Plant Physiology
Plant Physiology asks how plants move water and sugar, capture light, respire and regulate growth. Numericals usually involve water potential, respiratory quotient or photosynthetic rate; short notes cover C4 and CAM, phytohormones and mineral deficiency.
Photosynthesis
- Light reaction in thylakoids: PSII (P680) splits water, electrons pass through plastoquinone, cytochrome b6f and plastocyanin to PSI (P700), reducing NADP+. The proton gradient drives ATP synthase.
- Non-cyclic photophosphorylation gives ATP, NADPH and O2; cyclic gives ATP only.
- Calvin cycle: RuBP plus CO2, catalysed by RuBisCO, gives 3-PGA; 3 ATP and 2 NADPH per CO2.
- C4 plants (maize, sugarcane): PEP carboxylase fixes CO2 in mesophyll, and decarboxylation in bundle-sheath cells suppresses photorespiration. CAM plants (Opuntia) fix CO2 at night.
- Blackman's law of limiting factors: one factor limits the rate at a time.
Respiration and transport
- Glycolysis in the cytoplasm, Krebs cycle in the matrix, electron transport chain on the inner membrane.
- Respiratory quotient: carbohydrate 1.0, fat about 0.7, organic acid above 1.
- Water moves down water potential: Ψw = Ψs + Ψp. Transpiration pull and cohesion-tension move water upward; root pressure causes guttation.
- Phloem transport follows the pressure-flow (Münch) hypothesis, with active loading.
Worked example
A cell has Ψs = −0.8 MPa and Ψp = 0.3 MPa, so Ψw = −0.8 + 0.3 = −0.5 MPa. Pure water at atmospheric pressure has Ψw = 0 MPa. Water enters the cell because it moves from higher (0) to lower (−0.5 MPa) water potential.
Traps that cost marks
- Deriving photosynthetic oxygen from CO2; it comes from photolysis of water.
- Reversing the respiratory quotient for fats.
- Saying ABA promotes growth; it closes stomata and promotes dormancy.