Edexcel IAL Biology Unit 4 — Energy, Environment, Microbiology and Immunity

I classified every question part from 17 papers, 2020 to March 2026 — 604 parts, 1,271 marks. Four topics carry about 60% of the marks: global warming (16%), microbiology (16%), ecology — sampling and succession (15%) and active immunity (14%). Photosynthesis light-dependent reactions add another 11%. Evolution, DNA profiling, the Calvin cycle and forensics are each 4–5%.

The command word tells you more than the topic. "Explain" leads with 28% of parts, then "Identify" (the multiple-choice items, 21%), "Describe" (11%), "Calculate" (10%) and "Suggest" (8%). "Suggest" is where Unit 4 separates the A from the B: an unfamiliar context, and you have to apply the biology rather than recall it. Twenty-eight percent of parts are high-difficulty. Revise the top four topics first, then practise every "Suggest" question in the solved papers until the reasoning pattern is automatic.

Photosynthesis is taught as two linked stages. The light-dependent reactions split water, make ATP by photophosphorylation and reduce NADP; the light-independent reactions fix carbon dioxide in the Calvin cycle with RUBISCO. You need chloroplast structure, absorption and action spectra, and chromatography with Rf values — CP 10 measures the rate against light, wavelength, temperature and CO₂.

Ecology moves from GPP and NPP through trophic efficiency to the abiotic and biotic factors that set a niche. CP 11, the habitat study with quadrats and transects, is the most examined practical in the whole A2. Succession is a short topic that appears almost every series.

Climate change is examined on evidence — CO₂ records, pollen in peat, dendrochronology — and on models, including why extrapolation has limits. The Q10 rule and the effect of temperature on development (CP 12) sit here. Evolution is covered as a change in allele frequency through mutation and selection, with allopatric and sympatric speciation. Finish with the reasons conclusions on controversial issues depend on who reaches them, and the case for reforestation, sustainable resources and biofuels.

Start with aseptic technique and the four ways to measure microbial growth, then the growth curve and the exponential growth-rate constant — a calculation that appears every year. CP 13 measures growth in liquid culture; CP 14 compares antibiotics.

Pathogens are handled by example: Ebola, TMV, HIV and phage λ for viruses, Mycobacterium tuberculosis for bacteria. Immunity is the largest single mark-earner in Topic 6: the non-specific barriers, phagocytosis and inflammation, then antigens, B and T cells, antibodies and the four types of immunity. Learn the evolutionary race between host and pathogen, bacteriostatic versus bactericidal antibiotics, and hospital-acquired infection codes of practice.

Forensics closes the unit: PCR, gel electrophoresis and DNA profiling, then time of death from decomposition, insect succession, body temperature and rigor.