THE CHEMISIRY OF PHOTOSYNTHESIS 135 



duced O2 is of no importance to those in favor of CO2 photolysis. However, 

 for those who accept water photolysis, all the Oo produced must of necessity 

 originate from water. 



Comparative biochemistry contributed greatly to the general acceptance 

 of water photolysis. However, the question arises whether it would not be 

 possible to apply CO2 photolysis to the photosynthetic sulfur bacteria. 



Light: CO2* ^ C + O, 



Dark: 2 HoS + Oo ^ 2 H.O + 2 S 



Dark: CO. -> CO2* 



Over-all: 2 H2S + CO2 -> C + 2 H2O + 2 S 



In the green plant a part of Oo is visibly produced. The remaining part is 

 used in the induced respiration providing additional energy. In the anaero- 

 bic sulfur bacteria it is, of course, impossible that the Oo produced is freely 

 developed. It must be used entirely for a reaction, specific and indispensable 

 to this organism, i.e., the oxidation of HoS. A comparison of this reaction 

 with that of induced respiration in plant photosynthesis shows some similarity 

 insofar as both reactions provide additional energy for the conversion of CO2 

 to CO2*. It is evident that these considerations are pure speculation. There 

 is no proof whatever that the reactions indicated above would proceed in 

 photosynthetic sulfur bacteria. The question is whether these reactions are 

 less probable than those discussed in § 48 and whether experimental evidence 

 instead of theoretical considerations will one day bring enlightenment. 



It is doubtful whether comparative biochemistry, i.e. the thesis of unity in 

 biochemistry, can serve as a reliable guide in all the fields of biochemical 

 research. In any case, Cohen (19a), Stanier (53) and others warn against 

 exaggerations and, thus, question the general validity of this thesis. Never- 

 theless, Bassham and Calvin (14) still take it for granted that Ruben's isotope 

 studies and van Niel's suggestion clearly indicate that the photosynthetic O2 

 has its ultimate origin in the O atoms of H2O. 



In Section D of this chapter the important role which must be imputed to 

 COo in the chlorophyll-polypeptide is discussed. It is becoming increasingly 

 apparent that the photosynthetic process centers around C-Oo. COo is not 

 merely a reaction partner but must be considered to be the primary substance. 

 In the green plant, CO2 is loosened in the photolyte so that it can be split 

 by the relatively weak light energy. The energy needed for the formation of 

 the photolyte is provided by the induced respiration. It is very probable 

 that a peroxide may be intermediately produced and that phosphorylations 

 take place (§ 69). 



It must be borne in mind that phosphorylation always occurs in respiration 

 or, in other words, no phosphorylation, no respiration. The converse, no 

 respiration, no phosphorylation, seems incorrect as Arnon found phosphoryl- 

 ation upon illumination independent of O2. Arnon does not take into con- 

 sideration the induced respiration which, being respiration, must be con- 



