How many turns of C3 are needed to yield one molecule of glucose?

Options

A. 8
B. 2
C. 6
D. 4

Explanation

To determine how many turns of the C3 cycle (Calvin cycle) are needed to yield one molecule of glucose, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Calvin Cycle**: The Calvin cycle, also known as the C3 cycle, is the process through which plants convert carbon dioxide into glucose. It involves a series of reactions that occur in the chloroplasts of plant cells.

2. **Carboxylation Phase**: The first step in the Calvin cycle is carboxylation, where carbon dioxide (CO2) is fixed to a 5-carbon sugar called ribulose bisphosphate (RuBP). This reaction produces a 6-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate (3-PGA), each containing 3 carbon atoms.

3. **Formation of Triose Phosphate**: In the subsequent reduction phase, ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), which is a 3-carbon sugar. Some of these G3P molecules can be used to form glucose.

4. **Glucose Composition**: A glucose molecule consists of six carbon atoms. Therefore, to produce one molecule of glucose, we need to incorporate six carbon atoms into the cycle.

5. **Carbon Dioxide Requirement**: Since each turn of the Calvin cycle incorporates one carbon atom from CO2, we need six turns of the cycle to fix six carbon atoms.

6. **Conclusion**: Thus, to yield one molecule of glucose, **six turns of the Calvin cycle are required**.

### Final Answer:
**Six turns of the C3 cycle (Calvin cycle) are needed to yield one molecule of glucose.**

---