The Electrical Resistance and Capacitance of the Membranes ofNitella translucens

Abstract

SUMMARY The resistance and capacitance of the membranes of Nitella translucens have been measured by direct current and alternating current methods. Current of the order of io~7 amp. was injected into the cell by means of a conventional Ag, AgCl-3N KC1 glass microelectrode inserted into the vacuole of the cell. The change of potential across the membrane was recorded by two other internal microelectrodes which had been inserted into the cell at known distances from the current-injecting electrode. In the direct-current experiments the input current was in the form of a square pulse, while sinusoidal currents of frequency 25 cycles per second were used in the alternating current experiments. The cell was treated as a short length of coaxial cable and from the measurements the following para meters could be obtained : the space constant (A), the membrane resistance (Rm) and the membrane capacitance (Cm). The values of Rm ranged from 6-7 to 36 K ohm cm.2 (mean of 214 K ohm cm.2) and those of A ranged from 1-5 to 5-7 cm. (mean of 2-6 cm.). The capacitance value was about 1 ixF cm.-2 These results are discussed within the framework of our knowledge of these parameters for other cells, particularly plant cells. The measured electrical resistance is shown to be at least ten times less than the value estimated from the passive fluxes of the principal ions K, Na, and CI. It is suggested that this dis crepancy, which is usually attributed to non-independent movement of these ions, could be partially explained on electro-osmotic grounds. The value of the capaci tance is very close to that which is usually obtained for other cell membranes. One exceptionally low value for Nitella has been quoted in the literature. The reason for the gross error in this particular measurement is given.