Unit 4 – Plant Physiology

 1. Photosynthesis

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It is the primary mode of autotrophic nutrition.

Site of Photosynthesis

• Occurs in chloroplasts, primarily in mesophyll cells of leaves.
• Pigments involved: Chlorophyll-a (main pigment), chlorophyll-b, carotenoids, and xanthophylls absorb light energy.

Phases of Photosynthesis

1. Photochemical Phase (Light Reactions):
• Takes place in the thylakoid membranes of chloroplasts.
• Involves photolysis of water, release of oxygen, and formation of ATP and NADPH via:
• Cyclic Photophosphorylation: Produces ATP only.
• Non-Cyclic Photophosphorylation: Produces ATP and NADPH.
2. Biosynthetic Phase (Dark Reactions):
• Takes place in the stroma of chloroplasts.
• Involves the Calvin Cycle (C3 pathway), where CO₂ is fixed into glucose using ATP and NADPH.

C3, C4, and CAM Pathways

• C3 Pathway: Calvin cycle; occurs in most plants; less efficient under high temperatures due to photorespiration.
• C4 Pathway: Found in plants like maize; minimizes photorespiration by spatial separation of carbon fixation and Calvin cycle.
• CAM Pathway: Found in desert plants like cacti; stomata open at night to fix CO₂, reducing water loss.

Chemiosmotic Hypothesis

• Explains ATP synthesis in chloroplasts during light reactions.
• Proton gradient across the thylakoid membrane drives ATP synthesis via ATP synthase.

Factors Affecting Photosynthesis

1. Light intensity and wavelength.
2. Carbon dioxide concentration.
3. Temperature (optimum range required).
4. Water availability.

2. Respiration

Respiration is the biological process by which energy is released from organic molecules like glucose.

Types of Respiration

1. Aerobic Respiration: Complete breakdown of glucose in the presence of oxygen.
2. Anaerobic Respiration (Fermentation): Partial breakdown of glucose without oxygen.

Stages of Cellular Respiration

1. Glycolysis:
• Occurs in the cytoplasm.
• Glucose is broken into 2 pyruvate molecules.
• Net gain: 2 ATP and 2 NADH.
2. Krebs Cycle (TCA Cycle):
• Occurs in mitochondrial matrix.
• Pyruvate is oxidized to CO₂, producing NADH, FADH₂, and GTP/ATP.
3. Electron Transport Chain (ETC):
• Occurs in the inner mitochondrial membrane.
• NADH and FADH₂ transfer electrons to generate a proton gradient for ATP synthesis.
• Produces ~34 ATP molecules per glucose molecule.

Energy Yield

• Aerobic respiration yields ~36-38 ATP per glucose molecule.
• Anaerobic respiration yields only 2 ATP per glucose molecule.

Amphibolic Pathways

• Respiration is amphibolic as it involves both catabolic (breakdown) and anabolic (biosynthetic) processes.

3. Plant Growth and Development

Plant growth refers to irreversible increase in size or mass, while development encompasses all changes leading to maturity.

Phases of Growth

1. Meristematic Phase:
• Active cell division occurs at root and shoot tips.
2. Elongation Phase:
• Cells elongate due to water uptake.
3. Maturation Phase:
• Cells differentiate into specific types for functional roles.

Conditions for Growth

1. Water availability.
2. Nutrient supply.
3. Temperature.
4. Light intensity.

Differentiation, Dedifferentiation, Redifferentiation

• Differentiation: Formation of specialized cells from meristematic cells (e.g., xylem).
• Dedifferentiation: Reversion of specialized cells back to meristematic state (e.g., cork cambium).
• Redifferentiation: Differentiated cells regain specialization for another function.

Sequence of Developmental Processes

1. Cell division → Cell elongation → Cell differentiation → Maturation → Senescence.

4. Plant Growth Regulators (PGRs)

PGRs are chemical substances that regulate growth and development in plants.

PGR	Functions
Auxins	Promotes cell elongation, apical dominance, root initiation; prevents leaf abscission (IAA).
Gibberellins	Stimulates stem elongation, seed germination, flowering (GA₃).
Cytokinins	Promotes cell division, delays senescence (kinetin).
Ethylene	Promotes fruit ripening, leaf abscission; inhibits stem elongation (ethylene gas).
Abscisic Acid (ABA)	Induces dormancy, closes stomata during water stress ("stress hormone").

Key Diagrams to Study

1. Structure of chloroplast showing thylakoids and stroma.
2. Glycolysis pathway with intermediate steps.
3. Krebs cycle with key products.
4. Electron transport chain mechanism.
5. Graph showing factors affecting photosynthesis.

Summary

This unit covers essential physiological processes in plants:

1. Photosynthesis as the basis for autotrophic nutrition and its biochemical pathways.
2. Cellular respiration as an energy-releasing process involving glycolysis, TCA cycle, and ETC.
3. Plant growth and development regulated by environmental conditions and growth regulators like auxins and gibberellins.