Question

In Calvin cycle, Rubisco incorporates CO\textsubscript{2 into ribulose 1,5 bisphosphate which rapidly splits into:}

• A. Glyceraldehyde 3-phosphate
• B. 2,3 phosphoglyceric acid
• C. 3 phosphoglycerate
• D. 1,3 diphosphoglycerate

Hint:

RuBP + CO\textsubscript{2} → 2 × 3-PGA in the first stable step of the Calvin cycle.

Answer 1

The Correct Option is C

In the Calvin cycle, Rubisco catalyzes the carboxylation of ribulose 1,5
bisphosphate (RuBP) with CO\textsubscript{2}, forming two molecules of 3
phosphoglycerate (3
PGA).

Answer 2

Carbon-counting derivation:
Ribulose 1,5-bisphosphate (RuBP) is a 5-carbon sugar. When Rubisco fixes one molecule of CO₂ onto RuBP, a transient 6-carbon intermediate forms. This unstable intermediate immediately hydrolyses into two smaller molecules. Counting atoms confirms the products must each contain 3 carbons, since the 6-carbon intermediate splits evenly in half, which corresponds to two molecules of 3-phosphoglycerate rather than a 3-carbon sugar-phosphate such as glyceraldehyde 3-phosphate, which only forms later after a reduction step, or any 2-carbon or 4-carbon species.
Hence, the split product is 3-phosphoglycerate.