Membrane structure as revealed by permeability studies 



pores does not lead to inherently nonsensical results. But after all, you may say, the 

 amphibian skin is a multicellular structure and the pores might be in the intercellular 

 substance rather than in the cell membranes proper. Prescott and Zeuthen (1952) 

 have, however, determined the diffusion permeability and the osmotic permeability 

 of eggs of various freshwater animals and have found the two to be distinctly differ- 

 ent. Some examples are shown in Table III. It will be seen that, particularly in the 

 ovarian eggs, the two permeability coefficients differ appreciably. We can therefore 

 say with some certainty that these cells have pores in the membrane, or else, perhaps, 

 something that experimentally shows up as pores. What I mean with this is that they 

 need not be permanent structures, but may form and close continuously. This possi- 

 bility cannot, however, be tested with our present techniques. 



Table III 



Permeability to water of various eggs (after Prescott and ^euthen) 

 Diffusion permeability coefficient: P d . 

 Filtration permeability coefficient: P { . 



It would be very interesting to obtain similar information concerning other cell 

 types. The technique of Prescott and Zeuthen (I.e.), making use of the diver balance 

 to determine the D 2 exchange rate as well as volume changes, lends itself to the 

 study of the permeability to water of many large and medium-sized cell types. 



The idea that living membranes have pores is of course not a new one. Thus 

 (Hollander's (1937) well-known lipoid-pore theory implies that small hydrophilic 

 molecules penetrate living membranes largely through pores. Davson and Danielli 

 (1943), however, have pointed out that the evidence given by (Hollander for his 

 theory is not quite conclusive. The existence of two different permeability constants 

 for water in many membranes can be taken as support for the lipoid-pore theory. 



If pores in living membranes do exist, this has certain obvious consequences with 

 respect to the way in which the flow of water through the pores influences the 

 diffusion of dissolved substances. Molecules which diffuse upstream will be slowed 

 down, whereas those diffusing downstream will be speeded up. Such 'drag effects' 

 may be of importance in processes of secretion and in apparently 'active' transport. 

 This effect will be greater for big molecules than for small ones, because the contri- 

 bution to the over-all movement will be less for slow molecules. On the other hand, 



39 



