Plant Physiology: Transport, Mineral Nutrition, Photosynthesis, Respiration and Growth
Domain 5: Plant Physiology - what is tested
This domain covers water and mineral transport, photosynthesis, respiration and plant growth regulation. Expect questions on water potential, transpiration, essential elements, C3/C4/CAM, the light and dark reactions, glycolysis/Krebs/electron transport, and phytohormones.
Core rules
- Water moves from higher to lower water potential. Root pressure, capillary action and transpiration pull contribute to ascent of sap; transpiration pull is the main force in tall trees.
- Essential elements are macro (N, P, K, Ca, Mg, S) or micro (Fe, Mn, Zn, Cu, B, Mo, Cl). Deficiency symptoms often appear first in older or younger leaves depending on mobility.
- Photosynthesis: light reaction in thylakoids produces ATP and NADPH and splits water; Calvin cycle in stroma fixes CO2 using RuBisCO. C4 plants use PEP carboxylase and bundle-sheath cells; CAM plants fix CO2 at night.
- Respiration: glycolysis in cytoplasm, Krebs cycle in mitochondrial matrix, electron transport on inner membrane. ATP yield is higher in aerobic respiration; fermentation yields much less.
- Growth regulators: auxin, gibberellin, cytokinin, abscisic acid and ethylene have specific promoting or inhibiting roles.
Worked example
A C4 plant is exposed to high temperature and bright light. Explain why its photosynthesis may outperform a C3 plant. In C4 plants, CO2 is first fixed in mesophyll cells by PEP carboxylase to form a four-carbon acid, which is transported to bundle-sheath cells. There, CO2 is released and fixed by RuBisCO. This concentrates CO2 around RuBisCO, reducing photorespiration, which is favoured by high temperature and high oxygen. Reasoning: the initial carboxylase has high CO2 affinity and no oxygenase activity, so carbon gain is more efficient under stress.
Traps
Saying the dark reaction occurs only at night; confusing transpiration with translocation; calling all minerals macronutrients; forgetting that RuBisCO can act as an oxygenase; assigning auxin only to cell division; mixing C4 and CAM anatomy.