Plants, algae and some bacteria use two photosystems, PSI with P700 and PSII with P680. Using light energy, PSII acts first to channel an electron through a series of acceptors that drive a proton pump to generate ATP, before passing the electron on to PSI. Once the electron reaches PSI it has used most of its energy in producing ATP, but a second photon of light captured by P700 provides the required energy to channel the electron to ferredoxin, generating reducing power in the form of NADPH. The ATP and NADPH produced by PSII and PSI, respectively, are used in the light-independent reactions for the formation of organic compounds. This process is non-cyclic, because the electron from PSII is lost and is only replenished through the oxidation of water. Hence, there is a constant flow of electrons and associated hydrogens from water for the formation of organic compounds. It is this stripping of hydrogens from water that produces the oxygen we breathe.