MEMBRANES AND CELL-PROCESSES 



i45 



as they do that of uncharged molecules. The ions formed by the dis- 

 sociation of any electrolyte have as a rule unequal diffusion-velocities, 

 and presumably unequal solubilities and other physical properties, in 

 correspondence with their chemical differences ; and hence we may infer 

 that they possess unequal abilities to pass through membranes. If this 

 is so, a membrane separating two electrolyte-solutions becomes the seat 

 of a potential-difference; i. e., a potential-difference, which may be 

 considerable, will exist between its opposite faces. This suggestion, 

 first made by Ostwald in 1890, has formed the basis of the chief pre- 

 vailing view — the so-called " membrane-theory " 

 of the nature of the bioelectric processes. Ost- 

 wald's suggestion, modified to suit the conditions 

 in cells, was essentially as follows. Imagine the 

 cell enclosed in a plasma-membrane freely per- 

 meable to the cations (positive ions, e. g., hydro- 

 gen ions or potassium ions) and impermeable 

 to the anions (negative ions) of a certain elec- 

 trolyte (which we may suppose to be lactic or 

 carbonic acid) contained in the protoplasm (Fig. 

 2). The cations then pass outward, carrying 

 their positive charges, while the anions remain 

 behind; this will proceed until the potential- 

 difference thus arising is sufficient to compen- 

 sate the diffusion-tendency (equivalent to the 

 osmotic pressure) of the cations. A condition 

 of equilibrium with outer surface positive and 

 inner negative thus results. The membrane 

 becomes the seat of an electrical polarization (normal or physi- 

 ological polarization) which is dependent on its impermeability to 

 anions. If the permeability of such a membrane were to increase 

 sufficiently to transmit the anions, a fall of the potential-difference 

 between the exterior and the interior of the cell would at once follow. 

 An effect of just this kind is seen in muscle and nerve during stimula- 

 tion, and is attributed by Bernstein and other upholders of the mem- 

 brane-theory to the changing ionic permeability of the membrane. The 

 selective permeability to ions of different sign, on which the potential- 

 difference between exterior and interior depends, disappears along with 

 the general increase in permeability accompanying stimulation: hence 

 a negative electrical variation is always associated with this process. 

 The precise arrangement imagined by Ostwald has not yet been 

 satisfactory realized, although, according to Briinings, precipitation- 

 membranes of copper ferrocyanide show sorre of the properties required 

 by this theory. But certain natural membranes present a much closer 

 approach to the theoretical requirements; thus the surface-membranes 

 of apples, which Beutner and Loeb have recently studied, behave as if 



Fig. 2. Illustra- 

 ting the supposed 

 conditions of polar- 

 ization of the plasma 

 membrane. The elec- 

 trolytes are lactic 

 and carbonic acids ; 

 the membrane is sup- 

 posed to be perme- 

 able only to H-ions. 



