The rationale behind this research is the need to define the conditions under which some plants over-accumulate trace elements (thus enabling phyto-remediation of contaminated sites) and other plants under-accumulate those elements serving as micro-nutrients. The principal research objective is to determine the mode of binding of trace elements and the extent of their electrostatic attraction to the root surface under several environmental conditions. The experimental data, with the aid of proper mathematical tools, make it possible to define a procedure for predicting metals uptake. Sorption of Cu2+ and Zn2+ to the plasma membrane (PM) of wheat root (Triticum aestivum L cv. Scout 66) vesicles was measured at different pH values and in the presence of organic acids and of various metal ions. The results were analyzed using a Gouy-Chapman-Stern model for competitive sorption to a negative binding site. The binding constants for the two investigated cations as evaluated from the sorption experiments were 5 M-1 for Zn2+ and 400 M-1 for Cu2+. Thus, the sorption affinity of Cu2+ to the PM is considerably larger than that of Ca2+, Mg2+ or Zn2+. The greater binding affinity of Cu2+ was confirmed by experiments in which competition with La3+ for sorption sites was followed. The amount of sorbed Cu2+ decreased with increasing K+, Ca2+, or La3+ concentrations, suggesting that all these cations competed with Cu2+ for sorption at the PM binding sites, albeit with considerable differences among these cations in effectiveness as competitors with Cu2+. The sorption of Cu2+ and Zn2+ to the PM decreased in the presence of citric acid or malic acid. Citric acid affected the sorption of Cu2+ or Zn2+ to PM more strongly then did malic acid. At lower pH values, proton competition resulted in both a lower organic acid-metal ion complexation and a lower sorption to the PM.