286 PROTOPLASM 



side of) its isoelectric point. In this condition, cations penetrate 

 the membrane more readily than do anions. In other words, the 

 alkaline membrane retards the movement of those ions which 

 are of the same electric sign as itself and permits the movement 

 of ions of the opposite side. On the acid side of the isoelectric 

 point, conditions are reversed. The membrane is now positively 

 charged, and anions pass through more readily than do cations. 



Mond carried out a similar experiment with red blood cells. 

 Assuming that the membranes of erythrocytes are electropositive, 

 he succeeded in changing their natural anion permeability into 

 cation permeability by changing the sign of the membrane. By 

 adding hydroxyl ions until the pH of the surrounding solution 

 is above 8, the previous permeability of the membrane for 

 chlorine and bicarbonate anions was changed to permeability 

 for the cation potassium (through exchange with the external 

 sodium ions). As the isoelectric point of globine is also about 8 

 (pH 8.1), this amphoteric protein is assumed by Mond to be 

 the permeability-controlling substance in the membrane of red 

 blood cells. The deduction is in keeping with certain (protein) 

 theories of protoplasmic and membrane structure. 



Permeability studies have, for the most part, been concerned 

 with the passage of the ions and molecules of dissolved sub- 

 stances; less attention has been paid to the passage of water 

 through membranes. The extraordinary fact has been men- 

 tioned that certain membranes appear to be more readily per- 

 meable to water in one direction than in another. Such behavior 

 can be explained on the basis of electroendosmosis. Since the 

 time of Helmholtz, chemists and biologists have followed him 

 in interpreting the passage of water through membranes in terms 

 of charge involving a typical Helmholtz double layer (Fig. 163), 

 which leaves the membrane negative or positive. Through such 

 a negative membrane water travels to the cathode; when the 

 membrane is positive, the water travels to the anode. In the 

 discussion on electroendosmosis, we considered only the one 

 case of a glass capillary which selectively adsorbs the negative 

 (0H-) ion of water, leaving the positive (H+) ion free to move. 

 Other types of capillaries, e.g., those of proteins, such as the 

 pores in a gelatin membrane, may adsorb the other, positive 

 ion and leave the negative ion of the water free to move; in this 

 case, the water would migrate to the anode. Which ion a 



