Which is not a step in Calvin cycle ?

Options

A. Carboxylation
B. Glycolytic reversal
C. Regeneration
D. Photophosphorylation

Answer

### Step-by-Step Solution:

1. **Understand the Calvin Cycle**: The Calvin cycle is a series of biochemical reactions that occur in the stroma of chloroplasts in photosynthetic organisms. It is crucial for converting carbon dioxide and other compounds into glucose.

2. **Identify the Steps of the Calvin Cycle**: The Calvin cycle consists of three main steps:
- **Carboxylation**: The process where carbon dioxide is fixed into an organic molecule.
- **Reduction**: The conversion of the fixed carbon into a carbohydrate using ATP and NADPH.
- **Regeneration**: The process of regenerating ribulose bisphosphate (RuBP) to continue the cycle.

3. **Analyze the Options Given**:
- **Carboxylation**: This is a step in the Calvin cycle.
- **Glycolytic Reversal**: This term refers to the reversal of glycolysis, and while it has connections to the Calvin cycle, it is not a distinct step within it.
- **Regeneration**: This is also a step in the Calvin cycle.
- **Photophosphorylation**: This is the process of converting ADP to ATP using light energy, but it is not a step in the Calvin cycle.

4. **Conclusion**: Among the options provided, **photophosphorylation** is not a step in the Calvin cycle.

### Final Answer:
The correct answer is **photophosphorylation**.