In `C_4` pathway, the `CO_2`-fixation in mesophyll cells is carried out by the enzyme

Options

A. pyruvate dehydrogenase
B. pyruvate decarboxylase
C. PEP carboxylase
D. RuBisCO

Answer

### Step-by-Step Solution:

1. **Understanding the C4 Pathway**: The C4 pathway is a photosynthetic process that occurs in certain plants, allowing them to efficiently fix carbon dioxide (CO2) in conditions where it might otherwise be lost through photorespiration.

2. **Location of CO2 Fixation**: In C4 plants, the initial fixation of CO2 occurs in the mesophyll cells. This is a crucial step in the pathway.

3. **Identifying the Enzyme**: The enzyme responsible for the fixation of CO2 in the mesophyll cells of C4 plants is **PEP carboxylase**. This enzyme catalyzes the reaction where CO2 is combined with phosphoenolpyruvate (PEP) to form oxaloacetate.

4. **Understanding the Products**: The first stable product formed in the C4 pathway is oxaloacetate, which is then converted into other compounds that eventually lead to the production of glucose.

5. **Comparison with C3 Pathway**: In contrast to the C4 pathway, the C3 pathway fixes CO2 directly in the mesophyll cells using the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco). However, C4 plants have evolved to minimize photorespiration, making them more efficient under certain environmental conditions.

6. **Conclusion**: Therefore, the enzyme responsible for CO2 fixation in mesophyll cells during the C4 pathway is **PEP carboxylase**.

### Final Answer:
In the C4 pathway, the CO2 fixation in mesophyll cells is carried out by the enzyme **PEP carboxylase**.

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