Plant Physiology: Photosynthesis, Respiration, Water Relations and Mineral Nutrition
This domain rewards stoichiometric accuracy: how many ATP and NADPH per CO₂, where each step happens, and which way water moves. Diagram-labelling of the Z-scheme and the Calvin cycle is standard.
Core rules
- Light reaction on thylakoid membranes: PSII (P680) splits water, electrons travel the electron transport chain to PSI (P700). Non-cyclic photophosphorylation yields ATP, NADPH and O₂; cyclic yields only ATP.
- Calvin cycle in the stroma: RuBP + CO₂ → 3-PGA, catalysed by RuBisCO; per CO₂ fixed, 3 ATP and 2 NADPH are consumed.
- C₄ (Hatch–Slack): PEP carboxylase in mesophyll, Kranz anatomy, Calvin cycle in bundle sheath, little photorespiration. CAM: CO₂ fixed at night as malate, decarboxylated by day.
- Respiration: glycolysis in cytoplasm (net 2 ATP, 2 NADH); link reaction and Krebs cycle in the matrix (2 ATP, 8 NADH, 2 FADH₂); electron transport chain and oxidative phosphorylation on the inner mitochondrial membrane.
- Water relations: water potential = solute potential + pressure potential; water moves from higher to lower ψ. Transpiration pull, cohesion–tension, root pressure and guttation. Stomatal opening follows K⁺ influx and malate formation.
- Seventeen essential elements; deficiency appears as chlorosis (N, Mg, Fe), necrosis, or stunting.
Worked example
ATP from one glucose: glycolysis 2 ATP + 2 NADH; link reaction 2 NADH; Krebs 2 ATP + 6 NADH + 2 FADH₂. Counting NADH as 3 ATP and FADH₂ as 2 ATP gives 2 + 6 + 6 + 18 + 4 + 2 = 38. Many texts give 36 because cytosolic NADH enters through a shuttle worth 2 ATP. State your assumption.
Traps that cost marks
Placing RuBisCO in C₄ mesophyll cells; claiming photorespiration generates ATP; reversing the direction of water movement; using chlorosis and necrosis interchangeably.