In Calvin cycle which step involve utilisation of 2 molecules of ATP for phosphorylation and two of `NADPH_(2)" per "CO_(2)` molecule fixed?

Options

A. Regeneration
B. Carboxylation
C. Reduction
D. All of above

Answer

To answer the question regarding which step in the Calvin cycle involves the utilization of 2 molecules of ATP for phosphorylation and 2 molecules of NADPH for each CO2 molecule fixed, we can break down the process step by step.

### Step-by-Step Solution:

1. **Understanding the Calvin Cycle**: The Calvin cycle consists of three main phases: carboxylation, reduction, and regeneration. Each phase has specific roles and requirements.

2. **Carboxylation Phase**: In this initial phase, carbon dioxide (CO2) is fixed by combining with ribulose bisphosphate (RuBP) to form 3-phosphoglycerate (3PGA). This step does not require ATP or NADPH.

3. **Reduction Phase**: This is the critical step where the utilization of ATP and NADPH occurs.
- In this phase, the 3PGA molecules are phosphorylated using 2 molecules of ATP, converting them into 1,3-bisphosphoglycerate (1,3-BPG).
- Subsequently, 2 molecules of NADPH are used to reduce 1,3-BPG into glyceraldehyde-3-phosphate (G3P), a sugar precursor.
- Therefore, for each CO2 molecule fixed, this phase requires 2 ATP and 2 NADPH.

4. **Regeneration Phase**: In this final phase, RuBP is regenerated from G3P. This step requires ATP but only 1 molecule of ATP per RuBP regenerated, which does not meet the requirement of 2 ATP.

5. **Conclusion**: The step in the Calvin cycle that involves the utilization of 2 molecules of ATP for phosphorylation and 2 molecules of NADPH for each CO2 molecule fixed is the **Reduction Phase**.

### Final Answer:
The step in the Calvin cycle that involves the utilization of 2 molecules of ATP for phosphorylation and 2 molecules of NADPH per CO2 molecule fixed is the **Reduction Phase**.

---