The 'Z-scheme' is the prevailing model for photosynthetic electron transport, in which electrons from Photosystem II (PS II) are transferred to Photosystem I (PS I) with the participation of the plastoquinone pool, the cytochrome b₆f complex, and plastocyanin. PS II can accomplish water oxidation due to the low potential of its oxidized primary donor, and PS I reduces ferredoxin by virtue of its low potential iron-sulfur centers. There have been several claims over the last two decades that PS II can carry out the reduction of ferredoxin, lately using Chlamydomonas reinhardtii mutants defective in synthesis of PS I (Greenbaum et al., 1995; Lee et al., 1996). However, recent studies performed on mutants harboring deletions of genes encoding PS I subunits demonstrated that photoautotrophic growth and C0₂ fixation require the presence of PS I (Cournac et al., 1997). When observed in 'PS I-deficient' mutants, photoautotrophic growth and C0₂ fixation are most likely attributable to small amounts of PS I in those mutants. We discuss thermodynamic and evolutionary implications of the Z-scheme and possible exceptions to it.