Cyclic Electron Transfer Around Photosystem I

Cyclic electron transport around Photosystem I remains one of the last great enigmas in photosynthesis research. Although first described in 1955 by Arnon and coworkers, the molecular details of the pathway, its physiological role and even its very occurrence remain in question. Nevertheless, significant progress is starting to be made in our understanding of this process. At least two pathways of cyclic electron transport appear to operate, one involving the transfer of electrons from NADPH to plastoquinone and the other operating via the donation of electrons from ferredoxin to plastoquinone. The relative importance of these two pathways seems to vary between cyanobacteria, unicellular green algae and higher plants as do many details concerning the regulation of the pathway and its functional organization in the thylakoid membrane. Two distinct functions for cyclic electron transport can be defined — the generation of ATP and, in higher plants, the generation of ΔpH to regulate light harvesting. These two functions give rise to the need for different regulatory processes to control the ratio of cyclic and linear electron flow. We discuss recent findings that cast new light on how cyclic electron transport is regulated under a range of physiological conditions.