CHAPTER II. — SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 723 



the normal oxidative catabolic processes ; bat they carry on other 

 processes of decomposition into which free oxygen does not enter, 

 provided that suitable material is accessible ; the processes are 

 termed fermentations. Thus, Bacteria cause putrefaction and 

 other similar fermentations in the most various organic substances 

 with which they happen to come into contact. Similarly Yeast is 

 the cause of the alcoholic fermentation of sugar, which may be re- 

 presented by the equation 



C6Hi206 = 2C2H60 + 2C02. 

 These fermentative catabolic pi-ocesses involve, like the oxida- 

 tive catabolic processes, an evolution of kinetic energy, but this 

 is, generally speaking, attained by a greater initial expenditure. 

 Yeast, for instance, certainly thriKfig, better when it can obtain 

 free oxygen than when it cannot ; but many of the Bacteria have 

 become so completely adapted to a life based upon fermentative 



tcatabolism, that they die if they come into relation with free 

 oxygen (anaerobia). 



Yeast, Bacteria, and other plants which can excite fermentation 

 are termed organised ferments, inasmuch as they are complete 

 living organisms and as their power of exciting fermentation 

 depends upon their being alive, in contradistinction to the un- 

 organised ferments or enzymes already mentioned, which are not 

 organisms, but simply chemical substances which may be pre- 

 pared and kept under conditions which would be fatal to life. 

 The peculiar properties of the unorganised ferments can, however, 

 be very readily destroyed, as, for instance, by boiling or otherwise 

 heating. 



The chief kinds of enzymes which have been found in plants 

 are : — 



1. Those that act on carbohydrates, converting the more 

 complex and less soluble carbohydrates into others of simpler 

 composition and greater solubility. 



2. Those that act on fats, decomposing them into glycerin and 

 fatty acid. 



8, Those that act on glucosides, glucose being a constant product. 



4. Those that act on the more complex and less soluble proteids, 

 converting them into others which are more soluble and probably 

 less complex, or decomposing them into non-proteid nitrogenous 

 substances (amides, etc.). 



The chemical action of some of these enzymes are illustrated by the following 

 equations : — 



