Photosynthesis cyclic photophosphorylation

Van Rensen, J.J.S. (1971). Action of some herbicides in photosynthesis of Scenedesmus, as studied by their effects on oxygen evolution and cyclic photophosphorylation. Wageningen H. Veenman. 80 p. 

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. 

Two h-type cytochromes are present in green plant photosynthesis 6559 and buz or bt. The former has traditionally been placed between plastoquinone and cytochrome / in the electron chain, and 69 has been placed between ferredoxin and cytochrome / in cyclic photophosphorylation (291). Both of these assignments have been challenged recently (292-295), and we must simply note the controversy and drop the issue for the purposes of this chapter. 

It could be that the break between respiration and photosynthesis in these bacteria is more recent than we think. Cytochrome Ca has been suggested to have a respiratory as well as a photosynthetic role in R. spheroides (S72) and R. capsulata (372a-c) and no alternative respiratory chain has yet been identified in any of the Athiorhodaceae. In some of these organisms a situation may exist as in Fig. 46 with electrons flowing to both from light-excited bacteriochlorophyll and from external donors, and then from c either to an electron-depleted bacteriochlorophyll or to an oxidase molecule. This would account for the observed control mechanism in the purple nonsulfur bacteria. Under aerobic conditions in the dark, bacteriochlorophyll would not be electron-defi.cient, whereas the oxidase would be in its oxidized state and capable of accepting electrons from c. Under anaerobic conditions, electrons would reduce the oxidase, and further electron transfer down that path would be blocked. Light then would promote electrons away from bacteriochlorophyll and set cyclic photophosphorylation in motion. 

Figure 28 The Z-scheme of photosynthesis for cyanobacteria (from Cornet et al., 1998). The cyclic photophosphorylation pathway enabling values of P/2e greater than 1.0 is indicated.

First, when a low concentration of PMS was added to wheat, rice, soybean or spinach leaves to increase the ATP content in the leaves by increasing cyclic photophosphorylation, the rate of photosynthesis was often increased (Fig.l). This enhancement was higher in young spinach leaves, whose coupling efficiency, as measured in isolated chloroplasts, was lower than that of mature ones(2). 

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. 

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... 

Arnon and his group have definitively established that ferredoxins (iron-sulfur proteins noted for their strongly electronegative redox potentials) are the primary electron acceptors in photosynthesis, and that they are essential electron carriers for the light-induced generation of reducing power and ATP formed in the processes of cyclic and non-c clic photophosphorylation. Reducing power—either reduced ferredoxin or reduced nicotinamide adenine dinucleotides, NAD(P)H—and ATP constitute the assimilatory power required for the further assimilation in the dark of carbon dioxide, nitrate and sulfate. ... 

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