Plant Water Relations, Mineral Nutrition and Photosynthesis
What the exam tests
This domain connects plant water relations, mineral nutrition, and photosynthesis. Expect questions on water potential, transpiration, essential elements, deficiency symptoms, light reactions, the Calvin cycle, and C4/CAM adaptations.
Core rules
- Water moves from higher to lower water potential. Water potential = solute potential + pressure potential. Solute potential is negative; pressure potential is positive in turgid cells.
- Transpiration pull, cohesion, and adhesion move water through xylem. Root pressure and guttation occur when transpiration is low.
- Essential elements are macronutrients (N, P, K, Ca, Mg, S) or micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl, Ni). Deficiency symptoms include chlorosis (N, Mg, Fe) and necrosis.
- Photosynthesis: light reactions split water, produce O2, ATP, and NADPH; the Calvin cycle fixes CO2 using RuBP and rubisco. C4 plants use PEP carboxylase and bundle-sheath cells; CAM plants fix CO2 at night.
Worked example
In a C4 plant such as maize, CO2 is first fixed in mesophyll cells to oxaloacetate (4C) by PEP carboxylase. The 4C acid moves to bundle-sheath cells, releases CO2, and enters the Calvin cycle. Reasoning: PEP carboxylase has a high affinity for CO2 and does not bind O2, so photorespiration is minimized. This is an advantage in hot, dry conditions.
Traps
Do not say transpiration and guttation are the same; guttation is loss of liquid water through hydathodes. Do not confuse solute potential (always negative) with water potential. In C3 plants, the first stable product is 3-PGA; in C4, it is OAA. Cyclic photophosphorylation produces ATP only, not NADPH.