DIV. ii PHYSIOLOGY 275 



Such organisms are called anaerobes or anaerobionts in contrast to 

 the typical aerobes or aerobionts. All intermediate stages connect the 

 two extremes. The true anaerobionts decompose large amounts of 

 organic substances, and this decomposition, which is in principle the same 

 as the process of intramolecular respiration, is termed FERMENTATION. 

 As in intramolecular respiration, these processes are concerned with 

 obtaining combined oxygen. 



The prototype of fermentation is the alcoholic fermentation brought 

 about by the yeast fungus. In this sugar is split up into alcohol 

 and carbon dioxide, and the process has great technical importance 

 in the production of beer, wine, and brandy. The chemical process 

 is the same as that of intramolecular respiration in a green plant; 

 in contrast to this the yeast plant obtains in the fermentation a 

 complete substitute for respiratory activity. It is, however, only 

 independent of oxygen when it is supplied with a suitable ferment- 

 able material (sugar). In the absence of sugar, oxygen is indis- 

 pensable, and normal respiration takes place. When both sugar 

 and oxygen are supplied, respiration and fermentation go on simul- 

 taneously ; part of the sugar is transformed into C 2 H 6 O and C0 2 and 

 another part into H 2 and C0 2 . Obviously, the transformation of 

 sugar into alcohol and carbon dioxide will provide much less energy 

 than the complete combustion to carbon dioxide and water. It is 

 thus easy to understand that yeast utilises enormous quantities of 

 sugar. Only about 2 per cent of the sugar in the nutrient solution is 

 used in the construction of the substance of the plant, i.e. is assimilated; 

 the rest is fermented. For effecting this extensive decomposition of 

 the sugar, yeast employs a specific enzyme (zymase), the existence of 

 which was demonstrated by E. BUCHNER ( 49 ). 



Many other carbohydrates undergo fermentations, and this also 

 * holds for proteids. In the latter case the process is termed putre- 

 faction when it takes place in the absence of oxygen, and decay when 

 oxidation is possible. In nature aerobic bacteria occur first in the 

 fermentation of albuminous substances, and these prepare the way for 

 anaerobic forms, so that a sharp distinction between decay and putre- 

 faction is impossible. In all cases the proteids are first hydrolytically 

 dissociated with the production of the substances already mentioned, 

 especially amino-acids. These are further changed, first by the separa- 

 tion of XH 2 , and then more profoundly ; ill-smelling substances such as 

 indol and skatol are often, but not in all cases of proteid fermentation, 

 formed. It is impossible to draw a sharp line between those decom- 

 positions which go on without the assistance of atmospheric oxygen 

 and those in which oxygen plays a part. We are obliged to class as 

 fermentations all those metabolic processes by which energy is obtained, 

 which differ from typical oxygen respiration. In this sense the oxida- 

 tion of alcohol to acetic acid effected by the acetic acid bacteria and 

 also the production of acids in the higher plants, especially in succulent 



