The present position in the field of facilitated diffusion and selective active transport 



formamide, water, and urea, frequently penetrate cells faster than is compatible 

 with permeation through a strictly homogeneous lipoid layer. In this group of mole- 

 cules the only specific feature seems to be that the molecules must be very small, and 

 hydrogen-bonding. With the occasional rapidly permeating large polar molecules, 

 such as glucose, mentioned in the previous paragraph, the specific features are ability 



Table III 



Comparison oj calculated and experimental permeability constants. All values multiplied 



by io 16 



to form hydrogen bonds and a very sharp structural specificity: for example, the 

 exceptional rate of entry of glucose into human red cells is not shown by either 

 methyl glucoside or sorbitol. 



(3) The effect of temperature on permeability constants 



When the change of permeation rate with temperature is considered, it is again 

 found that the membranes of many cells are homogeneous lipoid membranes, to a 

 first approximation. And again, the same molecules which were exceptionally fast in 

 penetrating, by comparison with other molecules, provide evidence of inhomogeneity 

 in the membrane. 



Thus glycerol penetrating ox red cells has a permeability constant of 0-002 and a 

 Clio of 3 5, both of the order of magnitude to be expected for glycerol permeating a 

 homogeneous lipoid layer. But glycerol permeates rabbit red cells much more rapidly, 

 with a permeation constant of 005 and a £^ 10 of 2. When the special mechanism 

 which permits this is poisoned with a trace of Cu ++ , the permeability constant falls 

 to about o 002 and the Q,io r i ses to 3 5. The values for the poisoned membrane show 

 that its general structure is that of a homogeneous lipoid layer, but that this is norm- 

 ally masked, in the case of glycerol, by a small proportion of areas highly and specific- 

 ally permeable to this substance. 



It is also found that the very small molecules which penetrate abnormally fast 

 also have abnormal Q^ Q values. 



