A study of dielectric membrane breakdown in theFucus egg

Summary

Unfertilized eggs and zygotes of the marine brown alga,Fucus serratus, have been subjected to single external electric field pulses of 1 to 1760 μsec duration (τ _p ) and 50 to 400 V field strength (U _p cm⁻¹). During exposure, the difference in electric potential across the plasmalemma (V _m ) was recorded intracellularly from single eggs, and the efflux of⁸⁶Rb⁺(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 cytoplasmic⁸⁶Rb⁺(K⁺). The dependence of these responses uponU _p and τ _p is fully consistent with the assumption that the membrane undergoes a localized reversible dielectric breakdown and reseals within <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 thresholdV _m for membrane breakdown (V _c ) of 0.58 and 0.51 V is estimated for the turgorless undertilized eggs and the turgescent (4.8 bar) zygotes, respectively. Using these values forV _c , and a reasonable value of the membrane's elastic modulus (i.e.,Y _m ≈10⁶ Nm⁻²), possible sites of membrane breakdown are dicussed in terms of membrane thickness and relative permittivity.