Plant Physiology | IISER Smart Prep

1. Core Concept

Plants are the primary energy transducers of the biosphere. Photosynthesis stores solar energy in carbon-carbon bonds (Anabolic), while Respiration releases that energy to form ATP (Catabolic). Growth Regulators (PGRs) act as chemical signals to efficiently coordinate these metabolic activities with the plant’s entire life cycle.

2. Photosynthesis in Higher Plants

Light Reaction (Photochemical Phase)

Location: Grana Thylakoids.
Pigments: Chlorophyll a (Primary), Chl b, Xanthophylls, Carotenoids (Accessory - protect Chl a from photo-oxidation).
Non-Cyclic (Z-Scheme): PS II (P₆₈₀) → Electron Transport → PS I (P₇₀₀). Produces ATP, NADPH, and O₂.
Cyclic: Only PS I involved. Occurs when only light >680 nm is available or CO₂ is extremely low. Produces only ATP.
Chemiosmotic Theory: H⁺ ions accumulate inside the Thylakoid Lumen. The gradient breaks strictly through CF₀-CF₁ particles to synthesize ATP.

Dark Reaction (Biosynthetic Phase)

C₃ Cycle (Calvin): Occurs universally in all plants.

Carboxylation: CO₂ + RuBP (5C) → RuBisCO → 2 molecules of 3-PGA (3C).
Cost: For 1 Glucose, 18 ATP and 12 NADPH are consumed.

C₄ Pathway (Hatch-Slack): Found in tropical plants (e.g., Maize, Sorghum).

Kranz Anatomy: Bundle sheath cells have thick walls and no intercellular spaces.
Mechanism: CO₂ is fixed as OAA (4C) in mesophyll → transported to bundle sheath → released for the Calvin cycle.
Advantage: Completely suppresses Photorespiration.

Photorespiration (C₂): RuBisCO binds O₂ instead of CO₂. Occurs in 3 organelles: Chloroplast → Peroxisome → Mitochondria. It is a highly wasteful process. (Mnemonic: Cheap Pizza Mart)

C3 vs C4 Plants: Quick Comparison

C3: First stable product = 3-PGA (3C). Photorespiration is Present. Found in most plants.
C4: First stable product = OAA (4C). Photorespiration is Absent. Found in tropical plants (Maize, Sorghum).
Efficiency: C4 plants are more efficient in low CO₂ and high light/temp but require more energy (30 ATP/glucose).

Factors Affecting Photosynthesis

Light: Light intensity, quality (wavelength), and duration. Light is rarely a limiting factor except in dense forests.
CO₂ Concentration: Major limiting factor. C4 plants reach saturation at ~360 ppm, while C3 plants only at ~450 ppm.
Temperature: Dark reactions are enzymatic and more temperature-sensitive. C4 plants have a higher temperature optimum.
Water: Indirect effect via stomatal closure (reduces CO₂ availability) and leaf wilting.

Blackman's Law of Limiting Factors

If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value; it is the factor which directly affects the process if its quantity is changed.

Hill Reaction

Robin Hill (1939) demonstrated that chloroplasts can produce oxygen in the absence of CO₂, provided they are supplied with an artificial electron acceptor (Hill reagent). This proved that the oxygen evolved in photosynthesis comes from the photolysis of water.

3. Respiration in Plants

Glycolysis (EMP Pathway)

Location: Cytoplasm. Entirely Anaerobic (No O₂ required).
Reaction: 1 Glucose (6C) → 2 Pyruvic Acid (3C).
Net Yield: 2 ATP and 2 NADH+H⁺.

Aerobic Respiration

Link Reaction: Pyruvate enters mitochondria → Acetyl CoA + CO₂ + NADH.
TCA Cycle (Krebs): Occurs in Mitochondrial Matrix.
Acetyl CoA (2C) + OAA (4C) → Citric Acid (6C).
Yield per Glucose: 6 NADH, 2 FADH₂, 2 ATP/GTP.
ETS (Oxidative Phosphorylation): Inner Mitochondrial Membrane.
O₂ acts as the terminal electron acceptor; forming metabolic H₂O.
Energy conversion: 1 NADH = 3 ATP; 1 FADH₂ = 2 ATP.

Fermentation (Anaerobic Respiration)

Alcoholic Fermentation: In yeast, pyruvic acid → acetaldehyde → Ethanol + CO₂. (Enzymes: Pyruvate decarboxylase, Alcohol dehydrogenase).
Lactic Acid Fermentation: In muscle cells (during exercise) and some bacteria, pyruvic acid → Lactic acid. (Enzyme: Lactate dehydrogenase).
Note: Less than 7% of energy in glucose is released. No extra ATP is produced beyond the 2 ATP from glycolysis.

Respiratory Balance Sheet (Per Glucose)

Theoretical yield assumes a perfectly efficient system:

Glycolysis: 2 ATP (Direct) + 2 NADH (6 ATP in ETS) = 8 ATP.
Link Reaction: 2 NADH (6 ATP in ETS) = 6 ATP.
Krebs Cycle: 2 GTP/ATP (Direct) + 6 NADH (18 ATP) + 2 FADH₂ (4 ATP) = 24 ATP.
Total Gross Yield: ~36 to 38 ATP.

Respiratory Quotient (RQ) =

Vol CO₂ evolvedVol O₂ consumed

Carbohydrates = 1.0 | Fats = 0.7 | Proteins = 0.9 | Organic Acids > 1.0

4. Plant Growth and Development

Growth Dynamics

Differentiation: Cell becomes specialized (e.g., Tracheids).
Dedifferentiation: Specialized cell becomes meristematic again (e.g., Cork Cambium formation).
Redifferentiation: Dedifferentiated cells lose division power to specialize again (e.g., Cork/Phelloderm).

Plant Growth Regulators (PGRs)

Auxin: Apical dominance, initiates rooting in stem cuttings, widely used weedicide (2,4-D).
Gibberellins (GA): Bolting (internode elongation), significantly increases grape stalk length, speeds up malting in brewing industry.
Cytokinins: Cell division, lateral growth (overcomes apical dominance), delays leaf Senescence (Richmond-Lang effect).
Ethylene: Fruit ripening, "triple response" in seedlings, heavily breaks seed/bud dormancy.
Abscisic Acid (ABA): Stress hormone, stimulates stomatal closure, induces seed dormancy (direct Antagonist to GA).

Growth Curve

Under favorable conditions, most plants and plant organs show a Sigmoid (S-shaped) curve when growth is plotted against time.

Lag phase: Initial slow growth.
Log (Exponential) phase: Rapid growth at a constant rate.
Stationary phase: Growth slows down due to limited resources or senescence.

Photoperiodism & Vernalization

Photoperiodism: Effect of light duration (day length) on flowering.
- Long-day plants (LDP): Flower when day length > critical period.
- Short-day plants (SDP): Flower when day length < critical period.
- Day-neutral plants (DNP): Flowering not dependent on photoperiod.
Vernalization: Requirement of a cold period (low temperature) to shorten the vegetative phase and induce flowering. (e.g., Winter varieties of Wheat).

5. Conceptual Insights & Visual Intuition

Z-Scheme Intuition: Think of it as an "energetic uphill/downhill" ride. Light kicks electrons uphill (PS II), they ride downhill via ETS (making ATP), get kicked uphill again (PS I), and finally land securely on NADP⁺.
Proton Pumping: In Photosynthesis, H⁺ gets pumped IN (to the Lumen). In Respiration, H⁺ gets pumped OUT (to the Intermembrane space).

6. Common Mistakes

The Glucose Trap: Remember that 1 single turn of the Krebs cycle is for 1 Pyruvate. For an entire 1 Glucose molecule, you must double all products (NADH, FADH₂, ATP).
O₂ Source: In photosynthesis, the released O₂ comes exclusively from the photolysis of Water, absolutely not from CO₂.
Photorespiration Organelles: IAT often asks the sequence: Chloroplast → Peroxisome → Mitochondria. (Use Mnemonic: Cheap Pizza Mart).

7. IAT Exam Focus Points

Chemiosmotic Locations: High yield concept. Lumen (where H⁺ accumulates) vs Stroma (where there is low H⁺).
C₄ vs C₃ ATP cost: C₄ is energetically more "expensive" (requires 30 ATP per glucose compared to 18 ATP for C₃) but it efficiently prevents wasteful photorespiration.
RQ Values: Direct match-the-following questions frequently appear on Carbohydrates, Tripalmitin, and Organic acids.
Hormone Antagonism: GA (breaks dormancy) vs ABA (induces dormancy); Auxin (promotes apical dominance) vs Cytokinin (promotes lateral growth).
Action vs Absorption Spectrum: Know that Chl a is the main core pigment, but accessory pigments critically help widen the spectrum range of light that can be absorbed.

8. Practice Mock Test

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