i METABOLISM 17 



other. The same process of gaseous diffusion is continually 

 going on between the carbon dioxide in the interior of 

 Amoeba and the oxygen in the surrounding water, the proto- 

 plasm acting as the porous partition. In this way the carbon 

 dioxide is got rid of, and at the same time a supply of 

 oxygen is obtained for further combustion. 



The taking in of oxygen might be looked upon as a kind 

 of feeding process, the food being gaseous instead of solid 

 or liquid, just as we might speak of " feeding " a fire both 

 with coals and with air. Moreover, as we have seen, the 

 giving out of carbon dioxide is a process of excretion. It 

 is, however, usual and convenient to speak of this process 

 of exchange of gases as respiration or breathing, which is 

 therefore another function performed by the protoplasm of 

 Amoeba. 



The oxidation of protoplasm in the body of an organism, 

 like the combustion of wood or coal in a fire, is accompanied 

 by an evolution of heat. That this occurs in Amoeba can- 

 not be doubted, although it has never been proved. The 

 heat thus generated is, however, constantly being lost to the 

 surrounding water, so that the temperature of Amoeba, if we 

 could but measure it, would probably be found, like that of 

 a frog or a fish, to be very little if at all above that of the 

 medium in which it lives. 



We thus see that a very elaborate series of chemical pro- 

 cesses is constantly going on in the interior of Amoeba. 

 These processes are divisible into two sets : those which 

 begin with the digestion of food and end with the manufac- 

 ture of living protoplasm, and those which have to do with 

 the destruction of protoplasm and end with excretion. 



The whole series of processes are spoken of collectively 

 as metabolism. We have, first of all, digested food diffused 

 through the protoplasm and finally converted into fresh 



