METABOLISM OF ORGANISMS 



41 



and simpler forms until 'dust' remains. Although undoubtedly 

 many of these compounds would automatically, so to speak, tend 

 to simplify, nevertheless this is not only hastened, but chiefly 

 carried out by organisms of decay such as the Bacteria. Through 



Animal 

 Proteins 



Bacterial 

 Decay 



Proteins 

 of Green Plants 



\ 



Nitrates Nitrate 



Bacteria 



Nitrite 

 Bacteria. 



Ammonia 



Fig. 16. 



N-Fixing Atmosphere Denitrifying 

 Bacteria | Bacteria 



The Nitrogen Cycle. A schematic representation of the circulation 

 of nitrogen in nature. 



enzymes, or ferments, which they form, fermentation occurs. 

 The carbohydrates and fats are resolved into carbon dioxide and 

 water, and the proteins are reduced to carbon dioxide, water, and 

 ammonia (NH 3 ) or free nitrogen, while the nitrogenous waste 

 (urea, etc.) of animals is similarly broken down. (Fig. 17.) 



A B C D 



Fig. 17. — Yeast, a unicellular colorless plant, very highly magnified. A, cell 

 showing granular cytoplasm and a large vacuole; B, showing nucleus; C, cell 

 budding; D, mother cell and bud after division is comnleted. 



Practically all of these long series of chemical reactions are 

 carried on by different kinds of Bacteria. Most green plants, how- 

 ever, take their nitrogen chiefly in the form of nitrates, and ac- 

 cordingly we find that another type of Bacteria (Nitrite Bac- 



