DISCUSSION 567 



\ilamiii K l)ut rather an appreciable amount of Coenzyinc Q272. Now, 

 would you conuiiciit on this? 



Dr. Arnon: \'c's, as I said at the beginning, what we need there is a 

 quiuoue. Now we said that it was vitamin K, because we accepted the work 

 of Dr. Dam. He finds \ itamin K specifically concentrated in the chloroplasts. 

 If it turns out that plastocjuinone gives the same effect as vitamin K we 

 will accept that result without altering our scheme in any essential respect. 



Dr. Thomas: I would like to apologize to Dr. .-\rnon with regard to 

 measurement of oxygen evolution in isolated chloroplasts. In a paper I 

 mentioned that Dr. Arnon measured photosynthetic oxygen evolution as a 

 Hill reaction. This has been done in many cases and I must confess that 

 I did not read tlie section on methods too well. For in the second paper, I 

 think, in the section on methods he measured oxygen evolution in photo- 

 synthesis. The point I made is that Dr. Arnon stated that oxygen evolution 

 in the Hill reaction is the same as oxygen evolution in photosynthesis. Now 

 in some work as we did in Utrecht we tried to find oxygen evolution by 

 isolated chloroplasts in photosynthesis, not in the Hill reaction. In the 

 paper describing this study, we claimed to be the first to find a true photo- 

 synthetic oxygen evolution by chloroplast suspensions. As the misinterpre- 

 tation of Dr. .Arnon's results was brought to our attentioii after publication 

 of our paper, I am glad to find an opportunity here to state that, contrary 

 to our previous remark, Dr. Arnon and coworkers were the first to obtain 

 photosynthetic oxygen evolution with isolated chloroplasts. 



Dr. Kok: I am rather impressed by this idea of Chromatium making only 

 ATP in the light, using hydrogen as the reductant. But I was so brought 

 up in van Niel's thinking that it is hard to digest. If you only need light 

 to form ATP and not to form the reductant, why do you need so many 

 quanta? Why is this process so inefficient? You would think that one 

 quantum per CO2 would be enough to work. 



Dr. Arnon: I am not prepared to discuss quantum efficiency in photo- 

 synthesis from our own data. But if I am asked to speculate I would say 

 that the minimum that we would need, according to our scheme, for green 

 plants, based on our work with cell-free systems, is about 4 and a fraction 

 quanta. Let me make another point that I forgot to mention. This Chro- 

 matium photosynthesis with hydrogen is, I believe, nothing else than Dr. 

 Gaffron's photoreduction in hydrogen-adapted algae, where he simply 

 forced the algae to take a march back in evolution X million years, and in 

 this way they have learned again to live as they used to in the old days, 

 reducing pyridine nucleotide in the dark and using light only for ATP 

 formation. 



Now as far as TPN reduction in chloroplasts is concerned we need at 

 least two quanta to reduce 1 TPN. If we get the electron from OH" to the 

 cytochrome-chlorophyll complex and then to TPN, we would need two of 



