Plant Physiology (Photosynthesis, Respiration, Transport, Plant Growth)
Plant Physiology tests how plants capture energy, respire, move water and minerals, and regulate growth through hormones and environmental signals.
Photosynthesis and respiration
- Overall: 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O. Oxygen comes from water, not carbon dioxide.
- Light reactions occur on thylakoids: PSII (P680) and PSI (P700), the Z-scheme, photolysis of water, and non-cyclic photophosphorylation yielding ATP and NADPH. Cyclic photophosphorylation yields ATP only.
- The Calvin cycle occurs in the stroma; RuBisCO fixes CO2, and each CO2 needs 3 ATP and 2 NADPH.
- C4 plants use PEP carboxylase in mesophyll and decarboxylation in bundle-sheath cells, avoiding photorespiration; CAM plants open stomata at night.
- Respiration: glycolysis in cytoplasm (net 2 ATP, 2 NADH), link reaction and Krebs cycle in the matrix, electron transport on the inner mitochondrial membrane; about 38 ATP per glucose aerobically.
Transport and growth
- Water moves by apoplast and symplast pathways; the Casparian strip forces symplastic entry. Cohesion–tension with transpiration pull explains tall-tree water transport; root pressure is minor.
- Stomata open when guard cells accumulate K+ and become turgid. Phloem translocation follows the pressure-flow hypothesis.
- Auxin causes apical dominance and phototropism; gibberellin elongates stems; cytokinin promotes cell division; ABA closes stomata; ethylene ripens fruit.
Worked example
For one glucose, count ATP: glycolysis gives 2 ATP + 2 NADH; two pyruvates give 2 NADH; two Krebs cycles give 2 ATP, 6 NADH and 2 FADH2. With 10 NADH × 3 = 30 and 2 FADH2 × 2 = 4, total = 2 + 2 + 30 + 4 = 38. The reasoning assigns 3 ATP per NADH and 2 per FADH2.
Traps
O2 is released from water; NADPH, not NADH, is the photosynthetic reductant; cyclic flow produces no O2; transpiration pull, not root pressure, lifts water in tall trees; auxin accumulates on the shaded side.