Membrane conductance ofChara measured in the acid and basic zones

Abstract

A sinusoidal AC current (usually at 1 Hz) was injected into either the acid or basic zone present in illuminated internodal cells ofChara. The “cable” properties of the cell were then studied by two techniques. The first involved measuring the dependence of the transverse AC membrane current (measured in the external medium) upon the distance away from the point of current injection. The second measured the AC membrane voltage at two different positions along the cell. It was found that AC current injected into an illuminated basic region left the cell close to the point of current injection, the effective cable lengthλ B being typically 3 to 5 mm. For AC current injected into an illuminated acid zone however, the cable lengthλ A was significantly longer (≈ 10 to 15 mm). From considerations of the properties of electrically inhomogeneous cables it was possible to estimate the individual area-specific conductances of the acid and basic zones. In strong illumination the conductance of the acid zones was typically 0.8 to 1.0 S/m2, whereas that of the basic zones was much higher (≈ 5 to 8 S/m2). Upon removal of illumination the conductance of each zone was found to slowly decrease, and after several hours the membrane conductance was relatively homogeneous over the cell surface. The large degree of longitudinal spatial inhomogeneity in the membrane conductance of illuminatedChara cells has important consequences for the interpretation of experimental results, and can produce large errors in the estimated membrane area-specific conductance if these inhomogeneities are not explicitly recognized and accounted for. An examination of the literature suggests that previous measurements utilizing point current injection might indeed have systematically underestimated the area-specific membrane conductance ofChara.