ACTION OF ENZYMES 53 



the resistance to reaction of the substances concerned, or rather 

 the reciprocals of these resistances (that is, the chemical con- 

 ductivities). The velocity of change of each substance is then 

 proportional to the product of the osmotic pressure or molecular 

 concentration of that substance and the constant l which repre- 

 sents the reciprocal of the resistance to change. For (as is shown 

 in deducing the conditions of equilibrium) the energy set free by 

 the reaction will vary with the osmotic pressure, and the resist- 

 ance is inversely proportional to the constant, and accordingly the 

 product of these two is proportional to the velocity of reaction. 



In all cases the tendency to react is proportional to the osmotic 

 pressure or molecular concentration of each substance in solution, 

 and this, the fundamental law of chemical kinetics, is called the 

 law of mass action. When two or more substances tend to unite 

 to form a single substance, the tendency on the part of each sub- 

 stance is by the law of mass action proportional to its pressure 

 or concentration in the solution, and hence the velocity of forma- 

 tion of the combined substance will be proportioned to the product 

 of the concentrations of the uniting constituents. Accordingly, 

 in any equation of velocity, the velocity of formation of a substance 

 may be set down as equal to the product of the concentrations 

 of its constituents in the solution, multiplied by a constant (k) 

 which represents, and is the reciprocal of, the resistance to the 

 reaction. The value of k will hence vary with the amount of 

 catalyst present, with the temperature and other factors which 

 alter the resistance, but these factors being supposed kept constant, 

 the value of (k) will remain constant throughout the reaction. 



We are now in a position to state equations for the velocity 

 of reactions. 



Let two substances, A and B, in solution react to form two others, 

 C and D, according to the equation 



Further, let the molecular concentrations of the four substances (or 

 their osmotic pressures which are proportional to these concentra- 



1 It will be shown later (p. 78 et seq.) that the assumption that the resistance 

 is constant throughout the reaction is only an approximation, and that the 

 resistance really varies somewhat with the ratio between enzyme and sub- 

 stratum. 



