THE ROLE OF AMMONIA IN NITROGEN METABOLISM 447 



2. Glycoproteins or Glucoproteins are proteins combined with a carbo- 

 hj'drate radical. Thej' are not known to occur in plants. 



3. Phosphoprotcins are compounds of proteins and an unknown non-pro- 

 tein complex which contains phosphoric acid in a different linkage from 

 those found in nucleic acids or the phospholipids. The casein of milk 

 is a familiar example of this type of protein. 



4. Chrojnoproteins are proteins that are combined with a non-protein 

 group which is colored. The hemoglobin of the blood belongs in this 

 group as do the colored proteins present in certain marine algae. 



5. Lecithoproteins are combinations of proteins with lecithin or some other 

 phospholipid. They occur as constituents of the cytoplasm. 



III. The Derived Proteins. — This group includes products that are 

 formed as a result of the partial hydrolysis or decomposition of the simple 

 proteins. The proteoses, peptones and peptides are among the better known 

 representatives of the derived proteins. The coagula obtained upon heating 

 many proteins are other examples of this group. 



Colloidal Properties of Proteins. — Although some proteins can exist in 

 true solution or in the form of crystals most of them occur in living organisms 

 in the colloidal state. The individual molecules of some proteins are so large 

 as to bring them within the size range of colloidal micelles. The colloidal 

 micelles of other protein sols are probably aggregates of a number of molecules. 

 The significant point, however, is not whether the micelles are composed of 

 one molecule or many, but that they are micelles. In general the properties 

 of protein sols and gels are those of hydrophilic sols and gels in general, as 

 discussed in Chap. V. 



Other Compounds Playing an Important Part in the Nitrogen Metab- 

 olism of Plants. — I. Ammonia. — This compound is supposed to play a 

 central role in the nitrogen metabolism of plants, although ordinarily it does 

 not occur in plant cells in more than traces. There are three possible sources 

 of ammonia in plants: (i) Reduction of nitrates produces ammonia, (2) 

 Ammonia may be absorbed directly from the soil, principally in the form of 

 ammonium ions, (3) Ammonia is also sometimes set free in plant cells upon 

 the oxidation of amino acids or related compounds. As described later am- 

 monia released in this way is usually tied up in the synthesis of asparagine, 

 glutamine, and urea, although if the cells are markedly deficient in carbohy- 

 drates ammonia may accumulate in them. 



2. Ammonium Salts. — Ammonium salts do not commonly occur in plant 

 cells in significant concentrations. However, in species with a distinctly acid 



