`C_(4)` plants are more expensive in terms of ATP required for `CO_(2)` fixation, the additional number of ATP required by `C_(4)` plants in comparison to `C_(3)` plants is

Options

A. `6 ATP`
B. `12 ATP`
C. `18 ATP`
D. `24 ATP`

Answer

To determine the additional number of ATP required by C4 plants in comparison to C3 plants for CO2 fixation, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Photosynthetic Pathways**:
- C3 plants primarily use the Calvin cycle for photosynthesis, where ribulose bisphosphate (RuBP) fixes CO2 to produce 3-phosphoglycerate (3-PGA).
- C4 plants utilize a modified pathway that involves two types of cells: mesophyll cells and bundle sheath cells. In mesophyll cells, CO2 is initially fixed into a 4-carbon compound (oxaloacetic acid) before being transported to bundle sheath cells where the Calvin cycle occurs.

2. **ATP Consumption in C3 Plants**:
- For the synthesis of one glucose molecule in C3 plants, a total of 18 ATP molecules are consumed.

3. **ATP Consumption in C4 Plants**:
- In C4 plants, the process is more complex. The initial fixation of CO2 into oxaloacetic acid in mesophyll cells requires additional ATP.
- The total ATP required for the production of one glucose molecule in C4 plants is 30 ATP.

4. **Calculate the Additional ATP Requirement**:
- To find the additional ATP required by C4 plants compared to C3 plants, we subtract the ATP used by C3 plants from that used by C4 plants:
\[ \text{Additional ATP} = \text{ATP in C4} - \text{ATP in C3} = 30 \text{ ATP} - 18 \text{ ATP} = 12 \text{ ATP} \]

5. **Conclusion**:
- Therefore, the additional number of ATP molecules required by C4 plants in comparison to C3 plants for CO2 fixation is **12 ATP**.

### Final Answer:
The additional number of ATP required by C4 plants in comparison to C3 plants is **12 ATP**.

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