What to Study in the Second Year of A Level Biology: No Optional Topics—A Chapter-by-Chapter Look at Cambridge Chapters 12–19 and Edexcel Units 4 and 5

First, a quick note: A-Level Biology has no optional topics. All chapters in the A Level section of Cambridge 9700 (Chapters 12–19) are compulsory, and all six units of Edexcel IAL are also mandatory. In our article on How A-Level Biology is Assessed, we only listed the topic names for the A Level section; for practicals and extended response questions, see articles 184 and 198. This article breaks down what is studied in the second year (A Level stage) chapter by chapter: Cambridge Chapters 12–19 and Edexcel Unit 4 and Unit 5, followed by a comparison of the differences between the two exam boards. This is based on the Cambridge 9700 syllabus (2025–2027 edition; the official statement for the 2028–2030 edition indicates no major changes affecting teaching) and the Pearson IAL Biology syllabus.

1. Papers Taken in the Second Year

• Cambridge: Paper 4 A Level Structured Questions (2 hours, 100 marks, 38.5% of A Level, requires AS content) and Paper 5 Planning, Analysis and Evaluation (see article 184);
• Edexcel: Unit 4 and Unit 5 are each 1 hour 45 minutes, 90 marks, each accounting for 20% of IAL; each paper tests at least 9 marks of Level 2+ mathematical skills; there may be synoptic questions requiring knowledge from Unit 1 and 2; a scientific article is released before the Unit 5 exam, with a question based on it in the exam (see article 198 for writing tips). For the Unit 6 practical, see article 184.

2. Cambridge A Level: Chapters 12–19

1. Chapter 12 Energy and Respiration: Why organisms need energy; features of ATP and its two synthesis methods (substrate-level phosphorylation, chemiosmosis); energy comparison of carbohydrates, lipids, and proteins as respiratory substrates; calculation of the respiratory quotient (RQ); glycolysis, link reaction, Krebs cycle, oxidative phosphorylation, roles of NAD and FAD; mitochondrial structure and function; lactate fermentation in mammals and ethanol fermentation in yeast; how rice adapts to waterlogged roots; experiments using redox indicators and respirometers;
2. Chapter 13 Photosynthesis: Chloroplast structure and function; roles of chlorophyll a, b, and carotenoids, absorption and action spectra, pigment separation using chromatography; cyclic and non-cyclic photophosphorylation; three stages of the Calvin cycle; light intensity, carbon dioxide concentration, and temperature as limiting factors; experiments using redox indicators and whole plants;
3. Chapter 14 Homeostasis: Principles of homeostasis and negative feedback; urea production; kidney structure, urine formation in the nephron; hypothalamus, posterior pituitary, and antidiuretic hormone regulating water; cell signaling using glucagon as an example; insulin and glucagon regulating blood glucose; principles of glucose test strips and biosensors; stomatal regulation in plants, guard cells, and the role of abscisic acid in stomatal closure;
4. Chapter 15 Control and Coordination: Comparison of endocrine and nervous systems; sensory and motor neurons; generation and conduction of action potentials, saltatory conduction in myelinated nerve fibers, refractory period; cholinergic synapses; neuromuscular junctions and the sliding filament model of striated muscle contraction; rapid responses in the Venus flytrap in plants, auxin promoting elongation growth, gibberellin's role in barley seed germination;
5. Chapter 16 Inheritance: Haploid and diploid, meiosis and its role in generating variation; genetic diagrams: monohybrid and dihybrid crosses, involving dominance, codominance, multiple alleles, sex linkage, autosomal linkage, and epistasis (no need to memorize epistatic ratios); chi-squared test (formula provided); relationships of TYR, HBB, F8, HTT genes to albinism, sickle cell anemia, hemophilia, and Huntington's disease; gibberellin and stem elongation (Le allele); structural and regulatory genes, lac operon; transcription factors;
6. Chapter 17 Selection and Evolution: Continuous and discontinuous variation; t-test (formula provided); stabilizing, directional, and disruptive selection; founder effect and genetic drift; antibiotic resistance in bacteria; Hardy-Weinberg principle; artificial selection (disease-resistant breeding in wheat and rice, inbreeding and hybridization in corn, milk yield in dairy cows); DNA sequence data and evolutionary relationships; speciation (sympatric and allopatric);
7. Chapter 18 Classification, Biodiversity, and Conservation: Three species concepts; three-domain classification and the four kingdoms of eukaryotes; virus classification; ecosystems and niches; random sampling, quadrats, transect lines, transect belts, mark-release-recapture (Lincoln index); Spearman's rank correlation and Pearson's correlation; Simpson's Diversity Index; causes of extinction, reasons for conserving biodiversity, roles of zoos, botanical gardens, reserves, and seed banks, assisted reproduction; controlling invasive alien species; roles of IUCN and CITES;
8. Chapter 19 Genetic Technology: Recombinant DNA and genetic engineering, roles of restriction enzymes, DNA ligase, plasmids, DNA polymerase, and reverse transcriptase; promoters; confirming expression using marker genes; gene editing; PCR and gel electrophoresis; microarrays; bioinformatics databases; recombinant human proteins (insulin, factor VIII, adenosine deaminase); genetic screening (BRCA1, BRCA2, Huntington's disease, cystic fibrosis) and gene therapy (severe combined immunodeficiency, inherited eye diseases) and their ethics; transgenic salmon, herbicide-resistant soybeans, insect-resistant cotton, and ethical and social issues of GMOs in food production.

3. Edexcel Unit 4: Energy, Environment, Microbiology, and Immunity

Topic 5 Energy Flow, Ecosystems, and the Environment:

• Photosynthesis: light-dependent reactions, photophosphorylation, light-independent reactions, chloroplast structure, absorption and action spectra, pigment chromatography;
• Ecology: gross and net primary production, energy transfer efficiency; populations, communities, habitats, ecosystems, biotic and abiotic factors, niches; succession;
• Climate Change: evidence and causes; carbon cycle and emission reduction; limitations of predictions by data extrapolation; impacts on rainfall, species distribution, and life cycles; effects of temperature on enzyme activity; why scientific conclusions on controversial issues may depend on who produces them;
• Evolution: changes in allele frequencies, isolation and speciation; reforestation and sustainable resources (including biofuels).

Topic 6 Microbiology, Immunity, and Forensics:

• Microbial culture techniques, methods of measuring growth, stages of the bacterial growth curve;
• Structural comparison of bacteria and viruses; infection processes of Mycobacterium tuberculosis and HIV; pathogen entry routes; non-specific immunity; antigens and antibodies, B cells and T cells; natural, artificial, active, passive immunity; the "evolutionary arms race" between pathogens and hosts;
• Bacteriostatic and bactericidal antibiotics; hospital-acquired infections and prescribing guidelines;
• Forensics: PCR, gel electrophoresis, DNA profiling; determining time of death through degree of body decomposition, succession stages, forensic entomology, body temperature, and muscle rigidity.

Core Practicals: Factors affecting the rate of photosynthesis in aquatic plants (10), habitat ecological survey (11), effects of temperature on organism development (12), growth rate of microorganisms in liquid culture (13), effects of different antibiotics on bacteria (14).

4. Edexcel Unit 5: Respiration, Internal Environment, Coordination, and Genetic Technology

Topic 7 Respiration, Muscles, and the Internal Environment:

• Aerobic respiration (glycolysis, link reaction, Krebs cycle, oxidative phosphorylation); fate of lactate after anaerobic respiration; respiratory quotient;
• Structure of skeletal muscle and sliding filament mechanism; automaticity of cardiac muscle; adrenaline and the "fight or flight" response;
• Negative and positive feedback; homeostasis; kidney structure, urea production, reabsorption in renal tubules; water regulation by osmoreceptors in the pituitary and hypothalamus; regulation of gene expression by transcription factors.

Topic 8 Coordination, Response, and Genetic Technology:

• Sensory, relay, and motor neurons; reflex arcs; conduction of action potentials and saltatory conduction in myelin sheaths; synapses, effects of drugs on synapses; receptors; habituation;
• Phytochromes, auxins, gibberellins in plants; nervous and hormonal coordination; functions of different brain regions; applications of MRI, fMRI, PET, CT in medical diagnosis and brain research;
• Genetic technology: producing drugs using GMOs; production and introduction of recombinant DNA; microarrays; bioinformatics; risks and benefits of GMOs.

Core Practicals: Investigating yeast respiration using artificial hydrogen carriers (15), measuring respiration rate and respiratory quotient using a respirometer (16), investigating the effects of exercise using spirometry data (17), production of amylase in germinating grains (18).

5. Comparison of the Two Exam Boards (Excerpts)

The practical implication is: when using textbooks or exercises from another exam board, you must select content according to your own syllabus. For example, Cambridge students in their second year need to specifically practice genetic diagrams (including epistasis), statistical tests, and ecological sampling; Edexcel students should focus on climate change, forensics, and microbiology.

6. How to Study

Here are some empirical suggestions:

1. Compare and contrast respiration and photosynthesis: Both involve ATP synthesis, electron transport, and chemiosmosis. Draw a comparison chart labeling the sites, reactants, products, and energy carriers;
2. Answer process questions step-by-step: The Krebs cycle, action potentials, muscle contraction, urine formation, and PCR are all multi-step processes. List the keywords for each step in order;
3. Draw diagrams for genetics questions: For Cambridge's dihybrid crosses, linkage, and epistasis, write out the parents, gametes, offspring, and ratios, then use a chi-squared test to analyze;
4. Practice statistics using the syllabus checklist: The Cambridge syllabus states that the formulas for chi-squared and t-tests will be provided. Focus on choosing the right test, stating your hypotheses, and drawing a conclusion;
5. Master the examples specified in the syllabus: Be able to explain the details of examples named in the syllabus (e.g., rice, four genes and genetic diseases, transgenic salmon, tuberculosis, and HIV);
6. Don't forget Year 1 content: Both exam boards may test Year 1 knowledge in Year 2 papers.

Finally

A-Level Biology has no optional topics. In Year 2, respiration and photosynthesis, homeostasis, nervous coordination, inheritance and evolution, ecosystems and conservation, and genetic technology are all compulsory. The two exam boards are broadly similar, but topics like immunity, Hardy-Weinberg, and sex-linked inheritance are taught in different years, while epistasis, gene therapy, climate change, forensics, and brain imaging are only covered by one board. Go through your syllabus chapter by chapter, practice the core practicals and past papers, and you will have a clear checklist for your Year 2 revision. If you need A-Level Biology tutoring, feel free to chat with our teachers.

Verification note: This article was verified in September 2026: Cambridge 9700 Chapters 12–19 content and notes (including that chi-squared and t-test formulas will be provided, and ratios for epistasis are not required), as well as immunity, antibiotics, and tuberculosis belonging to AS Chapters 10 and 11, are from Cambridge International's "Biology 9700 syllabus for 2025, 2026 and 2027"; the statement that there are "no significant changes affecting teaching" in the 2028–2030 version is from the 2028–2030 syllabus; the duration, marks, and weighting of Paper 4 were verified in our 169th article on this site. The duration, marks, minimum math marks requirement, synoptic question details, pre-released articles, topic content, and Core Practicals 10–18 for Edexcel Units 4 and 5, as well as sex-linked inheritance, three-domain classification, and Hardy-Weinberg belonging to Units 1 and 2 respectively, are from the Pearson IAL Biology syllabus. In the comparison table, "Not in syllabus content" means we could not find the corresponding item in the syllabus content list. The content is our abridged translation. The study methods are our empirical recommendations.

References:

• Cambridge International: Biology 9700 Syllabus 2025–2027 (PDF)
• Pearson Edexcel: International A Level Biology Specification (PDF)