CHAPTER VIII 

 THE UTILIZATION OF FOOD 



Living Things Require Food for several reasons. In 

 the first place, their own bodies, which are made up in 

 part of living protoplasm and in part of material which 

 the living protoplasm manufactures and lays down, must 

 have a source of supply for making protoplasm and these 

 other products; this source is in the food. In the second 

 place, living things are like machines in that they use 

 energy, and energy cannot come from nothing but must 

 be developed from some source, which in this case is also 

 the food. In respect to its energy requirement a living 

 organism is in exactly the same situation as any familiar 

 non-living machine which is operated through the burn- 

 ing of fuel. By way of illustration, recur to the automo- 

 bile, to which reference was made in Chapter III. Auto- 

 mobiles are driven by the explosion of an air-gas mixture 

 in the cylinders. The reason gasoline can be used as a 

 source of power for automobiles is that it has high energy 

 value which energy is released when the gasoline is 

 burned. An automobile is a device for converting this 

 chemical energy of burning into the mechanical energy 

 of motion. Any living thing which does any kind of 

 work must expend energy obtained from food in doing 

 its work, being in this respect in exactly the same situa- 

 tion as the automobile. 



For the manufacture of materials, then, and as a source 

 of energy, foods are necessary. Whichever of these ends 

 they may serve, they have to undergo chemical changes 

 in the process. These changes are of various kinds, some 

 of which will be described presently. The term metab- 



73 



