C. ADRIAN M. HOGBEN 177 



lined solely in terms of the elect rochemislry of the two solutions bathing a 

 complex biological membrane. The principal argument for accepting these 

 detlnitions is that we can proceed to analyze ionic movement without making 

 any restricting assumptions concerning the nature of the membrane. 



Tlie lack of unanimity in defining passive diffusion arises primarily from two 

 sources. The driving force, the electrochemical potential gradient, may be in- 

 sufhcientl}' stated. Not infrequently, it is assumed that a necessary charac- 

 teristic of passive diffusion is linear proportionality between net movement 

 and the driving force, or that, in other words, the diffusion coefficient through 

 a comple.x membrane should be constant. Saturation of a lipoid phase or of 

 fixed charges will bring about a striking departure from a linear relation be- 

 tween penetration and concentration. These aspects of ionic passive diffusion 

 have recently received critical consideration (80, 96) with theoretical treat- 

 ments developed in terms of the membrane constitution. 



Much of our present insight into movement through biological membranes 

 derives from the use of isotopes. H. H. Ussing utilized this tool to redefine pas- 

 sive diffusion. Quite apart from the increased sensitivity that accrues from use 

 of isotopic tracers, we obtain new information. We can measure the two op- 

 posite unidirectional movements through an unknown complex membrane. 

 The rates of these unidirectional movements are designated fluxes. Without 

 any regard to the structure of a membrane other than it being uniform at right 

 angles to the direction of movement, it has been shown by Ussing (98) that if 

 an ion moves independently in response to its own electrochemical gradient at 

 every point, the flux ratio of a passive ion is given by: 



M/M' = (c/c')(f/f')e<^-°-^^''/^^ 



The flux from one of the two solutions and the ion concentration and activity 

 coefficient in that solution are distinguished by primes. The flux (M) ratio is 

 equal to the product of the concentration (c) ratio, the activity coefficient (f) 

 ratio and an exponential function of the electrical potential difference (P.D.), 

 with zF/RT having their conventional physical chemical meaning (98). The 

 above equation constitutes the working definition of passive diffusion. 



There are several current usages of the term active transport that lead to 

 confusion. A majority of biologists would accept this term to imply that move- 

 ment is regulated by the biochemical activity of the cell. If a correlation be- 

 tween variations in the rate of transfer and metabolic factors were accepted as 

 the criterion of active transport, then sufficient distinction is not drawn be- 

 tween passive diffusion and active transport. A challenge to the biochemical 

 integrity of a cell, often gauged by the injurious effect of metabolic inhibitors, 

 may not only modify the source of energy transferred to material moving 

 through the membrane, but it may also modify structural features which de- 

 termine the rate of passive diffusion. 



