Electrical gradients across an ion-exchange membrane in student's artificial cell

Ion permeability of artificial membranes evaluated by diffusion potential and electrical resistance measurements

In the present article, a novel model of artificial membranes that provides efficient assistance in teaching the origins of diffusion potentials is proposed. These membranes are made of polycarbonate filters fixed to 12-mm plastic rings and then saturated with a mixture of creosol and n-decane. The electrical resistance and potential difference across these membranes can be easily measured using a low-cost volt-ohm meter and home-made Ag/AgCl electrodes. The advantage of the model is the lack of ionic selectivity of the membrane, which can be modified by the introduction of different ionophores to the organic liquid mixture. A membrane treated with the mixture containing valinomycin generates voltages from -53 to -25 mV in the presence of a 10-fold KCl gradient (in to out) and from -79 to -53 mV in the presence of a bi-ionic KCl/NaCl gradient (in to out). This latter bi-ionic gradient potential reverses to a value from +9 to +20 mV when monensin is present in the organic liquid mixture. Thus, the model can be build stepwise, i.e., all factors leading to the development of diffusion potentials can be introduced sequentially, helping students to understand the quantitative relationships of ionic gradients and differential membrane permeability in the generation of cell electrical signals.

ARTMEM--an interactive graphical program simulating membrane potential measurements across artificial membranes

This paper describes ARTMEM, an interactive graphical simulation program, and its use for teaching students the concept of membrane potentials and the measurement of electrical potentials across an ion-selective membrane. The program, written in Borland C++, and specifically designed for IBM-PC-compatible equipment, can also run on fast Macintosh computers using SoftPC emulation software. It demonstrates how an ion-selective membrane separating two different salt solutions automatically generates a potential across the membrane, as well as how such electrical potentials can be measured simply and the underlying permeability ratio determined. The program was developed to replace a previous experiment with actual artificial resin membranes; virtually every feature of the experiment has been simulated (e.g., unstirred-layer effects, solution-mixing mixing contamination, liquid junction potential effects). In addition, a number of these features and the procedure for data fitting are more clearly demonstrated using the computer simulation. The efficacy of such a simulation is discussed in comparison with the real experiment and other types of simulations.