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. Regard- 

 ing the complexity of reaction presented by living protoplasm as deter- 

 mined 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 ir. 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, 

 nitrogen ; carbon, and sulphur. The proportion of these elements in the 

 various proteins may be represented as follows : 



C 50-6-54-5 per cent. 

 H 6-5-7-3 

 N 15-0-17-6 ; , 

 S 0-3- 2-2 

 21-5-23-5 

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