As discovered by Calvin, when algal cells are exposed to radioactive `CO_(2)` in the light for only 1 second, the compound that is most quickly and extensively labeled with radioactive C has how many carbon atoms ?

Options

A. 2
B. 3
C. 4
D. 5

Answer

To solve the question regarding the compound that is most quickly and extensively labeled with radioactive carbon when algal cells are exposed to radioactive \( CO_2 \) for only 1 second, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Context**: The question refers to the Calvin cycle, which is the process of photosynthesis where carbon dioxide is fixed into organic compounds.

2. **Identifying the Initial Compound**: In the Calvin cycle, the first compound that reacts with carbon dioxide (\( CO_2 \)) is Ribulose bisphosphate (RuBP). RuBP is a 5-carbon compound.

3. **Carbon Fixation**: When RuBP combines with \( CO_2 \), it forms a 6-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate (3-PGA), which is a 3-carbon compound.

4. **Labeling Process**: The question specifically asks for the compound that is most quickly and extensively labeled with radioactive carbon. Since RuBP is the first compound to accept \( CO_2 \), it will be the first to be labeled.

5. **Conclusion**: Therefore, the compound that is most quickly and extensively labeled with radioactive carbon is RuBP, which has **5 carbon atoms**.

### Final Answer:
The compound that is most quickly and extensively labeled with radioactive carbon has **5 carbon atoms**.

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