48 THE ACTION OF THE LIVING CELL 



changes incident to the life process are conditioned 

 as regards both their velocity and specificity by bio- 

 chemical catalysts or enzymes. The action of these 

 remarkable substances is as yet but imperfectly under- 

 stood; nevertheless the evidence for their existence 

 is irrefutable. For example, starches may be hydro- 

 lyzed to glucose by means of strong mineral acid or 

 by sprouted barley. In 1832 Payen and Persoz ob- 

 served that an extract of barley which was free from 

 living cells would, when added to starch, efiPect its 

 hydrolysis with great ease. 



For many years it was believed that the action of 

 such extracts differed materially from the enzymatic 

 activities of living cells. It remained for Buchner 

 in 1897 to establish beyond a doubt that the chemical 

 activities of cells were actually controlled by endocel- 

 lular enzymes. This worker, in his now classic ex- 

 periment, subjected ground yeast cells to a tremen- 

 dous pressure, thus obtaining a yeast juice free of 

 living cells. This expressed fluid was found to possess 

 all the fermentative properties of the original yeast 

 cells. 



Since Buchner's time the presence of a great many 

 enzymes has been demonstrated in somewhat analo- 

 gous fashion. Indeed enzyme preparations have been 

 obtained from various cells, thus enabling us to ac- 

 count for the majority of the chemical transformations 

 effected by living matter. Nevertheless the chemical 

 structure of the individual enzymes still remains one 

 of Nature's secrets. 



Now these enzymes for the most part exist within 

 the cells and if an insufficient quantity of proper food- 

 stuffs is available to the cell, they readily attack the 



