These protons, as well as additional protons pumped across the thylakoid membrane coupled with the electron transport chain, form a proton gradient across the membrane that drives photophosphorylation and thus the generation of chemical energy in the form of adenosine triphosphate (ATP). The electrons reach the P700 reaction center of photosystem I where they are energized again by light. They are passed down another electron transport chain and finally combine with the coenzyme NADP and protons outside the thylakoids to form NADPH. Thus, the net oxidation reaction of water photolysis can be written as: