GENERAL PRINCIPLES 193 



actions are covered more or less comprehensively in textbooks in biochemis- 

 try, and since physical processes are very numerous in the living animal but 

 usually receive very little attention from the kinetic point of view, most of 

 the effort is put on the kinetics of physical processes. The presentation em- 

 phasizes the formal similarity of all these processes, and the fact that there 

 are many common factors upon which the rates depend. Unfortunately we 

 do not know enough at this time to achieve very much of the ultimate goal 

 mentioned in the Foreword. 



GENERAL PRINCIPLES 



Rate-Controlling Step 



If any physical or chemical process goes from initial state to final state 

 through a series of intermediate steps, usually one of those steps is inherently 

 slower than the others and controls the rate of the over-all process. For 

 example, a bucket brigade passing pails of water hand to hand from the river 

 to the burning house can transport water no faster than the little old lady 

 who forms the slowest link. The principle is true for chemical and physical 

 processes as well. In most processes in which we are interested, the over-all 

 process involves physical transport as well as chemical reaction. One of the 

 physical steps or one of the chemical steps may be rate-determining. 



A measurement of the over-all rate or speed is always a measure of the 

 speed of the slowest step. Consider the chain of events: 



If the reaction B — * C is the slowest, then the over-all rate is the rate of 

 B — C. (As an exercise, apply this principle to the over-all event of free air 

 becoming dissolved in the blood stream. What would you expect to be the 

 slowest step?). 



Equilibrium 



If a process can proceed forward or backward, starting as either reactants 

 or products and produce products or reactants, respectively, the process will 

 move spontaneously (although perhaps slowly) in a direction toward mini- 

 mum free energy for the over-all reaction materials: The reaction will "stop" 

 when the concentrations are such that the work the reactants can do equals 

 the work the products can do, and then apparently the reactions in both 

 directions cease. The materials have then reached thermodynamic equi- 

 librium. 



The rate of the forward reaction will depend upon the inherent attraction 

 the reactants have for each other, and upon the concentrations of the reac- 



