Class 12th Biology Plant Physiology Notes

🌱 Introduction

Plant physiology deals with how plants function, including processes like photosynthesis, respiration, transpiration, and transport of nutrients. These processes help plants grow, develop, and survive.

☀️ Photosynthesis – The Life Process of Plants

👉 Definition: The process by which green plants make their own food using sunlight, carbon dioxide, and water.

🟢 Equation:

\text{6CO}_2 + \text{6H}_2O + \text{Light Energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + \text{6O}_2

👉 Steps of Photosynthesis:
1️⃣ Light Reaction (in Grana of chloroplast) – Uses sunlight to split water (photolysis) and release oxygen.
2️⃣ Dark Reaction (in Stroma of chloroplast) – Uses ATP and NADPH to convert CO₂ into glucose (Calvin Cycle).

📝 Key Concept: Chlorophyll absorbs light, and stomata regulate gas exchange.

🌬️ Respiration in Plants

👉 Definition: The process of breaking down glucose to release energy (ATP).

Types of Respiration:

• Aerobic Respiration (with oxygen) – Produces more energy (ATP).
  • Equation:

    \text{C}_6\text{H}_{12}\text{O}_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)}

📝 Key Concept: Plants respire day and night, but photosynthesis occurs only in the presence of sunlight.

💦 Transpiration – Water Loss in Plants

👉 Definition: The process by which plants lose water vapour through stomata.

Types of Transpiration:

• Stomatal Transpiration – Through stomata (90%).
• Cuticular Transpiration – Through waxy cuticle.
• Lenticular Transpiration – Through lenticels in bark.

📝 Key Concept: Transpiration creates a pulling force (transpiration pull) that helps water move up the plant.

🌊 Transport in Plants

👉 Function: Movement of water, minerals, and food throughout the plant.

🔹 Xylem – Water Transport

• Moves water and minerals from roots to leaves.
• Works by capillary action, root pressure, and transpiration pull.

🔹 Phloem – Food Transport

• Moves food (sugar, nutrients) from leaves to other parts (translocation).
• Uses active transport (energy required).

📝 Key Concept: Xylem moves only upward, while phloem moves both up and down.

🧪 Plant Hormones – Growth Regulators

👉 Function: Control growth, development, and responses to stimuli.

Types of Plant Hormones:

• Auxins – Promote cell elongation (root and stem growth).
• Gibberellins – Help in stem elongation, flowering, seed germination.
• Cytokinins – Promote cell division and delay aging in leaves.
• Abscisic Acid (ABA) – Inhibits growth, causes leaf fall.
• Ethylene – Helps in fruit ripening.

📝 Key Concept: Auxins, gibberellins, and cytokinins promote growth, while ABA and ethylene slow it down.

🌱 Tropisms – Plant Movements

Plants respond to stimuli like light, gravity, water, and touch.

Types of Tropisms:

• Phototropism – Growth towards light (shoots grow toward light).
• Geotropism – Growth in response to gravity (roots grow downward).
• Hydrotropism – Growth towards water (roots move toward moisture).
• Thigmotropism – Growth in response to touch (climbing plants).

📝 Key Concept: Positive tropism – growth toward stimulus; Negative tropism – growth away from stimulus.

💡 Key Points to Remember

✅ Photosynthesis provides food and oxygen.
✅ Respiration occurs all the time, while photosynthesis happens only in daylight.
✅ Transpiration helps in water movement and cooling.
✅ Xylem carries water; phloem transports food.
✅ Plant hormones regulate growth and responses.
✅ Tropisms help plants respond to environmental changes.