Catalysis or the Speeding-up of Chemical Reactions 47 



Another good example of enzyme action is found in the case 

 of the extraction of indigo from the indigo-plant. This plant 

 contains a compound indican which is a combination of the 

 active principle of indigo with glucose. On steeping the plant 

 in tepid water for 12-15 hours an enzyme acts on the indican 

 and the indigo principle passes into solution from which it 

 subsequently settles out on oxidation by atmospheric oxygen. 



In the case of the animal organism enzymes are found to be 

 very abundant. These natural catalysts have each a specific action. 

 One set assists a particular reaction, another induces a different 

 chemical change, and others accomplish hydrolysis of different 

 types. In the saliva, for instance, an enzyme ptyalin is found 

 which acts on starch to break it down into sugars. In the 

 stomach pepsin is present which acts in the acid solution of the 

 gastric juice to break down proteins into simpler bodies. In the 

 alkaline pancreatic juice another enzyme trypsin occurs, which 

 also acts on proteins. Fat-splitting enzymes are also present. 

 So the work of the digestion and assimilation of food goes on, 

 being accomplished by the agency of these catalytic compounds. 



Attention might again be drawn to the gentle way in which 

 the enzymes bring about the chemical changes for which they are 

 responsible. A fat can be split into two parts in the laboratory 

 by hydrolysing it (causing it to add on water). This is only 

 brought about by boiling for a considerable time with dilute 

 sulphuric acid or strong caustic soda solution. (The latter re- 

 action is the basis of the change that takes place in soap manu- 

 facture). On the other hand a fat may be broken down into 

 glycerine and the fatty acid by the action of the enzyme, lipase. 

 This change goes on without appreciable rise of temperature, 

 whereas with the inorganic catalyst considerable boiling is 

 necessary. 



The action of an enzyme has been compared to the unlocking 

 of a door by a key : in contradistinction the action of an inorganic 

 catalyst has been likened to the breaking down of the door by 

 shatterings and smashings. In this we get the contrast between 



