4.1 Photosynthesis in Higher Plants

The Basics

• Equation: 6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂
• Site: Chloroplasts (in mesophyll cells).
• Phases:
  1. Photochemical Phase (Light Reactions): Thylakoid membranes.
  2. Biosynthetic Phase (Calvin Cycle): Stroma.

Photochemical Phase (Light Reactions)

• Photosystems: PS I (P700) and PS II (P680).
• Photophosphorylation: ATP synthesis using light energy.
  • Non-Cyclic: Involves PS I and PS II; produces ATP, NADPH, and O₂.
  • Cyclic: Involves only PS I; produces only ATP.
• Chemiosmotic Hypothesis: Explains ATP synthesis via a proton gradient.

Biosynthetic Phase (Calvin Cycle / C3 Cycle)

1. Carboxylation: CO₂ is fixed to RuBP by the enzyme RuBisCO.
2. Reduction: 3-PGA is reduced to G3P using ATP and NADPH.
3. Regeneration: RuBP is regenerated from G3P using ATP.

Photorespiration

• A wasteful process in C3 plants where RuBisCO binds to O₂ instead of CO₂.
• Occurs in low CO₂ and high O₂ conditions.
• Reduces the efficiency of photosynthesis.

The C4 Pathway (Hatch-Slack Pathway)

• An adaptation to hot, dry climates to minimize photorespiration.
• Kranz Anatomy: Specialized leaf structure with bundle sheath cells.
• Process:
  1. CO₂ is first fixed into a 4-carbon compound in mesophyll cells (by PEP carboxylase).
  2. This compound is transported to bundle sheath cells, where CO₂ is released and enters the Calvin cycle.

C3 vs. C4 Plants

Factors Affecting Photosynthesis

• Blackman's Law of Limiting Factors: The rate is limited by the factor in shortest supply.
• Key Factors:
  • Light Intensity
  • Carbon Dioxide Concentration
  • Temperature
  • Water