The photosynthetic light reactions are effective due to cooperation of four functional complexes of thylakoid membrane. Within the framework of this cooperation PS I mediates light-driven electron transport from plastocyjanin to ferredoxin which then reduces NADP+ to NADPH. The second function of PS I is mediating cyclic elctron flow in cooperation with cytochrome b6/f complex. PS I domain directly engaged in electron transport along with a group of proteins performing structural role comprise a PS I core.