In respiration, substrate level phosphorylatin (direct ATP synthesis) occurs in the reaction

Options

A. Dihydroxy acetone 3-P `rarr` Glyceraldehyde 3-P
B. 3-Phosphoglyceric acid `rarr` 2-Phospho-glyceric acid
C. 1, 3-bishosphoglyceric acid `rarr` 3-Phospho-glyceric acid
D. All the above

Answer

To solve the question regarding substrate level phosphorylation in respiration, we will analyze each option provided in the question step by step.

### Step-by-Step Solution:

1. **Understanding Substrate Level Phosphorylation**:
- Substrate level phosphorylation refers to the direct synthesis of ATP from ADP and a phosphorylated intermediate. This process occurs during specific reactions in glycolysis and the citric acid cycle.

2. **Analyzing Each Option**:
- **Option 1**: Dihydroxyacetone 3-phosphate to Glyceraldehyde 3-phosphate.
- This reaction does not produce ATP. It is an isomerization reaction and does not involve substrate level phosphorylation.
- **Conclusion**: Incorrect.

- **Option 2**: 3-Phosphoglyceric acid to 2-Phosphoglyceric acid.
- This reaction is also not associated with ATP production. It is a conversion step that does not involve substrate level phosphorylation.
- **Conclusion**: Incorrect.

- **Option 3**: 1,3-Bisphosphoglyceric acid to 3-Phosphoglyceric acid.
- This reaction is crucial because it involves the transfer of a phosphate group to ADP, resulting in the formation of ATP. This is a classic example of substrate level phosphorylation.
- **Conclusion**: Correct.

- **Option 4**: All of the above.
- Since options 1 and 2 are incorrect, this option cannot be correct.
- **Conclusion**: Incorrect.

3. **Final Answer**:
- The correct answer is **Option 3**: When 1,3-bisphosphoglyceric acid converts to 3-phosphoglyceric acid, substrate level phosphorylation occurs.