Photosynthesis cyclic electron transport

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. 

Collective name for polyprenylated 2,3-dimethylben-zoquinone derivatives, [e.g., PQ-9 (n=9), C5jH ,02, Mr 749.22, bright yellow platelets, mp. 48-49 C, u max 314 nm (petroleum ether)], isolated from chloro-plasts. The P. play a role as redox substrates in photosynthesis for cyclic and non-cyclic electron transport where they are converted reversibly into the corresponding hydroquinones (plastoquinols). The biosynthesis of P. proceeds from homogentisic acid, a product of L-tyrosine degradation, through prenylation and methylation (methyl group from L-methionine) to the plastoquinols which are dehydrated to the P.. ... 

In the course of photosynthesis water must be cleaved. All of the processes that are related to the cleavage of water-photolyse, are called the primary processes of photosynthesis. To these belong not only photolysis but also the noncyclic electron transport, which is tightly coupled to it, and cyclic electron transport. 

Fig. 28. Ionization of chlorophyll. The ejected electron can return to the chlorophyll molecule via several redox systems. A cycle of this kind is found in the cyclic electron transport of photosynthesis.

Table 2 summarizes the effects of inhibitors of photosynthesis and PS-I electron transport mediators on the accumulation of inorganic carbon and evolution. The accumulation of inorganic carbon was insensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea(DCMU), an inhibitor of PS-II and non-cyclic electron transport. This indicates that the HCD3 transport does not require non-cyclic electron flow from PS-II, being consistent with the action spectra for HCO3 transport. The HCO transport was accelerated by the mediator of PS-I cyclic electron transport and cyclic photophosphorylation, phenazine... 

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. 

We have become acquainted with the redox systems that participate in the primary processes of photosynthesis. We have also learned that two pigment collectives are involved in the primary processes and in both cases chlorophyll a, present in a distinctive form, is the active component. We are now left with the problem of explaining the interplay of these components in the photolysis of water and the subsequent electron transport, both noncyclic and cyclic. To do this we shall draw up an energy diagram in which we arrange the individual components in the right order and according to their redox potential (Fig. 32). This sounds... 

Fig. 32. Electron transport in the primary processes of photosynthesis. Red— noncyclic, black dotted line == cyclic (modified from Levine 1969).

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