300 D. I. AHNON, M. H. ALLEN, F. R. WHATLEY 



Krall : Has the effect of cyanide or carbon monoxide been studied in your sys- 

 tem? 



Arnon: Cyanide inhibits CO-, fixation much more tlian it inhif)its photosyn- 

 thetic phosphoryhition, and botii of these more than photolysis, whicli is hardly 

 inhibited at all by cyanide. So there is a differential effect the same as with the 

 other inhibitors. However, I think the results of the cyanide inhibition should be 

 taken with great caution. 



Bassham: I just want to point out that Dr. Arnon suggested in his chloroplast 

 preparation there are separate pathwa3^s for electron transport to the carbon- 

 reduction cycle and to the i)hosphorylation system. This is not surprising, since 

 vitamin K can be a Hill oxidant and more or less short-circuit the path of the 

 electrons from the j)hol()chemical reaction to photosynthetic phosphorylation. 

 However, since two separate paths are indicated, and since in photosynthesis in 

 whole plants there is probably a different path for the electrons from the photo- 

 chemical reaction to carbon dioxide reduction from the pathway to phosphoryla- 

 tion here. I don't think one can draw any conclusions from these data regarding 

 the matter of the electron transport system between the photochemical part and 

 the carbon-reduction part. 



Anion : I am sorr}- but I do not understand the last point. What is the basis foi' 

 the statement that in the intact cell there would be a different electron transport? 



Bassham : It is quite possible that in the intact cell we don't known as yet the 

 path between the photochemical reaction and the reductant that is used in the 

 carbon-reduction cycle. But in your chloroplast preparations you may be short- 

 circuiting this path by putting in vitamin K or FMN, which accepts the electrons 

 directly from the Hill reaction and then uses these for the photosynthetic phos- 

 phorylation. If there is such a short-circuit as that, one cannot draw anj' conclu- 

 sions regarding the nature of the electron transport system in photosynthesis 

 between the photochemical and the carbon-reduction cycle. 



Arnon : We don't regard this as a short-circuit at all. 



Bassham : Do you have any reason for not regarding it as a short-circuit? 



Brown : It is a basic disagreement — not a matter of misunderstanding. 



Arnon : No, there is some misunderstanding which I would like to clear up. We 

 believe that in photosynthesis, at least in photosynthesis by isolated chloroplasts, 

 there are two normal sinks for electrons, one to generate ATP and the other to 

 reduce CO2. The CO2 sink is dependent on ATP. I was trying to point out (this 

 is the point which is very important in our scheme) that in no case have we ever 

 had CO2 fixation by a chloroplast preparation which has lost the capacity for 

 ATP formation. We consider that ATP is a prerequisite for CO2 fixation. Thus 

 the formation of ATP is a normal path in our scheme, not a short-circuit. 



The only short-circuit which would occur in our scheme would be the Hill reac- 

 tion. When we substitute an artificial electron acceptor, such as quinone or ferri- 

 cyanide, for the phosphorylating pathway, we simply do not permit the electrons 

 to recombine with the oxidant and to carry out the coupled phosphorylation. 

 Instead we get the Hill reaction. 



Lucile Smith : I am wondering why you think it is necessary to add vitamin K 

 back to your preparation. Do you think that in the preparation of chloroplasts 



