292 Biological Chemistry. 



The study of the action of tissues or tissue extracts on 

 chemical substances virtually resolves itself to-day into the 

 study of the enzymes contained within the tissue. 



The Nature of Ferment Action. If a solution of 

 sucrose (cane-sugar) is treated with an extract of yeast 

 under suitable conditions it undergoes the change known 

 as " inversion " (see p. 192), and yields a mixture of dextrose 

 and laevulose in equal proportions - 



CnHnOu + H 2 = C 6 H I2 6 + C (i H 12 6 



Dextrose. Laevulose. 



In the above equation no account is taken of the ferment 

 " invertase," to the agency of which the reaction is due. 

 A very small amount of the ferment, furthermore, is able 

 to produce a relatively very large amount of chemical 

 change. An action of this description is described as 

 " catalytic," and the agent which brings it about is known 

 as a " catalyst," which has been defined by Ostwald as " a 

 substance which alters the velocity of a reaction but does 

 not appear in the end products." It may be claimed that 

 this definition does not accurately correspond with the 

 facts, for cane-sugar, for example, if left in contact with 

 water, does not undergo change ; the reaction only pro- 

 ceeds after the addition of the invertase. There are 

 reasons to suppose, however, that a small amount of inver- 

 sion of the cane-sugar does actually take place in the 

 absence of the enzyme, but that the amount is so small 

 that it cannot be detected by chemical methods even after 

 a prolonged interval. Instances of catalytic action are 

 not confined to products derived from living organisms. 

 Hydrogen and oxygen will not combine together to form 

 water vapour at ordinary temperatures except in the 

 presence of finely-divided platinum ; hydrogen peroxide 

 will not oxidize various organic substances except in the 





