Carbon fixation specificity: In the Calvin cycle, carbon dioxide is initially fixed by reacting with which sugar substrate to form 3-phosphoglycerate?

Introduction / Context:
Rubisco, the most abundant enzyme on Earth, catalyzes the carboxylation that initiates the Calvin cycle. Identifying its true substrate clarifies the entry point of CO2 into photosynthetic carbon assimilation.

Given Data / Assumptions:

• Rubisco recognizes ribulose 1,5-bisphosphate (RuBP) and CO2 in the chloroplast stroma.
• The immediate product is an unstable six-carbon intermediate that splits to two molecules of 3-phosphoglycerate.
• Alternative sugar phosphates listed are not the CO2 acceptor in this cycle.

Concept / Approach:
CO2 fixation in the Calvin cycle occurs when CO2 adds to RuBP at the 2-carbon position, followed by cleavage to 3-phosphoglycerate. Therefore, the correct CO2 acceptor is ribulose 1,5-bisphosphate (historically called ribulose diphosphate).

Step-by-Step Solution:

Verification / Alternative check:
Classical radiolabeling experiments show the earliest labeled stable product is 3-phosphoglycerate, consistent with RuBP carboxylation.

Why Other Options Are Wrong:

• Ribulose phosphate or ribose triphosphate: not the Calvin cycle CO2 acceptor.
• 3-phosphoglycerate is the product, not the CO2-accepting substrate.
• Fructose 1,6-bisphosphate is part of regeneration and other pathways, not initial fixation.

Common Pitfalls:
Confusing names like ribulose phosphate versus ribulose 1,5-bisphosphate; the 1,5-bisphosphate is essential for Rubisco recognition.

Final Answer:
Ribulose diphosphate (ribulose 1,5-bisphosphate, RuBP)