CHEMICAL CHANGES 153 



venient to commence consideration of the nitrogen cycle 

 with one of the more complex organic nitrogenous com- 

 pounds. Such compounds, for example, are the proteins 

 of plants and animals. These undergo decomposition 

 after the death of the plant or the animal, the more or 

 less insoluble compounds being converted into soluble 

 forms by a process of hydrolysis. This transformation is 

 termed proteolysis. Finally the nitrogen appears in the 

 form of ammonia, so that from this point of view the full 

 process may be termed one of ammonification. The first 

 step in the process apparently is the breaking down of 

 the proteins into the proteoses, differing somewhat in the 

 size of the molecule though still very large but showing 

 greater sohibility. The proteins are in turn transformed 

 into peptones, still very complex compounds. The pep- 

 tones in turn are split into peptids and the peptids finally 

 into alpha amino acids. Some eighteen or twenty dif- 

 ferent amino acids have been detected among the decom- 

 position products of proteins. Certain species of bacteria 

 and other microorganisms help to bring about the process 

 termed deaminization, in which nitrogen is given off in form 

 of ammonia. 



A considerable proportion of the nitrogen of animal 

 bodies is excreted in the form of urea or in some species 

 of animals as uric acid. These compounds are likewise 

 attacked by bacteria and are broken down into ammonia. 

 Eventually, therefore, practically all organic nitrogenous 

 compounds under the influence of bacterial action liberate 

 their nitrogen in the form of ammonia. The ammonia thus 

 produced in soil, for example, is readily available to many 

 species of plants and to various microorganisms. Most 

 of the ammonia, however, is rapidly converted by other 

 bacteria by a process termed nitrification. 



Nitrification. — Certain species of bacteria present in soil 

 oxidize ammonia to nitrous acid and to nitrites. Others 



