Photosynthesis consists of a series of endergonic redox reactions, with light as the source of energy, chlorophyll as the energy converter, and electrons flowing through membrane and soluble proteins. Here, we give an account of the most recent results on the structure-function relationships of the membrane-embedded complexes cytochrome b(6)-f and photosystem I and of the two soluble proteins (cytochrome c(6) and plastocyanin) that serve as alternative electron carriers between them. Particular attention is paid to the evolutionary aspects of the reaction mechanism and transient protein-protein interactions between the membrane complexes and their partners in cyanobacteria, eukaryotic algae, and plants.