Abstract
From our theoretical treatment which is an extension of the classical Donnan theory, we have estimated the rational selectivity coefficients of the carboxylic groups of the walls during exchanges of divalent ions against monovalent ones (i.e. calcium and potassium or calcium and sodium ions). These coefficients express the interactions between the different ions, those between the counter-ions and the ionized groups of the wall, and the influence of water. These quantitative values are consistent with the great affinity of the carboxylic groups for the calcium ions. They vary with the experimental conditions, showing a purely physicochemical mechanism of "regulation" of the exchanges in the cells walls of Lemna minor L.