Discussion
A suspension of photosynthetic green algae was illuminated in the presence of 14CO2 for few seconds. The first metabolite in the Calvin cycle to be radiolabeled will be
Answer: C
The first labelled metabolite is 3-phosphoglycerate.
In the carboxylation step of the Calvin cycle, rubisco adds \(CO_{2}\) to ribulose 1,5-bisphosphate, and the unstable six-carbon intermediate splits at once into two molecules of 3-phosphoglycerate, one of which carries the new carbon. Calvin's short-exposure experiments with algae found the label in 3-phosphoglycerate within seconds.
1,3-bisphosphoglycerate and glyceraldehyde 3-phosphate are formed from 3-phosphoglycerate afterwards, in the reduction steps. Ribulose 1,5-bisphosphate is labelled only later, when the cycle regenerates it.
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