Photosynthesis 387 



40 years, in different institutes throughout the world, very different photosynthetic 

 yields have been found— different not in percentages, but in hundreds of percent. 

 Even if the manometry and the light measurements had been correct everywhere, 

 not even approximate agreement would have been possible, owing to ignorance of 

 the essential conditions of culture and measurement. Thus, in the United States, 

 during the years 1938 to 1948, an average quantum requirement of 16 per mole- 

 cule of oxygen gas produced was found, corresponding to an energy yield of 18 

 percent in red light. This value is removed from the optimal value 1 by several 

 hundred percent. 



If one maintains the now-established conditions of good yield, one will obtain 

 good yields from now on, everywhere and always. Figure 3 shows an example of 

 oxygen evolution during constant illumination in a 5-hour experiment in which the 

 quantum requirement per molecule of 0-2 produced was approximately 3 for the 

 entire period. Any deviation from linearity with time was within the experimental 

 error. Figure 4 shows oxygen development in a 6-hour experiment in which the 

 quantum requirement per molecule of O2 produced was approximately 4. Table 1 

 contains the results of 23 six-hour experiments conducted on 23 days of the months 

 March to May, 1957, in which only a single instance of a poor yield, namely a 

 quantum requirement of 7.5, was obtained 3 . 



The quantum requirement of 3 per molecule of O2 signines that in red light 

 about 90 percent of the incident light energy can be converted into chemical 

 energy. Since light energy is freely transformable energy, this energy efficiency is 

 completely compatible with both the first and second laws of thermodynamics. 

 Thermodynamically incompatible with good yields were only those theories con- 

 cerning the chemical mechanism of photosynthesis that are today at long last 

 recognized as incorrect. 



In summary, one can say that, with the fixing of the conditions of culture and 

 measurement, the dispute concerning the efficiency of utilization of sunlight is 

 finally decided. It is a decision in favor of nature. The reaction by which nature 

 transforms the energy of sunlight into chemical energy, and upon which the 

 existence of the organic world is based, is not so imperfect that the greater part 

 of the applied light energy is lost; on the contrary, the reaction is, like the world 

 itself, nearly perfect. 



The Multiquanta Problem 



But how is it possible that carbonic acid can be split by the light quanta of visible 

 light, which are so deficient in energy that several quanta are necessary? In the 

 photochemistry of the inanimate world, no reactions are known in which several 

 quanta react with one molecule at one time, and, moreover, several-quanta reac- 

 tions are theoretically scarcely conceivable. 



The problem was solved several years ago at Dahlem by Dean Burk and us 4 . 

 By measuring photosynthesis under special conditions, a Splitting of photosynthesis 

 into two reactions was observed : a light reaction and a dark reaction. Normally these 

 two reactions overlap each other so that one cannot observe each one separately. 



