Three carbons of phosphoglyceric acid formed during carbon fixation are derived from

Options

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

Answer

To solve the question regarding the origin of the three carbons in phosphoglyceric acid formed during carbon fixation, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Context**: The question pertains to the process of carbon fixation in photosynthesis, specifically within the Calvin cycle (also known as the C3 cycle).

2. **Identify Key Components**: The main components involved in carbon fixation are ribulose 1,5-bisphosphate (RUBP) and carbon dioxide (CO2).

3. **Recognize the First Stable Product**: In the Calvin cycle, the first stable compound formed after the fixation of CO2 is phosphoglyceric acid (PGA), which is a 3-carbon compound.

4. **Analyze the Reaction**: When CO2 is fixed, it combines with RUBP (a 5-carbon compound). This reaction is catalyzed by the enzyme RuBisCO. The combination of 1 carbon from CO2 with the 5 carbons from RUBP results in a 6-carbon intermediate.

5. **Formation of PGA**: This 6-carbon compound is unstable and quickly splits into two molecules of phosphoglyceric acid (PGA), each containing 3 carbons.

6. **Conclusion**: Therefore, the three carbons in phosphoglyceric acid are derived from the combination of carbon dioxide and ribulose 1,5-bisphosphate (RUBP).

7. **Final Answer**: The correct option is D: RUBP plus carbon dioxide.