Plant Physiology
Domain 4 covers transport, photosynthesis, respiration and plant growth. The exam tests water potential, transpiration, phloem loading, C3 and C4 pathways, respiration stages and hormone effects.
Core rules
- Water moves from higher to lower water potential. Root pressure, cohesion-tension and transpiration pull drive xylem transport; stomata open when guard cells take up K+ and become turgid.
- Phloem transport follows the pressure-flow hypothesis: sucrose loaded at source lowers water potential, water enters, and pressure pushes sap to sink.
- Photosynthesis: light reactions in thylakoids produce ATP and NADPH; Calvin cycle in stroma fixes CO2 using Rubisco. C4 plants use PEP carboxylase and Kranz anatomy, reducing photorespiration.
- Respiration: glycolysis in cytoplasm, Krebs cycle in mitochondrial matrix, oxidative phosphorylation on inner membrane. RQ = CO2 released/O2 used.
- Growth is controlled by auxin, gibberellin, cytokinin, ABA and ethylene; photoperiodism and vernalisation affect flowering.
Worked example
A respiring tissue consumes 6 O2 and releases 6 CO2. Find RQ and identify the substrate. RQ = 6/6 = 1.0. A value of 1.0 indicates carbohydrate respiration, because glucose oxidation uses six O2 and releases six CO2. Fats have RQ below 1 because they are more reduced and require more O2; organic acids can have RQ above 1.
Traps
- Saying stomata open because of K+ efflux; opening involves K+ influx.
- Confusing light reaction site (thylakoid) with Calvin cycle site (stroma).
- Claiming C4 plants lack Rubisco; they use it in bundle-sheath cells.
- Mixing up ABA (stress hormone, closes stomata) with auxin (apical dominance).