566 LIGHT AND LIFE 



of water has been one of the most fruitful tlieories in biology for a very 

 important reason: it has led to experiments. It has guided the work on 

 photosynthesis for the last 20 years, including specifically our own research 

 up to the last year. Now we have gotten away from this hypothesis. Because 

 of our results with Chromatiurn and with green plants, we now visualize 

 the primary photochemical act not as a photolysis of water, where we split 

 water to an H and OH radical, but rather as the formation of an oxidized 

 cytochrome. Instead of photolysis we have an ionization of water as we 

 have described in our scheme published in Nature (5) . We use the proton 

 from the water, not the hydrogen atom, and we use the OH". By a reaction 

 which is really an analog of the anode reaction, we take the electron from 

 the OH", to a cytochrome to the chlorophyll to the pyridine nucleotide, with 

 a concomitant release of oxygen. 



Dr. Forti: I think that Dr. Vishniac reported recently that the hydrogen 

 is incorporated from water into the chlorophyll molecules. I wonder if Dr. 

 Arnon or Dr. Vishniac would comment on that. How would this fit with 

 Dr. Arnon's ideas about splitting of water? 



Dr. Arnon: I would like to defer this question to Dr. Vishniac. 



Dr. Vishniac: Would you repeat that question, please? 



Dr. Forti: Well, I know that you reported very recently that tritium is 

 incorporated from tritiated water into chlorophyll. It seems to me that this 

 would really imply a splitting of water. 



Dr. Vishniac: No, I don't think so. All I can say is that in the light we 

 get incorporation of tritium from water into chlorophyll a. This need not 

 be a direct incorporation of tritium, it could be a secondary effect of electron 

 transport. 



Dr. Forti: What about the subsequent transfer of tritium from chlorophyll, 

 also reported. 



Dr. Vishniac: This is more complicated than it might appear. I would 

 rather not discuss it right now. 



Dr. fA(;KNi)ORF: I was a little bit confused aiwut the ratio between the 

 anaeroi)ic and oxygen-requiring phosphorylation, which you find as 2.2. Can't 

 you alter that ratio cpiite at will by just varying the concentration of co- 

 factors? 



Dr. Arnon: The (oncentrations of cofactors in these experiments were 

 selected so that they were optimum for each type of reaction. That is, the 

 optimum concemration for the anaerobic type and the oxygen-catalyzed type 

 that Dr. Vennesland and you have studied. 



Dr. Jagendorf: How do you define the borderline between the two? 



Dr. Bishop: I have found no vitamin K at all in chloroplasts of spinach, 

 sugar beets or in Swiss chard. My techniques are perhaps not as definitive 

 as those of Dr. Zill and he also has not detected vitamin K in chloroplasts. 

 Also in photosynthetic bacteria, including Chromatiurn, I have not observed 



