Plant Physiology

Plant physiology is the branch of biology that deals with the functioning of different physiological processes in plants. It includes essential life processes like water transport, mineral nutrition, photosynthesis, respiration, and plant growth & development. Understanding these processes helps in comprehending how plants grow, develop, and respond to environmental conditions.

Transport in Plants

Plants absorb water and minerals from the soil and transport them to different parts through specialized tissues.

1. Means of Transport

• Diffusion: Passive movement of molecules from a higher to a lower concentration.

• Osmosis: Diffusion of water through a semi-permeable membrane.

• Facilitated Transport: Transport through specific proteins without energy expenditure.

• Active Transport: Movement of molecules against the concentration gradient, requiring ATP.

2. Water Absorption and Ascent of Sap

• Root Pressure: Pushes water upward due to active ion transport in roots.

• Capillary Action: Water rises due to surface tension and adhesion in narrow xylem vessels.

• Cohesion-Tension Theory: Water moves up through xylem due to transpiration pull and cohesive forces.

3. Transpiration

• Loss of water in vapor form from aerial parts of plants, mainly through stomata.

• Helps in cooling, nutrient transport, and maintaining water balance.

4. Phloem Transport – Translocation of Food

• Organic solutes (sucrose, amino acids) are transported through phloem from source (leaves) to sink (storage or growing organs).

• Pressure Flow Hypothesis explains this movement.

Mineral Nutrition

Plants require various minerals for proper growth and metabolism.

1. Essential Elements

• Macronutrients: Required in large quantities (C, H, O, N, P, K, Ca, Mg, S).

• Micronutrients: Required in small quantities (Fe, Mn, Zn, Cu, Mo, B, Cl, Ni).

2. Role of Minerals in Plant Growth

• Nitrogen (N): Essential for amino acids, proteins, and chlorophyll.

• Phosphorus (P): Component of ATP, DNA, and phospholipids.

• Potassium (K): Regulates stomatal movement and enzyme activation.

3. Nitrogen Metabolism

• Plants absorb nitrogen as nitrate (NO₃⁻) or ammonium (NH₄⁺).

• Nitrogen fixation: Conversion of atmospheric nitrogen into ammonia by bacteria like Rhizobium.

• Nitrification: Ammonia is converted into nitrites and then nitrates.

• Denitrification: Nitrates are converted back into nitrogen gas by bacteria.

Photosynthesis in Higher Plants

Photosynthesis is the process by which plants synthesize organic food using sunlight.

1. Pigments Involved in Photosynthesis

• Chlorophyll a & b: Main pigments absorbing light.

• Carotenoids & Xanthophylls: Accessory pigments.

2. Light Reactions (Photochemical Phase)

• Occurs in the thylakoid membranes of chloroplasts.

• Involves photosystem I (PSI) & photosystem II (PSII).

• ATP and NADPH are formed through cyclic and non-cyclic photophosphorylation.

3. Dark Reactions (Biosynthetic Phase)

• Occurs in the stroma of chloroplasts.

• Calvin Cycle (C₃ Pathway): Fixes CO₂ using RuBisCO enzyme.

• C₄ Pathway: Found in tropical plants; prevents photorespiration.

4. Factors Affecting Photosynthesis

• Light intensity, CO₂ concentration, temperature, and water availability influence the rate of photosynthesis.

Respiration in Plants

Respiration is the process by which stored food is broken down to release energy.

1. Types of Respiration

• Aerobic Respiration: Requires oxygen, occurs in mitochondria, produces ATP.

• Anaerobic Respiration: Occurs in absence of oxygen, produces alcohol/lactic acid.

2. Glycolysis (EMP Pathway)

• Takes place in the cytoplasm.

• Converts glucose into pyruvate.

3. Krebs Cycle (TCA Cycle)

• Occurs in mitochondria.

• Generates NADH and FADH₂ for ATP production.

4. Electron Transport Chain (ETC)

• Located in the inner mitochondrial membrane.

• Produces ATP via oxidative phosphorylation.

5. Respiratory Quotient (RQ)

• Ratio of CO₂ released to O₂ consumed during respiration.

• RQ varies based on the type of substrate used (carbohydrates, proteins, fats).

Plant Growth and Development

Plants grow throughout their life due to meristematic activity. Growth and development are regulated by internal and external factors.

1. Phases of Growth

• Meristematic Phase: Active cell division occurs.

• Elongation Phase: Cells increase in size.

• Maturation Phase: Cells attain functional maturity.

2. Plant Growth Regulators (PGRs)

• Auxins: Promote cell elongation, root initiation, and fruit growth.

• Gibberellins: Stimulate stem elongation, seed germination, and flowering.

• Cytokinins: Promote cell division and delay senescence.

• Abscisic Acid (ABA): Induces dormancy and stress responses.

• Ethylene: Involved in fruit ripening and leaf abscission.

3. Photoperiodism and Vernalization

• Photoperiodism: Effect of light duration on flowering.

• Vernalization: Cold treatment required for flowering in some plants.

This chapter is essential for NEET as it explains the fundamental physiological processes in plants that sustain life. Understanding these concepts helps in better grasping plant metabolism and adaptation strategies.