MECHANISMS OF ION TRANSPORT 



31 



(a) 



+ 



+ +++ + + 



(b) 



Fig. 6. Transport across membranes 

 a. Facilitated diffusion. Diagram illustrating the potential energy barriers 

 in a lipid membrane (redrawn from Davson and Danielli, 1943); b. Electro- 

 osmosis. For explanation, see text. 



diflfusion when the diameter of the channels through which move- 

 ment occurs is reduced. This is demonstrated by the well-known 

 fact that a dilute agar gel offers high resistance to mass flow, but 

 allows diffusion of substances through it at a rate not very much 

 lower than that of diffusion in water. Temperature affects the rate 

 of mass flow by its influence on the coefficient of viscosity; theoreti- 



