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

Abstract

Summary. A dielectric dispersion cell was modified to enable chloroplast suspensions placed between the electrodes to be illuminated directly. The dispersion (e' vs. 1/V-~-) for suspensions of intact spinach chloroplasts was linear between 0.2 and 5 MHz, the slope of the dispersion depending on the permeability of the internal membranes, the ionic concentration, and the chloroplast volume. The outer chloroplast envelope only contributed to the dispersion insofar as it retained ions within the stroma, thereby increasing the ionic concentration in the region of the internal membranes. When suspensions were illuminated, the slope of the dispersion decreased, due to chloroplast shrinkage. The permeability of the internal membranes remained constant following illumination, to within a 3 % margin of error. TerminologyThroughout the paper the following symbols are used: The permeability of a membrane may depend on the rates of both active and passive transport of ions through the membrane. 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,