130 LIFE: ITS NATURE AND ORIGIN 



constructed for experiments in vitro. Isolated enzymes, supposed 

 to be pure, have sometimes proved to be mixtures, and it is always 

 a question as to how far experiments with pure enzymes may be 

 applied to an intact biological unit. Liver slices energetically 

 oxidize certain of the lower fatty acids, but when the liver is 

 minced this power immediately disappears. 



As an illustrative instance, we may take the fermentation of glu- 

 cose by yeast, 81 widely practiced and long studied. Present experi- 

 mental data justify the belief that underlying the extremely naive 



equation 



C 6 H 12 O 6 =2C0 2 +2C 2 H 5 OH 

 the following catalyzed system is operative (end products shown in 

 rectangles): 



Glycogen (within the yeast cell) Glucose 



Glucopyranose 1 -phosphate (Cori ester) 



i 

 Glucopyranose 6-phosphate (Robinson ester) < — 



1 

 Fructofuranose 6-phosphate (Neuberg ester) 



Fructofuranose 1 , 6-diphosphate (Young-Harden ester) 



| Glyceraldehyde phosphate < 1 



2 "Triose phosphate" Dihydroxyacetone phosphate — ' 



I (Oxidation) 

 2 Glyceric acid 3-phosphate \ 



* \ 



2 Glyceric acid 2-phosphate \ 



1 H 2 



2 enol- Pyruvic acid phosphate \ 



1 . .\ 



(Phosphate removal by adenylic acid) \ 



2 Pyruvic acid / 



I / 



2 Acetaldehyde / 



(Reduction) y 



1 



2COo 



2 Ethyl alcohol 



The successive steps tabulated are catalyzed by specific enzymes, 

 and many of the changes have been shown to be reversible. Besides 

 the final products (ethyl alcohol and carbon dioxide), a small 

 amount of glycerin is also formed. Since the formation of Cori 

 ester from glycogen is inhibited by glucose, enzymic formation of 

 glycogen within the yeast cell may serve to restrict glucose concen- 

 tration as glucose diffuses into the yeast cell. 



If sodium sulfite is added to a yeast fermentation, the formation 

 of alcohol practically ceases and the small percentage of glycerin 

 which occurs in normal fermentation increases so that it becomes 

 a main product, with acetaldehyde and carbon dioxide. The 



