DESTRUCTIVE METABOLISM 409 



times it continues for a time; whence it is inferred that the energy re- 

 leased by anaerobic respiration is usually inadequate for growth. Life, 

 however, persists for a variable time, sometimes for weeks or months, 

 though in active parts the functions are much disturbed after a few hours, 

 and death shortly ensues. 



2. FERMENTATION 



Microorganisms. — The fact that anaerobic respiration gives rise, 

 among other things, to alcohol and carbon dioxid, suggests at once some 

 relation to a process long known to occur in sugary juices, like those of 

 grapes and apples, when they are allowed to stand unsterilized and un- 

 sealed. The sugar disappears, bubbles of gas (CO s ) rise through the 

 liquid, and considerable alcohol is formed in it. This process is known 

 as fermentation. It was shown long ago to be due to the presence of 

 yeast plants, for it does not occur when they are excluded. Further 

 study has shown that analogous changes which take place in organic- 

 substances, many of them (like the souring of milk and the spoiling of 

 meat) being familiarly known, are due to the action of other micro- 

 organisms. The application of the term fermentation has now been 

 extended to cover all these changes. 



Names. — Fermentations are named after the most prominent or de- 

 sirable substance produced, or sometimes after the substance destroyed. 

 Thus, the fermentation of glucose (grape sugar) is alcoholic fermenta- 

 tion; that of lactose (milk sugar) is lactic fermentation; that of alcohol 

 is acetic fermentation; because alcohol, lactic acid, and acetic acid, 

 respectively, are formed. On the contrary, the cellulose fermentation is 

 so named because cellulose is destroyed. When proteins are attacked, 

 evil-smelling gases are among the products, and such fermentations are 

 frequently distinguished as putrefactions; but they are not essentially 

 different from others. Only a few of the better known and more impor- 

 tant fermentations can be treated here. 



Alcoholic fermentation. — The alcoholic fermentation is produced 

 in different sugars by various organisms. The sugars that are now 

 known to be fermentable are only those the number of whose carbon 

 atoms is 3 or a multiple of 3; thus, the trioses (C 3 H 6 3 ), hexoses 

 (C 6 H 12 6 ), and nonnoses (C 9 H 18 9 ) are directly attacked; while the more 

 complex carbohydrates (di- and polysaccharides), such as cane and malt 

 sugar (C u ,Ho 2 0„) and starch [5«(C 6 Hi O 5 )], are fermented only after 



