CHAPTER XIV 

 ENZYMES AND THEIR ACTION 



THE characteristic difference between the reactions of inorganic 

 compounds and those of organic substances lies in the rapidity, or 

 velocity, of the chemical changes involved. Speaking generally, 

 chemical reactions take place between substances which are in 

 solution, so that they may come into sufficiently intimate contact 

 that chemical action between them can take place. There are, of 

 course, occasional examples of reactions between dry solids, such 

 as the explosion of gunpowder, etc., but the general rule is that 

 reacting materials must be in either colloidal or true solutions. 



Inorganic materials, when dissolved in water, usually ionize 

 very readily. That is, they are not only disintegrated into indi- 

 vidual molecules, but a considerable proportion of these molecules 

 separate into their constituent ions. When solutions containing 

 ionized compounds are brought together, conditions for chemical 

 interaction are ideal, and the reaction proceeds with such tre- 

 mendous rapidity as to be completed almost instantaneously, in 

 most cases. 



Organic compounds, on the other hand, ionize only very slowly, 

 if at all. Hence, reactions between organic compounds, even 

 when they are in solution, proceed very slowly unless carried on at 

 high temperatures, under^ncreased pressure, or under the influ- 

 ence of some catalytic agent. Even under the stimulation of these 

 reaction-accelerating agencies, most chemical changes in organic 

 compounds when carried on in the laboratory, require several 

 hours or even days and sometimes weeks, for then* completion. 

 But when similar reactions take place in living organisms, they 

 proceed with velocities which resemble those of inorganic com- 

 pounds in the laboratory. This difference between the velocity 

 of organic reactions when caried on under artificial conditions 

 in the laboratory (often spoken of as " in vitro ") as compared 

 with that of the same reactions when they take place in a living 



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