754 



SUMMARY 



is extended by experiments in which extraction of lipid com- 

 ponents, including plat toquinone by hexane, leads to inactiv- 

 ation of both O^ evolution and the 520 change. 



Although the original paper of Hill and Bendall suggest 

 cytochrome b^ as the primary reductant of Photosystera II, the 

 evidence so far has been discouraging (W. Bonner). Only under 

 abnormal conditions is a light induced reduction of this 

 carrier now clearly seen (J. Olson). 



In complete photosynthesis the reductant of the second 

 photoact reduces the oxidant of the first. The 0.4 volt 

 potential gap between these two is believed to be bridged by 

 exergonic electron flow, possibly coupled to ATP formation. 

 Indophenol and other quinoid dyes, reduced by ascorbic acid, 

 are able to donate electrons to some member of this region of 

 the chain between the two photoacts. In that case electron 

 flow to TPN or etc. is driven by photosystem I alone. Another 

 locus of entry for electron donors other than water appears to 

 be at the high potential end of the chain (Trebst, Vi/itt). 

 Such oxidations require both photoacts. A new approach in 

 these investigations (Levine, Bishop) is the use of biochemi- 

 cal mutants with lesions in the electron transporting enzymes, 

 combined with the application of specific electron donors, 

 oxidants, or different wavelengths of light. On the whole 

 these studies support the picture developed by other means. 



Electron transport over the chain now partially defined, 

 leads to phosphorylation of ADP. Net flow depends, of course, 

 on addition of some soluble redox component, to which recentJiy 

 "phosphodoxin" (Black) a natural low molecular weight com- 

 pound, can be added. The question why indophenol dyes were 

 photoreduced without concomitant phosphorylation seems to be 

 satisfactorily answered by the uncoupling effect of the 

 oxidized dye (Keister, Avron). 



Experience with phosphorylation accompanying the Kill 

 reaction continues to show a P/2e ratio of 1.0 as a maximal 

 limit. In view of the higher ratio demanded by the Calvin 

 scheme for COp fixation, and the demands in the cell for ATP 

 to be used in growth and other synthetic processes, it would 

 seem that in vivo phosphorylation should result in part from 

 cyclic or pseudo-cyclic electron flow as well. Further evi- 

 dence for photophosphorylation in whole algae, independent 

 of oxygen evolution, came at this meeting from studies of 

 acetate metabolism (Gaffron). Also, the photoinhibition of 

 respiration observed with the mass spectrometer can be ex- 



