The Electrical Properties of Plant Cell Membranes. V. The Effects of Net Longitudinal Currents induced with External Current Electrodes upon the Cable Properties of Charophytes

Abstract

Analytical solutions describing the electrical 'cable' properties of cylindrical, radially symmetric, cells with two concentric membranes are derived for experimental configurations whereby current is injected through the membranes via external electrodes located at different longitudinal positions, i.e. when a net longitudinal current is present. The spatial behaviour of the membrane voltages and longitudinal currents are calculated, using values that should be appropriate for Chara and Nitella. It is shown that the area-specific values of the conductances of the plasmalemma and tonoplast derived from such measurements can be significantly in error if the appropriate cable properties are not considered. It is also demonstrated how suitable location of the external current electrodes can significantly reduce any errors produced by the cable properties and concomitantly reduce the magnitude of the associated longitudinal variation in membrane voltage (thus substantially reducing the distortion produced by any non-linearities in the membrane I- V characteristics). In most configurations, location of the current electrodes two-thirds of the distance along the length of each cell section bathed in electrolyte will generally be close to optimum. It is also shown that measurement of the change in membrane voltage (in response to an injected current) approximately half-way along the section bathed in electrolyte will usually ensure that the measured conductance varies approximately linearly with that of the membrane.