CONCEPTS AND TERMS 



of ion concentrations. Since cells ordinarily contain substantially 

 higher levels of such anions than the surrounding solutions do, the 

 alkali metal cations tend to enter the cells against chemical gradients. 

 Were this tendency not opposed by active extrusion of sodium ion, 

 this uptake would continue to provoke water uptake and swelling 

 until the cells would be destroyed. The diagrammatic representation 

 of Figure 13 will serve to remind one of the origin of this osmotic 

 pressure difference. For this reason, destructive osmotic swelling of 

 cells tends to result whenever alkali metal transport is inhibited 

 (cf., for example, Wilson, 1954). As noted earlier in this discussion, 

 only a high degree of cation impermeability was at one time believed 

 to protect cells from destruction by the osmotic forces arising from 

 the tendency of ions and water to be taken up. The following words 

 by Harris (1941) show how this concept changed: 



It is believed that a cation impermeability of the erythrocytes 

 prevents their rupture by the osmotic force of the Gibbs-Don- 

 nan equilibrium. However, the same result can be achieved, al- 

 though in a different way, if the metabolic activity of the cells 

 controls the distribution of cations even though the membrane 

 be considered permeable to sodium and potassium. Indeed, be- 

 cause of this activity and the apparently very slow rate of per- 

 meation of cations, as compared to anions, the membrane may 

 be said to be functionally impermeable to the positive ions, at 

 least as regards such functions as the transport of COo. Thus, the 

 calculations of Van Slyke et al. (1923) and more recently of 

 Rapoport and Guest (1939), showing that the anions of blood 

 tend to distribute themselves according to the Donnan equilib- 

 rium (assuming cation impermeability), lose none of their sig- 

 nificance. 



In the case of muscle fiber, the potential differences involved 

 may possibly be sufficient to account for the cellular potassium con- 

 tent so that sodium-ion extrusion would account for alkali metal 

 distribution without invoking an active potassium-ion uptake. A 

 direct linkage between sodium- and potassium-ion transfer is never- 

 theless suspected. For the human red blood cell, the asymmetry of 

 the chloride distribution (the concentration being about 70 per cent 

 as great in the cell water as in the plasma water at pH 7.4) provides 

 an estimate of the potential difference across the membrane and 

 shows that its value cannot be great enough to account for the 

 30:1 distribution ratio for the potassium ion; hence, an active up- 



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