Biology: Body Systems, Reproduction and Heredity, Cells, Ecosystems and Food Chains
Cells
The exam tests cell structure, transport and how cells specialise. Plant cells have a cellulose cell wall, chloroplasts and a large permanent vacuole; animal cells lack these. Both have a nucleus, cytoplasm, cell membrane and mitochondria. Diffusion moves particles from high to low concentration; osmosis is the movement of water across a partially permeable membrane from a dilute to a more concentrated solution.
Body systems
Digestion breaks food into soluble molecules absorbed in the small intestine, where villi increase surface area. Arteries carry blood away from the heart, veins return it, and capillaries exchange materials with tissues. Alveoli exchange oxygen and carbon dioxide. The kidneys filter blood and reabsorb useful substances such as glucose and water.
Reproduction and heredity
Gametes form by meiosis and carry one copy of each chromosome; fertilisation restores the diploid number. Genes exist as alleles, and a dominant allele shows even when only one copy is present. A cross Tt × Tt gives 3 tall : 1 short. In humans the father's sperm (X or Y) determines sex.
Ecosystems and food chains
Producers make food by photosynthesis, consumers eat, and decomposers recycle nutrients. Arrows show the direction of energy flow. Only about 10% of energy passes to the next trophic level, so food chains are short and top predators are few. Carbon cycles through photosynthesis, respiration, combustion and decay.
Worked example
Grass contains 20 000 kJ. Primary consumers receive about 2 000 kJ, secondary consumers 200 kJ, and tertiary consumers 20 kJ. A tertiary consumer needing 50 kJ per day cannot be supported, so this chain supports roughly four levels.
Traps
Energy and biomass pyramids can have different shapes. Arrows point from the eaten organism to the eater. A dominant allele is not necessarily the commonest. Plant cells respire too, taking in oxygen as well as releasing it. Villi absorb nutrients; they do not digest them.