In an experiment, the carbon dioxide available to a `C_(3)` plant was labelled with a radioactive isotope and the amount of radioactivity in the chloroplast was measured.As photosynthesis proceeded, in which of the following molecules did the radioactivity first appear ?

Options

A. PGAL
B. PEP
C. PGA
D. RuBP

Answer

To determine in which molecule the radioactivity first appears during photosynthesis in a C3 plant, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Experiment**: The experiment involves a C3 plant where carbon dioxide (CO2) is labeled with a radioactive isotope. This means that the carbon atoms in the CO2 are tagged, allowing us to track their incorporation into organic molecules during photosynthesis.

2. **Photosynthesis Overview**: In C3 plants, photosynthesis occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). The Calvin cycle is where CO2 is fixed into organic molecules.

3. **Key Molecules in the Calvin Cycle**: The main molecules involved in the Calvin cycle include:
- Ribulose bisphosphate (RuBP)
- Phosphoglycerate (PGA)
- Phosphoenolpyruvate (PEP)
- 1,3-diphosphoglycerate
- Glyceraldehyde-3-phosphate (G3P)

4. **Carbon Fixation**: In the Calvin cycle, CO2 is fixed by RuBP to form PGA. This reaction is catalyzed by the enzyme RuBisCO.

5. **Radioactivity Tracking**: Since the CO2 is labeled with a radioactive isotope, the first molecule that will incorporate this labeled carbon during the Calvin cycle is PGA.

6. **Conclusion**: Therefore, as photosynthesis proceeds, the radioactivity will first appear in phosphoglycerate (PGA).

### Final Answer:
The radioactivity first appears in **phosphoglycerate (PGA)**.

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