36 Introduction 



be considered explained, until the 1-quantum reaction on which it is based has 

 been discovered. 



Working in Dahlem, in 1950, Dean Burk and the author discovered the 1-quan- 

 tum reaction of photosynthesis, a reaction in which 1 light quantum develops 1 

 molecule of oxygen, as follows : 



X0 2 -i- 1 hv = X + 2 



where XO2 is the substance from which the oxygen develops. Obviously, XO2 

 cannot be carbon dioxide, since several quanta of visible light are needed to liberate 

 the oxygen from carbon dioxide. 



This is the way the 1-quantum reaction was discovered. We were making yield 

 determinations with Chlorella and were calculating the yields, as usual, from the 

 dirTerence in manometric readings obtained in light and in darkness. During this 

 work we noticed that yields improved when the intervals between the light and 

 dark periods were shortened. A limit was reached at time intervals of 1 minute 

 each. Then 1 light quantum developed 1 molecule of oxygen. 



In the course of these experiments we further observed that shorter light and 

 dark periods not only increased the amount of oxygen developed in light but 

 also the oxygen consumed in the dark. To the discovery of the 1 quantum reaction 

 in light was thus added the second discovery that the subtrate XO2 arises in a dark 

 reaction in which extra oxygen is consumed. This discovery led to an explanation 

 of the 1-quantum reaction energetically and chemically. 



Many favorable conditions contributed to these discoveries ; but a high carbon 

 dioxide pressure (at least 10" ,, of an atmosphere) was primarily responsible.* If the 

 carbon dioxide pressure had failed to exceed 2% of an atmosphere, the 1-quantum 

 reaction would not have been discovered. Another condition promoting these dis- 

 coveries was the Substitution of ammonium chloride for nitrate in the Suspension 

 medium of Chlorella; the use of large quantities of cells in the manometric vessels 

 was necessary because the 1-quantum reaction is not a catalytic but a stoichiometric 

 cell reaction; and, finally, a fully developed two-vessel method was required that 

 would suffice to measure the oxygen metabolism in one minute with an adequate 

 degree of accuracy. As in other areas of cell physiology, the decisive discoveries 

 were made by the use of the two-vessel method. 



Later on the techniques for measuring the 1-quantum reaction were simplified 

 and improved. In the course of years, use of the new methods brought confirmation 

 of all results of the first work, experiments as well as conclusions. 



5. Is Water the Photolyte in Photosynthesis ? 



The theory that photosynthesis is the photolysis of water originated by the use 

 of isotopes in experiments with Chlorella, in which the oxygen of the water was 



* 1961 Addition. High carbon dioxide pressures are necessary to saturate Chlo- 

 rella with photolyte : that means to saturate the Chlorophyll with photolyte. Only 

 from Chlorophyll that is combined with photolyte can absorbed light develop 

 oxygen. 



