MEMBRANE PERMEABILITY AND DRUG STRUCTURE 51 



tremely slowly. However, as in most instances substances 

 which penetrate very slowly will not have a drug action, 

 these more slowly penetrating substances are probably 

 of little practical importance. 



Table iv shows the permeability of a number of dif- 



TABLE V 



THE PERMEABILITY OF DIFFERENT CELLS TO UREA AND 



TO GLYCEROL 



ferent cells to various substances. In Table v some 

 figures are given for the permeability of certain cells to 

 urea and to glycerol. It will be seen from these results that 

 the permeability of different cells to the same substance 

 may vary by a factor of 100 or even 1000-fold. Moreover 

 the relative order of permeation of substances into dif- 

 ferent cells is not always the same. Thus some cells are 

 more permeable to urea than to glycerol, whereas others 

 are more permeable to glycerol than to urea. From this 

 it is apparent that considerable therapeutic advantages 



Cell Physiology 4 



