PHOTOSYNTHESIS 



phosphorylated compounds participate at all in the reduction of car- 

 bon dioxide, they must be either drawn from a reservoir filled during 

 preceding anaerobic periods or produced by a cycle belonging to the 

 assimilatory system. No indication in favor of the first alternative 

 has been obserxed. Special experiments to test it, however, have yet 

 to be performed. 



Another approach to the question of whether phosphorylations 

 occur within the assimilatory mechanism would be by way of investigat- 

 ing further the metabolism of the photosynthetic purple bacteria 

 (6,19,20). Studies on purple bacteria have proved extremely useful 

 in the past for elucidating the similarities between photoreductions 

 and the photosynthesis of green plants {cj. references 19 and 20). We 

 now arrive at the point at which it would be of interest to analyze 

 in detail the differences in their mechanisms. 



An important difference between the green plants and the 

 purple bacteria from the point of view of the possible role of phos- 

 phorylated compounds is the fact that the metabolism of the plant 

 centers around carbohydrates, whereas most of the purple bacteria 

 decline to utilize them in any way either in light or dark. They do not 

 respire and they do not ferment glucose and consequently cannot grow 

 in glucose media. The metabolism of purple bacteria revoh'es, if we 

 consider only organic substrates, mainly around aliphatic acids and, 

 in a few cases, simple alcohols. Acetate, propionate, butyrate, croto- 

 nate, malate, etc., are favorite substrates. The elementary analysis 

 of an entire green plant yields, in general, data indicating the pre- 

 dominance of compounds of carbohydrate nature. The available 

 elementary analyses of some purple bacteria have yielded figures 

 indicating the presence of more hydrogen and of less oxygen than is 

 found in carbohydrates and a composition very close to that of an 

 extracted substance having the formula (C4H602)„. The latter 

 proved to be a polymer of crotonic acid. The green plants store 

 carbohydrates in more or less pure form whenever photosynthesis 

 has lasted for a while in strong light, for respiration proceeds at a much 

 slower pace. Some purple bacteria {Athiorhodaceae) do not seem to 

 accumulate photoproducts in excess of what can be used for immediate 

 •synthesis of integrated cell material, that is to say, for growth of more 

 bacteria. Others {Thiorhodaceae) accumulate unknown photosynthetic 

 l^roducts that break down anaerobically in the dark by a sort of back 



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