16 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



circumstance that we cannot create living matter 

 would be no proof that such matter did not originally 

 come into existence through the play of physical and 

 chemical agencies or other natural forces. 



We may now consider the second feature which 

 distinguishes living beings from artificial machines 

 the power of metabolism. The metabolic power 

 consists of the ability of the living protoplasm 

 simultaneously to build up some materials (living 

 or dead) and to break down others. These processes 

 of metabolism must be pictured as keeping the cell 

 contents in a continual state of flux. In youth the 

 constructive processes preponderate over the de- 

 structive; in old age the destructive processes are 

 more active than the constructive; in adult life 

 there is an approximate balance between the forces 

 that upbuild and those that break down. It has 

 been already mentioned that ferments or enzymes 

 play a large part in the chemical processes of the 

 body. This holds true both of the upbuilding or 

 synthetic processes and of the destructive ones. 

 Physiologists are now agreed that there are enzymes 

 which break down sugars, enzymes which break 

 down fats, and enzymes which break down proteins. 

 In recent years there has come to light a singular and 

 unpredicted property of these enzymes. This is 

 the reversibility of their action under special condi- 

 tions. Croft Hill showed that under certain condi- 

 tions the sugar-splitting enzymes can combine two 

 molecules of a simple sugar into one molecule of a 



