Biology: cells, genetics, evolution and ecosystems
The biology section tests cell structure and transport, enzyme and energy metabolism, Mendelian and molecular genetics, evolution by natural selection, and energy and nutrient flow through ecosystems.
Core rules
- Cells: the membrane is a fluid mosaic of phospholipid bilayer with embedded proteins; osmosis moves water toward the higher solute concentration; active transport needs ATP and can run against the gradient.
- Enzymes: specificity by active-site shape; rate rises with temperature then falls sharply as the enzyme denatures; optimum pH is enzyme-specific.
- Genetics: DNA base pairing A–T, G–C; transcription then translation; meiosis halves chromosome number and generates variation by crossing over and independent assortment.
- Evolution: natural selection acts on existing variation; Hardy–Weinberg p²+2pq+q²=1 assumes random mating, large population, no selection, mutation or migration.
- Ecosystems: roughly 10% of energy passes to the next trophic level; carbon and nitrogen cycle between organisms and the abiotic environment.
Worked example
In a population, 4% show a recessive trait. Then q²=0.04, so q=0.2 and p=0.8. Heterozygote frequency is 2pq=2×0.8×0.2=0.32, i.e. 32% of individuals are carriers. State the Hardy–Weinberg assumptions when asked to justify the estimate.
Traps that cost marks
Confusing allele frequency with genotype frequency; assuming a dominant allele is commoner; mixing mitosis and meiosis outcomes; reversing the direction of osmosis; drawing food-chain arrows toward the eater; overestimating energy transfer efficiency.
Key skills to drill
Identify the variable being asked for (allele, genotype or phenotype frequency) before choosing a formula.