PHOTOSYNTHESIS 



independently; they supplement or exclude each other. In a very 

 interesting quantitative experiment, van Niel (20) found that the 

 bacteria simply cease to take up oxygen when exposed to a sufficiently 

 intense radiation. Here w^e have to search for a direct interrelation, 

 an intermediate metabolic link. 



We may sum up as follows. Whether phosphorylated com- 

 pounds participate in photosynthesis must be considered in the light 

 of two sets of observations. First, the assimilatory mechanism in 

 green plants and in purple bacteria must be "self-supporting" as far 

 as phosphorylations are concerned, since it functions under conditions 

 in which neither respiration nor fermentation of carbohydrates seem 

 capable of providing enough ready-made phosphorylated compounds. 

 Second, in purple bacteria a carbohydrate appears perhaps as the 

 primary product of the photochemical reaction, but instead of being 

 stored it is converted into cell material of different elementary com- 

 position. These organisms are incapable of oxidizing or fermenting 

 ordinary plant carbohydrates. 



From the evolutionary point of view it is interesting that the 

 "respiration" of the purple bacteria corresponds to the oxyhydrogen 

 reaction and related oxidations in anaerobically adapted algae and not 

 to the "normal" respiration in plants. We may speculate that the 

 liberation of oxygen from "hydroxylated" compounds could be com- 

 bined effectively with the reduction of carbon dioxide only after the 

 synthesis and the utilization of sugars had become separated. Under 

 aerobic conditions, the back reactions with free oxygen in the assimila- 

 tory system had to be prevented. We know that in adaptable algae 

 this is brought about by the oxidative inactivation of one or two of the 

 catalysts involved. Apparently we have here a parallel to the well- 

 known case in which anaerobic fermentations are prevented from con- 

 tinuing in air by a special oxidation, the so-called "Pasteur reaction." 



References 



(1) Brown, N. C, Forest Products. 2nd ed., Wiley, New York, 1927. 



(2) Emerson, R. L., Stauffer, T. F., and Umbreit, W. W., "Relation- 

 ship between phosphorylation and photosynthesis in Chlorella" Am. J. 

 Botany, 31, 107 (1944). 



(3) Forestry Depletion in Outline. Northwest Regional Council, Portland, 

 1940 (25^). 



47 



