Ferredoxin (Fd) is a soluble protein that facilitates reduction of to NADPH. Fd moves to carry an electron either to a lone thylakoid or to an enzyme that reduces . Thylakoid membranes have one binding site for each function of Fd. The main function of Fd is to carry an electron from the iron-sulfur complex to the enzyme ferredoxin– reductase. Ferredoxin– reductase (FNR) This enzyme transfers the electron from reduced ferredoxin to to complete the reduction to NADPH. FNR may also accept an electron from NADPH by binding to it. Plastocyanin Plastocyanin is an electron carrier that transfers the electron from cytochrome b6f to the P700 cofactor of PSI in its ionized state P700. Ycf4 protein domain The Ycf4 protein domain found on the thylakoid membrane is vital to photosystem I. This thylakoid transmembrane protein helps assemble the components of photosystem I. Without it, photosynthesis would be inefficient. Evolution Molecular data show that PSI likely evolved from the photosystems of green sulfur bacteria. The photosystems of green sulfur bacteria and those of cyanobacteria, algae, and higher plants are not the same, but there are many analogous functions and similar structures. Three main features are similar between the different photosystems. First, redox potential is negative enough to reduce ferredoxin. Next, the electron-accepting reaction centers include iron–sulfur proteins. Last, redox centres in complexes of both photosystems are constructed upon a protein subunit dimer. The photosystem of green sulfur bacteria even contains all of the same cofactors of the electron transport chain in PSI. The number and degree of similarities between the two photosystems strongly indicates that PSI and the analogous photosystem of green sulfur bacteria evolved from a common ancestral photosystem. See also Biohybrid solar cell References External links Photosystem I: Molecule of the Month in the Protein Data Bank Photosystem I in A Companion to Plant Physiology James Barber FRS Photosystems I & II Photosynthesis Light reactions EC 1.97.1 Protein complexes