Plant Biology
Plant Biology
This domain tests photosynthesis, water and solute transport, plant hormones, and responses to the environment. Questions frequently ask you to interpret limiting-factor graphs or trace a pathway through plant tissue.
Core rules
- Photosynthesis has light-dependent reactions in the thylakoid membranes, producing ATP and NADPH, and the Calvin cycle in the stroma, fixing CO2 via rubisco.
- Net photosynthesis = gross photosynthesis − respiration. Light compensation point is where net uptake is zero.
- Water moves through xylem by cohesion–tension: transpiration at leaves pulls a continuous water column upward.
- Sugars move in phloem from source to sink by pressure flow; loading at the source raises solute concentration and draws water in.
- Plant hormones include auxin for cell elongation and apical dominance, gibberellin for stem elongation and germination, cytokinin for cell division, abscisic acid for stomatal closure and dormancy, and ethylene for ripening and senescence.
Worked example
At a light intensity of 200 µmol m⁻² s⁻¹, a leaf shows net CO2 uptake of 8 µmol m⁻² s⁻¹; in darkness, respiration releases 2 µmol m⁻² s⁻¹. Gross photosynthesis = net + respiration = 10 µmol m⁻² s⁻¹. Reasoning: respiration continues in the light, so net measurements underestimate carbon fixation.
Traps
- Xylem transport is essentially one-way; phloem transport can be bidirectional between sources and sinks.
- Stomata close to conserve water but also restrict CO2 entry, linking water status to photosynthesis.
- In C4 and CAM plants, initial carbon fixation differs, reducing photorespiration.
- Do not confuse chlorophyll absorbing light with the Calvin cycle using light directly; the Calvin cycle uses ATP and NADPH.