FERMENTATION 153 



particularly associated with yeast and is responsible for the 

 alcoholic fermentation of sugar. It has been described as 

 occurring in higher plants such as the beetroot, potato, 

 lupin, and others,* and has been isolated from germinated 

 seeds ; in general the presence of a zymase like enzyme is 

 concluded from the appearance of alcohol in plant tissues 

 kept in anaerobic conditions. A large quantity of zymase, 

 comparable to that which occurs in yeast, is not to be ex- 

 pected in the higher plants, since its function is apparently 

 only that of preparing material for oxygen respiration. 



Carboxylase has the property of removing carbon dioxide 

 from carboxyl groups, thereby converting pyruvic acid, for 

 example, into carbon dioxide and acetic aldehyde ; it occurs 

 in yeast, where it is associated with zymase, and, according to 

 Bodnar,f in the beetroot and potato tuber. 



On an earlier page mention has been made of the formation 

 of lactic and butyric acids in addition to alcohol from sugar, 

 Kostytschev considers that these fermentative mechanisms 

 are fundamentally similar and are based on one and the same 

 initial stage, the breaking of a molecule of sugar into two mole- 

 cules of pyruvic acid with the liberation of four atoms of 

 hydrogen — 



C 9 H I2 6 = 2CH3 . CO . COOH + 4 H. 

 Pyruvic acid 



The subsequent reactions varies according to the conditions 

 which determine the formation of the three different end 

 products. 



1. Alcoholic Fermentation. — This will result if the pyruvic 



acid is decomposed before the hydrogen has time to exert 



its reducing action — ■ 



2CH3 . CO . COOH = 2CH 3 CHO + 2C0 2 . 

 Acetaldehyde 



The hydrogen then reacts with the acetaldehyde to form 

 alcohol — 



2CH3CHO + 4 H = 2CH 3 CH,OH. 



* Palladin and Kostytschev : " Ber. deut. bot. Gesells.," 1906, 24, 273. 

 Stoklasa and Chocensky : id., 1907, 25, 122. 



t Bodnar : " Biochem. Zeitsch.," 1916, 73, 193. See also Zaleski : 

 " Ber. deut. bot. Gesells.," 1913, 31, 349. 



