METABOLISM 39 



sarily used in the securing of additional food, but some is also used in 

 growth, in reproduction, and in carrying on other activities. Among 

 the lower animals the portion of energy used in the securing of additional 

 food is much larger than in the higher animals. The development of 

 efficiency among the latter is, to a considerable degree, connected with 

 the possession of more effective food-securing devices, which leaves a 

 proportionately larger part of the total energy of the organism to be used 

 in other ways. Man has solved this problem far more successfully than 

 any animal below him, and the advance he has made to a dominating 

 position in the animal kingdom may to a considerable degree be attrib- 

 uted to this fact. 



64. Uses of Different Foods. — The different foods serve different 

 purposes in the organism. The protein food is in part used to replace the 

 protein of living tissue when that is used up. Carbohydrates furnish 

 the mechanical energy expended in muscular movements. Fats are 

 used chiefly as a source of heat. All dissimilative changes in the body 

 liberate heat, but from fats, owing to the fact that they contain a very 

 small amount of oxygen and are therefore susceptible of a great deal of 

 oxidation, may be produced more heat than from any other food. Water 

 must be maintained in large amount in the organism, both because it is 

 needed to give the required consistency to the protoplasm and because 

 it serves as a vehicle for other substances in solution. Salts are essential 

 constituents of protoplasm, also participating in the metabolic changes 

 and exerting a regulatory effect upon them. Oxidation processes take 

 place in all of the cells of the organism, the extent of such processes in any 

 given cell determining the amount of activity carried on by the cell. 

 They do not occur in the blood except in the blood corpuscles, which are 

 cells. 



65. Storage. — The organism does not in all cases make immediate use 

 of the food absorbed, in which case it may be stored against future need. 

 Fats are thus accumulated in the form of fat. Since carbohydrates are 

 the chief sources of muscular energy and since the body must at all 

 times have not only a ready supply but also a large volume in storage to 

 be used as needed, there is in the Hver an abundant supply of stored 

 carbohydrate ready to be given out to the blood and circulated to all 

 parts. An excess of carbohydrates may be changed to fats and stored 

 as such. In the chemical changes in the organism, carbohydrates may be 

 derived from substances resulting from protein decomposition, and fats 

 may in some cases be changed to sugar, but neither of them can be con- 

 verted into proteins, since these contain nitrogen, which is lacking in 

 carbohydrates and true fats. Proteins are not stored, but any excess is 

 immediately broken down and the waste products eliminated. Storage 

 should not be confused with growth, since the stored food is not a part 

 of the protoplasmic organization. 



