Photosynthesis cyclic electron-transport chain

Ghanotakis, D.F., Yocum, C.F. and Babcock, G.T. 1986. ESR spectroscopy demonstrates that cytochrome b-559 remains low potential in Ca2+-reactivated, salt-washed PS II particles. Photosynthesis Research, 9,125-134. Hervas, M., Ortega, J.M., de la Rosa, M.A., de la Rosa, F.F. and Losada, M. 1985. Location and function of cytochrome b-559 in the chloroplast non-cyclic electron transport chain. Physiol. Veg., 23,593-604. 

The electron transport chain process of photoreactions I and II is noncyclic photophosphorylation. Cyclic photophosphorylation, which may proceed in the case of oxygen deficiency and can be considered as a shortcircuiting of electron transport, presumably does not play a role in the normal photosynthesis energy storing of the cells. 

Cyclic photophosphorylation In photosynthesis, Photophosphorylation (light-dependent ATP synthesis) that is linked to a cyclic flow of electrons firom photosystem 11 down an electron transport chain and back to photosystem II it is not coupled to the oxidation of H2O or to the reduction of NADP. Compare non-cyclic photophosphorylation. 

Rieske protain/centar an iron-sulphur protein first isolat from Complex III of the mitochondrial electron transport chain, in which it occurs with cytochromes b and C) [J.S, Rieske el al. Biochem. Biophys Res Commun. IS (1%4) 338-344], but which has now been found in the equivalent cytochrome be complexes in the bacterial plasma membrane and the chloroplast thylakoid membrane. The latter, known as the cytochrome bff complex, partidpates in cyclic and noncyclic electron flow in the light phase of photosynthesis (see Photosynthesis). All Rieske proteins are one-electron redox systems with a standanl redox potential in the + 0.2 to + 0.3V range and have a (2Fe-2S] center, a single membrane-spanning a-helix, and a characteristic electron spin resonance (ESR) spectrum. The chloroplastidic R.p/c, with a M, of - 20,000, is smaller than that of the mitochondnon. It is encoded in the nucleus, synthesized in the cytoplasm and translocated to the chloroplast, where it is inserted into the thylakoid membrane. Within the thylakoid membrane its [2Fe-2S] redox centre (near to its C-terminus) can readily pass electrons to cytochrome /, a c-type cytochrome that projects from the luminal surface cytochrome / then passes electrons to plastocyanin (see) dissolved in the aqueous milieu of the thylakoid lumen. 

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