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Photosystem I
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 centre, photosystem I.

As with photosystem II and the bacterial reaction centre, a pair of chlorophyll a molecules initiates photoinduced charge separation. This pair is referred to as P700. 700 is a reference to the wavelength at which the chlorophyll molecules absorb light maximally. The P700 lies in the centre 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 left on the P700 is neutralized by the transfer of an electron from plastocyanin. Thus the overall reaction catalyzed by photosystem I is:

$$\begin{matrix} \ & light & \ \\ Pc(Cu^+ ) + Fd_{ox} & \Longrightarrow & Pc(Cu^{2+}) + Fd_{red} \end{matrix}$$

The cooperation between photosystems I and II creates an electron flow from H2O to NADP+. This pathway is called the 'Z-scheme' because the redox diagram from P680 to P700 resembles the letter z.