The present position in the field of 

 facilitated diffusion and selective active transport 



by 



J. F. DANIELLI 



Z°°l°g.y Department, Kings College, London 



DEFINITION AND CHARACTERIZATION OF PROCESSES 



It is desirable to distinguish, as accurately as is possible at the present time, between 

 several processes: diffusion, facilitated diffusion, and selective active transport. This 

 contribution is concerned with selective active transport, i.e. active transport which is 

 selective for a limited range of molecular species. It is not concerned with unselective 

 active transport; for instance a process whereby environmental fluid is accumulated 

 unchanged in a vacuole on one side of a membrane, and discharged unchanged from 

 the vacuole on the other side of the membrane, would be active transport, but un- 

 selective and therefore not of significance in this discussion. 



Diffusion is brought about by the driving force of thermal agitation. In a homo- 

 geneous fluid the rate at which a given molecular species diffuses may be calculated, 

 at constant temperature and pressure, if the viscosity of the fluid and the molecular 

 weight of the diffusing species are known*. As a result of a diffusion, the free energy 

 of the system is lowered, and there is usually a decline in gradients of chemical poten- 

 tial! ^ no force other than that of thermal agitation is acting upon the molecules 

 (i.e. if gravitational, electrical and other forces have no significant effect upon the 

 final distribution of molecules). Thus diffusion is selective in terms of molecular 

 weight (or linear dimensions j - ), but is unselective in terms of structural and steric 

 factors. 



Facilitated diffusion also occurs under the driving force of thermal agitation, but 

 differs from diffusion in that the rate at which molecules diffuse is strongly influenced 

 by structural and steric factors. It is a process commonly found in studies of the 

 permeability of plasma membranes, and in the past has usually been included in the 

 category of active transport. But it is better separated as a special type of diffusion, 

 since the equilibria attained by facilitated diffusion are the same as those achieved 

 by diffusion. The difference between the two processes is essentially that, by facilitated 

 diffusion, some molecular species may reach diffusion equilibrium much more rapidly 

 than would be possible by non-facilitated diffusion. 



* This is true for molecules of a molecular weight of up to about 1,000. When the molecules diffusing are 

 very large compared with the solvent molecules, the rate of diffusion is more accurately calculable from the 

 linear dimensions than from the molecular weight. 



t This is not always so, for if the diffusion of two species is linked, as in Osterhout's well-known guaiacol 

 model for K + accumulation, there may be an increase in the chemical potential gradient of one species achieved 

 at the expense of a decline in the gradient of another species. 



