BALANCED REACTIONS 



125 



as enter it. Now alter the conditions, (1) open the bottle to a dry 

 atmosphere, i.e. to unlimited air containing inlinitely little mois- 

 ture. The reaction will proceed almost entirely in one direction — 

 evaporation. (2) If the bottle be opened to an air supersaturated 

 with moisture, the reverse process, condensation, will predominate. 

 The effect of the removal of the maltose in increasing the speed 

 of digestion of starch is shoAvn very clearly in the following 

 experiment in which the course of the digestion of the starch was 

 followed by the iodine reaction. In one case, the digestion was 

 carried on in a beaker, and in the other in a dialysing tube 

 immersed in running water so that the maltose dialysed out. 



TABLE XXI 



Ecto-enzymes generally have a catabolic function. The by- 

 products of their activity are removed as rapidly as they are 

 formed. Many endo-enzymes have for the most part an anabolic 

 activity. They are brought into contact with simple compounds 

 and proceed to build them into more complex substances. All 

 enzymes have both breaking-down and building-up functions. 

 The conditions under ivhich they zvork determine their function. 



A peculiar phenomenon has been noted in this connection, 

 namely, that the substance built up by an enzyme may not be 

 quite the same as the complex substance originally broken down 

 by it. Maltase, for example, splits maltose into molecules of 

 glucose, but the substance formed by the action of maltase on 

 glucose is not maltose but its /3 form, isomaltose. On the other 

 hand, isomaltose is split by emulsin into glucose, while emulsin 

 causes two glucose molecules to unite to form maltose. This is 

 explained by supposing that the nature of the enzyme-substrate 

 complex influences the rate of the reaction. If HCl is used as 

 catalyst, the equilibrium point is reached with glucose, maltose, 

 and isomaltose present in the same pr()j)()rti()ns, irrespective of 

 the original proportions present in the substrate. The point of 

 equilibrium is changed by the enzyme. If maltase is used, the 

 proportion of maltose is diminished, but if emulsin is the enzyme 



