Plant Physiology and Biochemistry
Plant physiology and biochemistry in the RBSE Senior Secondary Agricultural Biology paper tests how crop plants capture energy, move water and minerals, and regulate growth. Expect pathway-level questions, factor-effect questions, and simple numericals on gas exchange and energetics.
Core processes
- Photosynthesis has light-dependent thylakoid reactions producing ATP and NADPH, and carbon fixation in the stroma. C3 plants fix CO2 via RuBisCO to 3-PGA; C4 plants first form oxaloacetate (4C) in mesophyll and decarboxylate in bundle-sheath cells; CAM plants separate carboxylation and decarboxylation in time.
- Photorespiration consumes O2 and releases CO2, lowering net photosynthesis, especially in C3 plants at high temperature.
- Aerobic respiration: glycolysis (cytoplasm), Krebs cycle (mitochondrial matrix), electron transport chain (inner membrane). The respiratory quotient (RQ) is CO2 released/O2 consumed; carbohydrate RQ ≈1, fat ≈0.7, organic acid >1.
- Water moves by transpiration pull; stomata open when guard cells accumulate K+ and become turgid.
- Growth regulators: auxin (apical dominance, rooting), gibberellin (stem elongation, bolting), cytokinin (cell division), ABA (stress, stomatal closure), ethylene (ripening, abscission).
Worked example
How many ATP and NADPH are needed for one glucose molecule in the Calvin cycle? Three turns fix three CO2 and yield one G3P; two G3P make one glucose, so six turns are needed. Each turn uses 3 ATP and 2 NADPH. Total = 6×3 = 18 ATP and 6×2 = 12 NADPH. Reasoning: do not count the 3 ATP/2 NADPH per CO2 as per glucose; multiply by six, not three.
Traps
Confusing photorespiration with dark respiration; saying C4's first stable product is 3-PGA; treating CAM as spatial separation; using RQ=1 for all substrates; ignoring that stomatal closure reduces both CO2 entry and transpiration.