Which of the following isomeric reaction occurs in regeneration phase
I) Ribose 5 -phosphate `rarr` Ribulose 5 - phosphate
II) Xylulose 5 - phosphate `rarr` Ribose 5 phosphate
III) GAP `rarr` DHAP
IV) DHAP `rarr` GAP
IV) DHAP rarr GAP

Options

A. I & III
B. II & III
C. III & IV
D. II & IV

Answer

To determine which isomeric reactions occur in the regeneration phase of photosynthesis, we will analyze each of the given reactions step by step.

### Step-by-Step Solution:

1. **Identify the Reactions**:
- I) Ribose 5-phosphate → Ribulose 5-phosphate
- II) Xylulose 5-phosphate → Ribose 5-phosphate
- III) GAP (Glyceraldehyde 3-phosphate) → DHAP (Dihydroxyacetone phosphate)
- IV) DHAP → GAP

2. **Analyze Each Reaction**:
- **Reaction I**: Ribose 5-phosphate to Ribulose 5-phosphate is an isomerization reaction. This reaction is part of the regeneration phase of the Calvin cycle.
- **Reaction II**: Xylulose 5-phosphate to Ribose 5-phosphate is not an isomerization reaction; it involves a change in the carbon skeleton and does not fit the criteria for isomerization.
- **Reaction III**: GAP to DHAP is an isomerization reaction. This reaction also occurs during the regeneration phase.
- **Reaction IV**: DHAP to GAP is essentially the reverse of Reaction III and is also an isomerization reaction. However, since it is the same reaction written differently, it does not need to be counted again.

3. **Conclusion**:
- The isomeric reactions that occur in the regeneration phase are:
- I) Ribose 5-phosphate → Ribulose 5-phosphate
- III) GAP → DHAP (and its reverse, DHAP → GAP, is the same reaction).
- Therefore, the correct answer is that the isomeric reactions occurring in the regeneration phase are I and III.

### Final Answer:
The isomeric reactions that occur in the regeneration phase are:
- I) Ribose 5-phosphate → Ribulose 5-phosphate
- III) GAP → DHAP

Thus, the answer is **Option I and III**.