CHAPTER V 

 PROTEINS 



Introductory. The group of the proteins is of great im- 

 portance in nature, both in plants and in animals. Members of 

 this group are found in every living cell, and are necessary 

 for the life of the organism. An animal may get on very well 

 for a long time on a diet containing no carbohydrate or fat, 

 but if fed on a diet containing no protein, the animal will die. 

 Plants build up their own proteins from nitrogen compounds of 

 the soil or air, but animals are dependent upon plants or other 

 animals for the materials out of which they construct their 

 proteins. Plant structures contain large amounts of carbohy- 

 drates along with protein and other materials, but the tissues 

 of animals are made up largely of proteins. Thus muscle, 

 nerve, ligament, skin, bone, blood, lymph, hair, nails, feathers, 

 eggs, etc., all contain much protein. Often it is their chief 

 solid constituent. In plants we find stores of protein laid away 

 in seeds, such as the grains, and in many other places. 



Elementary Composition. The members of the group of pro- 

 teins differ very widely among themselves in many properties, 

 but they are all alike in certain respects, as for example in 

 their elementary composition, for all proteins contain carbon, 

 hydrogen, oxygen and nitrogen. Some contain also sulphur 

 and some phosphorus. In addition to these elements we find 

 sometimes others, such as iron, iodine, copper, manganese, etc. 

 The proteins show individual variations in composition, but an 

 average percentage is as follows: 



C 50% N 16% 



H 1% S 0.3% 



22% P 0.4% 



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