Plant Physiology: Photosynthesis, Transport and Growth Regulation
Plant Physiology covers photosynthesis, respiration, water and mineral transport, plant growth regulators and developmental responses such as photoperiodism and vernalisation.
Photosynthesis
- The light reaction occurs in thylakoids: PSII splits water, electrons pass to PSI, producing ATP and NADPH by non-cyclic photophosphorylation.
- Calvin cycle: RuBisCO fixes CO2 onto RuBP; each CO2 costs 3 ATP and 2 NADPH.
- Photorespiration happens in C3 plants because RuBisCO can bind O2. C4 plants use PEP carboxylase in mesophyll and Kranz anatomy, so RuBisCO sees high CO2 in bundle-sheath cells and photorespiration is suppressed.
Respiration and transport
- Glycolysis in the cytoplasm, Krebs cycle in the mitochondrial matrix, electron transport on the inner membrane.
- Respiratory quotient: about 1 for carbohydrates, below 1 for fats, above 1 for organic acids.
- Water rises by root pressure and the cohesion–tension pull of transpiration; stomata open when guard cells take up K+ and become turgid.
Growth regulators
- Auxin: apical dominance and rooting. Gibberellin: bolting and seed germination. Cytokinin: cell division. ABA: stress hormone that closes stomata. Ethylene: ripening.
Worked example
In a C4 leaf, CO2 is first fixed in mesophyll cells into a four-carbon acid, which is shuttled to bundle-sheath cells and decarboxylated there. The released CO2 keeps RuBisCO saturated, so even on hot, dry days little carbon is lost to photorespiration and the plant out-yields C3 crops.
Traps
- Water is split at PSII, not PSI.
- ABA closes stomata; auxin does not.
- RQ values say nothing about ATP yield.
- Photoperiodism controls flowering; vernalisation is a cold requirement.