Biology - Cells, Genetics and Human Systems
The biology paper covers cell structure and function, transport across membranes, enzymes, photosynthesis and respiration, genetics and inheritance, human body systems (digestion, circulation, gas exchange, excretion, nervous coordination), reproduction, and ecology. Questions mix labelled-structure recall with explanation, and the explanations carry the higher marks.
Core rules
- Prokaryotic cells lack a true nucleus and membrane-bound organelles; eukaryotic cells have both.
- Plant cells have a cellulose cell wall, chloroplasts and a large permanent vacuole; animal cells do not.
- Osmosis is the movement of water across a partially permeable membrane from a region of higher water potential to lower water potential.
- Enzymes are proteins that catalyse reactions; they are denatured by high temperature or extreme pH because the active site changes shape.
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, requiring light and chlorophyll. Respiration releases the energy.
- Magnification = image size ÷ actual size; both measured in the same unit.
Worked example
In tomato plants, tall (T) is dominant over short (t). Cross two heterozygous tall plants and state the genotype and phenotype ratios. Reasoning: each parent produces gametes T and t in equal numbers (alleles separate at meiosis). A Punnett square of 4 boxes gives TT, Tt, Tt, tt. Genotype ratio 1 TT : 2 Tt : 1 tt. Phenotype ratio 3 tall : 1 short, because TT and Tt both show the dominant allele. Count squares in the grid, not letters written in the margin.
Traps that cost marks
- Giving a phenotype ratio where the genotype ratio is asked, or vice versa.
- Confusing mitosis (growth, genetically identical cells) with meiosis (gametes, chromosome number halved).
- Reversing the direction of osmosis.
- Stating a magnification without units or with mismatched units.
- Describing an enzyme as 'killed' by heat - enzymes are denatured, not killed.