SECTION V 



THE PROTEINS 



As sources of energy to the organism all three classes of food-stuffs are 

 valuable in proportion to their heat equivalents, and it is often a matter of 

 indifference whether the main bulk of the energy required is supplied at 

 the expense of fat or at the expense of carbohydrate. The proteins however 

 form the most important constituent of living protoplasm. On this account 

 protein must always be present in the food to supply the material necessary 

 for building up new protoplasm in the growing animal and for replacing 

 the waste of living material which is taking place in the discharge of its 

 normal functions. Regarding the complexity of reaction presented by living 

 protoplasm as determined in the first instance by the chemical and physical 

 complexity of this material itself, we should expect to find that the proteins 

 forming its main constituents would themselves partake of some of this 

 quality. The carbohydrates and fats, although in many cases made up of 

 huge molecules, are nevertheless built up on a very simple type. Starch, 

 for instance, with a molecular weight of over 30,000, is formed simply by the 

 polymerisation of glucose molecules. The ordinary fats, stearin and 

 palmitin, consist of fatty acids with long straight chains of CH 2 groups 

 combined with the glyceryl radical. Their molecular weight is very large, 

 but their molecules are simple in structure. When however we break up a 

 protein molecule we meet with a great number of subsidiary groups, the 

 presence of which is essential to the making of a nutritive protein. 



Owing to this complexity of structure it is not easy to give a simple 

 definition in chemical terms of what we mean by the term ' protein.' It is 

 necessary rather to describe certain of the qualities presented by this group, 

 the possession of which we regard as essential to the conception of a protein. 

 Elementary Composition. All proteins contain oxygen, hydrogen, nitro- 

 n, carbon, and sulphur. The proportion of these elements in the various 

 roteins may be represented as follows : 



C SO'6-54'5 per cent. 

 H 6-5- 7-3 

 N 15/0-17 '6 

 S : 3- 2'2 

 21-5-23-5 



Nearly all the proteins contain a small trace of phosphorus varying from 



ft to 0'8 per cent. It is doubtful however how far this phosphorus forms 

 integral part of the protein molecule. 

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