After the electron has left Photosystem II it is transferred to a cytochrome b6f complex and then to plastocyanin, a blue copper protein and electron carrier. The plastocyanin complex carries the electron that will neutralize the pair in the next reaction center, Photosystem I.
As with Photosystem II and the bacterial reaction center, a pair of chlorophyll a molecules initiates photoinduced charge separation. This pair is referred to as P700, where 700 is a reference to the wavelength at which the chlorophyll molecules absorb light maximally. The P700 lies in the center of the protein. Once photoinduced charge separation has been initiated, the electron travels down a pathway through a chlorophyll α molecule situated directly above the P700, through a quinone molecule situated directly above that, through three 4Fe-4S clusters, and finally to an interchangeable ferredoxin complex. Ferredoxin is a soluble protein containing a 2Fe-2S cluster coordinated by four cysteine residues. The positive charge on the high-energy P700 is neutralized by the transfer of an electron from plastocyanin, which receives energy eventually used to convert QH back to Q. Thus the overall reaction catalyzed by Photosystem I is:
:Pc(Cu) + Fd + hν → Pc(Cu) + Fd
The cooperation between Photosystems I and II creates an electron and proton flow from HO to NADP, producing NADPH needed for glucose synthesis. This pathway is called the Z-scheme because the redox diagram from HO to NADP via P680 and P700 resembles the letter Z.