For fixation of one `CO_2` molecule in `C_3` plants the energy required in the form of ATP and `NADPH_2` are

Options

A. 2 ATP and 2 `NADPH_2`
B. 5 ATP and `3 NADPH_2`
C. 3 ATP and `2 NADPH_2`
D. 18 ATP and `12 NADPH_2`

Answer

To determine the energy required for the fixation of one CO2 molecule in C3 plants, we need to analyze the Calvin cycle, which consists of three main stages: carbon fixation, reduction, and regeneration.

### Step-by-Step Solution:

1. **Understanding the Calvin Cycle**:
- The Calvin cycle is the process by which C3 plants fix carbon dioxide (CO2) into organic molecules. It occurs in three main stages: carbon fixation, reduction, and regeneration.

2. **Carbon Fixation**:
- In the first step of the Calvin cycle, CO2 is fixed into a 5-carbon sugar (ribulose bisphosphate or RuBP) by the enzyme RuBisCO.
- **Energy Requirement**: This step does not require any ATP or NADPH.
- **Total ATP and NADPH used so far**: 0 ATP, 0 NADPH.

3. **Reduction Phase**:
- In the second step, the fixed carbon compounds are reduced to form glyceraldehyde-3-phosphate (G3P).
- **Energy Requirement**: This step requires 2 ATP and 2 NADPH.
- **Total ATP and NADPH used so far**: 2 ATP, 2 NADPH.

4. **Regeneration of RuBP**:
- In the final step, some of the G3P molecules are used to regenerate RuBP, allowing the cycle to continue.
- **Energy Requirement**: This step requires 1 ATP and does not require NADPH.
- **Total ATP and NADPH used so far**: 3 ATP (2 from reduction + 1 from regeneration), 2 NADPH.

5. **Final Calculation**:
- To summarize the total energy requirements for the fixation of one CO2 molecule:
- **Total ATP**: 3 ATP
- **Total NADPH**: 2 NADPH

### Conclusion:
The total energy required for the fixation of one CO2 molecule in C3 plants is **3 ATP and 2 NADPH**.