Calvin cycle carboxylation step During the carboxylation phase of the Calvin–Benson cycle in photosynthetic organisms, carbon dioxide (CO2) combines with which acceptor molecule?

Introduction / Context:
The Calvin–Benson cycle fixes inorganic CO2 into organic carbon. The initial carboxylation reaction determines the cycle’s stoichiometry and is catalyzed by the most abundant enzyme on Earth, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).

Given Data / Assumptions:

• Pathway: Calvin–Benson cycle in chloroplasts or in some bacteria.
• Key enzyme: RuBisCO.
• Question: identity of the CO2 acceptor.

Concept / Approach:
In the carboxylation phase, CO2 reacts with ribulose 1,5-bisphosphate (RuBP, a five-carbon ketose phosphate) to form an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate. Other metabolites listed are products or intermediates in different pathways (C4 fixation uses PEP to form oxaloacetate prior to entering the Calvin cycle but the carboxylation within the Calvin cycle itself is RuBP-dependent).

Step-by-Step Solution:
Identify the CO2 acceptor in the Calvin cycle: RuBP.Recall enzyme: RuBisCO catalyzes RuBP + CO2 → 2 × 3-phosphoglycerate.Exclude C4 preliminary steps (oxaloacetate formation from PEP) which are upstream and not the Calvin carboxylation.

Verification / Alternative check:
Photosynthesis texts consistently depict RuBP as the five-carbon CO2 acceptor inside the Calvin cycle.

Why Other Options Are Wrong:

• Phosphoglyceraldehyde (G3P): downstream product of reduction phase, not the acceptor.
• Pyruvate/citrate: TCA cycle intermediates, unrelated to Calvin carboxylation.
• Oxaloacetate: product of C4/PEP carboxylation outside the Calvin cycle proper.

Common Pitfalls:
Confusing C3 Calvin cycle carboxylation with C4/ CAM preliminary CO2 fixation strategies.

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