A Level Biology 9700 — all 19 topics, free.

Three routes. AS Level only = Papers 1, 2, 3. A Level staged over two years = Papers 1, 2, 3 in year 1, then Papers 4 and 5 in year 2. A Level in one series = all five papers.

1.1The microscope in cell studies

Magnification is how many times larger the image is than the object. Resolution is the ability to distinguish two points as separate, and it is limited by the wavelength of the radiation used. A light microscope resolves to about 200 nm; an electron microscope, using electrons of much shorter wavelength, resolves to about 0.5 nm (TEM).

1.2Cells as the basic units of living organisms

Know each organelle's structure and function: nucleus and nucleolus, rough and smooth ER, Golgi body, mitochondria, ribosomes (80S in eukaryotes, 70S in prokaryotes and in mitochondria and chloroplasts), lysosomes, centrioles, chloroplasts, cell surface membrane, cell wall, large permanent vacuole and tonoplast, microvilli, cilia and flagella.

Viruses are non-cellular: nucleic acid (DNA or RNA) inside a protein capsid, sometimes with a lipid envelope. They have no cytoplasm, no ribosomes and no metabolism, so they can only replicate inside a host cell.

2.1Testing for biological molecules

Semi-quantitative Benedict's: the final colour, or a colorimeter reading, estimates concentration when compared against a calibration curve made from known standards.

2.2Carbohydrates and lipids

Monosaccharides (glucose, fructose, ribose) join by condensation, forming a glycosidic bond and releasing water; hydrolysis reverses it. α-glucose and β-glucose differ only in the orientation of the OH on carbon 1 — and that tiny difference produces starch versus cellulose.

Lipids: a triglyceride is glycerol + three fatty acids joined by ester bonds. Saturated fatty acids have no C=C and pack closely (solid at room temperature); unsaturated have C=C kinks (liquid). Lipids are excellent energy stores — more energy per gram than carbohydrate — and are also thermal insulators, buoyancy aids and a metabolic water source. Phospholipids have a hydrophilic phosphate head and hydrophobic tails, which is why they form bilayers.

2.3Proteins

Amino acids join by peptide bonds formed in condensation reactions. Four levels of structure:

1. Primary — the sequence of amino acids, held by peptide bonds.
2. Secondary — α-helix and β-pleated sheet, held by hydrogen bonds between the backbone.
3. Tertiary — the overall 3D fold, held by hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bonds.
4. Quaternary — two or more polypeptides together, as in haemoglobin (four chains, each with a haem group).

Globular vs fibrous: haemoglobin is globular — compact, soluble, hydrophilic residues outward, functional. Collagen is fibrous — three chains wound into a triple helix, staggered and cross-linked, giving huge tensile strength and insolubility.

2.4Water

Water's properties all follow from its polarity and hydrogen bonding: it is a near-universal solvent for ions and polar molecules; it has a high specific heat capacity (thermal stability for organisms and habitats); a high latent heat of vaporisation (evaporative cooling); high cohesion and surface tension (the transpiration stream, and insects walking on water); and it is less dense as ice, so ponds freeze from the top and life survives beneath.

3.1Mode of action of enzymes

Enzymes are globular protein catalysts that lower the activation energy of a reaction. The induced fit model — the modern one — says the active site changes shape slightly as the substrate binds, straining the substrate's bonds. The lock-and-key model is the older, simpler version, and questions often ask you to explain why induced fit replaced it.

Enzymes may be intracellular (catalase, in cells) or extracellular (amylase, trypsin, secreted). Specificity comes from the complementary shape and charge of the active site.

3.2Factors that affect enzyme action

Temperature: rate rises with kinetic energy up to an optimum, then falls sharply as hydrogen and ionic bonds break, the tertiary structure and hence the active site change shape — denaturation, which is irreversible. pH: a sharp optimum; extremes alter the charges on R groups and denature the enzyme. Substrate concentration: rate rises then plateaus when all active sites are saturated. Enzyme concentration: proportional, provided substrate is in excess.

Inhibitors. A competitive inhibitor is a similar shape to the substrate and binds the active site; its effect is overcome by more substrate, so V_max is unchanged but K_m rises. A non-competitive inhibitor binds elsewhere, changing the active site's shape; more substrate does not help, so V_max falls.

4.1Fluid mosaic membranes

A phospholipid bilayer with proteins embedded — fluid because the phospholipids move laterally, a mosaic because proteins are scattered through it. Components: phospholipids, cholesterol (regulates fluidity and adds stability), glycolipids and glycoproteins (cell recognition, receptors, antigens), channel and carrier proteins, and enzymes.

4.2Movement into and out of cells

Water moves from higher (less negative) to lower (more negative) water potential. In plant cells: hypotonic surroundings → water enters → the cell becomes turgid; hypertonic → water leaves → the protoplast shrinks from the wall, which is plasmolysis. Animal cells lack a wall, so they burst (haemolysis) or crenate.

5.1Replication and division of nuclei and cells

The cell cycle: interphase (G₁ growth, S DNA replication, G₂ growth and preparation) → mitosis → cytokinesis. A chromosome consists of DNA wound around histone proteins; after S phase each chromosome has two identical sister chromatids joined at the centromere.

Telomeres protect chromosome ends and prevent loss of genes at each replication. Stem cells are unspecialised cells that divide repeatedly and can differentiate. Cancer arises when mutations in genes controlling the cell cycle cause uncontrolled division; a mutagen increases mutation rate and a carcinogen is any agent that causes cancer.

5.2Chromosome behaviour in mitosis

Prophase — chromosomes condense and become visible, nuclear envelope breaks down, spindle forms. Metaphase — chromosomes line up on the equator, attached by centromeres to spindle fibres. Anaphase — centromeres divide and sister chromatids are pulled to opposite poles. Telophase — chromosomes decondense, nuclear envelopes re-form; then cytokinesis.

The result is two genetically identical daughter nuclei, which is why mitosis is used for growth, repair, replacement and asexual reproduction.

6.1Structure of nucleic acids and replication of DNA

A nucleotide is a pentose sugar + phosphate + nitrogenous base. DNA is a double helix of two antiparallel strands, joined by hydrogen bonds between complementary bases: A–T (two H bonds) and C–G (three). The backbone is phosphodiester bonds. RNA is single-stranded, has ribose, and uracil in place of thymine.

Semi-conservative replication: DNA helicase unwinds and separates the strands by breaking hydrogen bonds; DNA polymerase adds free nucleotides to each template strand in the 5'→3' direction; each daughter molecule has one original and one new strand. The Meselson–Stahl experiment with ¹⁵N proved this.

6.2Protein synthesis

Transcription (in the nucleus): RNA polymerase makes mRNA complementary to the template strand of the gene. Translation (at a ribosome): tRNA molecules with anticodons complementary to each mRNA codon deliver amino acids, which are joined by peptide bonds.

The genetic code is a triplet code, non-overlapping, and degenerate (most amino acids have more than one codon). A gene mutation is a change in the base sequence: substitution may be silent (degeneracy), missense (a different amino acid) or nonsense (a premature stop); insertion or deletion causes a frameshift, which changes every codon downstream and is usually far more damaging.

7.1Structure of transport tissues

Xylem vessels are dead, hollow tubes with no end walls, lignified walls (support and waterproofing) and pits for lateral movement. Phloem sieve tube elements are living but have no nucleus and few organelles, with perforated sieve plates; each is supported by a companion cell packed with mitochondria, connected through plasmodesmata.

7.2Transport mechanisms

Water pathway: root hair cell → cortex by the apoplast (through cell walls) and symplast (through cytoplasm and plasmodesmata) routes → the Casparian strip in the endodermis blocks the apoplast route, forcing water through the cell membranes so uptake can be controlled → xylem → up the stem → leaf → stomata.

Cohesion–tension theory: transpiration from the leaves lowers water potential, creating tension; water molecules cohere by hydrogen bonding into a continuous column and adhere to the vessel walls, so the column is pulled up.

Transpiration rate increases with light intensity (stomata open), temperature (faster evaporation and diffusion) and air movement (steeper gradient), and decreases with high humidity. Xerophytes reduce water loss with thick cuticles, sunken stomata, rolled leaves, hairs and reduced leaf area; hydrophytes have air spaces (aerenchyma) and stomata on the upper surface.

Translocation is explained by the mass flow hypothesis: at the source, sucrose is actively loaded into the phloem, lowering water potential so water enters from the xylem and raises hydrostatic pressure; at the sink, sucrose is unloaded and water leaves; the pressure gradient drives the flow.

8.1The circulatory system

Mammals have a closed double circulation: blood passes through the heart twice per circuit, so systemic pressure can be kept high while pulmonary pressure stays low enough not to damage the lungs.

Tissue fluid forms at the arterial end of a capillary because high hydrostatic pressure exceeds the opposing (negative) water potential due to plasma proteins; at the venous end hydrostatic pressure has fallen, so most fluid returns. The excess drains into the lymphatic system.

8.2Transport of oxygen and carbon dioxide

The oxygen dissociation curve is S-shaped because of cooperative binding: binding the first oxygen changes haemoglobin's shape, making subsequent binding easier.

The Bohr shift: a higher carbon dioxide concentration lowers the pH, reducing haemoglobin's affinity for oxygen and shifting the curve right — so more oxygen is released exactly where respiring tissue needs it most.

Fetal haemoglobin has a higher affinity for oxygen (curve to the left) so that it can take up oxygen from maternal blood across the placenta.

Carbon dioxide transport: about 85% as hydrogencarbonate ions (CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻, catalysed by carbonic anhydrase in red blood cells, with the chloride shift maintaining charge balance), about 10% bound to haemoglobin as carbaminohaemoglobin, and about 5% dissolved in the plasma.

8.3The heart

The left ventricle wall is much thicker than the right, because it pumps blood around the whole body at high pressure while the right pumps only to the lungs at low pressure. Atrioventricular valves (with tendinous cords) and semilunar valves prevent backflow; they open and close because of pressure differences.

The cardiac cycle: atrial systole → ventricular systole (AV valves close — the first heart sound; semilunar valves open) → diastole (semilunar valves close — second sound; ventricles refill).

Myogenic control: the sinoatrial node initiates each beat; excitation spreads through the atria, is delayed at the atrioventricular node (allowing the atria to finish emptying), then passes down the bundle of His and Purkyne tissue to make the ventricles contract from the apex upwards.

9.1The gas exchange system

Trachea → bronchi → bronchioles → alveoli. Cartilage holds the airways open, ciliated epithelium sweeps mucus (made by goblet cells) upwards, and smooth muscle adjusts airway diameter.

Alveoli are adapted for rapid diffusion: enormous total surface area; walls one flattened cell thick, as is the capillary endothelium, giving a very short diffusion distance; a dense capillary network and continuous ventilation maintaining a steep concentration gradient; and a moist surface with surfactant preventing collapse.

10.1Infectious diseases

HIV infects and destroys T-helper lymphocytes, so the immune system cannot mount effective responses and the person dies of opportunistic infections rather than of HIV directly. Malaria control is difficult because Plasmodium has resistant strains, mosquitoes develop insecticide resistance, and no fully effective long-term vaccine has been simple to deploy.

10.2Antibiotics

Antibiotics such as penicillin work by inhibiting bacterial cell wall synthesis, so the cell takes in water by osmosis and bursts. They have no effect on viruses, which have no cell wall and no metabolism of their own.

Resistance arises by random mutation; antibiotic use then acts as a selection pressure, so resistant bacteria survive, reproduce and pass on the allele — often on plasmids, which can transfer even between species. Reduce it by prescribing only when necessary, completing the course, rotating antibiotics, and strict hospital hygiene.

11.1The immune system

Phagocytes (macrophages and neutrophils) engulf pathogens non-specifically and immediately. An antigen is a molecule that triggers an immune response; self antigens are recognised as the body's own, non-self antigens are not.

B-lymphocytes mature in the bone marrow and produce antibodies. T-lymphocytes mature in the thymus: T-helper cells release cytokines that activate B cells and macrophages; T-killer cells destroy infected cells. Memory cells of both types persist, giving a secondary response that is faster, larger and longer-lasting — the basis of immunity.

11.2Antibodies and vaccination

An antibody is a Y-shaped glycoprotein with two variable regions forming antigen-binding sites specific to one antigen, and a constant region. Antibodies work by agglutination, neutralisation and by marking pathogens for phagocytosis.

Active immunity involves your own lymphocytes and memory cells, so it is slow to develop but long-lasting. Passive immunity is immediate but temporary — no memory cells are made.

Herd immunity protects the unvaccinated because transmission chains are broken when enough of the population is immune. Vaccination programmes can fail through antigenic variation (influenza), poor cold-chain storage, difficulty reaching populations, and vaccine refusal.

Monoclonal antibodies — identical antibodies from a single clone of cells — are used in diagnosis (pregnancy tests, disease markers) and treatment (targeting cancer cells directly, or delivering drugs to them).

12.1Energy

Organisms need energy for active transport, synthesis of macromolecules, movement, cell division and (in mammals and birds) maintaining body temperature. ATP is the universal energy currency: it releases a usable, immediate quantity of energy when hydrolysed to ADP + P_i, and it is easily regenerated.

12.2Respiration

Chemiosmosis: electrons pass along the electron transport chain, releasing energy used to pump protons into the intermolecular space; the proton gradient drives protons back through ATP synthase, which phosphorylates ADP.

Mitochondrial structure fits function: cristae give a large surface area for the electron transport chain and ATP synthase; the matrix holds the enzymes of the link reaction and Krebs cycle; the small intermembrane space allows a steep proton gradient to build quickly.

Anaerobic respiration gives far less ATP because only glycolysis proceeds, and its purpose is simply to regenerate NAD so glycolysis can continue — lactate in mammals (reversible in the liver), ethanol and CO₂ in yeast (irreversible).

13.1Photosynthesis as an energy transfer process

Light-dependent stage (thylakoid membranes): light excites electrons in chlorophyll; photolysis of water (H₂O → 2H⁺ + ½O₂ + 2e⁻) replaces them and releases oxygen; electron transport drives chemiosmotic ATP synthesis and reduces NADP.

Light-independent stage / Calvin cycle (stroma): CO₂ combines with RuBP (5C), catalysed by rubisco, giving two molecules of GP (3C); GP is reduced to TP using ATP and reduced NADP; most TP regenerates RuBP, the rest becomes carbohydrate and other products.

Chloroplast structure fits function: stacked thylakoids give a large surface area for pigments; the stroma contains the Calvin cycle enzymes; chloroplast DNA and 70S ribosomes allow some proteins to be made in place.

13.2Investigation of limiting factors

Light intensity, carbon dioxide concentration and temperature can all limit the rate. On a rate graph, the factor being varied is limiting while the line rises, and something else is limiting once it plateaus.

Chromatography separates photosynthetic pigments; calculate R_f values and compare with known values to identify chlorophyll a, chlorophyll b and the carotenoids. An absorption spectrum shows which wavelengths a pigment absorbs; an action spectrum shows the rate of photosynthesis at each wavelength — their close match is the evidence that those pigments drive the process.

14.1Homeostasis in mammals

Negative feedback is the principle: a receptor detects a deviation from the set point, a coordination centre processes it, and effectors act to restore the set point.

The kidney. Ultrafiltration at the glomerulus is driven by high hydrostatic pressure across the basement membrane, which acts as the molecular filter. Selective reabsorption in the proximal convoluted tubule recovers all the glucose and amino acids by active transport and co-transport — hence the microvilli and abundant mitochondria. The loop of Henle acts as a hairpin counter-current multiplier, building a high solute concentration in the medulla; a longer loop allows more concentrated urine, which is why desert mammals have very long loops.

Osmoregulation: osmoreceptors in the hypothalamus detect a fall in blood water potential; the posterior pituitary releases ADH; ADH makes the collecting duct walls more permeable by inserting aquaporins, so more water is reabsorbed and urine is more concentrated.

Blood glucose: the islets of Langerhans — β cells release insulin when glucose is high (increasing uptake and glycogenesis), α cells release glucagon when it is low (glycogenolysis and gluconeogenesis). Type 1 diabetes results from destruction of β cells; type 2 from reduced insulin sensitivity of target cells.

14.2Homeostasis in plants

Stomatal control. Guard cells open the stoma when they become turgid: potassium ions are actively pumped in, lowering water potential, so water follows by osmosis; the unevenly thickened inner wall makes the cells curve apart. Abscisic acid (ABA) is released under water stress and causes stomatal closure, conserving water.

15.1Control and coordination in mammals

Nervous versus endocrine: nervous signals are electrical, fast, short-lived and precisely targeted; hormonal signals are chemical, slower, longer-lasting and widespread.

The resting potential (about −70 mV) is maintained by the sodium–potassium pump (3 Na⁺ out for 2 K⁺ in) and by the membrane's greater permeability to K⁺. An action potential follows: a stimulus depolarises the membrane to threshold → voltage-gated Na⁺ channels open, Na⁺ floods in, reaching about +40 mV → Na⁺ channels close and K⁺ channels open, repolarising and briefly hyperpolarising the membrane → the pump restores the resting potential.

The refractory period ensures action potentials are discrete and travel in one direction only, and it sets the maximum frequency. Action potentials are all-or-nothing: a stronger stimulus does not give a bigger action potential, it gives a higher frequency. Conduction is faster in myelinated axons because of saltatory conduction between the nodes of Ranvier, and faster in axons of larger diameter.

Synapses: depolarisation opens voltage-gated Ca²⁺ channels; calcium entry makes vesicles fuse and release acetylcholine by exocytosis; it diffuses across and binds receptors on the postsynaptic membrane, opening cation channels; acetylcholinesterase then breaks it down. Synapses ensure one-way transmission and allow summation and integration.

Muscle contraction — the sliding filament model: an action potential travels down T-tubules, calcium is released from the sarcoplasmic reticulum and binds troponin, moving tropomyosin off the binding sites; myosin heads bind actin, the power stroke pulls the actin filament, ATP binds and detaches the head, and hydrolysis re-cocks it. The sarcomere shortens; the filaments themselves do not.

15.2Control and coordination in plants

Rapid responses: the Venus fly trap closes when trigger hairs are bent twice in quick succession, generating an action potential that causes rapid changes in cell turgor.

Auxin (IAA) causes cell elongation by loosening cell walls; it accumulates on the shaded side of a shoot, so that side elongates more and the shoot bends towards the light — phototropism.

Gibberellin in germinating cereal seeds stimulates the aleurone layer to synthesise amylase, which hydrolyses stored starch to maltose for the growing embryo — a well-known example of a plant hormone switching on a gene.

16.1Passage of information from parents to offspring

Haploid (n) cells have one set of chromosomes, diploid (2n) two. Meiosis halves the chromosome number and generates variation in three ways: crossing over at chiasmata in prophase I, independent assortment of homologous pairs at metaphase I (and of chromatids at metaphase II), and random fertilisation.

16.2The roles of genes in determining the phenotype

Learn the vocabulary precisely: gene, locus, allele, dominant, recessive, codominant, linkage, test cross, F₁, F₂, phenotype, genotype, homozygous, heterozygous.

Handle monohybrid and dihybrid crosses, codominance, multiple alleles (ABO blood groups), sex linkage (haemophilia, colour blindness), autosomal linkage and epistasis. A test cross with the homozygous recessive reveals an unknown genotype.

16.3Gene control

Structural genes code for proteins; regulatory genes control whether they are transcribed. The lac operon in E. coli is the model: without lactose, the repressor protein binds the operator and blocks RNA polymerase; with lactose present, lactose binds the repressor, changing its shape so it cannot bind the operator, and the genes for lactose metabolism are transcribed.

In eukaryotes, transcription factors bind to promoter regions to switch genes on or off; gibberellin's effect on amylase synthesis in germinating seeds works this way.

17.1Variation

Continuous variation (height, mass) is polygenic and strongly influenced by the environment, giving a normal distribution. Discontinuous variation (blood group) is controlled by one or few genes with little environmental effect, giving distinct categories.

Genetic variation arises from mutation, crossing over, independent assortment and random fertilisation. Use standard deviation and standard error to describe the spread, and the t-test to compare the means of two samples.

17.2Natural and artificial selection

Natural selection occurs because populations over-reproduce, resources are limited, individuals vary, and those with advantageous alleles survive and reproduce more — so the frequency of those alleles rises. Three patterns: stabilising (favours the mean, e.g. human birth mass), directional (favours one extreme, e.g. antibiotic resistance), and disruptive (favours both extremes).

Artificial selection: selective breeding of dairy cattle for milk yield, and of wheat for yield and disease resistance. The mechanism is the same, but the selection pressure is applied by humans.

17.3Evolution

The equations assume a large population, random mating, no mutation, no migration and no selection. Speciation follows reproductive isolation: allopatric speciation from a geographical barrier, sympatric from behavioural, temporal or mechanical isolation within the same area.

Molecular evidence — comparing DNA base sequences, mitochondrial DNA and amino acid sequences — reveals evolutionary relationships and has revised many classifications built on morphology alone.

18.1Classification

The taxonomic hierarchy: domain, kingdom, phylum, class, order, family, genus, species. The three domains are Bacteria, Archaea and Eukarya. Within Eukarya: Protoctista, Fungi, Plantae and Animalia. Viruses are not placed in any of these, having no cellular structure.

A species is a group of organisms with similar morphology and physiology that can interbreed to produce fertile offspring. That definition fails for asexual organisms and for extinct species — a standard evaluation question.

18.2Biodiversity

Biodiversity spans ecosystem, species and genetic levels. Sample with random quadrats for a uniform area, systematic transects where there is an environmental gradient, and mark–release–recapture for mobile animals.

Test correlations with Pearson's linear correlation (for linear relationships between normally distributed variables) or Spearman's rank correlation (for ranked or non-normal data). Remember that correlation does not imply causation.

18.3Conservation

Reasons to maintain biodiversity: ecological (stability, ecosystem services), economic (crops, medicines, tourism), ethical and aesthetic. Threats: habitat destruction, over-exploitation, pollution, invasive species, climate change.

Methods: protected areas and national parks; sustainable harvesting; ex situ conservation in zoos, botanic gardens and seed banks; captive breeding and reintroduction; assisted reproduction; and international agreements such as CITES and the Convention on Biological Diversity. In situ conservation is generally preferred because it maintains the species in its natural habitat, preserving its ecological role and a larger gene pool.

19.1Principles of genetic technology

Recombinant DNA is DNA formed by joining DNA from two different sources. The standard toolkit:

PCR amplifies DNA: denaturation at about 95 °C separates the strands, annealing at about 55 °C attaches the primers, and extension at about 72 °C lets Taq polymerase — heat-stable, from a thermophilic bacterium — build new strands. Each cycle doubles the DNA.

Gel electrophoresis separates fragments by size: DNA is negatively charged, so it moves towards the anode, and shorter fragments travel further. Compare against a ladder of known sizes.

19.2Genetic technology applied to medicine

Recombinant human proteins — insulin, factor VIII, ADA — avoid the risks of animal or donor sources (immune reactions, disease transmission), can be made in unlimited quantity, and are acceptable to people with religious or ethical objections to animal products.

Genetic screening uses DNA probes and microarrays to detect alleles associated with disease. Gene therapy introduces a functioning allele into a patient's cells — somatic gene therapy affects only that individual, while germ-line therapy would be heritable and is not permitted. Social and ethical issues include cost, access, consent, insurance discrimination and the risk of the vector triggering harmful effects.

19.3Genetically modified organisms in agriculture

Examples: Bt crops carrying a bacterial gene for an insecticidal protein; herbicide-resistant soya; Golden Rice engineered to make β-carotene to address vitamin A deficiency; GM salmon with accelerated growth.

Benefits: higher yields, less pesticide use, improved nutrition, tolerance of drought or salinity. Concerns: transfer of transgenes to wild relatives, effects on non-target species, loss of crop genetic diversity, farmers' dependence on seed companies, and uncertainty about long-term effects.

GivenFormulae provided in Papers 4 and 5

Degrees of freedom are not provided and must be worked out: for χ², categories − 1; for a t-test, (n₁ + n₂) − 2. Tables of critical values are supplied.

ChooseWhich test, and how to word the conclusion

The conclusion formula, worth several marks and easily rehearsed:

1. State the null hypothesis: "there is no significant difference between…".
2. Compare the calculated value with the critical value at p = 0.05 for the correct degrees of freedom.
3. If calculated > critical → reject the null hypothesis; the difference is significant, and there is a 5% or less probability that it is due to chance.
4. If calculated < critical → accept the null hypothesis; any difference is due to chance.
5. Put the conclusion back into the biological context.

P3Paper 3: Advanced Practical Skills

Two or three questions totalling 40 marks: an investigation and a light-microscope activity. Centres provide microscopes for half the candidates at a time, so half start on each. Questions are set in different areas of AS Biology and may use unfamiliar contexts.

Decisions you are marked on: identifying the independent and dependent variables; choosing a suitable range for the independent variable; choosing the number of values — Cambridge states a minimum of five — and the intervals between them; deciding what to control and how; and deciding when to repeat.

Microscope work: prepare a temporary slide, focus on low power first, draw a plan diagram (tissue outlines only, no individual cells, drawn in clean continuous lines with no shading), draw individual cells at high power, label with straight uncrossed lines, add a scale or magnification, and calculate actual sizes using an eyepiece graticule calibrated with a stage micrometer.

P5Paper 5: Planning, Analysis and Evaluation

Two or more questions totalling 30 marks, written. It may draw on both AS and A Level material and on unfamiliar contexts.

Planning checklist:

1. State a hypothesis linking the independent and dependent variables, with a biological reason.
2. Identify the independent, dependent and controlled variables explicitly.
3. Describe how to vary the independent variable, with the range and at least five values.
4. Name the apparatus for each measurement and justify the choice on precision.
5. Say how each control variable is kept constant — a water bath for temperature, a buffer for pH, the same volume and concentration each time.
6. Include replicates and say you will calculate a mean.
7. State the risks and precautions that are specific to this investigation.
8. Say how the data will be analysed — the graph to plot and the statistical test to apply.

Analysis and evaluation: process the data (rates, percentage change, means), calculate standard deviation or standard error, plot with error bars, identify anomalies and say what you would do about them, distinguish random errors (reduced by repeats) from systematic errors (not reduced by repeats — they need recalibration or a change of method), and state the limitations of the conclusion.

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• Cambridge International — 9700 subject page: syllabus, specimen papers, past papers, mark schemes and examiner reports.
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How to revise this subject

1. Learn the structures with their functions attached. "Many mitochondria" is not an answer; "many mitochondria to supply ATP for active transport" is.
2. Practise the mathematics. Magnification, actual size, percentage change in mass, rates from gradients, water potential, chi-squared, standard error. Biology loses more marks to arithmetic than to biology.
3. Use the command word. "Describe" wants what you see; "explain" wants why; "compare" needs both similarities and differences with explicit linking words ("whereas", "both").
4. Give named examples. Cholera, TB, malaria, HIV/AIDS and measles are the named diseases; learn the pathogen, transmission, and control for each.
5. Draw genetic diagrams in full. Parental phenotypes, genotypes, gametes in circles, the Punnett square, offspring genotypes, phenotypic ratio. Marks are allocated line by line.
6. Do not memorise the statistical formulae. They are given. Memorise instead when to use each test and how to state the conclusion with its null hypothesis.