PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CON^^RSION PROCESS 409 



Arnon: DPN is not reduced at all by light in chloroplasts. Chloroplasts are 

 specific for TPN ; DPN does not work. 



LooMis: I would say that one of your slides indicates that oxygen was being 

 released too, because light affecting the chloroplasts would release an oxygen. 



Arnox : Quite so. This is a difference between the oxidativ^e phosphorylation 

 by mitochondria in the Vishniac and Ochoa system and non-cyclic photophos- 

 phorylation by chloroplasts. In their system oxygen had to be supplied, in our 

 system oxygen is an excreted by-product. 



Vernon : This morning you said there was some evidence for photophos- 

 phor>-lation accompanying DPN reduction. Could you expand on this ? 



Arnon: Yes, we have evidence, which is not yet as extensive as we wish, that 

 non-cyclic electron transfer in photosynthetic bacteria is accompanied by phos- 

 phorylation, as would be expected from our postulation ; our basic view is that we 

 get a phosphorylation whenever cytochrome is oxidized. When we supply electrons 

 from an exogenous electron donor through cytochromes to chlorophyll and thence 

 to DPN, the cytochromes get oxidized by chlorophyll as the electron transfer 

 occurs. We now have evidence that phosphorylation is also coupled with these 

 reactions in photosynthetic bacteria. 



GoLDACRE : In Xitel/a, chloroplasts which are free in the cytoplasm can often 

 be seen rotating at the rate of several rotations a second even in expressed cytoplasm 

 outside the cell. This conceivably can be the result of a flow of current tangential 

 to the surface. We have heard a lot about the movement of electrons, and the 

 evolution of hydrogen and oxygen, and potential differences, and I was wondering 

 if under any conceivable arrangement of the components of chloroplasts you could 

 get current flowing over part of the surface of the chloroplast. 



Arnon : I would not wish to speculate so far afield. Suffice it to say that accord- 

 ing to our present view what is really important is that we form chemical energy 

 from light. Once the cells form ATP, the cell can use it for different metabolic 

 purposes. With the availability of ATP as a general kind of cellular currency other 

 energy-requiring cellular phenomena are possible and would not necessarily have 

 to be connected with specific electron transfer reactions. 



Frenkel : Warburg in recent publications, has stressed the importance of COo 

 for the Hill chloroplast reaction. I am, therefore, interested in Dr. Arnon 's views 

 as he indicated that CO., plays no role in the Hill reaction. 



Arnon: May I just make this comment. Warburg has proposed that COo 

 reacts catalytically in the Hill reaction and that CO^ cancels out in the overall 

 balance of the reaction. There is nothing in our work which excludes this possi- 

 bility. W^hat we do maintain, however, is that there is no uet COo fixation because 

 our experiments were done without added COo and in the presence of KOH. 



