ENERGY TRANSFORMATION 409 



When carbohydrates are used as sources of energy, the organisms 

 have to obtain their nitrogen from another source, either inorganic or 

 organic. In the case of proteins, both the energy and nitrogen are 

 obtained from the same source. In view of the fact that the proto- 

 plasm synthesized by microorganisms has about the same carbon con- 

 tent as the original protein and usually a lower nitrogen content and 

 since only a part of the energy liberated is utilized by the organism 

 and often a larger part converted into kinetic energy, the use of pro- 

 teins as sources of energy results in a liberation of inorganic nitrogen, 

 usually in the form of ammonia. 



Two methods are commonly employed for determining the energy 

 transformation of microorganisms, (1) the differential method which 

 determines the heat of combustion of a medium before and after the 

 growth of microorganisms; (2) the direct method which measures the 

 heat formed during the life processes of the microorganisms. Rub- 

 ner 48 found that both of these methods gave comparable results. A 

 third method is also possible. It is based upon indirect calorimetry 

 or the calculation of energy utilization from the chemical changes 

 produced in the metabolism. This method has been used largely in 

 the study of energy changes by autotrophic bacteria where the chemical 

 substances involved are relatively simple in composition. In the case 

 of the reactions brought about by heterotrophic bacteria, we are still 

 in the dark as to a number of chemical and thermochemical reactions 

 involved. 



Most nutrients are richer in oxygen than the microbial protoplasm. 

 This is particularly true when nitrate forms the nitrogen source. Syn- 

 thesis of protoplasm can, therefore, be looked upon in heterotrophic 

 processes more as a process of reduction rather than of oxidation. 

 The intake of oxygen, or respiration, serves the purpose of energy 

 transformation, rather than for synthetic purposes. Oxygen oxidizes 

 the nutrients available, the degree of oxidation determining the 

 final products. The complete oxidation of carbohydrates gives car- 

 bon dioxide and water; in the case of nitrogenous substances ammonia 

 (or nitrate) and sulfuric acid are also produced. In the case of sub- 

 stances low in oxygen (hydrogen-oxygen ratio more than 1), such as 

 fats, fatty acids and alcohols, the amount of oxygen necessary for 

 oxidation is so great that only aerobic organisms, such as fungi, are 

 capable of using them as sources of energy. It must be emphasized 



48 Rubner, 1903 (p. 419). 



