Of the 15 chemical reactions that make up the Calvin cycle, 12 occur in the last step, the regeneration of RuBP (Fig. 8.5). A large number of reactions is needed to rearrange the carbon atoms from five 3-carbon triose phosphate molecules into three 5-carbon RuBP molecules. ATP is required for the regeneration of RuBP, raising the Calvin cycle’s total energy requirements to two molecules of NADPH and three molecules of ATP for each molecule of CO₂ incorporated by rubisco.

The Calvin cycle does not use sunlight directly. For this reason, this pathway is sometimes referred to as the light-independent or even the “dark” reactions of photosynthesis. However, this pathway cannot operate without the energy input provided by a steady supply of NADPH and ATP. Both are supplied by the photosynthetic electron transport chain, in which light is captured and transformed into chemical energy. In addition, several Calvin cycle enzymes are regulated by cofactors that must be activated by the photosynthetic electron transport chain. Thus, in a photosynthetic cell, the Calvin cycle occurs only in the light.

Quick Check 2 The Calvin cycle requires both ATP and NADPH. Which of these molecules provides the major input of energy needed to synthesize carbohydrates?