CHAPTER XIV 

 PROTEINS 



All things change, nothing perishes. 



— Ovid. 



Composition. — The proteins are organic substances composed 

 of carbon, hydrogen, oxygen, nitrogen, and sulphur. They are 

 used chiefly in building up the living material, protoplasm, and 

 their name (Gr., fundamental) indicates their importance as the 

 basis of the protoplasm itself. The vital phenomena take place in 

 the protoplasm, and seem to consist largely of the reactions between 

 the various parts of this proteinaceous material. Naturally pro- 

 teins must occur in every living cell. Reproductive cells are almost 

 entirely protein. As storage products they are found especially 

 in seeds, those of leguminous plants being exceptionally rich. 



In ordinary cells, the protein may occur in solution but, in 

 storage organs, it is commonly found in small solid masses called 

 protein or aleurone grains, which may be amorphous, crystalline, 

 or partly both. Grains of the latter type are found in the castor 

 bean (Ricinus). In the center of the amorphous protein matrix 

 is a crystal of protein, at the end of which is a spherical " globoid" 

 composed of a double phosphate of calcium and magnesium. The 

 crystals of different proteins may be cubical, hexagonal, or needle- 

 shaped ; while many aleurone grains are much simpler and contain 

 neither a crystal nor a globoid. 



As mentioned above, the proteins are composed of five elements 

 of which the sulphur occurs in the smallest proportions. Plant 

 proteins are fairly uniform in their composition and the analysis 

 of about twenty different ones gives the following average percent- 

 ages of their constituents: carbon 52%, hydrogen 7%, nitrogen 

 17%, oxygen 22%, and sulphur 1%. 



General Characters. — The general properties of the proteins 

 may be catalogued and discussed under the following heads : 



I. Chemical properties. II. Physical properties. 



a. amino-acid constitution. a. indiffusibility. 



b. precipitation reactions. b. coagulation. 



c. color reactions. c. optical activity. 



d. precipitation without change. 



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