ASPECTS OF SALT ABSORPTION IN CELLS 99 



Space penetrated presumably includes a thin layer of water adhering 

 to the root surface, and any water-filled spaces in and around the 

 cell walls. Its value depends very much on the extent to which the 

 intercellular spaces of the material are impregnated with water when 

 the determinations are made. 



If the freely accessible space within a cell or tissue contains 

 immobile electrical charges or adsorption sites, the actual volume of 

 tissue penetrated by salt may be different from that calculated by the 

 method just described, because the concentration of ions in the space 

 will be different from that in the external medium. For example, if 

 Donnan equilibria are established in a space containing immobile 

 anions the concentration of mobile cations will be higher, and that 

 of mobile anions lower than in the medium, resulting in the first 

 case in an over-estimation, and in the latter, an underestimation of 

 the actual volume penetrated. For this reason it is often necessary 

 to refer to the "apparent" free space (AFS) rather than the actual 

 free space of a tissue or cell (Briggs and Robertson, 1957). AFS is 

 therefore defined as the apparent volume of a cell or tissue penetrated 

 passively by salt assuming that salt reaches the same concentration 

 there as in the medium. From measurements of passive salt uptake 

 from salt solutions of different concentration, it is possible to deduce 

 that the apparent free space consists of two components, the WFS 

 and a "Donnan-free space" (DFS) containing immobile anions. The 

 lower the external concentration of salt, the greater the relative 

 concentration of cations in the Donnan-free space and the greater 

 the value of the apparent free space for cations. Conversely, the 

 lower the external concentration, the lower the relative concentration 

 of anions and the lower the calculated value of AFS for anions. 

 Hope (1953) demonstrated that experimentally-determined values of 

 AFS for chloride in bean {Vicia faba) roots placed in solutions of 

 potassium chloride, increased with increasing external concentration 

 in a manner consistent with the establishment of Donnan equilibria 

 (Fig. 34b). It is sometimes possible to make a reasonable estimate 

 of the actual volume occupied by free space, e.g. by microscopic 

 observations assuming that penetration into the cytoplasm occurs; 

 the concentration of immobile anions can then be calculated. By 

 assuming that the Donnan-free space represents free space in the 

 cytoplasm occupying 14 per cent of the tissue volume, Hope 

 calculated that the average concentration of immobile anions in the 



