The process of photosynthesis involves two stages, namely, the light reactions and the dark reactions or light independent reactions.
The following is the summary of the events that occur during the two stages:
• Light reactions:
During the light reactions, light is captured by specialized pigment molecules. This oxidizes them and they initiate a series of electron-transfer reactions. The electron-transfer reactions terminate with the reduction of NADP+ to NADPH. This generates a transmembrane proton gradient, which is coupled to the enzymatic synthesis of ATP from ADP and Pi. The pigments that were oxidized are reduced by H2O, which produces O2.
Thus, during the light reactions, the light energy is harnessed to form NADPH and ATP.
• Light independent reactions:
The NADPH and ATP that are produced during the light reactions are utilized to bring about the reduction of CO2. The CO2, thus fixed gets incorporated into the three-carbon precursors of carbohydrates, to form various carbohydrates during the dark reactions.