Plant Structure, Function and Physiology
This domain tests how plants transport water and solutes, how they photosynthesise, and how they regulate gas exchange and growth. Expect questions on xylem and phloem, stomata, limiting factors, mineral nutrition and plant hormones.
Transport
Xylem carries water and mineral ions upward in the transpiration stream. Cohesion between water molecules and adhesion to vessel walls maintain a continuous column under tension created by evaporation from leaves. Root pressure is a minor contributor. Phloem carries sucrose from source to sink by pressure flow: sucrose is loaded actively, water follows osmotically, and the resulting pressure drives flow. Transpiration rate rises with temperature, wind and light and falls with high humidity.
Photosynthesis
The light-dependent reactions occur on thylakoid membranes: water is photolysed, oxygen is released, and ATP and NADPH are produced. The light-independent Calvin cycle occurs in the stroma, where rubisco fixes CO₂ onto RuBP using ATP and NADPH. Light intensity, CO₂ concentration and temperature can each limit the rate. C4 plants concentrate CO₂ and reduce photorespiration.
Worked example
A potted plant is moved from still, humid air to moving dry air. Transpiration increases because the water-vapour gradient between the leaf air spaces and the atmosphere becomes steeper, so diffusion out of the stomata speeds up. Still humid air reduces the gradient and slows water loss.
Traps
Do not say phloem transport uses transpiration pull. The Calvin cycle does not require darkness; it requires ATP and NADPH. Stomata are not always open in light, and mineral ions are not taken up mainly by transpiration.
Plant hormones and responses
Auxins promote cell elongation and apical dominance; gibberellins promote stem elongation and germination; abscisic acid closes stomata under water stress. Phototropism and geotropism depend on unequal auxin distribution.