Structure of chloroplasts, light and dark reactions

1. Introduction to Photosynthesis

1. Photosynthesis is the process by which green plants synthesize organic compounds using light energy.
2. It occurs in the chloroplasts of plant cells.
3. The process involves the conversion of carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂).
4. The overall reaction is: 
   6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂
5. It consists of two major phases:
   • Light-dependent reactions (occur in thylakoids).
   • Light-independent reactions or Calvin cycle (occur in stroma).

2. Structure of Chloroplasts

1. Chloroplast is a double-membrane organelle found in plant cells.
2. Inside the chloroplast, there are thylakoids, which form stacks called grana.
3. The fluid-filled space around the grana is called the stroma, where dark reactions occur.
4. Thylakoid membranes contain chlorophyll, the green pigment responsible for capturing light.
5. Other pigments like carotenoids and xanthophylls help in light absorption.

3. Light Reactions (Light-Dependent Reactions)

1. Occurs in the thylakoid membranes.
2. Uses light energy to produce ATP and NADPH, which are used in the Calvin cycle.
3. Involves two photosystems:
   • Photosystem I (PSI) - absorbs light at 700 nm.
   • Photosystem II (PSII) - absorbs light at 680 nm.
4. Key steps in the light reaction:
   • Photolysis of water: Water splits into oxygen, protons, and electrons.
   • Electron transport chain (ETC): Electrons move through proteins, generating ATP.
   • Formation of ATP by chemiosmosis (via ATP synthase enzyme).
   • Formation of NADPH by the reduction of NADP⁺.
5. Oxygen is released as a byproduct.
6. There are two types of photophosphorylation:
   • Cyclic photophosphorylation - Produces ATP only.
   • Non-cyclic photophosphorylation - Produces both ATP and NADPH.

4. Dark Reactions (Light-Independent Reactions / Calvin Cycle)

1. Occurs in the stroma of the chloroplast.
2. Does not require light directly but depends on ATP and NADPH from the light reaction.
3. Fixes carbon dioxide (CO₂) into organic molecules.
4. Consists of three phases:
   • Carbon fixation: CO₂ is fixed into a 3-carbon compound using the enzyme RuBisCO.
   • Reduction: ATP and NADPH convert the 3-carbon compound into G3P (glyceraldehyde-3-phosphate).
   • Regeneration: Some G3P molecules regenerate RuBP, allowing the cycle to continue.
5. Six turns of the Calvin cycle produce one glucose molecule.

5. Factors Affecting Photosynthesis

1. Light intensity: More light increases the rate of photosynthesis until a saturation point is reached.
2. Carbon dioxide concentration: Higher CO₂ levels enhance the Calvin cycle.
3. Temperature: Optimal temperature is required for enzyme function.
4. Water availability: Water is essential for photolysis in the light reaction.
5. Chlorophyll concentration: More chlorophyll enhances light absorption.

6. Importance of Photosynthesis

1. Produces oxygen, essential for life on Earth.
2. Forms the basis of the food chain as plants store energy in organic molecules.
3. Reduces atmospheric CO₂, helping to balance global carbon levels.
4. Provides raw materials for plant growth and energy storage.

7. Conclusion

1. Photosynthesis is the process by which plants convert light energy into chemical energy.
2. It occurs in chloroplasts and involves light-dependent and light-independent reactions.
3. It is essential for oxygen production, energy storage, and carbon fixation.