

CHA-PTER IV. 



PROTEINS. 1 THEIR DECOMPOSITION AND 

 SYNTHESIS. 



THE proteins are a class of substances, which in the light of our 

 present knowledge, consist, in the main of combinations of a- 

 amino-acids or their derivatives. These protein substances form 

 the chief constituents of many of the fluids of the body, constitute 

 the organic basis of animal tissue, and at the same time occupy a 

 decidedly preeminent position among our organic food-stuffs. They 

 are absolutely necessary to the uses of the animal organism for the 

 continuance of life and they cannot be satisfactorily replaced in the 

 diet of such an organism by any other dietary constituent either 

 organic or inorganic. Such an organism may exist without pro- 

 tein food for a period of time, the length of the period varying 

 according to the specific organism and the nature of the substitu- 

 tion offered for the protein portion of the diet. Such a period is, 

 however, distinctly one of existence rather than one of normal life 

 and one which is consequently not accompanied by such a full and 

 free exercise of the various functions of the organism as would be 

 possible upon an evenly balanced ration, i. e., one containing the 

 requisite amount of protein food. These protein substances, are, 

 furthermore, essential constituents of all living cells and therefore 

 without them vegetable life as well as animal life is impossible. 



The proteins, which constitute such an important group of sub- 

 stances, differ from the carbohydrates and fats very decidedly in 

 elementary composition. In addition to containing carbon, hy- 

 drogen, and oxygen, which are present in fats and carbohydrates, 

 the proteins invariably contain nitrogen in their molecule and gen- 

 erally sulphur also. Proteins have also been identified which con- 

 tain phosphorus, iron, copper, iodine, manganese, and zinc. The 

 percentage composition of the more important members of the 

 group of protein substances would fall within the following limits : 

 = 50-55 per cent, 11=6-7.3 P er cent > = 19-24 per cent, 

 N = 15-19 per cent, 5 = 0.3-2.5 per cent, P = 0.4-0.8 per cent 



term proteid has been very widely used by English-speaking scientists 

 to signify the class of substances we have called proteins. 



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