Calvin–Benson cycle in C3 plants: which molecule serves as the immediate acceptor of CO2 during carbon fixation?

Introduction / Context:
The Calvin–Benson cycle fixes CO2 into carbohydrates in the chloroplast stroma of C3 plants. Understanding the first acceptor of CO2 clarifies how rubisco initiates the cycle and why photorespiration competes with productive carbon fixation under certain conditions.

Given Data / Assumptions:

• Focus is on C3 photosynthesis (Calvin cycle), not the C4 Hatch–Slack pathway.
• Rubisco catalyzes carboxylation of a 5-carbon sugar.
• Question tests identification of the immediate CO2 acceptor molecule.

Concept / Approach:
Rubisco adds CO2 to ribulose-1,5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate in the carboxylation step. Regeneration of RuBP from triose phosphates completes the cycle. Misidentifying PEP as the acceptor conflates C3 with C4 pathways, where PEP carboxylase fixes bicarbonate into oxaloacetate in mesophyll cells.

Step-by-Step Solution:

Verification / Alternative check:
Photosynthesis texts depict the carboxylation of RuBP as the entry point for CO2 into the Calvin cycle.

Why Other Options Are Wrong:

• A/C: Sugar phosphates within the pentose phosphate pool but not the direct CO2 acceptor in C3 fixation.
• D: TCA intermediate, not relevant to Calvin cycle entry.
• E: PEP is the initial acceptor in C4 and CAM mesophyll, not in C3.

Common Pitfalls:
Confusing C3 and C4 pathways; always tie RuBP to rubisco in C3 plants.

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