18 Hugh Davson 



impermeable to cations such as Na+ and K+. In this way the 

 cell would be able to fulfil its function in the maintenance of 

 the acid-base balance of the body, permitting the Cl~ — HCOg" 

 exchange that mediates the buffer action of haemoglobin in 

 the cell. 



It might be thought that by making the cell impermeable to cations, 

 such as Na +, we should be establishing conditions for a Gibbs-Donnan 

 equilibrium leading to a large excess of osmotic pressure ; however, the 

 concentrations of impermeable cations will be equal on both sides of the 

 membrane, so that any Donnan effect due to impermeable cations on 

 one side of the membrane will be counterbalanced by an equal effect due 

 to impermeable ions on the other side. 



It is easy to show that an osmotic equilibrium between the 

 inside and outside of the cell is possible, in spite of the high 

 concentration of indiffusible protein anions within the cell; 

 thus the impermeability of the cell membrane to cations such 

 as Na+ confers on it a stability that would be lacking in the 

 presence of a permeability to this ion; in other words, the 

 colloid osmotic pressure of the cellular proteins can only 

 operate in the presence of a permeability to both Na+ and 

 anions. It is now well known, however, that cell membranes 

 do not show an absolute impermeability to such ions as Na+ 

 or K+; the use of isotopes has permitted the demonstration of 

 an unequivocal exchange of these ions across the erythrocyte 

 membrane. The exchanges are very slow compared with the 

 exchanges of Cl~ and HCOg", but they do occur, so that we 

 must expect a constant movement of NaCl and NaHCOg into 

 the cell, associated with the migration of water, unless some 

 process prevents this. As is well known, the process that does 

 prevent it is an active transport of Na+ ions out of the cell; 

 the membrane is permeable to Na+ so that there is a continual 

 drift of this ion into the cell because of the demands of the 

 Gibbs-Donnan distribution, but by some process not under- 

 stood, metabolic energy of the cell is employed in driving the 

 salt out. Practically, in consequence, the cell may be des- 

 cribed as a cell impermeable to Na+ and therefore in stable 

 equilibrium with its environment. The total amounts of 

 water and electrolytes within the cell will be determined by 



