3.1 Photosynthesis

Definition

• Process by which green plants and some other organisms use sunlight to synthesize foods.
• Uses chlorophyll, carbon dioxide, and water.
• Converts light energy into chemical energy.

Basic Process

• Plants take in CO₂ from atmosphere and water from soil.
• Using sunlight and chlorophyll, they convert these into glucose (sugar) and oxygen.
• Word Equation: Carbon Dioxide + Water + Light Energy (Chlorophyll) → Glucose + Oxygen

Factors Affecting Photosynthesis

1. Light: Provides energy; rate increases with intensity.
2. Carbon Dioxide (CO₂): Raw material; increased concentration generally increases rate.
3. Water: Raw material; shortage can reduce rate.
4. Chlorophyll: Green pigment; absorbs light energy; essential for the process.

Significance of Photosynthesis

• Food Production: Primary source of food for almost all life on Earth.
• Oxygen Release: Releases oxygen into the atmosphere, essential for respiration.
• Carbon Cycle: Helps maintain balance of CO₂ and O₂ in the atmosphere.

Experiment to Demonstrate Photosynthesis (Necessity of Light)

• Aim: Show sunlight is necessary for photosynthesis.
• Procedure:
  1. Destarch potted plant (dark room 2-3 days).
  2. Cover part of a leaf with black paper/foil.
  3. Expose plant to sunlight (6-8 hours).
  4. Pluck leaf, remove cover.
  5. Boil leaf in water, then alcohol (to remove chlorophyll).
  6. Wash, then dip in dilute iodine solution.
• Observation: Exposed part turns blue-black (starch present); covered part remains yellowish-brown (no starch).
• Conclusion: Sunlight is necessary for photosynthesis to produce starch.