Photosystem I

Currently shown is photosystem I (Note: Very large molecule! Please wait for it to load).

As with photosystem II and the bacterial reaction centre, a 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 molecule situated directly above the P700, through a molecule situated directly above that, through three clusters and finally to an ferredoxin complex. 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:

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.

In chronic photoinhibition, damage can also occur at PSI because O₂ can be reduced to generate superoxide (O₂^-) from triplet chlorophyll or from the acceptor side of PSI. O₂^- can then be converted to H₂O₂.