CHAPTER VIII 

 MEMBRANE POTENTIALS 1 



We have seen that electrolytes influence the osmotic pressure, 

 swelling, and viscosity of protein solutions in a similar way, so 

 that we must think of the possibility that the cause of this influ- 

 ence is the same for all these properties. 



When a solution of a protein salt, e.g., I per cent gelatin 

 chloride, is separated from distilled water by a collodion mem- 

 brane, a potential difference exists across the membrane between 

 the gelatin chloride solution and the outside solution with which 

 it is in equilibrium. If this P.D. is measured with the aid of a 

 Compton electrometer with saturated KC1 calomel electrodes, 

 it is found that the P.D. is influenced in the same way by 

 electrolytes as the osmotic pressure, swelling, and viscosity (see 

 Fig. 41 in this chapter). This in itself would only mean the 

 addition of another property varying in the same characteristic 

 way as osmotic pressure, or swelling, or viscosity of proteins 

 under the influence of electrolytes, if it were not for the fact that 

 we can correlate the variations of the new property with the 

 Donnan equilibrium, and that we can calculate the P.D. with a 

 fair degree of accuracy on the basis of this equilibrium. This 

 then gives us a rational, quantitative theory of the influence of 

 the pH, the valency of ions, and of the concentration of neutral 

 salts on a colloidal property of proteins. 



It is necessary to. give a brief description of the method of 

 measuring the P.D. Suppose that the protein in solution is 

 gelatin chloride containing 1 gm. of originally isoelectric gelatin 

 in 100 c.c. solution. Such solutions of gelatin chloride are put 

 into collodion bags closed with rubber stoppers which are per- 

 forated with glass tubes serving as manometers, as described in 

 the osmotic pressure experiments. These collodion bags filled 



1 This chapter is based on LOEB, J., J. Gen. Physiol, vol. 3, pp. 557, 667, 

 1920-21; vol. 4, pp, 351, 463, 1921-22. 



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