Plant Physiology (Water Relations, Photosynthesis, Respiration)
What the exam tests
This domain tests water potential, osmosis, transpiration, stomatal regulation, photosynthesis, photorespiration, and respiration. Numeric water-potential questions and pathway-product questions are common.
Core rules and concepts
- Water potential: Ψw = Ψs + Ψp. Water moves from higher Ψw to lower Ψw.
- Solute potential is negative; pressure potential can be positive (turgid cell) or negative (xylem tension).
- Transpiration pull drives ascent of sap in xylem; cohesion and adhesion maintain the water column.
- Stomata open when guard cells become turgid; potassium ion uptake and malate formation contribute.
- Light reactions occur in thylakoids and produce ATP and NADPH; the Calvin cycle occurs in the stroma.
- C4 plants first fix CO2 into oxaloacetate in mesophyll cells; CAM plants fix CO2 at night.
- Glycolysis occurs in the cytoplasm; Krebs cycle in the mitochondrial matrix; electron transport on the inner membrane.
Worked example
Cell A has Ψs = -0.8 MPa and Ψp = 0.3 MPa. Cell B has Ψw = -0.6 MPa. Which way does water move? For A, Ψw = -0.8 + 0.3 = -0.5 MPa. Since -0.5 MPa is higher than -0.6 MPa, water moves from A to B. The lower (more negative) potential is the destination.
Traps that cost marks
Do not compare solute potential alone; include pressure potential. Do not say water moves from low to high water potential. Photorespiration is not the same as respiration. In C4 plants the first stable product is oxaloacetate, not 3-phosphoglycerate.