METABOLISM 59 



and hair, and for many of the autumnal 

 colors of leaves. In the decay of animals 

 and plants in the soil a dark coloration 

 is produced which is due to these pig- 

 ments. 



If an organism is killed by exposure 

 to boiling water the process of combus- 

 tion ordinarily stops, but if it is killed by 

 certain other methods (as by acetone) 

 the combustion continues after death (in 

 complete absence of bacterial action). 

 The explanation lies in the fact that com- 

 bustion in organisms depends on en- 

 zymes: these are ordinarily destroyed 

 by the heat of boiling water but not by 

 exposure to acetone. 



Enzymes play a very highly important 

 role in the chemical reactions of the or- 

 ganism. We suppose that the enzyme 

 does not initiate the reaction but merely 

 hastens it. It is often said that its 

 action is analogous to that of oil in 

 lubricating a machine and it need not, 

 theoretically at least, be itself used up in 

 the process. Any substance which acts 



