74 THE UTILIZATION OF FOOD 



olism is used by the biologist as a convenient general 

 term to include all these chemical changes of foods and 

 other substances made out of them. 



Metabolism in General. — Chemical processes in liv- 

 ing things involve, first, a preliminary stage, viz., the 

 taking in of the food materials; second, the metabolism 

 itself, or the transformations by which these are worked 

 over into cell-substance or products on the one hand, or 

 used up as sources of energy on the other; and third, the 

 getting rid of such substances or products resulting from 

 the chemical action as are not to be retained within the 

 protoplasm. Materials to be gotten rid of are either 

 manufactured things like enzyms, which both plant and 

 animal cells form and secrete, or waste products, like the 

 carbon-dioxid gas which is produced whenever food is 

 oxidized. The chemistry of living protoplasm is exceed- 

 ingly complicated, far too complicated for mankind to 

 hope to understand it completely within any reasonably 

 short future time. Enough is known about it, however, 

 that the different purposes served by the metabolic 

 processes can be set down, as is done in succeeding 

 paragraphs. 



Basic Metabolism. — Living protoplasm differs from 

 dead protoplasm in that in living protoplasm there is 

 going on continuously what we may think of as the ** life 

 process," which is made up of very complicated chemical 

 activities whose stoppage means death. It is true that 

 in most kinds of plants at some time in their history, 

 and in some kinds of animals, life is tided over from 

 season to season by a drying out of the protoplasm with 

 a corresponding cutting down of the chemical activity 

 going on within it; so seeds and spores manage to live 

 over from one season to the next, or for several seasons, 

 because the chemical processes fundamental to life are 

 cut down to the absolute minimum and not much ma- 

 terial is used up. But in actively living protoplasm 

 those chemical processes which make up the basis of life 



