Physiology (Transport, Nutrition, Respiration, Excretion, Coordination)
Physiology covers transport in plants and mammals, nutrition and digestion, gas exchange and respiration, excretion and osmoregulation, and coordination by nerves and hormones. The exam tests mechanisms and the consequences of changes in conditions, often with graphs.
Core rules
- Fick’s law states that diffusion rate is proportional to surface area and concentration gradient and inversely proportional to diffusion distance.
- In xylem, transpiration pull, cohesion and adhesion move water upward; in phloem, pressure flow moves assimilates from source to sink.
- Digestion hydrolyses polymers; absorption in the ileum uses villi, microvilli, co-transport and active transport.
- Gas exchange surfaces are thin, moist, vascularised and have a large area; ventilation maintains gradients.
- The nephron uses ultrafiltration, selective reabsorption, secretion and osmoregulation; ADH increases collecting-duct water permeability.
- Action potentials depend on Na+ influx and K+ efflux; synapses use Ca2+-dependent neurotransmitter release.
Worked example
An organism consumes 2.0 dm3 O2 and produces 1.4 dm3 CO2 in a closed respirometer. RQ = 1.4/2.0 = 0.70. Reasoning: an RQ near 0.7 indicates fat is the main respiratory substrate; carbohydrate would give about 1.0. The volumes must be measured under the same temperature and pressure, and CO2 must be absorbed only when measuring O2 uptake.
Traps
- Transpiration is water loss, not water uptake; potometers measure uptake, which may differ.
- Ultrafiltration is pressure-driven in the glomerulus; reabsorption is selective and active for glucose and amino acids.
- Oxygen dissociation curves shift right with high CO2, low pH and high temperature, reducing affinity and aiding unloading.