Biology: cells, reproduction and heredity, evolution and classification, ecosystems
What the biology section asks
The paper covers living things and cells, reproduction and heredity, evolution and classification, and ecosystems. Questions use diagrams of cells, crosses, food webs and classification keys, and often ask you to predict offspring ratios or explain energy flow.
Core rules
- Cells and living things: Both plant and animal cells have a nucleus, cytoplasm and cell membrane. Plant cells also have a cellulose cell wall, chloroplasts and a permanent vacuole. Mitochondria release energy by respiration; ribosomes build proteins.
- Reproduction and heredity: Mitosis produces genetically identical body cells for growth and repair. Meiosis produces gametes with half the chromosome number. In a monohybrid cross, a dominant allele is expressed when present; recessive only when two recessive alleles are present.
- Evolution and classification: Variation arises from mutation and sexual reproduction; natural selection favours individuals better adapted to the environment. Classification groups organisms by shared characteristics, from kingdom to species.
- Ecosystems: Producers make food by photosynthesis; consumers eat other organisms; decomposers break down dead material. Energy passes along food chains but about 90% is lost at each level as heat, movement and waste.
Worked example
In pea plants, round seeds (R) are dominant to wrinkled seeds (r). Two heterozygous round-seeded plants (Rr × Rr) are crossed. Predict the offspring. Set out the gametes: each parent gives R or r. A Punnett square gives RR, Rr, Rr, rr - one RR, two Rr and one rr. So 3/4 show round seeds and 1/4 show wrinkled seeds; the genotype ratio is 1 RR : 2 Rr : 1 rr. The recessive phenotype reappears even though both parents are round.
Traps
Saying meiosis produces identical cells; writing phenotype ratios as genotype ratios; assuming a dominant allele is always more common in a population; forgetting that plants respire as well as photosynthesise; reversing the direction of energy flow in a food chain; and placing an organism in the wrong kingdom because of one shared feature.