9.5: Photosystem I
Like Photosystem II (PS II), Photosystem I (PS I) captures photons and transports them through chlorophyll molecules into a reaction center. In PS I, the photons reenergize the electrons that have entered PS I from PS II. From the reaction center, the high energy electron is sent through an electron transport chain and ultimately joins with an additional electron and a proton to reduce NADP+ into NADPH. Thus, similar to PS II that captures energy to generate ATP, PS I captures energy to create NADPH.
The pigments of the light-harvesting complex in Photosystem I absorb photons and relay the energy to the reaction center (P700). Following oxidation, a high-energy electron is passed from the specialized pair of chlorophyll a to the primary electron acceptor. This time, however, the missing electrons from the chlorophyll a pair are replaced by the electrons traveling from Photosystem II (instead of splitting of water as in PS II). On their way from PS II to PS I, the electrons pass through the electron transport chain, comprising the carrier molecule plastoquinone, the dual-protein cytochrome complex, and plastocyanin.
Once the electron was excited in the reaction center of PS I, it enters a second electron transport chain—the protein complex ferredoxin. The single electron then joins with another electron and a proton (H+) that are all accepted by NADP+ reductase to form the product NADPH. Thus, PS II captures energy to create a proton gradient that is ultimately used to make ATP, and the energy captured in PSI is used to produce NADPH.