PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 4OI 



pyridine nucleotides by a non-cyclic electron flow mechanism. In organ- 

 isms which contain, or can adaptively form, hydrogenase or nitrogenase 

 (photosynthetic bacteria and algae), this phase of photosynthesis can also 

 be observed today as a photoproduction of molecular hydrogen or photo- 

 fixation of nitrogen gas. 



The mechanism of anaerobic cyclic photophosphorylation appears to 

 have remained essentially unchanged and constitutes today the common 

 denominator of all photosynthetic cells. The difl^erences between bacterial 

 and plant photosynthesis seem to have arisen from evolutionary trans- 

 formations of the non-cyclic electron flow mechanism. When it first 

 emerged, the non-cyclic electron flow mechanism was probably of the 

 bacterial type. It could accept electrons from several electron donor 



10 20 



minutes 



30 



Fig. 35. Pyridine nucleotide reductase from Chromatiiim. Experimental 

 conditions as in Table XIX, except that KoH^'-POj and ADP were omitted. 4 /xmoles 

 DPN or TPN were added as indicated. (Losada, Nozaki, Tagawa, and Arn( n 

 [155]; Whatley, Dieterle, and Arnon [161]). 



substances (thiosulphate, succinate, etc.) but not from water. Water 

 became an electron donor in the non-cyclic electron flow mechanism only 

 with the emergence of plant photosynthesis. 



As was already discussed, the use of water as an electron donor, and 

 the resultant evolution of oxygen, are not essential for the key events in 

 the non-cyclic electron transport, the reduction of TPX and the coupled 

 formation of ATP. When, under special experimental conditions, ascorbate 

 replaced water as an electron donor [i6o], chloroplasts formed TPNH., 

 and ATP without the oxidation of water, i.e. without oxygen evolution 

 (Table XIV). 



The basic similarity of the non-cyclic electron flow mechanisms in 

 bacteria and chloroplasts is strengthened by the recent isolation by 

 Losada et al. [155] of a photosynthetic pyridine nucleotide reductase from 



