PHOTOSYNTHESIS 



succinic acid from a-ketoglutaric acid and carbon dioxide. This 

 type of fixation reaction does not depend on energy-rich acyl phos- 

 phate. Our knowledge of phosphorylation and the "energy-rich" 

 phosphate bond is based entirely on the carbohydrate metabolism in- 

 volved in respiration and fermentation. We know that both types of 

 catabolic reactions make use of the same phosphorylated compounds. 

 If phosphorylated substances participate in photosynthesis, are these 

 intermediates related to, or identical with, those occurring in general 

 cell metabolism? Some special reaction must provide for the energy- 

 rich phosphate bond assumed to promote the initial fixation of carbon 

 dioxide in the dark. Is this special reaction part of the normal re- 

 spiratory system or something different? In this regard Ruben (15) 

 mentioned as a possible source of energy the dismutation to oxygen or 

 reduction to water of the intermediate hydrogen acceptors (hydrox- 

 ylated substances). The reduction of carbon dioxide by a purely 

 thermal oxidoreduction in green algae occurs under circumstances 

 which exclude normal respiration (7). Here the energy-rich phosphate 

 bond would present a welcome means of explaining not only the fixation 

 or carboxylation but also the coupling between the oxidation of 

 hydrogen and the reduction of carbon dioxide. Experimentally, 

 one should try therefore to establish the existence of an independent 

 phosphorylation cycle serving exclusively the oxyhydrogen or "Knall- 

 gas" reaction. 



While the oxyhydrogen reaction may be considered to be a 

 sort of respiration and consequently invites comparison with the 

 normal cell respiration in air, conditions become more unfamiliar when 

 we turn to photoreduction. We mentioned earlier in this article why 

 special back reactions in photosynthesis must be assumed. The 

 question arises whether such (hypothetical) back reactions contribute 

 to the formation of energy-rich phosphate bonds. The autonomy of 

 the mechanism of photochemical reduction becomes quite apparent 

 in the following observation (8) . A concentration of 0.001 M phthiocol 

 [also of methylnaphthoquinone (vitamin K) or of o-phenanthroline] 

 inhibits strongly the respiration of algae like Scenedesmus, prevents 

 completely normal aerobic photosynthesis, hinders the adaptation to 

 and the reversion from photoreduction under anaerobic conditions, 

 and severs the coupling between the oxyhydrogen reaction and the 

 reduction of carbon dioxide in the dark. But if the inhibitor is added 



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