METABOLISM OF PHOTOS YN THE TIC BACTERIA. II. 



CERTAIN ASPECTS OF CYCLIC AND NONCYCLIC 



PHOTOPHOSPHORYLATION IN 



RHODOSPIRILLUM RUBRUM 



M. NOZAKI,! K. TAGAWA and DANIEL I. ARNON^ 



Department of Cell Physiology 

 University of California, Berkeley 



Investigations with isolated chloroplasts led to the discovery of 

 photosynthetic phosphorylation comprising two photochemical re- 

 actions, now called cyclic (Eq. 1) and noncyclic (Eq, 2) photophos- 

 phorylation (1-4), which produce the assimilatory power (ATP and re- 

 duced pyridine nucleotides) to drive carbon assimilation during photo- 

 synthesis. 



hv 



ADP + Pi >- ATP (1) 



chloroplasts 



2TPN + 2ADP + 2Pi + 4H O *- 2TPNH 



^ 1-1 1 X " 



hv 

 chloroplasts 



+ 2ATP + ©2 + 2H2O (2) 



The terms cyclic and noncyclic photophosphorylation were suggested 

 to denote the difference between a "closed" and an "open" electron 

 transport pathway (coupled with ATP formation) that are envisaged 

 for reactions 1 and 2, respectively (4,5). In cyclic photophosphoryla- 

 tion, the electrons cycle in a "closed" system within the photorecep- 

 tor particle, whereas in noncyclic photophosphorylation the photo- 

 receptor particle mediates an "uphill" electron transfer in an "open" 

 system from an external electron donor to an external electron ac- 

 ceptor. 



1 Present address: Department of Biochemistry, Medical School, Kyoto Univer- 

 sity, Kyoto, Japan. 



2 Aided by grants from the National Institutes of Health and the Office of Naval 

 Research. 



175 



