The light reactions of photosynthesis produce two main products that are subsequently used in the Calvin cycle: ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
During the light reactions, which occur in the thylakoid membranes of chloroplasts, sunlight is harvested to convert water and carbon dioxide into oxygen and energy-rich molecules. The energy captured from sunlight is used to generate ATP through a process called photophosphorylation, while NADPH is produced from the reduction of NADP+. Both ATP and NADPH serve as energy carriers that power the next stage of photosynthesis.
In the Calvin cycle, which takes place in the stroma of the chloroplasts, ATP provides the energy required for the conversion of carbon dioxide into glucose, while NADPH supplies the necessary reducing power to convert the carbon compounds into a stable form of sugar. Thus, without these products of the light reactions, the Calvin cycle would not be able to function effectively, emphasizing the interdependence of these two stages of photosynthesis.