MECHANISM OF CHEMICAL CHANGES 167 



This reaction is brought about within the cells of certain 

 species of bacteria, and probably to a less degree by certain 

 species of yeasts and molds. The enzyme laetacidase, how- 

 ever, has apparently never been secured outside the bac- 

 terial cell. 



Oxidizing enzymes which combine organic compounds 

 with oxygen from the air are comparatively common in 

 microorganisms. Probably most of them are intracellular. 

 But little detailed work has been done upon them. Much 

 more is known concerning the oxidases developed by higher 

 forms of life, particularly the higher plants. One of the 

 most striking of the oxidative changes is that produced by 

 members of the genus Acetobacter in the oxidation of alcohol 

 to acetic acid. This may be illustrated by the following 

 equation : 



C2H5OH + O2 = CH3COOH + HjO 

 Alcohol Acetio acid 



Many color reactions are the result of the presence of 

 oxidases. This is particularly true among higher forms. 

 The bruised surfaces of many fruits, for example, are 

 known to change color because of the action of oxidases in 

 the cells which are exposed to the air. It is quite possible, 

 though by no means demonstrated, that the oxidizing 

 activity of organisms belonging to the genera Nitrosomonas 

 and Nitrobacter is due to the presence of enzymes of this 

 group. Probably all cells contain oxidizing enzymes capable 

 of releasing energy from the food, particularly from carbo- 

 hydrate substances. 



Fermentation 



Fermentation may be very broadly defined as any 

 chemical change brought about directly by microorganisms 

 or indirectly as the result of the action of enzymes or other 

 substances produced by them. It is sometimes somewhat 

 more narrowly defined to include only chemical changes 



