BIOLOGICAL TRANSPORT 



ject to discriminating enzymatic attack. Elsewhere the author 

 (1960b, 1961) has emphasized the possibility that transport sites 

 occur in the matrix of the membrane structure rather than in small 

 shuttling carriers. The specificities observed are those to be ex- 

 pected of sites formed by three or more chemical groups in a three- 

 dimensional array. The membrane matrix could maintain a more 

 closely determined arrangement of such groups than that possible 

 for small molecules in free solution. Furthermore, translocation of 

 such sites, and a decrease of affinity, or even an inversion in the 

 order of affinities (as for the sodium and potassium ions), could be 

 produced by close steric control of such three-dimensional binding 

 sites, through structural changes akin to protein denaturation. 



70 



