson develops two ethical standpoints for discussing that conflict, then uses evidence, alternatives and animal welfare measures to examine a proposed study. A reasoned position depends...
8.13, Core practical 18 - Habituation and repeated-stimulus investigationsA protective response is useful when danger is real, but responding to every harmless touch wastes time and energy. In 8.13 you will learn how animals, including humans, reduce their response through habituation. Core practical 18 uses repeated gentle stimulation to investigate this learning, whi...
8.19 - Nature, nurture and brain evidenceBrain development depends on inherited information and on the conditions in which a brain develops. You will examine how newborn abilities, animal experiments, brain damage, twins and cultural comparisons help scientists investigate these contributions. The challenge is to decide what each observ...

Neurochemistry, genomes and applications

8.14, 8.15 - Neurochemistry and drugs at synapsesChanging a chemical signal at a synapse can change the activity of a whole brain circuit. In 8.14, dopamine in Parkinson’s disease and the investigated relationship between serotonin and depression show how neurochemistry guides drug research. In 8.15, L-Dopa and MDMA show two different ways that...
8.16 - Genome sequencing and personalised medicineTwo people with the same diagnosis may respond differently to the same medicine. Genome sequencing helps explain some of these differences and can guide treatment and prevention, but interpreting and sharing a person's sequence also creates difficult choices.
8.17, 8.18 - GM organisms, medicines and decisionsA gene can provide instructions for a protein medicine, but making a useful treatment requires much more than transferring DNA. In 8.17, you will follow medicine production in microorganisms, plants and animals. In 8.18, you will compare the benefits and risks of GM organisms using the biological...
Paper 3 - Reading scientific articles and synoptic evidenceA scientific article can bring familiar biology into an unfamiliar setting. This Paper 3 skills lesson uses a complete original practice article to connect molecular mechanisms, experimental design and numerical evidence. You will learn how to decide what a study supports and how to develop a con...