BALANCED REACTIONS 99 



Similarly, lactose and melibiose are both glucose galactosides 

 differing only in the hydroxyl of the glucose molecule united to 

 the galactoside. As galactosides, both are slowly hydrolysed by 

 crude emulsin (known to be a mixture of at least three enzymes). 

 Lactase is, however, without action on melibiose, and melibiase 

 does not split milk sugar. Till further experimental work has 

 been done attempted explanation of these facts is mere guesswork. 

 Fischer has suggested that the enzyme is to its substrate as a key 

 is to its own particular lock. The evidence at present available 

 does not altogether lend itself to this explanation. It looks as 

 if a careful study of the alterations brought about in the con- 

 figuration of colloids by slight modifications of the surrounding 

 conditions might lead towards an acceptable explanation of 

 specificity. (See also Optical Activity, p. 101.) 



Weight is given to this suggestion by examination of the 

 synthesising power of enzymes. Since enzymes accelerate re- 

 actions that would take place without them and all reactions are 

 theoretically reversible, the synthesis of complex bodies from their 

 constituents might be expected by the aid of the same enzyme as 

 brought about the splitting of the complex to simple. That is, 

 a lipase should not only split a fat into fatty acid -f-glycerol, but 

 should regenerate fat from fatty acid -fglycerol. 



A reversible or balanced reaction is one in which, under definite 

 conditions, there is a certain equilibrium point at which the 

 amount of material being broken down is exactly balanced by the 

 amount being built up. For example, take a stoppered bottle 

 half full of water. Two processes are going on simultaneously. 

 (a) Liquid water is undergoing vaporisation and the gaseous 

 hydrol is passing into the air, (b) Gaseous water particles are 

 passing from the air into the water to form, say, dihydrol. When 

 the air is saturated with humidity for that particular temperature, 

 exactly the same number of water molecules will leave the water 

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

 atmosphere, i.e. to unlimited air containing infinitely little 

 moisture. The reaction will proceed entirely in one direction 

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

 saturated 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 shown very clearly in the following 

 experiment by Lea, in which the course of the digestion of the 

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



