Photosynthesis

1. Definition:

• Photosynthesis is the process by which green plants convert light energy into chemical energy, forming carbohydrates from CO₂ and H₂O.
• It is the most fundamental process supporting life on Earth.
• Equation: 6CO₂ + 12H₂O →   chlorophyll + light  → C₆H₁₂O6 + 6O₂ + 6H₂O

2. Site of Photosynthesis

• Occurs in chloroplasts of mesophyll cells in leaves.
• Chloroplast contains chlorophyll pigments which absorb light energy.

3. Photosynthetic Pigments

• Chlorophyll a – Primary pigment, essential for light absorption.
• Chlorophyll b – Accessory pigment, expands wavelength range.
• Carotenoids (carotene, xanthophyll) – Protect chlorophyll from photo-oxidation.
• Pheophytin – Acts as an electron carrier.

Absorption Peaks:

• Chlorophyll a – Blue (430 nm) and Red (662 nm)
• Chlorophyll b – Blue (453 nm) and Red (642 nm)

4. Two Phases of Photosynthesis

1. Light Reaction (Photochemical phase)
2. Dark Reaction (Biochemical phase / Carbon fixation phase)

🌞 Light Reaction

• Takes place in the thylakoid membranes of chloroplast.
• Involves absorption of light energy, splitting of water, and formation of ATP & NADPH.

Steps:

• Light absorption by chlorophyll molecules in Photosystem I (PSI) and Photosystem II (PSII).
• Photolysis of water – Splitting of H₂O into H⁺, e⁻, and O₂ (in PSII): 2H2O→4H++4e−+O22H_2O → 4H^+ + 4e^- + O_22H2​O→4H++4e−+O2​
• Electron transport chain (ETC): Electrons pass through plastoquinone → cytochrome b₆f → plastocyanin → PSI → ferredoxin.
• ATP formation – via photophosphorylation.

1. • Non-cyclic photophosphorylation: Involves both PS I & PS II; produces ATP, NADPH, and O₂.
   • Cyclic photophosphorylation: Involves only PS I; produces only ATP.

Products of Light Reaction:
→ ATP + NADPH + O₂

🌿 Dark Reaction (Carbon Fixation Reaction)

• Occurs in the stroma of chloroplast.
• Utilizes ATP and NADPH to fix CO₂ into carbohydrates.
• It does not require direct light, but depends on light-produced products.

Types of Carbon Fixation Pathways:

• i) C₃ Cycle (Calvin Cycle) – Occurs in most plants (rice, wheat, soybean).
• ii) C₄ Cycle (Hatch and Slack Pathway) – Occurs in tropical grasses (maize, sorghum, sugarcane).
• iii) CAM Pathway (Crassulacean Acid Metabolism) – Found in succulents (pineapple, cactus).

i) C₃ Pathway (Calvin Cycle)

• Discovered by Melvin Calvin and Benson (1948–1954).
• First stable product: 3-phosphoglyceric acid (3-PGA).
• Enzyme: Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).

Three Stages:

1. Carboxylation: CO₂ fixed with RuBP to form 3-PGA.
2. Reduction: 3-PGA reduced to glyceraldehyde-3-phosphate (G3P) using ATP & NADPH.

• Regeneration: RuBP regenerated for next cycle.

ii) C₄ Pathway (Hatch and Slack Pathway)

• First stable product: Oxaloacetic acid (OAA) (4 carbon).
• Occurs in plants with Kranz anatomy (bundle sheath cells + mesophyll cells).
• Key enzyme: PEP carboxylase (phosphoenolpyruvate carboxylase).

Advantages:

• High photosynthetic efficiency.
• Less photorespiration.
• Efficient under high temperature and light.

Examples: Maize, Sugarcane, Sorghum, Pearl millet.

iii) CAM Pathway (Crassulacean Acid Metabolism)

• Found in xerophytes (e.g., cactus, pineapple).
• Stomata open at night and CO₂ fixed into malic acid.
• During the day, CO₂ is released internally and used for photosynthesis.
• Adaptation to arid conditions for water conservation.

Factors Affecting Photosynthesis

• Light intensity and quality – Increases rate up to saturation point.
• CO₂ concentration – Optimum 0.03–0.04% for C₃, higher for C₄ plants.
• Temperature – Optimum 25–30°C (C₃) and 30–40°C (C₄).
• Water – Required for photolysis; deficiency reduces rate.
• Leaf age and chlorophyll content – Young leaves are more active.

Photorespiration

• Occurs in C₃ plants due to oxygenase activity of RuBisCO.
• Consumes O₂ and releases CO₂ without producing ATP or sugars.
• Reduces photosynthetic efficiency.
• Absent in C₄ plants.