The Electrical Properties of Plant Cell Membranes. III. The Effects of Insulated Regions upon the Cable Properties and Measured Membrane Conductance of Charophytes

Abstract

A theoretical treatment of the electrical 'cable' properties of cylindrical, radially symmetric, plant cells with two concentric membranes is presented, in which the longitudinal conductivities of the cell wall and cytoplasm are explicitly included in conjunction with the separate admittances of the plasmalemma and tonoplast. The effects of electrically isolating different longitudinal regions of the cell by means of insulating external barriers are examined, and it is shown that the presence of the cell wall will ensure that any such insulation is imperfect. Using parameters that should be appropriate for charophytes such as Chara and Nitella, it is demonstrated that an appreciable fraction of the injected current can flow through the cell membranes within such 'insulated' regions, and consequently increase the effective membrane area. If not allowed for, this will produce an overestimation of the electrical current density and the area-specific conductances of the plasmalemma and tonoplast. Indeed, under some experimental conditions, the consequent error in the 'membrane' (plasmalemma and tonoplast in series) parameters can be several hundred percent. Thus reducing the fraction of the cell surface area bathed in electrolyte by means of insulating external barriers will not necessarily remove problems associated with the cable properties but will, in effect, transfer them to the insulated regions as the membrane conductance decreases. It is discussed how experimental configurations can be optimized for minimum error.