A-Level Biology Complete Revision Guide

A-Level Biology is a content-rich subject that demands thorough understanding, precise terminology, and the ability to apply concepts to unfamiliar contexts. Whether you’re studying Cambridge International AS & A Level Biology (9700) or Edexcel International Advanced Level Biology, this guide covers every major topic to help you revise systematically and effectively.

For topic-specific deep dives, see our A-Level Biology overview and Edexcel A-Level Biology Revision Guide: Genetics and Inheritance.

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1. Cell Biology

Cell Biology is foundational and examined at AS level across all specifications.

1.1 Cell Structure

• Eukaryotic Cells: Nucleus, mitochondria, endoplasmic reticulum (rough and smooth), Golgi apparatus, lysosomes, ribosomes, chloroplasts (plant cells), cell wall, vacuole
• Prokaryotic Cells: No membrane-bound organelles, circular DNA, plasmids, 70S ribosomes, cell wall (peptidoglycan), flagella, pili
• Ultrastructure: Electron microscope images — recognise organelles from TEMs and SEMs
• Magnification and Resolution: Magnification = image size / actual size, understanding limits of light vs electron microscopy

Key Diagrams to Memorise

• Animal and plant cell ultrastructure (labelled)
• Prokaryotic cell structure
• Fluid mosaic model of cell membrane

1.2 Cell Membranes

• Fluid Mosaic Model: Phospholipid bilayer, integral and peripheral proteins, glycoproteins, glycolipids, cholesterol
• Transport Mechanisms: Diffusion, facilitated diffusion, osmosis, active transport, endocytosis, exocytosis
• Water Potential: Ψ = Ψₛ + Ψₚ, predicting direction of water movement
• Factors Affecting Membrane Permeability: Temperature, solvents, pH — practical investigation with beetroot

1.3 Cell Division

• Mitosis: Prophase, metaphase, anaphase, telophase — significance for growth, repair, asexual reproduction
• Meiosis: Two divisions producing four genetically different haploid cells, crossing over, independent assortment
• Cell Cycle: Interphase (G1, S, G2), mitotic phase, cytokinesis
• Cancer: Uncontrolled cell division, role of tumour suppressor genes and oncogenes

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2. Biological Molecules (Biochemistry)

2.1 Carbohydrates

• Monosaccharides: Glucose (α and β forms), fructose, galactose
• Disaccharides: Maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose) — formed by condensation, broken by hydrolysis
• Polysaccharides: Starch (amylose + amylopectin), glycogen, cellulose — structure related to function
• Tests: Benedict’s test (reducing sugars), iodine test (starch)

2.2 Proteins

• Amino Acids: General structure, peptide bond formation (condensation reaction)
• Protein Structure: Primary (amino acid sequence), secondary (α-helix, β-pleated sheet), tertiary (3D folding — ionic, hydrogen, disulfide, hydrophobic interactions), quaternary (multiple polypeptide chains)
• Enzymes: Lock and key model, induced fit model, factors affecting enzyme activity (temperature, pH, substrate concentration, inhibitors)
• Enzyme Inhibition: Competitive (binds active site), non-competitive (binds allosteric site), reversible vs irreversible
• Biuret Test: Purple colour indicates protein

2.3 Lipids

• Triglycerides: Glycerol + 3 fatty acids, ester bonds, saturated vs unsaturated
• Phospholipids: Hydrophilic head, hydrophobic tails — basis of cell membranes
• Cholesterol: Steroid structure, role in membrane fluidity
• Emulsion Test: For detecting lipids

2.4 Nucleic Acids

• DNA Structure: Double helix, complementary base pairing (A-T, G-C), antiparallel strands, hydrogen bonds between bases
• RNA: mRNA, tRNA, rRNA — single-stranded, uracil replaces thymine
• DNA Replication: Semi-conservative replication, role of helicase, DNA polymerase, ligase
• ATP: Structure, role as energy currency, ATP hydrolysis and synthesis

2.5 Water

• Properties: High specific heat capacity, solvent properties, cohesion and surface tension, high latent heat of vaporisation
• Biological Importance: Transport medium, metabolic reactions, temperature regulation

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3. Genetics and Inheritance

For a detailed treatment of genetics, see our Edexcel A-Level Biology Revision Guide: Genetics and Inheritance.

3.1 Gene Expression

• Transcription: DNA → mRNA in the nucleus, role of RNA polymerase
• Translation: mRNA → protein at ribosomes, role of tRNA, codons and anticodons
• The Genetic Code: Triplet, degenerate, non-overlapping, universal
• Gene Mutations: Substitution, insertion, deletion — effects on protein structure and function

3.2 Inheritance Patterns

• Monohybrid Inheritance: Dominant/recessive alleles, genotype vs phenotype, Punnett squares
• Dihybrid Inheritance: Independent assortment, 9:3:3:1 ratio
• Codominance and Multiple Alleles: ABO blood groups
• Sex Linkage: X-linked conditions (e.g., haemophilia, colour blindness)
• Epistasis (A2): One gene masking the expression of another
• Chi-Squared Test: Testing for significant differences between observed and expected ratios

3.3 Genetic Technology (A2)

• PCR: Amplifying DNA — denaturation, annealing, extension
• Gel Electrophoresis: Separating DNA fragments by size
• Genetic Engineering: Restriction enzymes, ligases, vectors, transformation
• Gene Therapy: Somatic vs germ line, ethical considerations
• CRISPR: Basic understanding of gene editing technology
• Genomics and Bioinformatics: Human Genome Project, applications in medicine

Key Diagrams to Memorise

• DNA structure with labelled components
• Transcription and translation process
• Punnett squares for mono- and dihybrid crosses

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4. Ecology

4.1 Ecosystems

• Biotic and Abiotic Factors: How they influence population distribution
• Food Chains and Webs: Trophic levels, energy transfer, ecological pyramids
• Energy Flow: Only ~10% energy transferred between trophic levels, GPP, NPP (NPP = GPP - R)
• Nutrient Cycles: Carbon cycle, nitrogen cycle — role of decomposers, nitrifying/denitrifying bacteria

4.2 Populations

• Population Growth: Exponential growth, carrying capacity, S-shaped curves
• Factors Affecting Population Size: Predation, competition, disease, abiotic factors
• Sampling Techniques: Quadrats, transects, mark-release-recapture (Lincoln index)
• Biodiversity: Species richness and evenness, Simpson’s diversity index (D = 1 - Σ(n/N)²)
• Conservation: In situ vs ex situ conservation, importance of maintaining biodiversity

4.3 Succession

• Primary Succession: Colonisation of bare substrate, pioneer species to climax community
• Secondary Succession: Recovery after disturbance
• Changes During Succession: Increasing biodiversity, biomass, soil depth, complexity of food webs

Key Diagrams to Memorise

• Carbon and nitrogen cycles
• Energy flow through an ecosystem
• Population growth curves

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5. Physiology

5.1 Gas Exchange

• Human Gas Exchange System: Alveolar structure, large surface area, thin walls, rich blood supply, ventilation mechanism
• Plant Gas Exchange: Stomata, spongy mesophyll, guard cell mechanism
• Fick’s Law: Rate of diffusion ∝ (surface area × concentration difference) / thickness
• Smoking and Disease: Chronic bronchitis, emphysema, lung cancer — mechanisms of damage

5.2 Transport in Animals

• Heart Structure and Function: Four chambers, valves, cardiac cycle, pressure changes
• Blood Vessels: Arteries, veins, capillaries — structure related to function
• Haemoglobin: Oxygen dissociation curve, cooperative binding, Bohr effect
• Blood Composition: Red blood cells, white blood cells, platelets, plasma

5.3 Transport in Plants

• Xylem: Transpiration stream, cohesion-tension theory, factors affecting transpiration rate
• Phloem: Translocation, mass flow hypothesis, source to sink
• Root Structure: Root hair cells, apoplast and symplast pathways, Casparian strip

5.4 Homeostasis (A2)

• Negative Feedback: Maintaining a stable internal environment, examples in temperature and blood glucose regulation
• Blood Glucose Regulation: Insulin and glucagon, role of pancreas (β and α cells), type 1 vs type 2 diabetes
• Thermoregulation: Hypothalamus as thermoregulatory centre, vasodilation/vasoconstriction, sweating, shivering
• Kidney Function: Ultrafiltration, selective reabsorption, osmoregulation, role of ADH
• Nervous System: Sensory, relay, and motor neurones, reflex arcs, synaptic transmission
• Hormonal vs Nervous Communication: Speed, duration, specificity

5.5 Photosynthesis (A2)

• Light-Dependent Reactions: Photolysis, electron transport chain, photophosphorylation, NADPH production
• Light-Independent Reactions (Calvin Cycle): Carbon fixation by RuBisCO, reduction of GP to TP, regeneration of RuBP
• Limiting Factors: Light intensity, CO₂ concentration, temperature
• Chromatography: Separating photosynthetic pigments

5.6 Respiration (A2)

• Glycolysis: Glucose → 2 pyruvate, 2 ATP, 2 NADH (in cytoplasm)
• Link Reaction: Pyruvate → acetyl CoA + CO₂ (in mitochondrial matrix)
• Krebs Cycle: Acetyl CoA → CO₂ + reduced coenzymes (in matrix)
• Oxidative Phosphorylation: Electron transport chain, chemiosmosis, ATP synthase (inner mitochondrial membrane)
• Anaerobic Respiration: Lactate fermentation (animals), ethanol fermentation (yeast)
• Respiratory Substrates: Lipids yield more ATP per gram than carbohydrates, RQ values

Key Diagrams to Memorise

• Heart structure and blood flow
• Oxygen dissociation curves (including Bohr shift)
• Nephron structure
• Light-dependent and light-independent reactions
• Glycolysis, Krebs cycle, and electron transport chain
• Synapse structure and transmission

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6. Control and Coordination (A2)

6.1 Nervous Communication

• Neurone Structure: Axon, dendrites, myelin sheath, nodes of Ranvier, Schwann cells
• Action Potential: Resting potential, depolarisation, repolarisation, refractory period
• Saltatory Conduction: How myelination increases speed of transmission
• Synapses: Neurotransmitter release, postsynaptic potential, summation, role of acetylcholine

6.2 Hormonal Communication

• Endocrine System: Glands and hormones — adrenal glands (adrenaline), pancreas (insulin/glucagon), pituitary, thyroid
• Plant Hormones: Auxins, gibberellins, ABA — roles in tropisms, germination, leaf abscission

6.3 Muscle Contraction

• Sliding Filament Theory: Actin, myosin, cross-bridge cycling, role of Ca²⁺, troponin, tropomyosin
• Neuromuscular Junction: How a nerve impulse triggers muscle contraction
• Types of Muscle: Skeletal, smooth, cardiac — structural and functional differences

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7. Immune Response (A2)

• Non-Specific Immunity: Physical barriers (skin, mucous membranes), phagocytosis, inflammation
• Specific Immunity: B lymphocytes (humoral), T lymphocytes (cell-mediated)
• Antibody Structure: Variable and constant regions, antigen-antibody specificity
• Vaccination: Active vs passive immunity, natural vs artificial, herd immunity
• HIV/AIDS: How HIV destroys the immune system, why it’s difficult to vaccinate against
• Autoimmune Diseases: When the immune system attacks self-antigens
• Monoclonal Antibodies: Production and medical applications

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Cambridge vs Edexcel: Key Syllabus Differences

Aspect	Cambridge (9700)	Edexcel IAL
Structure	5 papers (P1 MCQ, P2 Structured, P3 Practical, P4 Structured, P5 Planning)	6 units (3 AS + 3 A2)
Practical	Paper 3 (hands-on lab) + Paper 5 (planning/analysis)	Practical skills in written papers
Essay Component	No formal essay paper	Unit 6 includes an extended writing task
Ecology Depth	Detailed succession and conservation	Similar but with different emphasis
Statistics	Chi-squared, Spearman’s rank, t-test	Primarily chi-squared and standard deviation
Genetics Technology	Strong emphasis on applications	Similar coverage with more ethical discussion
Data Analysis	Paper 5 specifically tests experimental design	Integrated throughout unit papers

For exam-specific pitfalls, review our guides on Cambridge A-Level Biology common mistakes and Edexcel A-Level Biology common mistakes.

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Extended Response (Essay) Tips

Extended response questions are worth significant marks and separate top-grade students from the rest.

Planning Your Response

1. Read the question twice — identify exactly what’s being asked and the command word (describe, explain, evaluate, discuss)
2. Underline key terms — these must appear in your answer
3. Plan for 1 minute per mark — a 10-mark essay needs ~10 minutes
4. Create a quick outline — bullet point the key concepts to include before writing

Writing Your Response

• Use correct biological terminology — examiners award marks for precise language
• Be specific, not vague — say “insulin binds to receptors on liver cell membranes” not “insulin affects the liver”
• Link cause and effect — use phrases like “this results in,” “consequently,” “leading to”
• Include named examples — specific enzymes, hormones, organisms strengthen your answer
• Use data if provided — refer to specific numbers from graphs/tables
• Answer the question — relate every paragraph back to what was asked

Common Mistakes in Extended Responses

• Writing everything you know about a topic instead of answering the specific question
• Using vague language (“stuff,” “things,” “gets bigger”)
• Forgetting to conclude or evaluate when the question asks you to
• Not explaining mechanisms — just stating facts without showing understanding

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Revision Strategies for A-Level Biology

1. Make flashcards for key terms — Biology has an enormous vocabulary; you need to know precise definitions
2. Draw and redraw diagrams — From memory, label all parts, and annotate with functions
3. Create process flowcharts — For processes like protein synthesis, the cardiac cycle, immune response
4. Practice data analysis — Biology exams increasingly test your ability to interpret graphs, tables, and experimental data
5. Use past papers under timed conditions — This is the single most effective revision technique
6. Study examiner reports — Learn what examiners consistently penalise

Stay informed about what to expect this year by reviewing A-Level Biology exam trends for 2026 and the top 10 most tested A-Level Biology topics.

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