Coupled light-induced electron transport

It has been shown that the reverse transhydrogenase reaction works as an energy generator [2]. Rps. viridis chromatophores [12] has an active transhydrogenase which is coupled to cyclic electron transport as in the R. rubrum chromatophores [2]. The reverse trans-hydrogenase induces a A P of 25 mV and 33 mV in Rps. viridis and R. rubrum chromatophores respectively (not shown). The A P was 50% and 30% of the light-induced A in Rps. viridis and R. rubrum chromatophores respectively. The reverse transhydrogenase driven AV was sensitive to FCCP. 

The activated membrane-bound ATPase is functionally coupled to proton movements. Thus, a transmembrane pH gradient (acid inside) of a magnitude similar to that observed during light-induced coupled electron flow is developed during ATP hydrolysis. ATP hydrolysis is stimulated, while the coupled proton transport is inhibited, by the addition of uncouplers, indicating that the rate of ATP hydrolysis is also partially limited by the electrochemical gradient which it creates. Nevertheless, attempts to measure H /ATP ratios in this system yielded numbers much below the expected ratio of 3. 

Fig. 16. Effect on the light-induced absorbance change at 515 nm (AAsis) and photophosphoiylation (ATP) and on electron transport (measured by oxygen evolution [Oj], P700 photooxidation [P700] and plastoquinone reduction [PQ]) as a function of (A) ethanol and (B) desaspidin concentrations. Figure source Witt (1971) Coupling of quanta, electrons, fields, ions and phosphorylation in the functional membrane of photosynthesis. Results by pulse spectroscopic methods. Quart Rev Biophys 4 438.

As PPi dependent protein phosphorylation is light induced and electron transport dependent, and ATP dependent kinases exist in thylakoids (1,2), it is obligatory to check the possibility that Pi might be produced from hydrolyzed PPi and coupled to endogenous ADP to synthesize ATP by photophosphorylation in remaining intact chlorop-lasts. However, venturicidin, DCCD (N,N-dicyclohexylcarbodiimide) and Dio-9, which all inhibit ATP synthesis in thylakoids (10), do not affect the PPi dependent protein phosphorylation (Fig. 3, lanes C-E). 

We found that in chloroplasts, both triphenyltin (TPT) and DCCD are less inhibitory of flash-induced ATP synthesis than of phosphorylation in continuous light. Also, the pattern of inhibition by triphenyltin of ATP synthesis induced by increasing numbers of flashes was very similar for phosphorylation associated with PSI, PSII, or whole chain electron transport. These results indicate that in chloroplasts, there is probably not a special association between a particular electron transport complex and a specific coupling factor. Experimental procedures have been described previously (Graan, Ort 1982) or are provided in the figure legends. 

Fig. 13 Photo-induced processes occurring during photovoltaic energy conversion at the surface of the nanocrystalline titania films, 1 sensitizer (S) excitation by light, 2 radiative and nonradiative deactivation of the sensitizer, 3 electron injection in the conduction band followed by electron trapping and diffusion to the particle surface, 4 recapture of the conduction band electron by the oxidized sensitizer (S+), 5 recombination of the conduction band electrons with the oxidized form of the redox couple regenerating the sensitizer and transporting the positive charge to the counterelectrode. Grey spheres, titania nanoparticles, red dots, sensitizer, green and blue dots oxidized and reduced form of the redox couple. See Color Plates...

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