Plant Physiology
Plant physiology is the study of how plants function — how they capture energy, transport water and nutrients, grow, respond to environment and reproduce. It is the bridge between biochemistry and ecology.
The process by which green plants, algae and cyanobacteria use solar energy to synthesise organic compounds (mainly glucose) from carbon dioxide and water, releasing oxygen as a by-product. Overall equation: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂.
Photosynthesis
Occurs in chloroplasts, organelles with double membrane, grana of thylakoids and a fluid stroma.
Light-dependent reactions (thylakoid)
- Take place on thylakoid membranes.
- Two photosystems: PS II (P680) and PS I (P700) linked by the Z-scheme.
- Water splitting (photolysis) at PS II releases O₂.
- Electron transport produces NADPH and pumps H⁺ into the thylakoid lumen, generating a proton gradient that drives ATP synthase.
- Outputs: ATP and NADPH.
Light-independent reactions — Calvin cycle (stroma)
Three phases:
- Carbon fixation — RuBP + CO₂ → 2 × 3-PGA (catalysed by Rubisco).
- Reduction — 3-PGA reduced to G3P using ATP and NADPH.
- Regeneration — RuBP regenerated from G3P.
Six turns of the cycle produce one glucose; consumes 18 ATP and 12 NADPH.
C3, C4 and CAM photosynthesis
- C3 — most plants (wheat, rice). First product is 3-carbon 3-PGA. Photorespiration losses at high temperature.
- C4 — sugarcane, maize, sorghum. First product is 4-carbon oxaloacetate (in mesophyll). Kranz anatomy separates C4 fixation from Calvin cycle (in bundle sheath). Higher efficiency in hot climates.
- CAM (Crassulacean Acid Metabolism) — succulents, pineapple. Stomata open at night to fix CO₂ as malate; close by day. Adapted to arid environments.
Respiration
Plants respire 24 h/day. Three stages:
- Glycolysis (cytosol) — glucose → 2 pyruvate; net yield 2 ATP + 2 NADH.
- Krebs cycle (mitochondrial matrix) — pyruvate → CO₂; yields ATP, NADH, FADH₂.
- Electron transport / oxidative phosphorylation (inner mitochondrial membrane) — produces majority of ATP (~32–34 per glucose).
Total: ~36–38 ATP per glucose under aerobic conditions.
Water relations and transpiration
- Osmosis — water moves from higher to lower water potential.
- Water potential (Ψ) = pressure potential + solute potential.
- Transpiration — loss of water vapour mainly through stomata; drives the upward column of water in xylem.
- Cohesion-Tension theory (Dixon 1894) — water column held by cohesion (H-bonds) and pulled by transpiration tension.
- Stomatal regulation — turgor of guard cells controls opening; ABA closes stomata under water stress.
Translocation of solutes
Phloem transport — sucrose moves from source (leaf) to sink (root, fruit, growing tip) by the Münch pressure-flow hypothesis (1930).
Mineral nutrition
Plants need 17 essential elements:
| Group | Elements |
|---|---|
| From air/water | C, H, O |
| Macronutrients (soil) | N, P, K, Ca, Mg, S |
| Micronutrients | Fe, Mn, Zn, Cu, B, Mo, Cl, Ni |
Functions to know:
- N — amino acids, nucleic acids, chlorophyll.
- P — ATP, nucleic acids, phospholipids.
- K — stomatal regulation, enzyme activator.
- Mg — central atom of chlorophyll.
- Fe — cytochromes, electron transport.
Deficiency symptoms are diagnostic: yellowing (chlorosis) of older leaves indicates mobile-nutrient deficiency (e.g. N, Mg); younger-leaf chlorosis points to immobile-nutrient deficiency (e.g. Fe, Ca).
- Photosynthesis: light reactions in thylakoid; Calvin cycle in stroma.
- Rubisco is the most abundant protein on Earth and catalyses CO₂ fixation.
- C4 plants (sugarcane, maize) outperform C3 in hot, high-light environments.
- Cohesion-Tension theory explains water rise in tall trees.
- Five major plant hormones: auxin, gibberellin, cytokinin, abscisic acid, ethylene.
Plant hormones (phytohormones)
| Hormone | Discoverer / classical experiment | Functions |
|---|---|---|
| Auxin (IAA) | Charles & Francis Darwin (1880); Went (1928) | Apical dominance, cell elongation, root initiation |
| Gibberellin (GA) | Kurosawa (1926, rice bakanae) | Stem elongation, seed germination, bolting |
| Cytokinin | Skoog (1955, coconut milk) | Cell division, delay of senescence |
| Abscisic acid (ABA) | Addicott (1963) | Stomatal closure, seed dormancy, stress response |
| Ethylene | Neljubov (1901) | Fruit ripening, abscission, senescence |
Modern additions: brassinosteroids, jasmonates, salicylic acid, strigolactones.
Photoperiodism
The response of plants to relative lengths of day and night. Three categories:
- Long-day plants (e.g. spinach, wheat) flower when day length exceeds critical limit.
- Short-day plants (e.g. chrysanthemum, rice) flower when day length is below critical limit.
- Day-neutral plants (e.g. tomato, cucumber).
Mediated by phytochrome (red/far-red switch).
Vernalisation
Cold treatment that promotes flowering — important in winter wheat varieties.
For exam answers on photosynthesis, write the balanced equation, identify the two stages and their location, and mention Rubisco by name. For respiration, give the three stages and ATP yields (2 + 2 + ~34). For hormones, learn the classical experiments — Went's agar block for auxin and Kurosawa's bakanae work for gibberellins remain examiner favourites.
Plant movements
- Tropisms — directional responses: phototropism (light), gravitropism, hydrotropism, thigmotropism (touch).
- Nastic movements — non-directional, e.g. sleep movements of leaves in Mimosa pudica, sensitive plant.