Find the number of ATP required in the reduction step of the C3 cycle if the cycle is involved in the formation of one sucrose molecule.

Options

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

Answer

To determine the number of ATP molecules required in the reduction step of the C3 cycle (Calvin cycle) for the formation of one sucrose molecule, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the C3 Cycle**: The C3 cycle, also known as the Calvin cycle, is a process in photosynthesis where carbon dioxide is fixed into organic compounds. It primarily produces a 3-carbon compound (G3P).

2. **Identify the Carbon Content of Sucrose**: Sucrose is a disaccharide composed of two monosaccharides, glucose and fructose, each containing 6 carbon atoms. Therefore, sucrose has a total of 12 carbon atoms.

3. **Calculate the Number of C3 Cycles Needed**: Since each turn of the Calvin cycle produces one 3-carbon compound (G3P), to produce a molecule of sucrose (12 carbons), we need to determine how many cycles are required:
\[ \text{Number of cycles} = \frac{\text{Total carbon in sucrose}}{\text{Carbon produced per cycle}} = \frac{12}{3} = 4 \]
Thus, 4 cycles of the C3 cycle are needed to produce one molecule of sucrose.

4. **Determine ATP Requirement per Cycle**: In each turn of the Calvin cycle, a total of 6 ATP molecules are consumed during the reduction phase.

5. **Calculate Total ATP for 4 Cycles**: To find the total ATP required for 4 cycles, we multiply the ATP required for one cycle by the number of cycles:
\[ \text{Total ATP} = \text{ATP per cycle} \times \text{Number of cycles} = 6 \times 4 = 24 \]

6. **Conclusion**: Therefore, the total number of ATP molecules required in the reduction step of the C3 cycle for the formation of one sucrose molecule is 24.

### Final Answer:
The number of ATP required in the reduction step of the C3 cycle for the formation of one sucrose molecule is **24**.

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