Biology: Cells and Reproduction, Inheritance, Ecosystems
What this domain tests
The biology section checks cell structure, how organisms reproduce and pass on genes, and how energy moves through a food chain. Expect diagram labelling, Punnett squares, and questions asking you to explain rather than name.
Cells and cell division
Plant cells have a cellulose cell wall, chloroplasts and a large permanent vacuole; both plant and animal cells have a nucleus, cytoplasm, cell membrane and mitochondria. Mitosis produces identical diploid cells for growth and repair. Meiosis halves the chromosome number to make gametes, so fertilisation restores the diploid number.
Reproduction and inheritance
Sexual reproduction involves gamete fusion and gives genetic variety; asexual reproduction gives clones. Chromosomes come in pairs; a gene is a length of DNA and alleles are its versions. A dominant allele is expressed even when only one copy is present; a recessive allele shows only when two copies are present. Genotype is the allele pair (Tt); phenotype is the visible trait.
Ecosystems and food chains
Producers photosynthesise, consumers eat, decomposers break down dead material and recycle nutrients. Arrows show the direction of energy flow, so they point from the eaten organism to the eater. Only about 10% of energy passes to the next level; most is lost as heat through respiration, which is why food chains are short.
Worked example
Cross two heterozygous tall pea plants, Tt × Tt. Each parent makes T and t gametes in equal numbers. The Punnett square gives TT, Tt, Tt, tt - genotypes 1:2:1, phenotypes 3 tall : 1 short. Reasoning: alleles separate when gametes form, then combine at random at fertilisation, so every T must be paired with every t.
Traps that cost marks
Drawing food-chain arrows backwards. Thinking "dominant" means "more common" - it means "expressed when present". Confusing mitosis with meiosis. Saying energy is recycled (nutrients are; energy flows and is lost). Starting a food chain with a consumer.