Biology - Physiology and Ecology
Biology - Physiology and Ecology links organ-system function with population and ecosystem processes. Topics include circulation, gas exchange, homeostasis, neuronal and hormonal control, energy flow, trophic levels, and population growth.
Core rules
- Homeostasis uses negative feedback: a deviation triggers a response that reduces it.
- Mammalian circulation is a double circuit; the left ventricle drives systemic flow through the aorta.
- Gas exchange depends on partial-pressure gradients and a large, thin, moist surface.
- Ecosystems lose energy at each trophic level; roughly 10% is transferred as biomass.
- Population growth is exponential when resources are unlimited; logistic when carrying capacity K limits growth.
Worked example
A producer level fixes 20,000 kJ m⁻² yr⁻¹. Using a 10% transfer efficiency, find the energy available to the third trophic level above producers. Primary consumers receive 10% of 20,000 = 2,000 kJ. Secondary consumers receive 10% of 2,000 = 200 kJ. Tertiary consumers receive 10% of 200 = 20 kJ. So the third trophic level above producers has about 20 kJ m⁻² yr⁻¹. The repeated 90% loss explains why food chains are short and why top predators are scarce.
Traps
- Confusing energy pyramids with biomass or numbers pyramids.
- Saying energy is recycled; nutrients cycle, energy flows and is lost as heat.
- Reversing the direction of a negative-feedback loop.
- Treating carrying capacity as a fixed number that never changes.
- Forgetting that gas exchange surfaces must be moist.