THE PROTEINS. 



Proteins make up the greater part of the solid matter of all 

 animal cells and tissues and are present in various parts of plants. 

 Meat and eggs consist mainly of protein ; milk, seeds and some fruits 

 contain a large proportion of it. Protein is thus an essential in- 

 gredient of our food. 



Composition of Proteins. 



Proteins are composed of amino acids, which may be regarded as 

 the units of the protein molecule just in the same way as a poly- 

 saccharide is composed of monosaccharide units and a fat of glycerol 

 and various fatty acid units. The amino acids are obtained by the 

 hydrolysis of proteins with acids, alkalies or enzymes. Up to the 

 present time 18 amino acids have been found to occur in the 

 protein molecule hence its complexity but, though 1 8 units may 

 be present in some proteins, others contain fewer, and in some 

 cases a protein has been found to be composed oi only 2 or 3 units. 

 The percentage amounts of the various amino acids, which have 

 been obtained by the hydrolysis of some of the proteins, are given in 

 the following table : 



We may notice in particular that salmine contains 87 per cent, of 

 arginine, that haemoglobin contains 1 1 per cent, of histidine, that 

 silk-fibroin is composed chiefly of glycine, alanine and tyrosine. 

 Some vegetable proteins, those of the cereals, contain about 40 per 

 cent, of glutamic acid. Numerous other differences can be noted, e.g. 

 that glycine is present in serum globulin, but absent in serum albumin. 

 But no stress must be laid upon other small differences since the 

 method of analysis is not a quantitative one. The amounts actually 



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