ACTION OF ENZYMES AND CELLS 173 



The only point essential with the simple type of catalyst is that 

 it shall work towards the equilibrium point ; but it can cause move- 

 ment towards that point not occurring before, alter velocity of 

 reaction and may fall short of reaching the equilibrium point. 



The experimental evidence with regard to the action of enzymes 

 is entirely in accord with the view here expressed. 



If, as has already been pointed out, the appropriate solutions 

 on which the digestive enzymes act be kept in sterilised condition, 

 not the slightest change is observable in any one case, no matter 

 how long the solution is preserved, but if the enzyme is added its 

 effect is apparent in a few minutes. 



Further, the nature of the reaction and of the products formed, 

 as well as the relative amounts of the latter, are often determined 

 by the nature of the catalyst added to the same medium. As, for 

 example, in the catalysis of protein by pepsin, trypsin, acid, and 

 alkali respectively. Here, in all cases, the products and their 

 amounts are different. Are we, then, to suppose that all these 

 different reactions to as many different equilibrium points are 

 running concurrently in any given protein solution, but at so slow 

 a rate as not to be observable ? It is an interesting theoretical 

 speculation ; but it would appear more probable that these different 

 catalysts possess a specific affinity for attacking some definite 

 molecular groupings in the complex protein molecule, and in each 

 case started a reaction which was not possible until that particular 

 catalyst was present in the solution. 



EQUATIONS FOR VELOCITY OF REACTION. 



The various expressions deduced for the value of the energy set 

 free in the reaction give us the driving agent in the reaction, but 

 the unknown values of the resistances opposed to this, and the 

 effect of the amount of catalysts upon them, render the velocity 

 theoretically indeterminate from a knowledge of the energy set 

 free in the reaction only. We have hence, in order to obtain 

 formulae for the expression of the velocity of reaction under different 

 conditions, to introduce empirical constants to denote the resistance 

 to reaction of the substances concerned, or rather the reciprocals 

 of these resistances that is, the chemical conductivities. The 

 velocity of change of each substance is then proportional to the 

 product of the osmotic pressure or molecular concentration of that 



