THE POLARIZATION CAPACITY AND RESISTANCE OF VALONIA : I. ALTERNATING CURRENT MEASUREMENTS.
Alternating current measurements of the effective capacity and resistance of Valonia cells were undertaken to determine whether a static or polarization capacity was responsible for the large slow time curves of counter E.M.F. produced by the flow of direct current. (For this purpose it was necessary that the cells be in the regular state.) With external contacts at the ends of ceils, a large fall of impedance occurs over the frequency range from zero to 20,000 cycles, above which the impedance is low and essentially that of the cell interior. As a first approximation to the cell circuit, a simple series-parallel circuit was employed in the bridge balance, with a resistance setting to represent the cell wall (and protoplasmic leakage), shunting the protoplasmic capacity in series with a resistance (sap plus polarization resistance). Both the capacity and its associated series resistance fall off regularly with frequency, giving curved lines on logarithmic plots against frequency, the slope of the resistance plot being steeper, and approaching that of f(-1), although curved to it. These parallel roughly the behavior of a polarizing electrode, which is also shown. Before concluding that the cell's capacity is therefore due to polarization, a further complication of the circuit was considered. This was the effect of the protoplasmic capacity distributed along the wall between the contacts. Both logically and experimentally it was shown that a distributed capacity, as represented by its approximate T-network of resistances and capacities invariant with frequency, could give rise to changes of capacity and series resistance with frequency which simulate to some extent the cellular phenomena. Distributed capacity was therefore reduced in the cells by using shorter air-gaps between the contacts, or abolished by measuring impaled cells, in which the current flow across the protoplasm was entirely radial. These measurements showed a smaller, but still significant change of capacity and of equivalent resistance (in series with it) with frequency, somewhat less than with electrodes, and probably representing the true protoplasmic behavior. It is concluded that the cells display a certain degree of polarization capacity, possibly in parallel with a static capacity invariant with frequency. This might result from an insulating (e.g. lipoid) cell surface, having a residual differential permeability to ions. This structure is consistent with other evidence showing the cells to be chiefly permeable to non-ionized, lipoid-soluble materials, but still displaying electrical effects (conductance, potential difference, polarization) ascribable to ionic mobility.
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Laboratories of The Rockefeller Institute for Medical Research, New York, and the School of Biological Sciences, Stanford University.