80 THE UTILIZATION OF FOOD 



by virtue of a very small amount of metabolism which 

 suffices to keep the protoplasm alive but does not permit 

 it any real activity. The active life of a warm-blooded 

 animal is longer than the effective life of the cold-blooded 

 animal whose chronological age is the same, by just the 

 number of days that the cold-blooded animal has been 

 in hibernation. 



Waste Materials are Produced in Connection with 

 Metabolism. — This is more striking in animals than 

 in plants, although the greater ease with which we recog- 

 nize the production of wastes in animals may be due to 

 the higher key on which all the metabolism of animals 

 is pitched. At any rate, we realize that in animals waste 

 products are produced which must be gotten rid of. It 

 is a curious fact about the metabolism of rest, or basic 

 metabolism, as it occurs in animals, that it is accompanied 

 by steady waste of the very complex substances of which 

 living protoplasm itself is largely composed. Thus pro- 

 teins, which have been described in an earlier chapter as 

 the most important constituents of living protoplasm, 

 are being continually lost from the protoplasm and have 

 to be as continually replaced. Although we do not under- 

 stand precisely why this happens, the fact that it does 

 happen is of a good deal of importance. For one thing, 

 it means that all animals, including ourselves, must ac- 

 quire in the diet the necessary materials by which the 

 protein that is lost in basic metabolism can be replaced. 

 Since animals are so constituted that they can only re- 

 place proteins with proteins, this means that the diet 

 must always include proteins or there will be a wasting 

 away of the life substance, with its ultimate death if the 

 loss is fallowed to go too far. 



Waste in Growth Metabolism is less noticeable. 

 In fact, it would seem as though animals should be able 

 to put together the constituents of which protoplasm is 

 composed without much waste. It is true, however, thajb 

 if — as usually happens — the substances which they get 



