ELEMENTARY VITAL PHENOMENA 157 



of beer (Fig. 54), are ferment-organisms, decomposing grape-sugar 

 into alcohol and carbonic acid.i They produce, however, in 

 addition an enzyme, invertin, which is able to convert cane-sugar 

 into grape-sugar. The two actions can be separated from one 

 another. If the yeast-cells be killed by chloroform or ether, it is 

 no longer possible for them to decompose grape-sugar into alcohol 

 and carbonic acid ; but the power of the inverting enzyme continues 

 undiminished, so that the change of cane-sugar into grape-sugar 

 goes on as well as before. In ferment-organisms the living sub- 

 stance exercises the ferment-action only so long as it lives, i.e., 

 its ferment-action is associated with metabolism. This evidently 

 indicates that in ferment-organ- 

 isms there is realised the second 

 case mentioned above, that which 

 is analogous to the action of nitric 

 acid in the manufacture of sul- 

 phuric acid ; while the peculiar 

 fact that the action of the en- 

 zymes may be replaced by other fj^. bi.-Saccharomyces, yeast-cells. (After 



substances, e.g., metals, suggests Eemke.) 



the probability that they work 



also like finely divided metals by pure contact. At present, 

 naturally, this question cannot be decided with absolute certainty. 

 Like the organised ferments the enzymes are highly complex com- 

 pounds, all of which probably contain nitrogen and are derived 

 from the metabolism of proteids ; they are made ineffective by 

 substances that enter into combination with proteids, as well as 

 by boiling ; within certain limits, however, an increase of tem- 

 perature is favourable to ferment-action, because therebj' the 

 intramolecular vibrations of the atoms are increased. 



If the action of ferment-organisms depends actually upon a 

 continual destruction and rebuilding of their own substance, then 

 all living organisms may be regarded as ferment-organisms ; for all 

 living substance transforms food-stuffs in its metabolism while not 

 disappearing itself Hence the metabolism of living substance can 

 be compared with the metabolism of nitric acid in the above 

 case. 



3. Assimilation and Dissimilation 



a. Assimilation 



The digestion of food-stuffs by the action of ferments is only a 

 preparation for the process of assimilation. Only after the food- 

 stuffs have been brought into the condition in which they can do 

 chemical work, i.e., after they have become dissolved, can their 



1 Cf. p. 111. 



