STRUCTURE OF NERVE CELL MEMBRANES 



125 



TABLE I 



Based on values for crystal radii of Pauling (1940) and heats of hydration of Verwey 

 (1942). I am indebted to Dr. R. L. Platzman for this calculation. 



and K+ results from a larger "effective" size of the Na + in aqueous solution, and 

 that special chemical agents or "carriers" are necessary for the selective in- 

 creases in Na+ or K + permeability that occur in the active membrane are to be 

 contrasted with very different proposals, to be developed below, that for pores 

 of a size 4-5 A. in radius, Na+ and K+ have precisely specified sizes and will 

 only penetrate through pores that closely fit the ion. These notions are arrived 

 at by considering the forces acting on the water structure in the vicinity of an 

 ion. 



In Table I is shown the extent to which various layers of hydration are bound 

 to Na+ and K+ together with values for the sizes of the resulting hydrated ions. 

 The importance of these data lies in two points: first, neither of the ions can 

 have a size that does not represent some integral number of water-molecule 

 layers, and second, the energy with which water molecules are bound in the 

 first hydration shell of six molecules is so great that the water is in essentially 

 a non-deformable solid state, while the energy with which the second water 

 shell is held is still much greater than thermal energy. The sizes of ions with 3 

 or more shells of hydration are so very large compared with probable membrane 

 pore sizes that they need not be considered further. 



The potentialities for ion penetration through a membrane pore would appear 

 to be such that if an ion at a specified level of hydration closely approximates 

 the size and shape of a pore, it may exchange all its hydration, beyond the speci- 

 fied level, for the solvation afforded by the walls of the membrane pore. If the 

 pore is circular in cross-section, then only ions of 1 or 2 layers of hydration need 

 be considered because these are the only hydration arrangements that yield 

 spherical ions. If pores have a somewhat different shape, it is possible to con- 

 sider ions that have one complete hydration layer and a partial second layer. 



