What molecule provides the energy for the Calvin cycle? ATP provides that energy. The Calvin cycle uses ATP made during the light-dependent reactions to drive carbon fixation and the regeneration of its starting molecule, ribulose-1,5-bisphosphate (RuBP).
ATP is not acting alone. NADPH supplies the reducing power—the high-energy electrons and hydrogen needed to convert carbon compounds into carbohydrate. ATP mainly supplies chemical energy through phosphate transfer, helping power otherwise unfavorable reactions.
The cycle operates in the chloroplast stroma in plants, although related Calvin cycles also occur in algae and some photosynthetic bacteria. Its key carbon-fixing enzyme is RuBisCO, which attaches carbon dioxide to RuBP. The immediate carbohydrate product is glyceraldehyde 3-phosphate (G3P), not glucose directly.
A common mix-up is calling the Calvin cycle the “dark reaction.” It does not require darkness and is regulated by products of the light reactions. A typical net production of one G3P requires 9 ATP and 6 NADPH, while two G3P molecules can later be combined to form glucose.