ON DIFFUSION; OSMOSIS 207 



Generalization of Method 



Enzymes are not the only catalysts in the living system, of course. Sur- 

 faces, acid (H + ), base (OH ), and metallic ions are all important catalysts. 

 The general principles outlined above apply to these equally as well as to 

 enzymes. The factoring method of analyzing rates — that of extracting from 

 the proportionality constant one after another the variables and universal 

 constants upon which the rate of a process depends — in some ways has 

 reached its highest state of development in chemical kinetics; and it is scor- 

 ing rather remarkable successes with some very complicated biochemical re- 

 actions. Whether this method of analysis, which ultimately reduces to 

 analysis of the intermolecular forces and molecular movements of a biologi- 

 cal process, is properly termed "biophysical chemistry' 1 or "chemical bio- 

 physics," is often uselessly debated. It is a matter of definition; and no 

 definition has yet been generally accepted. We use this illustration of the 

 factoring method not only to discuss the velocity of biochemical reactions in 

 terms of molecular interactions, but also by analogy to discuss in the follow- 

 ing sections the velocities of the physical processes of transport, namely dif- 

 fusion, osmosis (a special case of diffusion), viscous flow, electrical con- 

 ductivity of solutions and tissue, and heat conduction. 



ON DIFFUSION; OSMOSIS 



Diffusion may be defined as the movement, in a preferred direction, of one 

 component relative to the other components, of a mixture or solution. The 

 preferred direction is from the place of higher concentration to the place of 

 lower concentration of diffusing substance. No flow of the whole fluid need 

 occur — no turbulence, nor even convection; no gravitation, no electrical field 

 is of importance to transport by pure diffusion. 



The fact that diffusion occurs is not surprising when one remembers that 

 all molecules are in a state of continuous motion. The more molecules of 

 type P there are present in a particular volume of solution, the greater the 

 likelihood that some of these will gain enough excess energy to find their way 

 out of this volume. Consider two unit volumes with a common face, one 

 with concentration P in Q higher than the other (Figure 8-8). Because all 

 molecules are in continuous motion (i.e., have kinetic or thermal energy), 

 on the average more P molecules from volume 1 pass into volume 2 than the 

 reverse. In fact, the greater the concentration difference (actually the gradi- 

 ent dc/dx), the greater the speed at which they diffuse, other things being 

 equal. Figure 7-7 was an earlier impression of this same idea. 



If, however, some sort of barrier to diffusion is placed between volumes 1 

 and 2, the rate at which P diffuses is slowed down; and the greater the thick- 

 ness of this barrier the lower the rate becomes. To a first approximation, 



