238 ANIMAL BIOLOGY 



amount of alcohol. The conversion of grape juice into 

 wine depends upon the fermentation of the sugar of the 

 fruit into alcohol and carbon dioxide. The fermenting 

 agent is here the yeast plants, minute plant organisms 

 which rapidly multiply in the liquid. Alcohol itself may 

 be fermented by other organisms that convert it into acetic 

 acid which is the acid of vinegar. 



There are a great many kinds of fermentation caused 

 by different species of yeast and bacteria. Fermentation 

 may be caused also by certain substances called ferments or 

 enzymes. Ferments are produced by organisms and they 

 have the remarkable property of converting a great many 

 times their own bulk of other substances. A very small 

 amount of an enzyme may convert a very large amount 

 of a material without suffering any appreciable loss. Hu- 

 man saliva contains a ferment, ptyalin, that converts 

 starch into sugar. In the gastric juice, which is a secre- 

 tion of the stomach, there is another ferment, pepsin, 

 which converts proteins into simpler compounds called 

 peptones. By fermentation complex substances are 

 split up into simpler ones and these again may be further 

 split up by other ferments. Heat is liberated during 

 fermentation and energy is thus supplied to the body. 

 Under certain conditions enzymes may build up more 

 complex compounds out of simpler ones, thus affording a 

 means of keeping up the supply of complex substances in 

 the body. 



The real living substance of an organism, or in 

 Huxley's phrase " the physical basis of life, " is commonly 

 called protoplasm. This is a semifluid substance of a 

 great chemical complexity, and it differs somewhat in 

 composition in different species of animals and plants.' 

 It contains, C, H, O, N, and frequently S, P and K. Unlike 

 the substances just described, protoplasm has the power 



