Plant Structure and Physiology
Plant Structure and Physiology
This domain tests root, stem and leaf structure, water and sugar transport, transpiration, photosynthesis and plant responses. Expect questions linking anatomy to function and interpreting limiting-factor graphs.
Core rules
- Root hairs increase surface area for water and mineral uptake. Xylem vessels are dead, hollow tubes carrying water and dissolved minerals upward. Phloem sieve tubes are living and carry sucrose and amino acids in either direction.
- Transpiration is water loss, mainly through stomata. Cohesion and tension pull water up the xylem. Guard cells open stomata when turgid and close them when flaccid.
- Photosynthesis has light-dependent reactions producing ATP and NADPH, and the Calvin cycle fixing CO2. Light intensity, CO2 concentration and temperature can limit the rate.
- Auxin causes phototropism: it accumulates on the shaded side, promoting cell elongation and bending toward light.
Worked example
A graph shows photosynthesis rising with light intensity, then plateauing. Reasoning: at low light, light is limiting; at the plateau, another factor-usually CO2 concentration or temperature-limits the rate. Adding more light cannot help until that factor increases.
Traps
Xylem does not transport sugars. Phloem is not a one-way pipe. Plants respire all the time, not only at night. Transpiration is not the same as guttation. Stomata close to conserve water but also reduce CO2 uptake.