AC Impedance Measurements on Chara corallina. III. Characterisation of the Plasma Membrane Coat Using a New Presentation of Impedance Spectra

Abstract

It has been shown that the measurement of electrical impedence can provide a non-invasive approach to detect the presence of the plasma membrane coat (PMC). Further, the measurements provide an estimate of the geometrical properties of plasmalemmasomes (charasomes) which are similar to estimates determined from electron micrographs. Impedance measurements were made in the range of 1-10⁴ Hz. A transfer (network) function with either four or five time constants was fitted to the spectra. We present transfer functions as spectra in which peaks are observed at the reciprocal of these time constants. We call the reciprocal time constants frequency constants.

Our analysis suggests that two peaks in the frequency range of 1-10² Hz arise from the series combination of the plasmalemma and tonoplast and a peak at very high frequencies (>>10⁴ Hz) arises from the static cytoplasm.

The remaining two peaks in the frequency range of 10²-10³ Hz we attribute to the presence of the PMC. The peak at the higher frequency we ascribe to the PMC along smooth sections of plasmalemma and that at the lower frequency we ascribe to the PMC in plasmalemmasomes (charasomes). We found that this peak was not present in spectra obtained from cells without plasmalemmasomes.

Our model for the ultrastructure of the PMC and plasmalemmasomes is in good agreement with electron micrographs of these regions. Additionally, our measurements give a physiological range for the electrical properties of the PMC. Conductance and capacitance ranges are 13.6-52 S m^-2 and 6.7-17 mF m^-2 respectively.