258 



ELECTRON TRANSPORT 



ATP was inactive, the subsequent illumination caused the immediate 

 photooxidation of DPIPH2 in the typical fast reaction. The possible ob- 

 jection to this reasoning is that ATP may not enter into the chromato- 

 phore and be able to affect the reactions as does ATP formed directly 

 in the photophosphorylation process. This argument would seem in- 

 valid since the chromatophores used in this experiment are capable of 

 coupling with external ADP and Pi to form ATP which is contained 

 in the medium. Thus the phosphorylation site is available to added 

 reagents and a reversed electron flow, if operative in the above ex- 

 periments, should have been capable of demonstration. 



3 

 MINUTES 



Fig. 15. Effect of ATP addition upon DPIPHg oxidation 

 with NAD in the presence of /?. r«6n<>» chromatophores. 

 The experimental conditions given for Fig. 1 were em- 

 ployed with 0.21 mg BChl present. At the arrow, 8 

 ^moles of ATP were added to the reaction system. 



The reasons given above for supporting a direct electron transfer 

 from DPIPH2 to NAD are based on the fact that ATP formation and 

 DPIPH2 photooxidation can be separated by means of inhibitors and 

 various treatments. The possibility remains, however, that the reac- 

 tion does proceed via reverse electron transfer, with the energy being 

 supplied not by ATP itself , but by some high energy intermediate which 

 under normal circumstances would lead to ATP formation. K the in- 

 hibitors act at a late stage in ATP formation, and allow the formation 

 of intermediate high energy compounds, then the conclusions based on 



