Interaction and Interdependence
This section covers the IB Biology SL domain Interaction and Interdependence. The exam tests how biological systems interact: enzymes and metabolism, respiration and photosynthesis, signalling, body systems, immunity, populations and the transfer of energy and matter.
Core rules
- Enzymes lower activation energy and are affected by temperature, pH, substrate and inhibitors; competitive inhibitors bind the active site, while non-competitive inhibitors change enzyme shape.
- Cell respiration releases energy from organic molecules: glycolysis occurs in the cytoplasm, the link reaction and Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation on the inner membrane.
- Photosynthesis uses light energy: the light-dependent reactions in thylakoids produce ATP, NADPH and oxygen; the Calvin cycle in the stroma fixes carbon dioxide.
- Neural signalling depends on ion movements that create a resting potential, action potential, depolarisation and repolarisation; synapses transmit signals between neurons.
- Immunity involves barriers, innate responses and adaptive responses including antibodies and T cells; vaccination produces immunological memory.
- Populations are limited by carrying capacity; communities form food webs; energy flows through trophic levels while matter is recycled.
Worked example
Grass contains 20,000 kJ m-2 yr-1. Primary consumers assimilate 2,000 kJ m-2 yr-1 and secondary consumers assimilate 200 kJ m-2 yr-1. Each transfer is (2,000 / 20,000) x 100 = 10%, then (200 / 2,000) x 100 = 10%. The missing energy is lost mainly as heat from respiration, with some lost in egestion and excretion. This explains why food chains rarely have more than four or five trophic levels.
Traps
- Respiration and photosynthesis are not simple opposites; both involve electron transport and ATP.
- ATP is a short-term energy currency, not a long-term energy store.
- Enzymes are not used up in reactions; they can catalyse repeatedly.
- Energy is not recycled in ecosystems; matter is recycled, but energy flows and is lost as heat.
- The 10% figure is a useful approximation, not a fixed law.