Which molecule acts as the CO2 acceptor in the Calvin cycle?

Ribulose bisphosphate (RuBP) serves as the primary CO2 acceptor in the Calvin cycle, a critical phase of photosynthesis that takes place in the stroma of chloroplasts. During this cycle, carbon dioxide (CO2) from the atmosphere combines with RuBP in a reaction catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase (often referred to as Rubisco). This reaction forms an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).

The importance of RuBP lies in its role in fixing carbon dioxide and facilitating the synthesis of organic molecules, which plants use for energy and growth. Without RuBP, the Calvin cycle would not effectively capture carbon, making it essential for the process of photosynthesis to proceed.

The other choices do not serve as CO2 acceptors in this context: glucose is a product of photosynthesis, ATP is an energy carrier, and NADPH is a reducing agent that provides electrons for the reduction of 3-PGA to glyceraldehyde-3-phosphate (G3P) in the subsequent steps of the Calvin cycle.