During dark reaction for fixation of carbon, the three carbon atoms of each molecule of 3-phosphoglyceric acid (PGA) are derived from

Options

A. RuBP Only
B. RuBP + `CO_(2)`
C. `CO_(2)` only
D. RuBP + `CO_(2)` + PEP

Answer

To answer the question regarding the source of the three carbon atoms in each molecule of 3-phosphoglyceric acid (PGA) during the dark reactions of photosynthesis, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Dark Reactions**:
- The dark reactions, also known as the Calvin cycle, occur in the stroma of chloroplasts and are responsible for the fixation of carbon dioxide into organic molecules.

2. **Identifying Key Components**:
- The main components involved in the Calvin cycle are Ribulose 1,5-bisphosphate (RUBP), carbon dioxide (CO2), and the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco).

3. **The Reaction Process**:
- In the Calvin cycle, RUBP (a 5-carbon compound) reacts with carbon dioxide (a 1-carbon molecule) in a reaction catalyzed by rubisco.
- This reaction produces an unstable 6-carbon intermediate that quickly splits into two molecules of 3-phosphoglyceric acid (PGA), each containing three carbon atoms.

4. **Determining the Source of Carbon Atoms**:
- Each molecule of PGA contains three carbon atoms. These carbon atoms come from the combination of the 5-carbon RUBP and the 1-carbon CO2.
- Thus, the three carbon atoms in PGA are derived from both RUBP and CO2.

5. **Conclusion**:
- Therefore, the correct answer to the question is that the three carbon atoms of each molecule of 3-phosphoglyceric acid are derived from RUBP plus carbon dioxide.

### Final Answer:
**The three carbon atoms of each molecule of 3-phosphoglyceric acid (PGA) are derived from RUBP plus carbon dioxide.**

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