The magnitude and mechanism of the passive permeability of cane chloroplast internal membranes to various ions, measured by dielectric dispersion

Abstract

Summary. The dielectric dispersion technique was used to investigate the permeability of chloroplast internal membranes to a variety of anions and cations. Selectivity sequences depended on the pH at which measurements were made, and results were consistent with transport via fixed charged sites in the membranes. The possibility that these charged sites could line "pores" in the membranes was also investigated. No relationship between chloroplast shrinkage ~nd membrane permeability was apparent. TerminologyThroughout the paper the following symbols are used:~o =frequency of applied potential (s-1) ~' = dielectric constant of chloroplast suspension So =permittivity for free space (F m-1) = conductivity (f~-1 m-1) co =concentration of ions in region of membranes (m-3) e = ionic charge (coulombs) p =volume concentration of chloroplasts in suspension A = total area of membranes per unit volume of suspension (m-1) k = Boltzmann's constant (J ~ 1) T= temperature (~ In recent papers (Gordon, 1972a, b) it was shown that the dielectric dispersion (s' vs. 1/~/co) was linear within the region 0.5 to 50 MHz. This was attributed to electrodiffusion, that is, to the passive permeation of ions across the chloroplast internal membranes under the opposing influences of an applied electric field and a charge concentration gradient (Cole, 1968).