Plasma membrane surface potential (ψpm) as a determinant of ion bioavailability: A critical analysis of new and published toxicological studies and a simplified method for the computation of plant ψpm

Abstract

Plasma membranes (PMs) are negatively charged, and this creates a negative PM surface electrical potential (psiPM) that is also controlled by the ionic composition of the bathing medium. The psiPM controls the distribution of ions between the PM surface and the medium so that negative potentials increase the surface activity of cations and decrease the surface activity of anions. All cations reduce the negativity of psiPM, and these common ions are effective in the following order: Al3+ > H+ > Cu2+ > Ca2+ = Mg2+ > Na+ = K+. These ions, especially H+, Ca2+, and Mg2+, are known to reduce the uptake and biotic effectiveness of cations and to have the opposite effects on anions. Toxicologists commonly interpret the interactions between toxic cations (commonly metals) and ameliorative cations (commonly H+, Ca2+, and Mg2+) as competitions for binding sites at a PM surface ligand. The psiPM is rarely considered in this biotic ligand model, which incorporates the free ion activity model. The thesis of this article is that psiPM effects are likely to be more important to bioavailability than site-specific competition. Furthermore, psiPM effects could give the false appearance of competition even when it does not occur. The electrostatic approach can account for the bioavailability of anions, whereas the biotic ligand model cannot, and it can account for interactions among cations when competition does not occur. Finally, a simplified procedure is presented for the computation of psiPM for plants, and the possible use of psiPM in a general assessment of the bioavailability of ions is considered.