ORGANIC NITROGEN. ITS NATURE. 49 



complicated than the original proteid of the food. Thus, in the 

 bodies of plants and animals alike the nitrogen reaches a condition 

 allied to proteid. But, while proteids may serve as food for animals 

 and for the great class of colorless plants (fungi) they are quite out 

 of the reach of the green plants, which are the great food producers 

 of nature. Our next problem, then, must be to learn how . these 

 proteids are reduced to their original condition of nitrate. 



Part of the proteid thus built up into the body of the plant or the 

 animal remains there until the animal or plant dies, and at death it 

 is still a proteid and as complex as ever. In this form it may be- 

 come incorporated into the soil when the animal or plant dies, or it 

 may become eaten as food and pass through the body of another 

 animal. But much of it will eventually reach the soil while still 

 in the form of proteid. 



A second portion of the proteid is used up in the animal's 

 body to furnish energy and heat; it is metabolized, as we say. 

 When it is thus used its complex chemical molecule is broken to 

 pieces, and it is reduced to much simpler compounds. But it is not 

 decomposed sufficiently to bring the nitrogen back within the reach 

 of plant life. The carbon in this proteid is in part removed from it 

 and combined with oxygen, to be exhaled as CO 2 . The molecule 

 falls to pieces and various simpler by-products arise; but in the 

 animal's body, practically all of it eventually assumes the form of 

 urea (CON 2 H 4 ). Though this urea is a nitrogen molecule far 

 simpler than proteid, still it is not simple enough for a plant food. 

 Urea, or a closely allied compound, is the form in which nearly all of 

 the nitrogenous material resulting from proteid metabolism in the 

 animal body is excreted. Urea thus represents one stage in the 

 destruction of proteid compounds, and to this stage the proteids are 

 brought as the result of the metabolism in the life processes of 

 animals. In some animals this urea is secreted as urine by the 

 kidneys, but in others (birds) it is mixed with the feces; in all cases 

 it contains the nitrogen which is no longer of any use to the animal 

 world. It is estimated that some 38,000 tons of urea are excreted 

 daily by the human race. To this quantity must be added the far 

 greater amount excreted by other animals, for all animals, large 



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