Photosystems I and II

Photosystems are very large multisubunit cofactor-binding complexes that act as the primary sites of energy conversion in the biosphere through efficient harvesting of solar radiation and the successive conversion of absorbed photons at specialized sites called reaction centers, where photochemical reactions take place. Primary photochemistry generates a radical pair which is then stabilized by a cascade of electron transfer reactions to yield an electrochemical potential that sustains cellular metabolism. As in all oxygenic organisms, two photosystems, Photosystem I and II, are present in the thylakoid membranes of Chlamydomonas. In the present chapter, the general function of photosynthetic reaction centers is introduced and followed by a more detailed description of structural organizations, light-harvesting processes, primary energy, and electron transfer reactions within both Photosystem I and II. The interaction of Photosystem I with diffusing electron shuttles, ferredoxin and cytochrome c6 at its donor side, and ferredoxin and flavodoxin at its acceptor side, are also described. The contributions of studies performed in Chlamydomonas mutants targeting specific protein-cofactor as well as protein–protein interactions, and hence uncovering the importance of these processes on the photosystems’ reactivity and functionality are discussed in detail. Chlamydomonas has indeed served as a robust model organism, being capable of growing heterotrophically and hence able to tolerate mutations which would otherwise be lethal in strictly photoautotrophic species. Finally, aspects concerning the biogenesis of both photosystems are briefly reviewed.