THE CHEMISTRY OF THE CELL 13 



The simplest kind of living organism is a bacterium or 

 what is popularly known as a microbe ; this is a unicellular 

 organism and as a rule specially fitted to bring about certain 

 defined chemical changes. In more complex organisms 

 separate cells are found to have separate functions ; thus the 

 cells of the hning of the stomach bring about changes which 

 take place best in an acid medium. In the pancreas, on the 

 other hand, chemical change takes place under alkahne con- 

 ditions. From many species of cells it is possible to isolate 

 the catalytic substance or enzyme which helps to bring about 

 the change. 



While a large number of fermentations are known which 

 can be produced by the action of enzymes, there are others 

 which so far have only been produced by the action of living 

 organisms, such as, e.g., the lactic acid fermentation of sugar 

 or the acetic acid fermentation of alcohol. The alcohoHc 

 fermentation of sugar was at one time thought to belong to 

 this class of fermentation. But the experiments of Buchner 

 showed that it was possible to extract a substance from 

 yeast cells which brought about the formation of alcohol and 

 carbonic acid when added to grape sugar ; this substance 

 he termed zymase, and recent researches by Harden and 

 others have elucidated in a very interesting way the con- 

 ditions of its activity. 



It is probable that other cases where the active enzyme 

 has not yet been discovered will be found on further investiga- 

 tion to resolve themselves in a similar manner. 



At the same time it should be pointed out that the activity 

 of the cell is of a complex nature, and it is probable that the 

 Uving organism is concerned in two distinct modes of activity, 

 i.e., in maintaining its body substance and in developing 

 energy for growth and reproduction. Thus, broadly speaking, 

 in the animal body the processes of digestion are concerned 

 with the maintenance of the body substance, the processes 

 of respiration with the maintenance of energy. In both 



