The primary `CO_2` acceptor in `CO_2` fixation in `C_4` pathway is

Options

A. oxaloacetic acid
B. citric acid
C. RuBP
D. Phosphoenol pyruvate

Answer

**Step-by-Step Solution:**

1. **Understanding the C4 Pathway**: The C4 pathway is a method of carbon fixation used by certain plants, known as C4 plants, during photosynthesis. This process allows these plants to efficiently fix carbon dioxide (CO2) in conditions where it may be limited.

2. **Location of CO2 Fixation**: In C4 plants, CO2 fixation occurs in two types of cells: mesophyll cells and bundle sheath cells. The initial fixation occurs in the mesophyll cells.

3. **Identifying the Primary CO2 Acceptor**: In the mesophyll cells, the primary CO2 acceptor is a three-carbon molecule called phosphoenolpyruvate (PEP).

4. **Role of Enzymes**: The reaction where PEP accepts CO2 is catalyzed by the enzyme phosphoenolpyruvate carboxylase (PEPCase). This enzyme facilitates the conversion of PEP and CO2 into a four-carbon compound.

5. **Formation of the First Stable Product**: The product formed from this reaction is oxaloacetic acid (OAA), which is a four-carbon compound. This is significant as it is the first stable product in the C4 pathway, hence the name "C4 plants."

6. **Conclusion**: Therefore, the primary CO2 acceptor in the CO2 fixation process of the C4 pathway is phosphoenolpyruvate (PEP).

**Final Answer**: The primary CO2 acceptor in CO2 fixation in the C4 pathway is phosphoenolpyruvate (PEP).

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