Plant Physiology – Comprehensive Study Notes
(Designed for IIT‑JEE / National Entrance Exams)
1. INTRODUCTION
| Term | Definition | Key Points |
|---|---|---|
| Plant Physiology | Study of functional attributes of plants (metabolism, transport, growth, response). | Integrates anatomy, biochemistry, genetics. |
| Homeostasis | Maintenance of internal equilibrium (water, ion concentration, turgor, pH). | Relies on feedback loops, transport mechanisms. |
| Hormonal Regulation | Chemical signals (auxins, gibberellins, cytokinins, ethylene, abscisic acid, brassinosteroids). | Modulate growth & stress responses. |
2. WATER, ION AND CARBON TRANSIT
2.1 Water Absorption & Transport
| Concept | Equation | Notes |
|---|---|---|
| Water Potential (Ψ) | Ψ = Ψ_s + Ψ_p + Ψ_m + Ψ_g | |
| Potential Components | - Ψ_s (solute) = –CRT (C=solute molality, R=R, T=K) - Ψ_p (pressure) - Ψ_m (matrix) - Ψ_g (gravitational) |
|
| Hydraulic Conductivity (K_h) | K_h = (Q·L)/(ΔΨ·A) | Q = volume flow, L=length, A=cross‑section. |
| Hagen–Poiseuille Law | Q = (π·ΔΨ·r⁴)/(8·η·L) | r=radius, η=viscosity. |
| Turgor Pressure (π) | π = k·(C_c – C_o) | k = constant, C_c = cellular solute, C_o = external. |
Tricky Point:
- Co‑transporters (H⁺‑ATPase) create electrochemical gradients that drive passive diffusion of solutes (e.g., K⁺, Ca²⁺).
- Aquaporins increase effective r in Poiseuille’s law; their gating is pH/ABA‑dependent.
2.2 Ion Transport
| Mechanism | Driving Force | Key Transporters |
|---|---|---|
| Passive diffusion | Concentration gradient | Non‑specific channels |
| Facilitated diffusion | Concentration gradient | Specific ion channels |
| Active transport | ATP / H⁺ gradient | H⁺‑ATPase, Na⁺/K⁺‑ATPase, Ca²⁺‑ATPase |
| Co‑transport | H⁺ gradient | Sucrose/H⁺ symporters, K⁺/H⁺ antiporters |
Derivation:
The Nernst equation gives equilibrium potential (E = \frac{RT}{zF}\ln\frac{[ion]{out}}{[ion]{in}}).
For H⁺‑ATPase, the pumping rate (J = P_{H⁺}·(E_{pump}-E_{cell})).
2.3 Carbon Assimilation & Photo‑respiration
| Process | Equation | Key Enzyme |
|---|---|---|
| Calvin Cycle | 3 CO₂ + 9 ATP + 6 NADPH → C₆H₁₂O₆ + 9 ADP + 7 Pi + 6 NADP⁺ | Ribulose‑1,5‑bisphosphate carboxylase/oxygenase (Rubisco) |
| Photo‑respiration | CO₂ + O₂ + 3 PGA → 2 G3P + 1 OAA | Rubisco (oxygenase reaction) |
Tricky Point:
- Rubisco’s dual affinity for CO₂/O₂ leads to photo‑respiration; plants adapt via C₃, C₄, CAM pathways.
3. PHOTOSYNTHESIS – DEEP DIVE
3.1 Light Reactions (Photosystem II & I)
| Parameter | Expression | Physical Meaning |
|---|---|---|
| Photochemical Efficiency (Φ) | Φ = (ΔI)/I₀ | Ratio of usable light to incident. |
| Quantum Yield (Φₑ) | Φₑ = ΔN_e / ΔN_photons | Number of electrons per photon. |
| Electron Transport Rate (ETR) | ETR = Φₑ·I_abs·qP | qP = fraction of open PSII reaction centres. |
Derivation of ETR from PAM data:
(ETR = \frac{F_v/F_m}{F_v/F_0}\cdot I_{act}) where (F_v/F_m) is maximal PSII efficiency and (I_{act}) is actinic light intensity.
3.2 Dark Reactions – Carbon Fixation
| Reaction | Rate Equation (Rubisco) | Variables |
|---|---|---|
| Carboxylation | (v_c = V_{cmax}\frac{[CO_2]}{[CO_2]+K_c(1+[O_2]/K_o)}) | Vcmax: maximum carboxylation rate; Kc, Ko: Michaelis constants. |
| Oxygenation | (v_o = V_{cmax}\frac{[O_2]}{[O_2]+K_o(1+[CO_2]/K_c)}) |
Tricky Point:
- Farquhar model integrates both carboxylation & electron transport limitations to predict net assimilation (A = \min(v_c, v_e) - R_d).
3.3 Photo‑respiration & Photo‑maintenance
| Step | Equation | Key Concept |
|---|---|---|
| O₂ uptake | (v_{O_2} = V_{cmax}\frac{[O_2]}{[O_2]+K_o(1+[CO_2]/K_c)}) | O₂ competes at Rubisco. |
| Glycine‑Serine conversion | (v_g = k_{gly}\cdot [Gly]\ |