ENERGY REQUIREMENTS IN CELLULAR NUTRITION 2OI 



entirely upon chemical energy obtained by the digestion of food. This 

 means that they require organic food almost entirely, since inorganic 

 food furnishes energy only in exceptional cases. In this respect they 

 resemble the animals very much. 



The source of energy in microbial life is always of chemical origin. 

 The simplest processes are the oxidations, and simplest among these 

 the inorganic oxidations. A number of different types feeding ex- 

 clusively on minerals has been discovered during the last twenty years, 

 and some of them are of great economic importance. They resemble 

 plants in as far as they build their cells exclusively from carbon 

 dioxide, nitrates and ash. The food used for building material is 

 quite different from the food used for the provision of energy. 



Two typical examples are the nitrifying organisms in soil which 

 oxidize ammonia to nitrates. This process, according to Winogradski, 

 is divided distinctly into two phases: the Nitrosomonas oxidizes the 

 ammonia to nitrous acid, 



NH 3 + 3<3 = HNO 2 + H 2 O + 78.8 Cal. 

 and the Nitromonas oxidizes the nitrous acid to nitric acid, 



HN0 2 + O = HNO 3 + 18.3 Cal. 



These oxidation processes yield a certain amount of energy which 

 enables the bacteria to build their cells from carbon dioxide, ammonia, 

 and certain mineral salts. Without ammonia or without nitrous acid, 

 respectively, these bacteria cannot grow for lack of energy; they would 

 be like a plant without light. It is evident in this case that the food for 

 energy is also used to some extent as food for growth. The nitrogen 

 necessary to the bacteria is supplied by the ammonia or the nitrous acid. 

 As an example distinguishing strictly between the food for growth 

 and the food for energy may be mentioned the hyposulphite bacterium 

 studied by Nathanson. This organism oxidizes hyposulphites to sul- 

 phates and sulphur, largely following the formula 



Na 2 S 2 O 3 + O = Na 2 SO 4 + S + x Cal. 



Hyposulphite Sulphate Sulphur 



Besides, some more complex compounds, like sodium tetrathionate 

 (Na 2 S 4 O6), are formed. The bacterium builds its cells exclusively from 

 nitrates, carbon dioxide, and mineral salts; organic food is rejected. 

 The hyposulphite can hardly be used for the construction of the cell, 

 and must be considered entirely a food for energy. 



