The first step of `CO_2` fixation in Hatch and Slack's pathway in plants is

Options

A. Formation of O.A.A by carboxylation of PEP in bundle sheath cells
B. Formation of O.A.A by the carboxylation of RUBP in mesophyll cells
C. Formation of PGA in mesophyll cells
D. Formation of O.A.A by carboxylation of PEP in mesophyll cells.

Answer

**Step-by-Step Solution:**

1. **Understanding the Context**: The question pertains to the Hatch and Slack pathway, which is a type of C4 photosynthesis primarily found in certain plants, particularly those adapted to arid environments.

2. **Identifying the First Step of CO2 Fixation**: In the C4 pathway, the first step of CO2 fixation involves the conversion of carbon dioxide into a four-carbon compound.

3. **Recognizing Key Molecules**: The primary molecule involved in this initial step is phosphoenolpyruvate (PEP), a three-carbon compound that acts as the primary CO2 acceptor.

4. **Carboxylation Reaction**: The carboxylation of PEP occurs in the mesophyll cells of the plant. This reaction is catalyzed by the enzyme PEP carboxylase.

5. **Formation of OAA**: The reaction results in the formation of oxaloacetic acid (OAA), which is the first stable product formed during CO2 fixation in the Hatch and Slack pathway.

6. **Selecting the Correct Option**: Based on the above steps, the correct answer to the question is "formation of OAA by carboxylation of PEP in mesophyll cells."

**Final Answer**: The first step of CO2 fixation in Hatch and Slack's pathway in plants is the formation of OAA by carboxylation of PEP in mesophyll cells.

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