568 LIGHT AND LIFE 



these electrons to reduce 1 TPN. Since we need four hydrogens to reduce 

 one molecule of CO., we would therefore need 4 quanta to get 2 TPNH. 

 Thus we arrive at a minimum of 4 quanta. In Chromatium the problem 

 doesn't arise, if one uses hydrogen gas. What remains then is to use light 

 energy only for making ATP. 



Dr. van Niel: But I thought that the point which Dr. Kok wanted to 

 make is exactly that it makes it very unintelligible. 



Dr. Arnon: Then I don't understand the question. 



Dr. van Niel: If you make ATP with light energy, a single quantum will 

 produce enough ATP to carry on a lot of photosynthesis. 



Dr. Arnon: In the non-cyclic photophosphorylation in chloroplasts we get 

 one phosphorylation per two electrons transferred. This is the stoichiometry 

 which we observe with ferricyanide. Now two einsteins at a wave length of 

 red light give roughly about 80 kilocalories. So from 80 kilocalories we can 

 theoretically get 6 ATP's using 12 kilocalories as the value for making 1 

 ATP at a physiological pH. This figure of 12 seems to go up and down 

 like the stock market. According to Dr. H. A. Krebs, under physiological 

 conditions 12 kilocalories is still the figure to use. In mitochondrial systems 

 one gets 3 ATP's formed in the transfer of two electrons from DPNH to 

 oxygen. So, if an analogy to oxidative phosphorylation is allowed, with 2 

 quanta of red light we would also expect to get only 3 ATP's. Photosynthetic 

 bacteria use quanta of even longer wavelengths than green plants so that 

 they receive even less energy, and therefore the efficiency would come out 

 even better. If one gets 3 ATP's requiring 36 kilocalories from two quanta 

 of light supplying 80 kilocalories, the efficiency is over 50%. Using light at 

 longer wavelengths, the efficiency is even better. As a historical sidelight, 

 let us not forget that when oxidative phosphorylation was first discovered, 

 experimentally, only slightly better than 1 ATP per two electrons was 

 observed and it took about 10 or 20 years to get up to 3. 



Dr. Lipmann: I would like to say that no one knows the quantum yield 

 of phosphorylation. I think these chloroplast preparations vary in their 

 photophosphorylation capacity, because they may be damaged. Therefore 

 one cannot get the true quantum yield from photophosphorylation from 

 chloroplasts. Many of the reactions can go on in chloroplasts which are 

 not capable of giving good photophosphorylation. Therefore, I do not 

 think it is a good idea to judge quantum yields now. 



Dr. Kok: One should recall the work of VVassink et nl. (Wassink, E. C, 

 E. Katz and R. Dorrestein, Enzymologia 10: 285, 1942) with Chromatium, 

 which showed equal quantum requirements (10 h /CO2) , with either hydro- 

 gen or thiosulfate as the reductant (this was later confirmed by Larsen 

 [Larsen, H., C. S. Yocum and C. B. van Niel, /. Gen. Physiol. 36: 361, 1952] 

 with Chlorobiutn) . Possibly even more striking was that the bacteria 

 showed no preference for hydrogen compared to thiosulfate when both 

 were fed simultaneously. Hardly any light energy is required to reduce CO2 



