In the first step of the Calvin cycle (Fig. 8.5), CO₂ is added to a 5-carbon sugar called ribulose 1,5-bisphosphate (RuBP). This step is catalyzed by the enzyme ribulose bisphosphate carboxylase oxygenase, or rubisco for short. An enzyme that adds CO₂ to another molecule is called a carboxylase, explaining part of rubisco’s long name.

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Before rubisco can act as a carboxylase, RuBP and CO₂ must diffuse into its active site. Once the active site is occupied, the addition of CO₂ to RuBP proceeds spontaneously in the sense that no addition of energy is required. The product is a 6-carbon compound that immediately breaks into two molecules of 3-phosphoglycerate (3-PGA). These 3-carbon molecules are the first stable products of the Calvin cycle.