2 BIOLOGICAL PROBLEMS 



the course of the exposition we shall have occasion to 

 consider all that is necessary to the due understanding 

 of all the problems that are likely to meet us subse- 

 quently, although naturally in their main aspects only. 



Proteins.- -When we grow we produce new body 

 substance. When this is subjected to chemical 

 analysis, what do we find that it contains? There 

 are the elements carbon, hydrogen, oxygen, nitrogen, 

 sulphur, and phosphorus in organic combination, 

 together with certain inorganic salts. 



As regards the salts, we do not require to give 

 special attention to them, because they are present in 

 all natural articles of diet, while sulphur and phos- 

 phorus are also present, in the comparatively small 

 amounts required, in the sources of nitrogen used. 

 The case is different with the other four elements. 

 We must have not less than some definite and fairly 

 large amount in order to live and work. 



Let us direct our attention to nitrogen in the first 

 place. Although this gas is present in the atmos- 

 phere in large quantities, animals cannot make use 

 of it in this form, although some plants can do so. 

 For our use it must be already combined with carbon, 

 hydrogen, and oxygen in some form, and the simplest 

 form actually found to serve is that of the amino-acids, 

 which may be regarded roughly as simple organic 

 acids to which is linked the residue of an ammonia 

 group. But although these in the pure state have 

 been shown to be able to serve as nitrogen food, the 

 actual source from which we obtain them, on account 

 of its convenience, is by the digestion of what are 

 known as proteins. These consist of a number of 

 amino-acids combined together. The action of the 

 digestive enzymes is to split the proteins up into their 

 constituent amino-acids. 



Proteins, then, being the food materials from which 



