8 THE ANIMAL ORGANISM 



yield their nitrogen in a form related to ammonia. The proteins 

 of the body are man}^ and even those of similar parts in different 

 animals are slightly different. That the food does not consist of 

 proteins identical with those of the body it is entering does not 

 matter, since in digestion proteins are resolved into the amino- 

 acids of which they are composed and the animal so recombines 

 these as to meet its own needs. The food must, however, supply 

 the right amino-acids in sufficient quantities. It is found, for 

 instance, that mice fed upon a diet in which the only protein 

 present is zein, the protein of maize, which does not contain the 

 important constituent tryptophane, are unable to support life. 

 Proteins are important in the food of all animals, because, 

 while, like other substances that we shall mention below, they 

 can be oxidised to provide energy, it is normally they alone that 

 can make good the protein matter that every living body contains 

 and loses by wear and tear and also that can provide such material 

 for growth. When they are to be used for fuel the nitrogen is 

 discharged from their molecules as ammonia. This is deamina- 

 tion ; it is the ultimate source of most of the nitrogenous com- 

 pounds which are mentioned below as forming part of the excreta. 



Besides these substances the food usually contains (4) carbo- 

 hydrates (sugars, starches, and related substances), (5) fats. It is 

 chiefly these two classes of substances that are oxidised to provide 

 energy. Both contain carbon, hydrogen, and oxygen. In carbo- 

 hydrates the oxygen is present in exactly the proportions to 

 oxidise the hydrogen, as in cane sugar and malt sugar or maltose, 

 which both have the formula C12H22O11, grape sugar or glucose, 

 CgH^gOg, and starch (CeHioOg);^. In fats there is relatively less 

 oxygen ; therefore they require for complete combustion more 

 of that element than is needed to oxidise the carbon, and their 

 potential energy is greater than that of carbohydrates. In 

 digestion, insoluble carbohydrates, such as starch, are dissolved 

 by conversion into glucose or other simple sugars, and fats are 

 partially split into soluble components — fatty acids and glycerol. 



Both these processes, and also the digestion of protein, are 

 hydrolyses — decompositions into smaller molecules with the 

 aid of water taken up. Thus : 



(Starch) (Maltose) 



C12H22O11 + H2O =2C6Hi206 



(Maltose) (Glucose) 



