Describe the mechanism of 'C4 cycle' (Hatch and Slack pathway) in plants.

The C4 cycle (Hatch–Slack pathway) is a photosynthetic carbon fixation pathway that evolved in certain plants (e.g., maize, sugarcane) to minimize photorespiration and improve efficiency in hot, dry climates. It operates alongside the Calvin cycle (C3 pathway) and involves two distinct cell types: mesophyll cells (outer layer) and bundle sheath cells (inner layer).

Mechanism of the C4 Cycle

1. Initial CO₂ Fixation in Mesophyll Cells

   • CO₂ is first fixed into a 4-carbon compound (oxaloacetate) using phosphoenolpyruvate (PEP) and the enzyme PEP carboxylase (highly efficient, no oxygenase activity).
   • Oxaloacetate is rapidly converted to malate (or aspartate in some plants) by malate dehydrogenase.

2. Transport to Bundle Sheath Cells

   • Malate (or aspartate) is transported to the bundle sheath cells via plasmodesmata.

3. Decarboxylation and CO₂ Release

   • In bundle sheath cells, malate is decarboxylated by NADP-malic enzyme (or other decarboxylases), releasing CO₂ and regenerating pyruvate.
   • The released CO₂ enters the Calvin cycle (C3 pathway) in bundle sheath cells, where it is reduced to 3-phosphoglycerate (3-PGA) and eventually glucose.

4. Regeneration of PEP

   • Pyruvate is transported back to mesophyll cells and converted back to PEP by PEP carboxykinase (ATP-dependent), completing the cycle.

Advantages of the C4 Pathway

• Reduces photorespiration by concentrating CO₂ in bundle sheath cells (O₂ inhibition is minimized).
• Increases photosynthetic efficiency in high light and temperature conditions.
• Enhances water-use efficiency due to reduced stomatal opening.

The C4 cycle ensures that CO₂ is supplied efficiently to the Calvin cycle, making it ideal for plants in arid environments. The spatial separation of initial CO₂ fixation and the Calvin cycle is a key innovation in C4 plants.