Light reaction occurs inside thylakoids. It is dependent upon light. It involves photolysis of water and production of assimilatory power.

Electrons released during photolysis of water are picked up by P680 photocenter of photosystem II. From here electrons pass over a series of carrier which include PQ, cytochrome b-f complex and PC.

While passing over cytochrome complex, the electron losses sufficient energy for creation of proton gradient and ATP from ADP and inorganic phosphate. From PC electron is picked by the trap centre P700 of photosystem I which pushes out electron after absorbing light energy.

Electron passes over carriers FeS, ferredoxin and NADP-reductase which gives electron to NADP⁺ for combining with H⁺ to produce NADPH.