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 solve the question about the energy required for the fixation of one CO₂ molecule in C₃ plants, we will analyze the Calvin cycle, which is the process involved in carbon fixation.

### Step-by-Step Solution:

1. **Understanding the Calvin Cycle**: The Calvin cycle, also known as the dark reaction or light-independent reaction, is the process through which CO₂ is fixed into organic molecules in C₃ plants. It consists of three main phases: carboxylation, reduction, and regeneration.

2. **Carboxylation Phase**: In this first step, CO₂ is fixed to a 5-carbon sugar (ribulose bisphosphate, RuBP) to form 3-phosphoglyceric acid (PGA). This phase does not require any ATP or NADPH.
- **Energy Used**: 0 ATP, 0 NADPH.

3. **Reduction Phase**: In this second step, PGA is reduced to glyceraldehyde-3-phosphate (G3P). This process requires energy in the form of ATP and reducing power in the form of NADPH.
- **Energy Used**: 2 ATP, 2 NADPH.

4. **Regeneration Phase**: In the final step, some of the G3P molecules are used to regenerate RuBP, allowing the cycle to continue. This phase requires additional ATP.
- **Energy Used**: 1 ATP, 0 NADPH.

5. **Total Energy Calculation**: Now, we can sum up the total energy required for the fixation of one CO₂ molecule:
- From the carboxylation phase: 0 ATP, 0 NADPH.
- From the reduction phase: 2 ATP, 2 NADPH.
- From the regeneration phase: 1 ATP, 0 NADPH.

Therefore, the total energy required is:
- **Total ATP**: 0 + 2 + 1 = 3 ATP
- **Total NADPH**: 0 + 2 + 0 = 2 NADPH

### Final Answer:
For the fixation of one CO₂ molecule in C₃ plants, the energy required is **3 ATP and 2 NADPH**.

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