BENT LEY GLASS 873 



against the iheniUKheinical gradient. Hill and his colleagues now 

 preler a scheme in which two light-driven reactions against the iher- 

 niocheniical gradient are connected by an exergonic reaction between 

 cytochromes / and I),-. Ihis implies an intermediate stage when cyto- 

 chrome / is oxidi/ed and cytochrome /;,, is reduced. Such a state has 

 i)een observed in illuminated pale yellow-green leaves, in agreement 

 \\ith this hypothesis; but it has not been observed to date in normal 

 green leaves or in chloroplast preparations. Nor have the spectral 

 changes to be expected during the postidated hydrogen transfer been 

 demonstrated so far. 



In general, algae and photosynthetic bacteria also possess both /-type 

 and b cytochromes, but their roles in photosynthesis remain unknown. 

 Lucile Smith reports on the properties and behavior of bacterial cyto- 

 chromes in the bacteria Rliodospirilhim nibrum and Rhodopseu- 

 domonas spheroides. Light absorbed by tlie bacteriochlorophyll re- 

 sults in oxidation of the cytochromes (characterized by reduction in 

 absorption of the Soret bands lying between 400 and 450 m^) , as well 

 as changes in the absorption spectra of the carotenoid pigments which 

 are present. The cytochromes oxidized in the light include those 

 oxidized when oxygen is added in the dark. These cytochromes are 

 those of the respiratory transport chain, the equivalent of the mito- 

 chondrial cytochromes of higher organisms. But in addition at least 

 one other cytochrome is oxidized, one with an absorption peak at 

 420 Yiifi, corresponding to the cytochrome Co of these bacteria isolated 

 by Vernon. Cell-free extracts of R. rubrum, which can both respire 

 in the dark and promote rapid photophosphorylation, do not exhibit 

 oxidation of the cytochromes upon illumination unless phosphate 

 acceptor (ADP) is added to the system. Cytochrome c^ is consequently 

 implicated in the process of electron transfer associated with photo- 

 phosphorylation. The action of substrates and of inhibitors also dis- 

 tinguishes between the respiratory and photophosphorylative cyto- 

 chrome chains in the bacteria. But both chains are conceived to be 

 very similar in properties, to include both c-type and 6-type cyto- 

 chromes, and to conduct phosphorylation in a manner similar to 

 "oxidative phosphorylation." Only, in the one case, the source of 

 electrons is the oxidizable substrate; in the other it is the photo- 

 reductant produced in the primary reaction of photosynthesis; and 

 in the former case the transfer may be either to oxygen or to the 

 photooxidant produced in photosynthesis, whereas in the second 

 case only the photooxidant acts as electron acceptor. 



Martin Kamen suggests that the role of the cytochromes or other 



