The role of plastocyanin and cytochrome f in photosynthetic electron transport

Abstract

These results show that in three different types of particles from spinach chloroplasts both plastocyanin (spinach) and cytochrome₅₅₂ (Euglena) are equally effective als electron donors for pigment system I of photosynthesis, coupled to NADP reduction. This conclusion follows from the fact that both are equally effective in stimulating NADP reduction at the expense of an artificial electron donor system. In sonicated chloroplasts plastocyanin seems to be the better electron acceptor for electrons coming from the photooxidation of water by light reaction II, since addition of plastocyanin to a system depending on oxygen evolution yields better rates than addition of cytochrome₅₅₂. In order to explain the result that there are two possible electron donors for pigment system I it is suggested that there are two-perhaps spatially separated-pigment systems I in photosynthesis which are participating in a non-cyclic or a cyclic electron transport system and which are either coupled to plastocyanin or to cytochrome f. The difference in rates mentioned above may indicate that plastocyanin is a component of non-cyclic and cytochrome f of cyclic electron flow. The cyclic system can be converted into a non-cyclic system by the addition of an artificial electron donor and NADP.