178 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



Another perhaps trivial example of the usage of active transport may be en- 

 countered when the rate of isotopic penetration into a cell is characterized 

 by an 'adsorption isotherm' and interionic competition. These features have 

 been considered by some to be sufficient criteria for postulating active trans- 

 port. Yet penetration through an ion exchange membrane may be charac- 

 terized by saturation kinetics and interionic competition, even though the 

 driving force leading to net movement of an ion is the electro-chemical potential 

 gradient between the two solutions. 



It is desirable to define active transport in terms of the solutions bathing an 

 intricate membrane without making any restricting assumptions concerning 

 the structure of the membrane. Considerable stress will be placed upon the 

 generation of electrical current by active ion transport. Active transport can 

 be thought of as that movement of a particle which is not attributable to forces 

 measurable in the bathing solutions and requires energy generated by the inter- 

 vening membrane. The interested reader is referred to several recent critical 

 discussions of active transport (89, 99, 100). Active transport will be con- 

 sidered, in this discussion, to occur when there is a net transfer against an 

 electro-chemical potential gradient or net transfer in the absence of a gradient 

 but in spite of the resistance offered by the membrane. The electro-chemical 

 potential (ji) difference between the two solutions will be defined (98) by: 



M - m' = RT In 1^, • |- j + zF(P.D.) 



with symbols and primes having the same meaning as in the flux ratio equation 

 above. 



In the further consideration of active transport, it will become desirable to 

 distinguish those examples of uphill transfer which are due to the drag of 

 another species which flows downhill because of an activity difference between 

 the two solutions for the latter species and to designate such instances as coupled 

 diffusion. Without recapitulating the detailed treatment given by Ussing to 

 solvent drag (100), it is possible to single out one consideration. If there is a 

 hydrostatic or osmotic pressure gradient between the two solutions bathing a 

 membrane sufficient for solvent flow in the same direction as net solute move- 

 ment, the solute movement will not be designated active transport unless sol- 

 vent flow has been excluded as the cause. 



While attention will be directed in this article to the role of active transport 

 in achieving net movement across a secretory membrane, a more useful defini- 

 tion of active transport would have to be expanded to deal with the more fre- 

 quent circumstance of a steady state activity difference without net movement, 

 as between cell interior and exterior. In general the maintenance of an electro- 

 chemical potential difference without net movement will require the expendi- 

 ture of energy by the membrane, for absolute impermeability is improbable. 



For some this usage of active transport will be too narrow and for others too 



