CHAPTER 5 



THE COMPARATIVE BIOCHEMISTRY OF 

 PHOTOSYNTHESIS AND OF CARBON DIOXIDE 



FIXATION 



Until the end of the nineteenth century there appeared to 

 be a sharp distinction between the green plants, on the one 

 hand, which were able to develop in the light on a completely 

 mineral medium — the autotrophs — and all other plants 

 together with animals which required a supply of organic 

 substances for growth — the heterotrophs. This simple dis- 

 tinction has become less useful as a result of the discovery of 

 the wide range of biochemical activity exhibited by bacteria. 



In 1887 Winogradsky discovered a group of organisms 

 which could assimilate carbon dioxide in the dark. This they 

 could do only in the presence of hydrogen sulphide and as 

 a result of the reaction refractile globules of sulphur were 

 deposited within the cells; in the absence of sulphide the 

 sulphur globules disappeared, sulphate was liberated into the 

 culture medium, and eventually the cells died. A few years 

 later Winogradsky discovered a different group of organisms 

 which grew only in the presence of carbon dioxide and 

 ammonia or nitrite. Both groups of organisms were colour- 

 less and, although they grew in the dark, were autotrophic. 

 The energy necessary for the reduction of carbon dioxide 

 was obtained from the simultaneous oxidation of a simple 

 inorganic chemical compound, such as sulphide or sulphur 

 or ammonia or nitrite, a type of metabolism described as 

 chemosynthetic or chemo-autotrophic. 



Prior to Winogradsky's discovery of colourless bacteria 

 utilizing sulphur, Englemann in 1883 had described pig- 

 mented bacteria which assimilated carbon dioxide in the 

 presence of sulphide but which apparently needed light. 

 In subsequent work several difficulties arose with regard to 

 these bacteria since it was difficult to understand why, if 



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