ENERGY METABOLISM 427 



ated, but the basis always remains unchanged. In particular, 

 we find everywhere the same catalytic mechanisms and 

 methods of obtaining energy and accumulating it in high- 

 energy bonds. The differences, as we shall see belo^v, only 

 represent different ways of using the hydrogen liberated by 

 the process of dehydrogenation and different ways of further 

 transforming the breakdown products which are used for 

 btiilding the protoplasm of the living cells. 



In the diagram of alcoholic fermentation given above, all 

 the hydrogen which is formed by the dehydrogenation of 

 glyceraldehyde-iig-diphosphate to the corresponding acid is 

 used, with the help of coenzyme I, for the reduction of 

 acetaldehyde to ethanol. In other cases the hydrogen may 

 be taken up by other intermediate products of metabolism 

 and used for reducing processes in the course of the bio- 

 synthesis of components of protoplasm, or oxidised to water 

 by the oxygen of the air in the course of respiration. 



In all forms of anaerobic degradation of carbohydrates, 

 and in all organisms which have been studied in this respect, 

 pyruvic acid and its immediate derivatives occupy a key 

 position in the processes of biosynthesis of important com- 

 ponents of protoplasm, proteins, lipids, nucleic acids, etc. 

 The nature of the organisation of these synthetic processes 

 may, however, vary to some extent as between different 

 representatives of the living world. The processes of break- 

 down and synthesis may be co-ordinated to a greater or lesser 

 degree. During evolution this internal co-ordination there- 

 fore increases and at the same time there is an increase in 

 the coefficient of useful activity, the completeness wath which 

 the nutrient substances entering the living things from the 

 outside medium are used. 



The anaerobic dissimilation of carbohydrates into alcohol 

 and CO2 is usually brought about by a number of different 

 sorts of bacteria in the way which has been described above 

 for yeasts. In some species of these micro-organisms, how- 

 ever, the individual links in the chain of fermentative 

 reactions may vary to a certain extent. We may cite, as an 

 example, Pseudomonas lindneri which, according to A. J. 

 Kltiyver and W. J. Hoppenbrouwers,®^ can form a larger 

 amotmt of alcohol. The general features of its metabolism 



