The C4 cycle, also known as the Hatch-Slack pathway, is a specialized carbon fixation process in certain plants where the first stable compound produced during carbon dioxide assimilation is a 4-carbon molecule called oxaloacetate.
Understanding the C4 Cycle
The C4 cycle is an adaptation evolved by plants in hot, arid environments to minimize photorespiration and efficiently fix carbon dioxide. Unlike C3 plants, C4 plants have a unique leaf anatomy that facilitates this process.
Key Features of the C4 Cycle:
- Spatial Separation: The C4 cycle involves two distinct cell types: mesophyll cells and bundle sheath cells.
- Initial CO2 Fixation: In mesophyll cells, carbon dioxide is initially fixed by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase) to form oxaloacetate (a 4-carbon compound).
- Conversion and Transport: Oxaloacetate is then converted to malate or aspartate and transported to the bundle sheath cells.
- Decarboxylation: In the bundle sheath cells, malate or aspartate is decarboxylated, releasing CO2.
- Calvin Cycle: The released CO2 is then fixed by RuBisCO in the Calvin cycle, which occurs within the bundle sheath cells.
- PEP Regeneration: The pyruvate produced during decarboxylation is transported back to the mesophyll cells, where it is converted back to PEP, regenerating the CO2 acceptor.
Steps of the C4 Cycle
| Step | Location | Enzyme Involved | Reaction |
|---|---|---|---|
| 1. CO2 Fixation | Mesophyll Cell | PEP Carboxylase | CO2 + PEP → Oxaloacetate |
| 2. Conversion | Mesophyll Cell | Malate Dehydrogenase/Transaminase | Oxaloacetate → Malate/Aspartate |
| 3. Transport | Mesophyll Cell to Bundle Sheath Cell | -- | Malate/Aspartate Transported to Bundle Sheath Cells |
| 4. Decarboxylation | Bundle Sheath Cell | NADP-Malic Enzyme/PEP Carboxykinase | Malate/Aspartate → CO2 + Pyruvate |
| 5. Calvin Cycle | Bundle Sheath Cell | RuBisCO | CO2 fixed in the Calvin cycle to produce sugars |
| 6. PEP Regeneration | Bundle Sheath Cell to Mesophyll Cell | Pyruvate Phosphate Dikinase | Pyruvate → PEP |
Benefits of the C4 Cycle:
- Reduced Photorespiration: By concentrating CO2 in the bundle sheath cells, the C4 cycle minimizes photorespiration, which is a wasteful process that occurs when RuBisCO binds to oxygen instead of carbon dioxide.
- Enhanced Water Use Efficiency: C4 plants can close their stomata more during the day, reducing water loss, because they can efficiently fix carbon dioxide even at low CO2 concentrations.
- Adaptation to Hot and Arid Climates: The C4 cycle is particularly advantageous in hot, arid environments where photorespiration rates are high and water is scarce.
Examples of C4 Plants:
Common examples of C4 plants include:
- Corn (maize)
- Sugarcane
- Sorghum
- Various grasses native to tropical and subtropical regions.
The C4 cycle is a critical adaptation that enhances photosynthetic efficiency in specific plant species, particularly those thriving in hot and dry environments, by minimizing photorespiration and improving water use efficiency.
