FIRST HETEROTROPHS AND ANAEROBES 415 



to fix carbon dioxide became wider and wider/^ although 

 the biological significance of the phenomenon remained 

 obscure. The fact is that fixation of any kind needs energy. 

 Taking it over all, photosynthetic organisms use the energy 

 of light for this purpose while chemosynthetic organisms use 

 energy derived from the oxidation of hydrogen sulphide, 

 ammonia, ferrous oxide, etc. By these means the amount of 

 organic substances is increased at the expense of the carbon 

 of CO2 and the stored potential energy of the living cells. 

 When heterotrophs fix co,, on the other hand, they use the 

 energy which they obtain from the degradation or oxidation 

 of ready-made organic substances and thus the .process is not 

 accompanied by any increase in either their stored organic 

 carbon or their stored energy. Their over-all balance is 

 negative in both respects, as may easily be shown by direct 

 determinations. 



In view of this, the widespread occurrence of heterotrophic 

 fixation of carbon seemed incomprehensible. Furthermore, 

 later studies, especially those involving the use of CO2 contain- 

 ing labelled carbon atoms, showed beyond doubt that the 

 ability to fix carbon heterotrophically was in fact possessed 

 by all known living cells," not only by microbes'^* but also 

 by animals*^^ and even by the colourless cells of higher plants 

 which can only live heterotrophically, e.g. the cells of roots. ^° 



As we now know, the fundamental protoplasmic mechan- 

 ism taking part in the initial stage of the fixation of CO2 in 

 both autotrophs and heterotrophs is coenzyme A. This was 

 discovered by F. Lipmann^^ during a study of processes of 

 acetylation in living tissues and also studied simultaneously 

 in several other laboratories (D. Nachmansohn and M. Ber- 

 man,'^^ W. Feldberg and T. Mann" and others). 



The significance of coenzvme A is, essentially, as follows. 

 Acetic acid plays a very important part in the metabolism of 

 every living organism. It seems to be a connecting link 

 between the metabolism of carbohydrates, fats and proteins. 

 By itself, however, it is chemically inert and before it can 

 enter into reactions of acetylation or condensation it must be 

 activated in some way. This is done by the formation of an 

 acetyl derivative of coenzyme A, which was first isolated in 

 the pure state from yeast by F. Lynen, E. Reichert and L. 



