METABOLISM AND THE DISTRIBUTION OF MATERIAL 251 



pendent upon the formation and removal of acids by oxidative proc- 

 esses. The concentration of the products of a reaction in the solution 

 brings the reaction to a standstill. If we are dealing with cleavages 

 the crystalloid products may be readily removed by diffusion. This 

 does not occur hi the case of synthetic colloid products. Nor is it 

 so essential because from the point of view of the law of mass action 

 they are not to be regarded as dissolved. 



We know enzymes to be the excreta of the stomach and intestines, 

 but we also know that cells themselves contain enzymes; we may 

 mention the uricolytic enzyme of the liver and zymase of yeast 

 which ferments sugar. 



We must also recall that enzymes are the most strongly adsorbed 

 of all the colloids of the body, and that the ability to be adsorbed is 

 largely dependent on the acid or alkaline character of the medium. 

 An enzyme may be so fixed (e.g., rennet by charcoal) that its very 

 existence is no longer determinable; a change of reaction recalls it 

 to life. It may also be released by the approach of another colloid 

 (in our example casern) by which it is adsorbed still more strongly. 

 We thus get an inkling of the great importance enzymes have for the 

 life of the cell, without as yet understanding the details. 



The conditions governing dissimilation are much more readily un- 

 derstood. Through enzymatic cleavage of colloids, there are formed 

 crystalloid products which pass into the circulating fluids of the or- 

 ganism by diffusion and leave as excreta, or, after oxidation to C02, 

 are expired. 



We must not conclude that in every instance the entire colloid 

 molecule breaks down into crystalloid cleavage products. In this 

 way, by the splitting off of individual "side chains" (P. EHRLICH) 

 there is permitted great variation in cell life, which we might assume 

 from our previous experiences. 



