Carbon fixation occurs during the light-independent reactions, also known as the Calvin cycle. These reactions take place in the stroma, the space outside of the thylakoids. A 5-carbon sugar, ribulose bisphosphate (RuBP) binds to carbon dioxide, producing an unstable 6-carbon sugar. The enzyme catalyzing this reaction is rubisco. The unstable 6-carbon sugar breaks into two 3-carbon molecules. For every one of these 3-carbon molecules that eventually leaves the Calvin cycle as sugar, 5 are used to regenerate RuBP. ATP and NADPH provide the energy for the necessary rearrangements of molecules in the Calvin cycle.