Edexcel A-Level Biology Revision
Explore Edexcel A-Level Biology revision topics. Check the official specification for complete requirements for your exam year and route.
Qualification code: 9BI0
Course details
Assessment The assessment includes Paper 1, Paper 2 and Paper 3; see the official specification for the full structure.
Tick a point after revising it. Progress is saved on this device and does not measure mastery.
Reviewed course guidance
Revision points, with the explanation beside them
Open a point to read what to know, how to use it in an assessment, a plain-language explanation and a self-check. Answers stay hidden until you choose to reveal them.
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Topic group
Molecular structure and bonds
- Core
Scope: Core · 9BI0 Topic 1 Biological molecules
Open revision guidance
What to know
- Monosaccharides join by condensation to form glycosidic bonds; hydrolysis adds water to break them. Different monosaccharides form storage or structural polysaccharides.
- Amino acids join through peptide bonds; primary structure determines interactions that create secondary, tertiary and sometimes quaternary protein structure.
What to do in an assessment
- Relate a change in sequence to altered folding and therefore function
- distinguish a bond-forming reaction from hydrolysis.
In plain language
Relate a change in sequence to altered folding and therefore function; distinguish a bond-forming reaction from hydrolysis.
Self-check
What reaction joins two amino acids?
Show the answer
A condensation reaction removes water and forms a peptide bond between the amino acids.
Topic group
Carbohydrates and lipid tests
- Core
Scope: Core · 9BI0 Topic 1 Biological molecules
Open revision guidance
What to know
- Starch is a coiled glucose polymer suited to storage; cellulose chains form hydrogen-bonded microfibrils that strengthen plant walls.
- Triglycerides contain glycerol and three fatty acids; phospholipids have hydrophilic heads and hydrophobic tails that form bilayers.
What to do in an assessment
- Use a named test and its positive result when interpreting a food sample
- iodine changes from orange-brown to blue-black with starch.
In plain language
Use a named test and its positive result when interpreting a food sample; iodine changes from orange-brown to blue-black with starch.
Self-check
Why does cellulose strengthen a plant cell wall?
Show the answer
Straight chains form bundles of microfibrils linked by many hydrogen bonds, providing high tensile strength.
Topic group
Protein function and enzymes
- Core
Scope: Core · 9BI0 Topic 1 Biological molecules
Open revision guidance
What to know
- Globular proteins fold into specific shapes; haemoglobin binds oxygen reversibly, while antibodies have variable regions complementary to antigens.
- Enzyme rate depends on active-site availability, substrate concentration and conditions; denaturation changes the tertiary structure and substrate fit.
What to do in an assessment
- When comparing enzyme data, calculate initial rate from the steepest early interval and control pH, temperature, enzyme and substrate concentrations.
In plain language
When comparing enzyme data, calculate initial rate from the steepest early interval and control pH, temperature, enzyme and substrate concentrations.
Self-check
How does a competitive inhibitor reduce enzyme activity?
Show the answer
It occupies the active site, reducing substrate binding; adding more substrate can lessen its effect by competition.
Topic group
DNA replication and expression
- Core
Scope: Core · 9BI0 Topic 7 Modern genetics
Open revision guidance
What to know
- DNA strands are antiparallel; complementary base pairing is A–T and C–G. Replication is semi-conservative because each new molecule contains one original strand.
- Transcription makes complementary mRNA; translation uses codons and tRNA anticodons to assemble an amino-acid sequence at a ribosome.
What to do in an assessment
- Track strand direction and complementary bases explicitly
- explain how a base substitution may be silent, alter one amino acid, or create a stop codon.
In plain language
Track strand direction and complementary bases explicitly; explain how a base substitution may be silent, alter one amino acid, or create a stop codon.
Self-check
Why is each new DNA molecule described as semi-conservative?
Show the answer
Each contains one parental strand retained from the original molecule and one newly synthesised complementary strand.
Topic group
Cell ultrastructure and microscopy
- Core
Scope: Core · 9BI0 Topic 2 Cells, viruses and reproduction
Open revision guidance
What to know
- Eukaryotic cells contain membrane-bound compartments; prokaryotes lack a nucleus and have 70S ribosomes, circular DNA and often plasmids.
- Resolution is the ability to distinguish nearby points; electron microscopy reveals ultrastructure but requires non-living prepared specimens.
What to do in an assessment
- Calculate actual size as image size ÷ magnification and check the units
- compare organelle abundance to the cell’s metabolic role.
In plain language
Calculate actual size as image size ÷ magnification and check the units; compare organelle abundance to the cell’s metabolic role.
Self-check
A 12 mm image at ×30 000 represents what actual length?
Show the answer
12 mm ÷ 30 000 = 0.0004 mm = 0.4 µm.
Topic group
Membrane transport and cell signalling
- Core
Scope: Core · 9BI0 Topic 4.2 Cell transport mechanisms
Open revision guidance
What to know
- A fluid mosaic membrane contains phospholipids, proteins and carbohydrate chains; channel and carrier proteins provide selective transport routes.
- Facilitated diffusion follows gradients through proteins; active transport uses ATP to move against gradients, and co-transport couples one gradient to another solute.
What to do in an assessment
- Explain a transport result using concentration or water potential, membrane permeability, and energy use
- do not call every membrane crossing diffusion.
In plain language
Explain a transport result using concentration or water potential, membrane permeability, and energy use; do not call every membrane crossing diffusion.
Self-check
How can glucose enter an intestinal epithelial cell against its gradient?
Show the answer
Sodium–glucose co-transport uses the sodium gradient maintained by ATP-driven pumps to bring glucose into the cell.
Topic group
Cell cycle, mitosis and meiosis
- Core
Scope: Core · 9BI0 Topic 2 Cells, viruses and reproduction
Open revision guidance
What to know
- During interphase DNA replicates; mitosis separates sister chromatids to maintain chromosome number and produce genetically identical daughter nuclei.
- Meiosis uses two divisions to produce haploid cells; crossing-over and independent assortment generate variation.
What to do in an assessment
- Distinguish homologous chromosomes from sister chromatids in diagrams and state the chromosome number before and after each division.
In plain language
Distinguish homologous chromosomes from sister chromatids in diagrams and state the chromosome number before and after each division.
Self-check
Why does meiosis halve chromosome number?
Show the answer
Homologous chromosomes separate in meiosis I and sister chromatids in meiosis II, producing cells with one chromosome of each pair.
Topic group
Exchange surfaces and mass transport
- Core
Scope: Core · 9BI0 Topic 4 Exchange and transport
Open revision guidance
What to know
- A high surface-area-to-volume ratio shortens diffusion distances relative to demand; multicellular organisms use specialised exchange surfaces and mass transport.
- Ventilation, perfusion and thin epithelium maintain steep gradients in lungs; counter-current flow in fish gills sustains oxygen uptake.
What to do in an assessment
- Use Fick’s law qualitatively: larger area and steeper gradient increase rate, while a thicker barrier decreases it.
In plain language
Use Fick’s law qualitatively: larger area and steeper gradient increase rate, while a thicker barrier decreases it.
Self-check
Why does blood flow help maintain oxygen diffusion into an alveolus?
Show the answer
It carries absorbed oxygen away and brings deoxygenated blood, preserving a steep concentration gradient.
Topic group
Digestion and absorption
- Core
Scope: Core · 9BI0 Topic 1/2 biological molecules and digestion
Open revision guidance
What to know
- Hydrolysis converts polymers to absorbable monomers; brush-border enzymes complete digestion close to epithelial transport proteins.
- Villus epithelium has a short diffusion path and large area; capillaries carry monosaccharides and amino acids while lacteals take up lipid products.
What to do in an assessment
- Explain a named adaptation in terms of the movement of a specific nutrient and its destination.
In plain language
Explain a named adaptation in terms of the movement of a specific nutrient and its destination.
Self-check
Where do absorbed fatty-acid products initially enter the villus?
Show the answer
They enter the lacteal and travel through the lymphatic system before reaching the blood.
Topic group
Circulation and respiratory exchange
- Core
Scope: Core · 9BI0 Topic 4.3 Gas exchange; Topic 4.4 Circulation
Open revision guidance
What to know
- Haemoglobin binds oxygen reversibly; at high oxygen partial pressure it loads oxygen, and in respiring tissues lower partial pressure promotes unloading.
- The Bohr effect describes increased haemoglobin oxygen release as carbon dioxide concentration rises and pH falls in active tissues.
What to do in an assessment
- Read a dissociation curve by locating partial pressure and saturation
- compare curves only after identifying which condition shifts them.
In plain language
Read a dissociation curve by locating partial pressure and saturation; compare curves only after identifying which condition shifts them.
Self-check
Why does haemoglobin unload more oxygen in an active tissue?
Show the answer
High CO₂ lowers pH and shifts haemoglobin towards lower oxygen affinity, so oxygen is released where respiration is high.
Topic group
Plant transport and water relations
- Core
Scope: Core · 9BI0 Topic 4 Exchange and transport
Open revision guidance
What to know
- Water potential gradients drive osmosis; root hair uptake and the cohesion–tension mechanism move water through xylem as transpiration removes water from leaves.
- Sucrose loading creates pressure differences that drive phloem mass flow from sources to sinks; sinks may be growing roots, fruits or storage tissue.
What to do in an assessment
- Explain a potometer as an estimate of water uptake, not a direct measurement of transpiration
- control leaf area, light, wind and temperature.
In plain language
Explain a potometer as an estimate of water uptake, not a direct measurement of transpiration; control leaf area, light, wind and temperature.
Self-check
Why is water uptake not identical to transpiration rate?
Show the answer
Some absorbed water is used in photosynthesis or growth, so uptake is only a proxy for water lost from leaves.
Topic group
Photosynthesis and respiration pathways
- Core
Scope: Core · 9BI0 Topic 5 Energy for biological processes
Open revision guidance
What to know
- Light-dependent reactions in thylakoid membranes produce ATP and reduced NADP; the Calvin cycle uses these to fix carbon dioxide into triose phosphate.
- Glycolysis occurs in cytoplasm; aerobic respiration then uses link reaction, Krebs cycle and oxidative phosphorylation, with oxygen as final electron acceptor.
What to do in an assessment
- Describe where each stage occurs and follow carbon or hydrogen carriers
- state that ATP is made by substrate-level and oxidative phosphorylation.
In plain language
Describe where each stage occurs and follow carbon or hydrogen carriers; state that ATP is made by substrate-level and oxidative phosphorylation.
Self-check
What is the role of reduced NADP in the Calvin cycle?
Show the answer
It donates hydrogen/reducing power used to reduce glycerate-3-phosphate to triose phosphate.
Topic group
Energy transfer and nutrient cycling
- Core
Scope: Core · 9BI0 Topic 10 Ecosystems
Open revision guidance
What to know
- Energy flows through ecosystems and is dissipated as heat; matter cycles. Decomposers mineralise organic nutrients for producer uptake.
- Net primary productivity is the energy stored in plant biomass after respiratory losses; secondary productivity is new consumer biomass.
What to do in an assessment
- Calculate productivity only with consistent units and explain a transfer efficiency as biomass gained divided by biomass available ×100.
In plain language
Calculate productivity only with consistent units and explain a transfer efficiency as biomass gained divided by biomass available ×100.
Self-check
Why is energy flow not recycled like carbon?
Show the answer
Energy is dissipated as heat at each transfer and cannot be reused by the ecosystem in the same way matter can.
Topic group
Nervous and endocrine coordination
- Core
Scope: Core · 9BI0 Topic 9 Control systems; Topic 9.7 Chemical control in plants
Open revision guidance
What to know
- Resting potentials arise from ion gradients and selective membrane permeability; depolarisation opens voltage-gated sodium channels, then potassium movement restores potential.
- Synaptic transmission uses calcium-triggered vesicle release; neurotransmitter binds postsynaptic receptors and is removed or broken down to stop signalling.
What to do in an assessment
- Interpret an action-potential graph by identifying threshold, depolarisation, repolarisation and refractory period
- relate myelin to saltatory conduction.
In plain language
Interpret an action-potential graph by identifying threshold, depolarisation, repolarisation and refractory period; relate myelin to saltatory conduction.
Self-check
Why is there a brief refractory period after an action potential?
Show the answer
Voltage-gated channels need to reset, preventing immediate refiring and helping impulses travel in one direction.
Topic group
Homeostasis and kidney function
- Core
Scope: Core · 9BI0 Topic 9.9 Osmoregulation and temperature regulation
Open revision guidance
What to know
- Negative feedback uses receptors, coordination centres and effectors to oppose a change; insulin and glucagon regulate blood glucose in opposing directions.
- Ultrafiltration forms glomerular filtrate; selective reabsorption returns glucose, ions and water. ADH alters collecting-duct permeability and urine concentration.
What to do in an assessment
- Compare diabetes treatments through the actual defect and risk: insulin deficiency differs from insulin resistance
- evaluate evidence and consequences.
In plain language
Compare diabetes treatments through the actual defect and risk: insulin deficiency differs from insulin resistance; evaluate evidence and consequences.
Self-check
Why does an increase in ADH make urine more concentrated?
Show the answer
More aquaporins increase collecting-duct water permeability, allowing more water to return to blood.
Topic group
Inheritance and linkage
- Core
Scope: Core · 9BI0 Topic 7 Modern genetics
Open revision guidance
What to know
- Alleles segregate into gametes; independent assortment applies to genes on different chromosomes or sufficiently far apart. Linkage means nearby genes tend to be inherited together.
- A chi-squared test compares observed and expected categories; degrees of freedom and a critical value determine whether deviation is statistically significant.
What to do in an assessment
- Show parental genotype, gametes and offspring classes
- for linkage, compare recombinant frequency with 50% and state the inference.
In plain language
Show parental genotype, gametes and offspring classes; for linkage, compare recombinant frequency with 50% and state the inference.
Self-check
Observed ratio differs from expectation but χ² is below the critical value. What conclusion follows?
Show the answer
The deviation is not statistically significant at that threshold; retain the null hypothesis that chance explains the difference.
Topic group
Evolution, populations and selection
- Core
Scope: Core · 9BI0 Topic 8 Gene pools
Open revision guidance
What to know
- Allele frequencies change through selection, mutation, migration and genetic drift; small isolated populations are especially vulnerable to drift.
- Speciation requires reproductive isolation and divergence; different selection pressures can cause separated populations to become unable to interbreed successfully.
What to do in an assessment
- Use population evidence and the correct time scale
- distinguish a change in individuals from a change in allele frequencies across generations.
In plain language
Use population evidence and the correct time scale; distinguish a change in individuals from a change in allele frequencies across generations.
Self-check
Why can a small founder population lose genetic diversity by chance?
Show the answer
A small sample may omit alleles present in the source population, so drift changes allele frequencies even without selection.
Topic group
Gene regulation and biotechnology
- Core
Scope: Core · 9BI0 Topic 7 Modern genetics
Open revision guidance
What to know
- Transcription factors and epigenetic modifications influence gene expression; methylation or histone changes can alter transcription without changing DNA sequence.
- Recombinant DNA uses restriction enzymes and ligase to insert a target gene into a vector; transformed cells are identified and grown to produce a product.
What to do in an assessment
- For a genetic technology question, identify the target, vector, enzyme, selection step and intended product
- then evaluate benefits, risks and controls.
In plain language
For a genetic technology question, identify the target, vector, enzyme, selection step and intended product; then evaluate benefits, risks and controls.
Self-check
Why does an inserted gene need a promoter that works in the host?
Show the answer
The host transcription machinery must recognise the promoter to transcribe the inserted coding sequence.
Topic group
Immunity and experimental evidence
- Core
Scope: Core · 9BI0 Topic 6 Microbiology and pathogens; Topic 6.7 Response to infection
Open revision guidance
What to know
- Antigen presentation activates specific lymphocytes; clonal selection expands cells with complementary receptors, and memory cells support a quicker secondary response.
- A controlled investigation tests a hypothesis with repeatable methods; uncertainty, sample size and confounders limit the strength of a biological conclusion.
What to do in an assessment
- Distinguish correlation from causation and evaluate whether a control group, randomisation, blinding and replication address the stated source of bias.
In plain language
Distinguish correlation from causation and evaluate whether a control group, randomisation, blinding and replication address the stated source of bias.
Self-check
Why is a rapid secondary antibody response evidence of immunological memory?
Show the answer
Memory lymphocytes formed after the first exposure rapidly proliferate and differentiate when the same antigen returns.
Topic group
Practical methods and mathematical reasoning
- Core
Scope: Core · 9BI0 Core practicals and Appendix 9 mathematics
Open revision guidance
What to know
- Independent variables are manipulated, dependent variables measured and controls held constant; calibrated equipment and repeats improve validity and reliability.
- Use standard deviation or confidence intervals when supported by data; percentage change is (final−initial)/initial ×100, with sign and units interpreted.
What to do in an assessment
- In practical answers specify a method another student could reproduce, state the safety step and explain how the data test the hypothesis.
In plain language
In practical answers specify a method another student could reproduce, state the safety step and explain how the data test the hypothesis.
Self-check
A count rises from 40 to 50. What is the percentage increase?
Show the answer
(50−40) ÷ 40 ×100 = 25%.
Topic group
Statistical evidence in biology
- Core
Scope: Core · 9BI0 Appendix 9 Mathematical skills
Open revision guidance
What to know
- A statistical test asks whether a pattern could plausibly arise by chance under a null hypothesis; the test choice depends on data type and question.
- A low p-value supports rejecting the null at the chosen threshold but does not measure the size or biological importance of an effect.
What to do in an assessment
- State the null hypothesis, test assumptions and degrees of freedom where relevant; distinguish significance from causation.
In plain language
State the null hypothesis, test assumptions and degrees of freedom where relevant; distinguish significance from causation.
Self-check
A result is statistically significant at p<0.05. What does that mean?
Show the answer
If the null hypothesis were true, the probability of data at least this extreme is below 5%; it does not prove the alternative explanation.
Topic group
Genetic crosses and linkage evidence
- Core
Scope: Core · 9BI0 Topic 7 Modern genetics
Open revision guidance
What to know
- A test cross reveals an unknown dominant genotype by crossing with a homozygous recessive; offspring ratios distinguish likely genotypes.
- Linked loci are inherited together more often than unlinked loci; recombinant frequency estimates the distance between loci.
What to do in an assessment
- Show allele symbols and gametes, calculate the recombinant fraction and state how sample size affects confidence.
In plain language
Show allele symbols and gametes, calculate the recombinant fraction and state how sample size affects confidence.
Self-check
What does a recombinant frequency close to 50% suggest?
Show the answer
The loci assort independently or are so far apart on the same chromosome that crossing-over occurs frequently.
Topic group
Population genetics and allele frequency
- Core
Scope: Core · 9BI0 Topic 8 Gene pools
Open revision guidance
What to know
- Allele frequency is the proportion of copies of an allele among all copies of that gene in a population; it can change through selection, migration, mutation or drift.
- The Hardy–Weinberg model provides an expectation under specified assumptions, not a description guaranteed for every real population.
What to do in an assessment
- Calculate frequency from allele counts and identify which assumption is violated before explaining a departure from expectation.
In plain language
Calculate frequency from allele counts and identify which assumption is violated before explaining a departure from expectation.
Self-check
Why is a small population more affected by genetic drift?
Show the answer
Random sampling of a few breeders can shift allele frequencies substantially when each individual represents a large fraction of the gene pool.
Topic group
Plant signalling and tropisms
- Core
Scope: Core · 9BI0 Topic 9 Control systems; Topic 9.7 Light detection
Open revision guidance
What to know
- Auxin distribution can cause unequal cell elongation; in shoots, more auxin on the shaded side promotes elongation and bends growth towards light.
- Tropisms are directional growth responses; distinguish them from rapid non-growth movements and link the signal to differential growth.
What to do in an assessment
- For a seedling experiment, control light direction and age, measure curvature and use a covered or unilateral-light comparison.
In plain language
For a seedling experiment, control light direction and age, measure curvature and use a covered or unilateral-light comparison.
Self-check
Why does a shoot bend towards unilateral light?
Show the answer
Auxin accumulates on the shaded side, stimulating greater elongation there so the shoot curves towards the light.
Topic group
Evolutionary trees and classification
- Core
Scope: Core · 9BI0 Topic 3 Classification and biodiversity; Topic 8 Gene pools
Open revision guidance
What to know
- Classification groups organisms using shared derived characteristics and evidence from morphology, proteins or DNA; molecular similarity can refine proposed relationships.
- A node on a phylogenetic tree represents a common ancestor; taxa sharing a more recent node are more closely related.
What to do in an assessment
- Read the branching order rather than the page position; compare sequence differences using a stated method and avoid treating a living species as an ancestor.
In plain language
Read the branching order rather than the page position; compare sequence differences using a stated method and avoid treating a living species as an ancestor.
Self-check
On a tree, what does a shared recent branching point indicate?
Show the answer
The taxa share a more recent common ancestor than taxa whose lineages split earlier.
Topic group
Practical validity and uncertainty
- Core
Scope: Core · 9BI0 Core practicals and Appendix 9 mathematics
Open revision guidance
What to know
- Validity asks whether a method tests the intended relationship; reliability concerns repeatability, and precision concerns the spread or resolution of measurements.
- Random error creates scatter; systematic error shifts measurements consistently and is not removed by repeating alone.
What to do in an assessment
- Identify the actual source of uncertainty, predict its effect and propose an improvement that addresses it rather than merely adding more repeats.
In plain language
Identify the actual source of uncertainty, predict its effect and propose an improvement that addresses it rather than merely adding more repeats.
Self-check
Why will repeating not correct a zero error in a balance?
Show the answer
A zero error is systematic and shifts every reading; the balance must be zeroed or readings corrected.
Topic group
Water potential and plant cells
- Core
Scope: Core · 9BI0 Topic 4 Exchange and transport; Topic 9.9 Osmoregulation and temperature regulation
Open revision guidance
What to know
- Water moves down a water-potential gradient through partially permeable membranes; cell walls resist expansion and generate turgor pressure.
- In a hypertonic solution, plant cells lose water and the plasma membrane pulls away from the wall; in a dilute solution, cells become turgid.
What to do in an assessment
- State which compartment has the higher water potential and connect net movement to cell appearance.
In plain language
State which compartment has the higher water potential and connect net movement to cell appearance.
Self-check
What happens to a plant cell in a solution with lower water potential than its cytoplasm?
Show the answer
Water leaves by osmosis; the vacuole shrinks and the protoplast may pull away from the cell wall.
Topic group
Data presentation and biological calculations
- Core
Scope: Core · 9BI0 Core practicals and Appendix 9 mathematics
Open revision guidance
What to know
- Rates are changes in a measured quantity per unit time; use the initial linear region where later substrate depletion or product accumulation changes the rate.
- Logarithmic scales and percentage change can clarify wide ranges, but axes, units and transformations must be stated.
What to do in an assessment
- Show substitution, unit conversion and rounding; explain whether plotted points are raw observations, means or derived rates.
In plain language
Show substitution, unit conversion and rounding; explain whether plotted points are raw observations, means or derived rates.
Self-check
Why use the initial gradient to estimate an enzyme reaction rate?
Show the answer
At the start, substrate is abundant and product has not accumulated, so the gradient best estimates initial reaction rate.
Earlier topic checklist
These earlier topic ticks are separate from the reviewed points above and do not count towards that progress.
Biology topic list
This is a topic overview, not a complete specification checklist. Check the official course for all assessed content, options and your exam year.
These headings relate to Biology B (9BI0), not Biology A (9BN0). Confirm your course before using them for another Biology specification.
Tick each item off as you revise. Ticks are saved on this device only.
These ticks keep your earlier topic checklist. They do not count towards the new reviewed requirements above. Its numbering may be internal; use the linked references above for the exact reviewed source requirements.
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