A study of dielectric membrane breakdown in the Fucus egg.

Abstract

Unfertilized eggs and zygotes of the marine brown alga, Fucus serratus, have been subjected to single external electric field pulses of 1 to 1760 musec duration (tau p) and 50 to 400 V field strength (Upcm-1). During exposure, the difference in electric potential across the plasmalemma (Vm) was recorded intracellularly from single eggs, and the efflux of 86Rb+(K+) from the cytoplasm was measured on egg populations. A given single pulse instantaneously depolarizes the plasmalemma by a few (i.e., 6) millivolts and releases a certain fraction (i.e., 5%) of the cytoplasmic 86Rb+(K+). The dependence of these responses upon Up and tau p is fully consistent with the assumption that the membrane undergoes a localized reversible dielectric breakdown and reseals within less than 3 sec. The data are treated in terms of the electro-mechanical model for a compressible membrane by H.G.L. Coster and U. Zimmermann (1975, J. Membrane Biol. 22:73) and verify this model on a nonvacuolated plant cell. A threshold Vm for membrane breakdown (Vc) of 0.58 and 0.51 V is estimated for the turgorless unfertilized eggs and the turgescent (4.8 bar) zygotes, respectively. Using these values for Vc, and a reasonable value of the membrane's elastic modulus (i.e., Ym approximately 10(6) Nm-2), possible sites of membrane breakdown are discussed in terms of membrane thickness and relative permittivity.