BIO-ELECTRIC PHENOMENA 133 



electric organ shows that it is built up serially of large numbers 

 of units. The cause of the potential differences in cells must be 

 sought for in the " selective " permeability of the cell membranes, 

 or in alterations of the content of the protoplasm in electrolytes. 



(i) Differential permeability. Bayliss separated a concen- 

 trated from a dilute solution of the sodium salt, congo red, by a 

 membrane of parchment paper which is permeable to the sodium 

 ion but not to the anion. He found that the dilute side became 

 electro-positive on account of the preponderance of cations on 

 that side. 



(ii) Differential permeability does not afford a sufficient ex- 

 planation for all bioelectric phenomena. Even when the membrane 

 is freely permeable to both ions of the salt and when the anion is 

 the faster of the two, the dilute side of the parchment becomes 

 electropositive. This brings us again to the charge on the 

 membrane. Parchment has a negative charge in water, and in 

 dilute solutions of neutral salts so has baked clay, wood, bone, 

 charcoal, natural gelatine, etc., and all these cause a positive 

 charge to develop on the dilute side. That is, the generation of 

 a potential difference is just the reverse of electrical endosmose 

 (Chap. X.). This may be confirmed by altering the charge on 

 the membrane. Gelatine may be induced to take up a positive 



/N\ 

 charge. Mines found that when a dilute solution V^/ was 



separated by a gelatine membrane from a more concentrated 



N\ 



j solution of sodium chloride, the dilute solution became electro- 

 positive. Gelatine is made positive by treatment with the ions 

 of polyvalent metals. When an electropositive gelatine membrane 

 was used, the dilute solution became negative. 



(iii) A slight alteration in hydrogen ion concentration occurring 

 on one side of a membrane will cause the development of E.M.F. 

 If two solutions, one of pH 7 and the other of pH 8 are separated 

 by a membrane more permeable to //+ ions, an E.M.F. of about 

 30 millivolts may be obtained. The living cell has a pH of about 

 7-4 and its E.M.F. is not usually greater than 30 millivolts. 

 Polarisation current. 



In all chemical processes, alterations in potential difference take 

 place. The living complex, known as a cell, is a system in which 

 chemical transformations proceed continually and, therefore, 

 electromotive force is being generated continuously. These 

 currents may be demonstrated if special arrangements are made 



