CHEMICAL CHANGES 151 



Many species of bacteria, particularly the forms known 

 as facultative anaerobes oxidize carbon compounds in the 

 absence of atmospheric nitrogen, providing some other 

 compound containing oxygen which may be reduced is 

 readily available. For example, nitrates when introduced 

 into broth enable many bacteria that would otherwise be 

 aerobic to live under anaerobic conditions. These reduce 

 the nitrates to nitrites, utilizing the oxygen thus secured 

 for the oxidation of carbon compounds. It is evident that 

 the energy yield secured by this process is not as high as 

 by direct oxidation with atmospheric oxygen. 



Many organisms are able to grow in the absence of free 

 atmospheric oxygen because of their ability to bring about 

 intramolecular rearrangement of atoms or intramolecular 

 oxidation of carbon in other compounds. Certain bac- 

 teria, for example, directly or indirectly are able to trans- 

 form the sugar dextrose into lactic acid. This may be 

 represented by the equation 



C 6 H 12 6 = 2C 3 H 6 3 



Dextrose Lactic acid 



If both dextrose and lactic acid are studied by the 

 methods of the physical chemist it will be found that the 

 number of calories of heat secured by the complete oxida- 

 tion of one gram of dextrose is greater than that secured 

 by the complete oxidation of the same weight of lactic 

 acid. In other words, inasmuch as energy is indestruct- 

 ible, when sugar is changed into lactic acid an intramole- 

 cular oxidation has occurred with the liberation of a cer- 

 tain amount of energy. It is apparent that this method 

 is least efficient of all, that is, much larger quantities 

 of materials are needed for the development of a given 

 amount of energy by organisms belonging to the third 

 group than by those belonging to the first. The complete 

 oxidation of a gram of glucose gives 3.76 large calories 

 of heat. Similar oxidation of a gram of lactic acid will 



