Number of carboxylation in `C_4` cycle is

Options

A. four
B. three
C. two
D. one

Answer

**Step-by-Step Solution:**

1. **Understanding the C4 Cycle**: The C4 cycle, also known as the Hatch-Slack pathway, is a photosynthetic process that occurs in certain plants (C4 plants) to efficiently fix carbon dioxide (CO2) in conditions where it may be less available due to high temperatures or light intensities.

2. **Initial Carboxylation**: In the C4 cycle, the first step involves the carboxylation of phosphoenolpyruvate (PEP), a 3-carbon compound. This reaction is catalyzed by the enzyme PEP carboxylase. The CO2 combines with PEP to form oxaloacetic acid (OAA), which is a 4-carbon compound.

3. **Second Carboxylation**: The oxaloacetic acid (OAA) can then be converted into malate or aspartate, both of which are also 4-carbon compounds. When these compounds are transported to the bundle sheath cells, they undergo decarboxylation, releasing CO2.

4. **Conclusion**: The C4 cycle involves two distinct carboxylation reactions:
- The first carboxylation occurs when CO2 is fixed to PEP to form OAA.
- The second carboxylation occurs when OAA is converted into malate or aspartate, which ultimately releases CO2 for the Calvin cycle.

5. **Final Answer**: Therefore, the number of carboxylation reactions in the C4 cycle is **2**.

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