Exchange and transport in plants and animals
Exchange and transport in plants and animals tests surface area to volume ratios, exchange surfaces, transport systems and the relationship between structure and function. Expect questions on Fick's law, xylem, phloem, blood vessels and the heart.
Exchange surfaces
- Efficient exchange surfaces have a large surface area, thin barrier, steep concentration gradient and good ventilation or blood supply.
- Small organisms rely on diffusion; larger organisms need specialised exchange organs and transport systems.
Transport in plants and animals
Xylem transports water and mineral ions upward; transpiration pull and cohesion-tension move water. Phloem translocates sucrose and amino acids bidirectionally by mass flow. Mammalian blood vessels differ in wall thickness, lumen size and valves. The heart has four chambers; the left ventricle has the thickest wall because it pumps blood to the body.
Worked example
Fick's law states rate of diffusion is proportional to surface area x concentration difference / thickness. If thickness doubles while area and gradient stay constant, the rate halves. This explains why alveoli and gill lamellae are thin.
Traps
Xylem is not living at maturity and flow is mainly unidirectional. Phloem is living and uses energy for loading. Transpiration is water loss, not translocation. Arteries carry blood away from the heart, not always oxygenated blood.