In case of `C_(4)`-pathway

Options

A. `CO_(2)` combines with `PGA`
B. `CO_(2)` combines with `PEP`
C. `CO_(2)` first combines with `RuBP`
D. `CO_(2)` combines with `RMP`.

Answer

### Step-by-Step Solution:

1. **Understanding the C4 Pathway**: The C4 pathway, also known as the Hatch-Slack pathway, is a process of photosynthesis that occurs in certain plants, known as C4 plants. This pathway is characterized by the fixation of carbon dioxide (CO2) into a four-carbon compound.

2. **Initial Reaction**: In the first step of the C4 pathway, carbon dioxide (CO2) combines with phosphoenolpyruvate (PEP), which is a three-carbon compound. This reaction occurs in the mesophyll cells of the plant.

3. **Enzyme Involvement**: The enzyme that catalyzes this reaction is called phosphoenolpyruvate carboxylase (PEP carboxylase). This enzyme facilitates the combination of CO2 with PEP.

4. **Formation of Four-Carbon Compound**: The reaction between CO2 and PEP results in the formation of oxaloacetic acid (OAA), which is a four-carbon compound. This is a key step in the C4 pathway.

5. **Evaluating Options**: Based on the information:
- **CO2 combines with phosphoglyceraldehyde**: Incorrect, as this is not part of the C4 pathway.
- **CO2 combines with PEP**: Correct, as this is the first step in the C4 pathway.
- **CO2 first combines with RUBP**: Incorrect, this is characteristic of the C3 pathway.
- **CO2 combines with RMP**: Incorrect, as RMP is not involved in the C4 pathway.

6. **Conclusion**: The correct answer is that CO2 combines with phosphoenolpyruvate (PEP) in the C4 pathway.