C. ADRIAN M. HOGBEN 1 79 



broad. On the one hand, movement through a membrane may involve chemical 

 reactions which can dissipate metabolic energy, yet when the electro-chemical 

 potential of the traversing particle is not increased the criterion for active 

 transport is not satistied. On the other hand, the definition requires no pre- 

 sumption of a specific mechanism and will embrace quite dissimilar phenomena. 



Confining our attention to ions, there are several valuable extensions to the 

 definitions of passive diffusion and active transport in terms of the electrical 

 parameters of a membrane. It is generally assumed that electrical conductivity 

 of biological membranes is due to passive ionic movement. This permits a 

 quantitative evaluation of fluxes of individual passive ions. The relation be- 

 tween passive ionic flux and partial conductivity has been given by Ussing 

 (loi). When there is no electro-chemical gradient between two solutions, the 

 opposmg fluxes wfll be equal and the passive ionic flux in /xEq hr.^^ will ap- 

 proximately equal m.mhos of partial conductance. 



With active transport of an ion there is net transfer of charge from one side 

 to the other of a membrane. The electrical current generated by a membrane 

 can be measured by short-circuiting the two faces (loi). When the membrane 

 is bathed by identical solutions, the algebraic sum of the several active trans- 

 ports will be equal to the membrane short-circuit current. An appropriate 

 balance sheet can then be prepared of net ionic transport and the total electrical 

 current, allowing quantitative evaluation of a given ion transport system. 



Between the extremes, passive diffusion characterized as movement which 

 obeys the flux ratio equation and active transport leading to net transfer in 

 the absence of a favorable electro-chemical potential gradient between two 

 solutions, other types of movement are possible. Particular attention will be 

 paid to 'exchange diffusion' (97, 100), In this instance, ionic movement is con- 

 sidered to involve a combination between the penetrating ion and a carrier 

 within the membrane but without there being an uphill movement of the ion. 

 The flux ratio of the ion will no longer conform to the equation for passive dif- 

 fusion. When the carrier is saturated, the portion of the two fluxes of a given 

 ion which is due to exchange diffusion will approach equality and independence 

 of the electro-chemical potential. 



A rigid distinction between passive diffusion and active transport may prove 

 to have limited application. As understanding of a given system proceeds, it 

 may become appropriate to select terms more fitted to the description of the 

 intimate mechanisms. Nevertheless, it does allow a broad classification when 

 viewing comparative aspects and does provide a first step in characterizing 

 movement through living membranes. 



COMPARATIVE ASPECTS OF CHLORIDE TRANSPORT 



The first critical evidence for active chloride transfer was developed by 

 August Krogh. In the course of his classical study of osmotic regulation, he 

 paid special attention to loss and gain of salt through the skin of living frogs 



