Plant Physiology: Transport, Nutrition, Photosynthesis, Respiration and Growth
Plant physiology questions test mechanisms (water potential, pressure flow, Z-scheme, electron transport) and quantitative facts (ATP and NADPH per glucose, respiratory quotient). Precise pathway names and locations carry the marks.
Transport in plants
- Water potential equals solute potential plus pressure potential; water moves from higher to lower water potential.
- The apoplast is the wall route, the symplast the cytoplasmic route through plasmodesmata; the Casparian strip forces symplastic entry at the endodermis.
- Root pressure causes guttation; the cohesion-tension (transpiration pull) theory explains ascent in tall trees.
- Phloem transport follows the pressure-flow model: active loading at the source, water entry, unloading at the sink.
Mineral nutrition
- Macronutrients: N, P, K, Ca, Mg, S. Micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl, Ni.
- Hydroponics identifies deficiency symptoms; mobile nutrients such as N, K and Mg show deficiency first in older leaves.
Photosynthesis
- Light reactions: PSII (P680) to plastoquinone to cytochrome to PSI (P700) to NADPH; water splits at PSII, releasing oxygen.
- Calvin cycle: RuBisCO fixes CO2 onto RuBP; each glucose needs 18 ATP and 12 NADPH.
- C4 plants use PEP carboxylase in mesophyll and release CO2 in bundle-sheath cells, avoiding photorespiration; CAM plants open stomata at night.
Respiration
- Glycolysis gives 2 pyruvate, 2 ATP and 2 NADH; the Krebs cycle runs in the matrix; the electron transport chain sits on the inner membrane.
- Respiratory quotient = CO2 released divided by O2 consumed; it is 1 for carbohydrate.
Growth and development
- Auxin causes apical dominance, gibberellin causes bolting, cytokinin promotes cell division, ABA closes stomata under stress, ethylene ripens fruit. Photoperiodism and vernalisation control flowering.
Worked example
For tripalmitin, C51H98O6 + 72.5 O2 gives 51 CO2 + 49 H2O, so RQ = 51/72.5, about 0.7. Fat is more reduced than carbohydrate and needs proportionally more oxygen, so its RQ is below 1.
Traps that cost marks
- Water splits at PSII, not PSI.
- In C4 plants the first CO2 acceptor is PEP, not RuBP.
- ABA closes stomata; ethylene does not.
- Do not write that the Krebs cycle occurs on the inner membrane.