An electrochemical experiment is presented to illustrate biosorption to second-year and upper-division undergraduate students. The extraction of Cd[superscript 2+], Pb[superscript 2+], and Cu[superscript 2+] by nonliving "Typha latifolia" (cattail) roots can be monitored in real time by potentiometry determinations. The open circuit potential (OCP) for the metal in equilibrium with its ion is measured before, during, and after plant addition to a solution. Decrease in OCP indicates a reduction in the metal concentration in solution because of the metal uptake by the plant. The quantity of metal accumulated in the plant and the variation in the OCP can be calculated from the Nernst equation. This experiment is simple and pedagogical. (Contains 1 figure.)