128 PERMEABILITY 



Theories of Membrane Permeability. — A number of hypotheses have 

 been proposed as an explanation of the mechanism of differential permeability 

 in both inorganic and living membranes. The discussion will be limited 

 largely to the bearing of these theories upon the permeability of the cyto- 

 plasmic membranes. The tw^o theories vv^hich have been most favorably re- 

 garded in all discussions of this topic are the sieve theory and the solubility 

 theory. A number of variants of each of these theories has been proposed. 



1. Sieve Theory. — Probably the simplest picture w^hich can be conceived 

 of the operation of a membrane is that it works as a molecular sieve. The 

 pores in the membrane may be imagined to be of such dimensions that micelles, 

 molecules or ions below a certain size can diffuse through them while those 

 exceeding this size limit cannot pass through. Certain types of inorganic 

 membranes such as collodion, parchment paper, and copper ferrocyanide un- 

 doubtedly owe their property of differential permeability largely to such a 

 sieve-like action. 



Although a sieve theory of the permeability of the cytoplasmic membranes 

 is strongly advocated by some authorities, notably Bayliss (1924), there are 

 some very grave objections to the acceptance of any such hypothesis as a com- 

 plete explanation of this phenomenon. It is difficult to conceive of a sieve- 

 like structure of the cytoplasmic membranes, except insofar as this is imparted 

 by the normal intermolecular distances which are of course, very minute. A 

 more serious difficulty, however, is found in the fact that the permeability of 

 the cytoplasmic membranes to certain classes of compounds actually increases 

 with increase in the size of the molecule, at least up to a certain point. This 

 is true in certain homologous series of organic compounds already cited. 



2. Solubility Theories. — A second generally advocated theory is that mem- 

 branes are permeable to substances which will dissolve in them and imperme- 

 able to others. The operation of an inorganic membrane of this type can be 

 readily demonstrated by a simple experiment. A test tube is about half filled 

 with chloroform on top of which is introduced a very thin layer of water, and 

 then nearly filled with ether. The water may be colored faintly with a water- 

 soluble dye such as methylene blue in order to aid in distinguishing it from 

 the other liquids. After stoppering, the tube is clamped in an upright position, 

 and the position of the water meniscus is marked accurately. A second tube 

 is prepared in exactly the same way, except that xylene is substituted for ether 

 as the upper layer of liquid. If examined after a period of several days, it 

 will be found that in the first test tube the level of the water meniscus has 

 risen, while in the second it has fallen, although the distance through which 

 the meniscus will move will not be as great in the second tube as in the first. 

 In the first tube ether is diffusing through the water membrane more rapidly 



