A dielectric dispersion technique for measuring the ionic permeability of internal membranes of isolated chloroplasts

Abstract

Summary.A method has been developed in which a chloroplast suspension is placed between electrodes to which a variable AC potential is applied. The dielectric constant of the suspension varies inversely as the square root of frequency within the range 0.5 to 50 MHz. Results are consistent with the view that this dielectric dispersion is due to ion movement across the chloroplast internal membranes, under the influence of the applied potential. The slope of the dispersion depends on the permeability of the membranes, and thus enables the mobility of externally added ions to be calculated. Results imply that the internal membranes of sugar cane chloroplasts are more permeable to K + than Na +, and more permeable to CI-than CH3COO-. Results also confirm the view that Triton X-100 increases the ionic permeability of membranes.

The effect of illuminating spinach chloroplasts on their membrane permeability, measured by a dielectric dispersion technique

“…Hope (1956) assumed that dispersions for intact Chara chloroplast suspensions were due to the capacitance of the membranes comprising the outer envelope, but the circular Cole-Cole plots necessary to support this view were not obtained for either cane or intact spinach chloroplast preparations, even when the frequency was reduced to 5 KHz. The unity loss tangent reported for the dispersions (Gordon, 1972) in fact yields linear Cole-Cole plots, indicating that electrodiffusion was the major factor in the dispersion even for intact chloroplasts, rather than a static capacitance.…”

“…For a suspension of cane chloroplasts at 10 ~ it was shown (Gordon, 1972) that between 0.5 and 50 MHz, 5' varied with frequency in a manner typical of electrodiffusion; that is, the movement of ions across a membrane under the opposing influences of an electric field and a charge concentration gradient (Cole, 1968). The slope S,, of the dispersion e' vs. 1/1/~ was shown to be…”

“…Passive transport through chloroplast internal membranes has been investigated by means of the dielectric dispersion technique described in a recent paper (Gordon, 1972), and is measured in terms of the mean ionic mobility V, that is,…”

“…The cell previously described (Gordon, 1972) was modified to enable chloroplast suspensions placed between the electrodes to be illuminated directly (Fig. 1).…”

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

“…As co is increased, V tends to decrease, as if the membranes are beginning to saturate (Gordon, 1972a). In this paper, the permeabilities of monovalent and divalent ions were compared at 30 mM and 15 mM, respectively, concentrations well below saturation at all pH values investigated ( Figs.…”