During the dark reaction, 3 - PGA is derived from

Options

A. RuBP only
B. `CO_2` only
C. RuBP + `CO_2`
D. RuBP + `CO_2`+PEP

Answer

To solve the question "During the dark reaction, 3-PGA is derived from," we need to analyze the process of the dark reaction (Calvin cycle) in photosynthesis step by step.

### Step-by-Step Solution:

1. **Understanding the Dark Reaction**: The dark reactions, also known as the Calvin cycle, occur in the stroma of chloroplasts and do not require light directly. They utilize the products of the light reactions (ATP and NADPH) to convert carbon dioxide into glucose.

2. **Key Components**: The main components involved in the dark reaction are:
- **Ribulose bisphosphate (RuBP)**: A 5-carbon sugar that is the starting molecule for the Calvin cycle.
- **Carbon dioxide (CO2)**: The gas that is fixed during the dark reaction.

3. **Carboxylation Process**: The first step in the dark reaction is the carboxylation of RuBP. This reaction is catalyzed by the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase).

4. **Formation of 3-PGA**: When CO2 is added to RuBP, it forms a 6-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate (3-PGA), which is a 3-carbon compound.

5. **Conclusion**: Therefore, 3-PGA is derived from the combination of RuBP and CO2 during the dark reaction.

### Final Answer:
The correct option is **RUBP plus CO2** (Option 3).

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