RESPIRATION AND FERMENTATION 265 



structure, the so-called 7-glucose, which may have the following 

 structure: 



CH2OH— C OH— (CHOH) 3 — CH 2 . 



I o 1 



In the free condition, 7-glucose is very unstable and assumes an 

 inactive form, but it may be isolated in several combinations. 



It is interesting to note that in the molecule of sucrose, the 

 glucose is present in active form, and that by inversion first is 

 obtained 7-glucose, which later is transformed into the inactive 

 form. It is possible that this capacity of producing the most 

 active of all hexoses, explains the wide distribution and high 

 importance of sucrose in the plant body, and the animal 

 organism. 



No matter what alterations in our ideas of the chemical proc- 

 esses of alcoholic fermentation may take place in the future, it 

 must at any rate be considered as a complex catalytic process 

 caused not by one, but by several enzymes. In the living cell all 

 these enzymes are working in harmony with each other and the 

 process of fermentation goes on as smoothly as if it were a simple 

 reaction. But in a killed cell, or in the expressed sap, the system 

 of enzymatic activity is disturbed and fermentation soon ceases. 

 Hence the total quantity of sugar fermented by various prepara- 

 tions of zymase is always insignificant if compared with the amount 

 fermented by living yeast cells. 



82. Acid Fermentation and Its Chemistry. — Besides alcoholic 

 fermentation, there exists a number of other types. The most 

 important of these are lactic, butyric, and acetic fermentation. 

 The first two consist of the breaking down of sugar accompanied 

 by the formation of the corresponding acids, while the third repre- 

 sents a process of oxidation of alcohol by oxygen of the air. It is 

 more akin to respiration than to alcoholic fermentation. Pasteur 

 was the first to investigate the various kinds of fermentation and 

 to establish their biological nature. He isolated in pure cultures 

 the microorganisms by which they are caused. 



Lactic-acid fermentation is produced by special bacteria. It 

 has received its name from the chief end product, lactic acid, 

 CH3CHOHCOOH. These bacteria fall into several groups. 

 One of these causes purely lactic fermentation, i.e., the splitting 



