The photosystem I (PSI) complex is a multisubunit protein-pigment complex embedded in the thylakoid membrane which acts as a light-driven plastocyanin/cytochrome c6-ferredoxin oxido-reductase. The use of chloroplast transformation and site-directed mutagenesis coupled with the biochemical and biophysical analysis of mutants of the green alga Chlamydomonas reinhardtii with specific amino acid changes in several subunits of PSI has provided new insights into the structure-function relationship of this important photosynthetic complex. In particular, this molecular-genetic analysis has identified key residues of the reaction center polypeptides of PSI which are the ligands of some of the redox cofactors and it has also provided important insights into the orientation of the terminal electron acceptors of this complex. Finally this analysis has also shown that mutations affecting the donor side of PSI are limiting for overall electron transfer under high light and that electron trapping within the terminal electron acceptors of PSI is highly deleterious to the cells.