THE PHYSICAL BASIS OF LIFE 25 



sciences. So it appears clear that the organism is a system for 

 transforming energy into work performed — transforming the po- 

 tential energy stored in chemical complexes of its own substance 

 into the various vital processes of life. And it is in this transfer of 

 energy from one kind to another that we find exhibited the activi- 

 ties which are most distinctive of living things. In these processes 

 of metabolism many complex substances rich in potential energy, 

 which have entered as food and have been, in whole or part, added 

 to the protoplasmic system, are reduced to simpler and simpler 

 conditions and finally, with their energy content nearly or entirely 

 exhausted, are eliminated as excretions. Obviously, if life is to 

 persist, this continual waste must be counterbalanced by a propor- 

 tionate intake of food in order to renew the supply of energy and 

 to provide the materials which, after preliminary changes, are made 

 into an integral part of the living organism. 



4. Maintenance and Growth 



Thus in living the animal or plant is partially consuming and 

 rebuilding itself continually — metabolism is a dual process. 

 When constructive metabolism, anabolism, keeps pace with de- 

 structive metabolism, katabolism, the individual remains essen- 

 tially unchanged — it balances its account physiologically — and 

 this maintenance is the normal condition of adult life. But one of 

 the most obvious results of metabolism is growth, or permanent 

 increase in the size of the individual. As a rule growth is most 

 rapid during the early part of the individual's existence, or youth. 

 Indeed, at birth a child is about a billion times larger than the egg 

 cell from which it has developed. Later in life, when a certain phys- 

 iological balance, or maturity, is attained, growth chiefly occurs in 

 making good, in so far as may be, the wear and tear incidental to 

 living, and in providing for reproduction. 



Growth, as well as maintenance, means that the organism takes 

 the materials which it receives in the form of food, transforms 

 them and fits them into the protoplasmic organization here and 

 there throughout as needed. This interstitial growth by chem- 

 ical synthesis is in striking contrast to the growth, for example, of 

 crystals that increase in size merely by the addition to the surface 

 of new material of the same kind from the saturated solution, the 

 mother liquid, in which they are suspended. Crystal growth is 

 passive; organic growth is active. Protoplasm, with materials 



