NADPH carries high-energy electrons from the light reactions to the Calvin cycle. It supplies the reducing power needed to convert carbon compounds into carbohydrate precursors.
During the light-dependent reactions, electrons ultimately come from water. As they move through the photosynthetic electron-transport chain, their energy helps generate ATP and drives the reduction of NADP+ to NADPH. NADPH then moves its high-energy electrons and hydrogen equivalents into the chloroplast stroma, where the Calvin cycle uses them during its reduction phase.
NADPH is not the same as NADH, although both are electron carriers. NADH is used mainly in cellular respiration and related metabolic pathways, while NADPH is especially associated with biosynthesis and photosynthetic carbon fixation. ATP also enters the Calvin cycle, but it provides chemical energy rather than serving as the main source of reducing electrons.
The Calvin cycle uses NADPH to help reduce 3-phosphoglycerate-derived intermediates toward glyceraldehyde 3-phosphate (G3P). NADP+ is regenerated and can return to the light reactions to accept electrons again, linking the two stages of photosynthesis.