Question

A suspension of photosynthetic green algae was illuminated in the presence of \(^{14}\)CO\(_2\) for few seconds. The first metabolite in the Calvin cycle to be radiolabeled will be 3-phosphoglycerate (3-PGA).

• A. glyceraldehyde
• B. 1,3-bisphosphoglycerate
• C. 3-phosphoglycerate
• D. ribulose 1,5-bisphosphate

Hint:

In CO$_2$ fixation studies (Calvin’s “lollipop experiment”), $^{14}$CO$_2$ is first incorporated into {3-phosphoglycerate}, not into RuBP or sugars.

Answer 1

The Correct Option is C

Step 1: Calvin cycle CO$_2$ fixation.
- CO$_2$ first reacts with ribulose 1,5-bisphosphate (RuBP) in the Calvin cycle.
- This reaction is catalyzed by the enzyme Rubisco. 
Step 2: First stable product.
- The unstable 6-carbon intermediate immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
- Therefore, 3-PGA is the first stable compound to incorporate the radioactive carbon. 
Therefore, the first radiolabeled metabolite is 3-phosphoglycerate.