Plant Physiology
Scope
Photosynthesis, respiration, water and mineral transport, plant hormones and photoperiodism are tested, with emphasis on sites, inputs, outputs and limiting factors.
Core rules
- Light reactions occur in thylakoid membranes and make ATP and NADPH; the Calvin cycle occurs in the stroma and fixes CO2.
- C4 plants concentrate CO2 in bundle-sheath cells via PEP carboxylase; they reduce photorespiration and often have Kranz anatomy.
- Water potential (Ψ) = solute potential (Ψs) + pressure potential (Ψp); water moves toward more negative Ψ.
- Transpiration pull, cohesion and adhesion move water up the xylem.
- Auxin promotes cell elongation and apical dominance; gibberellin promotes stem elongation and seed germination; ABA closes stomata under stress; ethylene ripens fruit.
- Photoperiodism: short-day plants flower when the night exceeds a critical length; long-day plants when night is shorter.
Worked example
A cell has Ψs = −0.8 MPa and Ψp = 0.3 MPa. What is its water potential, and in which direction will water move if placed next to pure water at 0 MPa? Reasoning: Ψ = −0.8 + 0.3 = −0.5 MPa. Pure water has Ψ = 0 MPa, which is higher than −0.5 MPa. Water moves from higher to lower water potential, so it enters the cell. The pressure potential is positive because the cell is turgid.
Traps
- Do not confuse light reaction products with the Calvin cycle: ATP and NADPH are made in the light reactions and used in the stroma.
- Stomatal opening is linked to K+ uptake and guard-cell turgor, not simply to CO2 loss.
- C4 does not mean more photosynthesis under all conditions; it is advantageous in hot, dry habitats.
- ABA is a stress hormone and generally inhibits growth.