Oxygenic Photosynthesis
Oxygenic photosynthesis uses two photosystems: one that generates a proton-motive force for making ATP, and one that generates low potential electrons for reducing power. Because the electrons end up on NADPH, oxygenic phototrophs need an initial electron source. These bacteria take electrons from H2O, producing O2 as a waste product.
The most prominent oxygenic phototrophic bacteria, the cyanobacteria, were so active 3 billion years ago, that they are credited for converting the global atmosphere from one the lacked oxygen gas, to one that contains huge amounts of oxygen (about 16% of our current atmosphere). In addition, genetic evidence shows that sometime, billions of years ago, eukaryotes developed a symbiotic relationship with cyanobacteria, which evolved into what we now know as chloroplasts.
Photosystem II Generates ATP
The reaction center of photosystem II has an extremely high reduction potential (E'o= +1.0 volts). It is one of the few molecules that can take electrons from H2O to produce O2 (E'o= +0.8 volts). Light energy is absorbed by the light-harvesting pigments and transferred to the reaction center (P680). This energy is used to excite the electrons to a lower reduction potential (about E'o= -0.2 volts). The electrons then move through a series of electron carriers (quinones, cytochromes and plastocyanin) generating a proton motive force, which is used by membrane-bound ATPases to make ATP. Because the electrons end up at the reaction center for Photosystem I (P700), this is referred to as "non-cyclic" photophosphorylation.
Photosystem I Makes Reducing Power
Phototrophs that fix CO2 need a source of electrons to reduce NADP+. The electrons in P700 reaction center (which originally came from H2O) have a relatively high reducing potential of about (E'o= +0.4 volts). Light energy is absorbed by the light-harvesting pigments and transferred to the reaction center of Photosystem I (P700). Light energy is used again to excite the electrons to a lower reduction potential (now about E'o= -0.7 volts). These low potential electrons are transferred to a ferridoxin, which can directly reduce NADP+ to NADPH (E'o= -0.32 volts).
Alternatively, if the bacterium has enough reducing power for CO2-fixation, the low-potential electrons from Photosystem I can be directed into the electron transport chain of Photosystem II to generate ATP (dotted line). This is considered a remnant from cyclic photophosphorylation because, in this case, the electrons end up back at the reaction center for Photosystem I.