2 



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Concepts and terms 



Diffusion 



Although not a very effective form of migration when large 

 distances are involved, diffusion becomes extremely effective for 

 short distances and is undoubtedly the most important way in which 

 metabolites move about in the cell. If one reaction produces a solute 

 at point A and another consumes it at point B, the movement at a 

 steady state can proceed almost as efficiently as if it were a directed 

 flow. Furthermore, diffusion no doubt plays a part in all the other 

 modes of transport. 



The characteristics of diffusion are rather obvious. For con- 

 venience we shall consider molecules bearing no net charge, so that 

 a contribution of an electromotive gradient to the total ("electro- 

 chemical") gradient may be ignored. The net rate of migration for 

 such molecules depends on the concentration gradient, S\ — S.>; the 

 flux in a given direction through a membrane depends on the con- 

 centration (strictly speaking, the activity) in the phase of origin. 

 (The term flux, we should emphasize, designates the rate of a one- 

 way movement; therefore, we cannot correctly speak of a rate of 

 flux.) 



The flux ratio between two phases, as Ussing (1949) pointed 

 out, therefore equals the ratio of their concentrations, Si/S 2 . At 

 equilibrium for two similar aqueous phases, S1/S2 becomes 1. The 

 linear relationship between concentration and flux will be found to 



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