In the Calvin cycle for the fixation of molecules of `CO_2` how many ATP and NADPH are required in the reduction step ?

Options

A. 18 ATP and 12 NADPH
B. 15 ATP and 10 NADPH
C. 10 ATP and 10 NADPH
D. 3 ATP and 2 NADPH

Answer

To solve the question regarding the number of ATP and NADPH required in the reduction step of the Calvin cycle for the fixation of CO2, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding the Calvin Cycle**:
- The Calvin cycle is a series of biochemical reactions that take place in the chloroplasts of photosynthetic organisms. It involves the fixation of carbon dioxide (CO2) into organic molecules.

2. **CO2 Fixation**:
- In one complete turn of the Calvin cycle, 3 molecules of CO2 are fixed. However, to produce one molecule of glucose (which requires 6 carbon atoms), two turns of the cycle are needed.

3. **ATP and NADPH Usage**:
- During the reduction phase of the Calvin cycle, ATP and NADPH are utilized to convert 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P).
- For each turn of the cycle (fixing 3 CO2), 6 molecules of ATP and 6 molecules of NADPH are consumed.

4. **Calculating for Two Turns**:
- Since two turns of the cycle are needed to produce one glucose molecule, we multiply the ATP and NADPH usage by 2:
- ATP: 6 ATP × 2 = 12 ATP
- NADPH: 6 NADPH × 2 = 12 NADPH

5. **Accounting for Losses**:
- However, during the process, there are losses. Specifically, 2 ATP and 2 NADPH are used in other processes and do not contribute to the net gain.
- Therefore, we subtract these losses from the total:
- Net ATP = 12 ATP - 2 ATP = 10 ATP
- Net NADPH = 12 NADPH - 2 NADPH = 10 NADPH

6. **Final Answer**:
- Thus, the total number of ATP and NADPH required in the reduction step of the Calvin cycle is:
- **10 ATP and 10 NADPH**.