Introduction 37 



labeled but the oxygen of the carbon dioxide was not. It was observed that a part 

 of the oxygen given off in light was tracer-labeled. In view of the rapid hydration 

 of carbon dioxide in Chlorella, however, these experiments in no way constitute 

 proof for the photolysis of water. 



Subsequently, R. Hill thought he had proved the photolysis ofwater, whenhe 

 discovered that ferric Oxalate develops oxygen in light from chloroplasts, without 

 addition of carbon dioxide. Hill's experiments, however, failed to prove the photo- 

 lysis of water, since ferric Oxalate produces carbon dioxide in light and since, in 

 other ways as well, carbon dioxide was not properly excluded from the experiments. 



It is a fact that for a long time, from 1944 to 1958, the only support for the the- 

 ory of water photolysis could be found in our studies 19 . This work showed that 

 quinone develops oxygen from green grana carefully washed with water, the reac- 

 tion taking place in the presence of light and according to the equation 



2 quinone — 2 HoO = 2 hydroquinone — Oo 



The inability of the grana to develop oxygen from added carbon dioxide seemed 

 to block the oxygen-from-carbon dioxide pathway even more effectively. 



In 1959 and 1960, however, application of new methods to the quinone reactions 

 made us realize that such reactions could not proceed without carbon dioxide. 

 Only a very small quantity of carbon dioxide was needed, because the effect of 

 carbon dioxide in these quinone reactions is catalytic rather than stoichiometric. 

 Actually the only difference between quinone reactions and photosynthesis is with 

 respect to the mechanism by which carbon dioxide is utilized : In quinone reactions, 

 the reduced carbon dioxide is completely reoxidized to carbon dioxide by excess 

 quinone; whereas, in photosynthesis, a part of the reduced carbon dioxide remains 

 in the form of "fixed carbon dioxide." Thus, in 1960, went down the only sup- 

 porting pillar of the theory of water photolysis, and with it, the entire edifice of 

 theories based on the photolysis of water. 



We never took it seriously that the reduction of phosphoglyceric acid by dihydro- 

 nicotinamide — a reaction we discovered (see page 30) in 1939 — should be the reac- 

 tion by which carbon dioxide is reduced in photosynthesis. The appearance of 

 phosphoglyceric acid in radiograms taken from Chlorella after exposure to light 

 is adequately explained by the occurrence of light-induced respiration. 



6. The real Photolyte of Photosynthesis 



If Chlorella is shaken with carbon dioxide, under aerobic conditions and in the 

 dark, carbon dioxide is fixed chemically. If the uptake of carbon dioxide is allowed 

 to proceed to completion and is then followed by illumination, the carbon dioxide 

 disappears, with evolution of oxygen. This latter reaction proceeds until there is 

 equilibration of the processes of degradation and uptake of carbon dioxide, that 

 is until photosynthesis has reached a steady State. We call the carbon dioxide 

 Compound accumulating in the dark, from which oxygen is liberated in light, the 

 photolyte of photosynthesis. 



The maximum concentration of photolyte that can be obtained in Chlorella 

 depends on the pressure of carbon dioxide and on the type of cells that are being 



