Final answer:
CO2 is initially linked to ribulose bisphosphate (RuBP) in the first step of the Calvin cycle, catalyzed by the enzyme RuBisCO.
Step-by-step explanation:
In the first step of the Calvin cycle, the compound to which CO2 is linked is called ribulose bisphosphate (RuBP). The enzyme responsible for this reaction is ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known as RuBisCO. CO2 reacts with RuBP to form a short-lived 6-carbon intermediate that immediately splits into two 3-carbon molecules of 3-phosphoglycerate (3-PGA). This process is known as carbon fixation, which is the initial stage of the Calvin cycle, followed by the reduction and regeneration stages to eventually produce glucose and regenerate RuBP.