How many PGAL molecules would regenerate 15 RuBP?

Options

A. 30
B. 25
C. 15
D. 20

Explanation

To determine how many PGAL (phosphoglyceraldehyde) molecules would regenerate 15 RuBP (ribulose bisphosphate), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Calvin Cycle**: The Calvin cycle is a series of biochemical reactions in photosynthesis where carbon dioxide is fixed and reduced to form glucose and other carbohydrates. During this cycle, RuBP is regenerated.

2. **Identifying the Relationship**: In the Calvin cycle, for every 3 molecules of CO2 that enter the cycle, 6 molecules of PGAL are produced. However, only 1 PGAL molecule is used to form glucose, while the remaining 5 PGAL molecules are used to regenerate 3 RuBP molecules.

3. **Calculating PGAL for RuBP Regeneration**: Since 5 PGAL molecules are required to regenerate 3 RuBP molecules, we can set up a proportion to find out how many PGAL molecules are needed to regenerate 15 RuBP molecules.

4. **Setting Up the Proportion**:
- If 5 PGAL → 3 RuBP
- Then to find PGAL for 15 RuBP, we can use the ratio:
\[ \text{PGAL required} = \left(\frac{5 \text{ PGAL}}{3 \text{ RuBP}}\right) \times 15 \text{ RuBP} \]

5. **Calculating the Number of PGAL**:
\[ \text{PGAL required} = \left(\frac{5}{3}\right) \times 15 = 25 \text{ PGAL} \]

6. **Final Answer**: Therefore, to regenerate 15 RuBP molecules, 25 PGAL molecules are required.

### Conclusion:
The answer to the question is **25 PGAL molecules**.