The calvin cycle proceeds in three stages: (A) Carboxylation, during which CO2 combines with ribulose-1,5-bisphosphate. (B) Reduction, during which carbohydrate is formed at the expense of the photochemically made ATP and NADPH. (C) Regeneration during which the CO2 acceptor ribulose-1,5-bisphosphate is formed again so that the cycle continues.