36 PROTOPLASM AND THE CELL 



the formation of glycerine, succinic acid, and carbon dioxide, 

 while i % is used by the yeast in its nutrition. 



In the process of making alcohol yeast cells form something 

 which produces fermentation, i.e., sets up processes in other 

 compounds which would not take place were the yeast products 

 absent. We have seen that substances of a chemical nature 

 which thus induce chemical changes in other bodies are fer- 

 ments or enzymes. Familiar examples of such substances are 

 the digestive ferments of the alimentary tract. The cells of the 

 salivary glands for example secrete a substance termed ptyalin 

 which brings about the conversion of insoluble starchy food 

 stuffs into soluble sugars. The cells of the stomach and pancreas 

 secrete ferments termed pepsin and trypsin which convert in- 

 soluble proteid into soluble proteoses or peptones. These fer- 

 ments of the digestive tract act independently of the cells which 

 secrete them and may act apart from such cells, thus maintain- 

 ing .their chemical character apart from the living protoplasm 

 from which they were derived. Yeast cells on the other hand 

 do not perceptibly secrete their alcoholic ferments in like man- 

 ner but give rise to them in such minute quantities that they 

 cannot be identified and this little is given only under the stimu- 

 lus of a suitable medium for active life. 



Because of these supposed differences ferments have been 

 divided into two groups organized and unorganized. The 

 latter, like pepsin, trypsin, etc., act independently of the living 

 cell. The organized ferments formerly called enzymes, like 

 those of yeasts and bacteria act in nature only when the living 

 organisms are present. This distinction however is entirely 

 artificial and of no value. Indeed, yeast, the classic example 

 of an organized ferment, and bacteria too, for that matter, by 

 drastic measures can be made to give up their ferments which 

 are then active apart from the living cells. Biichner in 1897 

 was the first to demonstrate this fact by grinding yeast cells in 

 diatomaceous earth, compressing the mass and thus getting a 

 clear fluid which brought about the alcoholic fermentation 

 without the presence of living cells. He called the ferment thus 

 extracted, zymase. Very often such enzymes similarly bound 



