Plant and Animal Physiology: Exchange, Transport and Control
What this domain tests
Plant and animal physiology tests how organisms obtain and transport gases, water, nutrients and heat, and how they coordinate these processes. Expect comparisons of exchange surfaces, transport tissues and control systems rather than lists of organs.
Core rules
- Photosynthesis has light-dependent reactions in the thylakoid membranes (photolysis of water, ATP and reduced NADP production) and light-independent reactions in the stroma (carbon fixation via the Calvin cycle).
- Transpiration is the loss of water vapour from leaves; the cohesion-tension theory explains how water moves up xylem as a continuous column under tension. Xylem is dead, lignified tissue; phloem is living and translocates sucrose.
- Effective exchange surfaces have a large surface area, a thin barrier, a steep concentration gradient and, in animals, ventilation and blood flow to maintain that gradient.
- Mammalian circulation is double and closed; the left ventricle has a thicker wall because it pumps blood at higher pressure around the body.
- Homeostasis uses negative feedback; receptors detect a change, a coordination centre processes it, and effectors restore the set point.
Worked example
Fick's law states that diffusion rate is proportional to surface area × concentration difference ÷ thickness. If an exchange surface doubles its area and halves its thickness while the concentration gradient stays constant, the diffusion rate becomes 2 ÷ 0.5 = 4 times greater. This is why alveoli are numerous, thin-walled and well supplied with blood.
Traps
Transpiration and translocation are not the same process. Plants respire continuously, not only at night. In countercurrent exchange, water and blood flow in opposite directions, maintaining a gradient along the whole gill; in concurrent flow the gradient falls to zero.