4 VETERINARY PHYSIOLOGY 



of the enzyme. Sometimes these enzymes lead to a complete 

 decomposition of the substance upon which they act, but this 

 is often prevented by a checking influence exerted by the 

 accumulation of the products of their action, e.g. by the alcohol 

 developed from the sugar. With certain enzymes at least, the 

 action may actually be reversed if the enzyme is brought in 

 contact with the final products of decomposition. Thus the 

 enzyme which splits esters into their components may cause a 

 linking of those components to form esters. In this respect they 

 simply aid the establishment of an equilibrium between the 

 component substances in the reaction. 



The general action of the enzymes has been termed kata- 

 lytie. It may be compared to the action of an acid in the 

 inversion of cane sugar 



Here the acid merely hastens a reaction which would go on 

 slowly in the presence of water alone. 



The precise way in which such katalytic actions are brought 

 about is still not quite clear, but there is evidence that the 

 agent acts as a middle-man between the reacting substances, in 

 the case of H 2 2 taking up the and then liberating it in the 

 case of the decomposition of cane sugar taking up H 9 and 

 handing it on ; just as in the oxidation of glucose, which occurs 

 when it is boiled with an alkali, a metallic oxide, such as 

 cuprous oxide, may take oxygen from the air and hand it on to 

 the glucose, thus making the oxidation more rapid. 



Living yeast differs from these dead substances simply in 

 the fact that it uses the energy liberated from the glucose. In 

 virtue of this, the yeast has the power of repair and of growth. 



But protoplasm is also constantly breaking down, and if 

 yeast be kept at a suitable temperature in water without any 

 supply of material for construction, it gives off carbon dioxide 

 and decreases in bulk on account of these disintegrative 

 changes. These are as essential a part of the life of living 

 matter as the building-up changes, and it is only when they 

 are in progress that the latter are possible. 



Protoplasm (living matter) is living only in virtue of its 

 constant chemical changes, metabolism, and these changes 



