372 DANIEL I. ARNON 



bacterial photosynthesis, not all of the high energy electrons would return 

 via the cyclic route to [Chl+]. Some of them would be passed on to 

 pyridine nucleotide and used for CO2 assimilation. 



The electrons so removed from the photoreceptor particle would be 

 replaced by electrons donated by thiosulphate or succinate. This electron 

 transfer would be mediated by cytochromes. Thiosulphate and succinate 

 would thus act as hydrogen donors that reduce bacterial cytochromes after 

 these are oxidized by chlorophyll in light. The cytochrome system in 

 photosynthetic bacteria would be a gateway for the entry of electrons of a 



400 



500 

 ^ im/i) 



600 



Fig. 14. Reduction of Chromatium cytochromes by thiosulphate in a cell-free 

 system. Reaction mixture included, in a final volume of 3 -o ml. of o ■ i M tris buffer, 

 pH 7-8, chromatophores (P) containing o-i mg. bacteriochlorophyll and super- 

 natant fluid (S) corresponding to 0-3 mg. bacteriochlorophyll. 20 ^umoles of thio- 

 sulphate were added to one of a pair of Thunberg-type cuvettes and the resulting 

 difference spectrum was measured in a Gary spectrophotometer after 20 min. at 

 room temperature. Gas phase, argon (Losada, Nozaki and Arnon [136]). 



relatively low reducing potential and for their transfer to chlorophyll, 

 where they would be raised to a higher reducing potential at the expense 

 of the energy of absorbed light. 



The proposed sequence of reactions in photosynthetic bacteria will be 

 collectively designated as the non-cyclic electron flow mechanism. The 

 three components of the non-cyclic electron flow mechanism are {a) an 

 external electron donor system (represented here by thiosulphate or 

 succinate), {h) the photoreceptor particle which raises the donated electron 

 to a higher reducing potential at the expense of the energy of light, and 

 [c) the electron acceptor system (exemplified by DPN or TPN). 



Experimental support for the non-cyclic electron flow mechanism in 

 bacterial photosynthesis has recently become available. First, it was 



