Sorption of copper and zinc to the plasma membrane of wheat root.

Sorption of Cu(2+) and Zn(2+) 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 other metals. The results were analyzed using a Gouy-Chapman-Stem model for competitive sorption (binding and electrostatic attraction) to a negative binding site. The binding constants for the two investigated cations as evaluated from the sorption experiments were 5 M(-1) for Zn(2+) and 400 M(-1) for Cu(2+). Thus, the sorption affinity of Cu(2+) to the PM is considerably larger than that of Ca(2+), Mg(2+) or Zn(2+). The greater binding affinity of Cu(2+) was confirmed by experiments in which competition with La(3+) for sorption sites was followed. The amount of sorbed Cu(2+) decreased with increasing K(+), Ca(2+), or La(3+) concentrations, suggesting that all these cations competed with Cu(2+) for sorption at the PM binding sites, albeit with considerable differences among these cations in effectiveness as competitors with Cu(2+). The sorption of Cu(2+) and Zn(2+) to the PM decreased in the presence of citric acid or malic acid. Citric acid (as well as pH) affected the sorption of Cu(2+) or Zn(2+) to PM more strongly then did malic acid.