Biology: Cells, Exchange and Inheritance Basics
Core ideas
Biology questions reward clear structure–function reasoning. Know that cells are the basic unit of life; plant cells have a cell wall, chloroplasts and a large permanent vacuole, while animal cells do not. Organelles have jobs: nucleus controls activity and holds DNA, mitochondria release energy in respiration, chloroplasts carry out photosynthesis, the cell membrane controls what enters and leaves.
Multicellular organisms need exchange surfaces because surface area to volume ratio falls as size increases. Effective exchange surfaces are thin, moist, and have a large area and a good blood or air supply - think alveoli, villi, root hair cells and leaves. Transport systems (xylem and phloem in plants; circulatory system in animals) connect exchange surfaces to the rest of the body.
For inheritance, distinguish genotype from phenotype, dominant from recessive alleles, and understand that gametes carry one allele per gene so offspring receive one from each parent. A simple Punnett square predicts ratios of offspring genotypes.
Traps to watch
- Saying plants do not respire; they respire continuously as well as photosynthesising.
- Confusing diffusion (passive, high to low concentration) with active transport (requires energy).
- Assuming a dominant allele is more common in a population - dominance is about expression, not frequency.
- Confusing genotype ratios (e.g. 1:2:1) with phenotype ratios (e.g. 3:1).
Worked example
Cross two heterozygous parents (Aa × Aa). Possible gametes are A or a from each. The square gives AA, Aa, Aa, aa: genotype ratio 1 AA : 2 Aa : 1 aa, phenotype ratio 3 dominant : 1 recessive. Rule of thumb: always state which allele is dominant before interpreting a ratio.