Chloroplast membrane characteristics

Abstract

1. 1.Isolated Spinacea oleracea chloroplasts respond as reversible osmometers over limited ranges of sucrose concentration. Comparison of albumin and sucrose spaces in centrifuge tube pellets indicates that sucrose penetrates more of the chloroplast space than does albumin. Furthermore, the evidence suggests that the grana (or thylakoid) compartment is the sucrose and salt impermeable, osmotically responsive space. 2. 2.Lowering the pH of a chloroplast suspension to the isoelectric point, i.e. pH 4.7, results in the same degree of shrinkage as obtained by extrapolation of the Boyle-Van't Hoff plots. This suggests that chloroplast volume is in part determined by the degree of ionization of fixed dissociable charge groups of the membranes and possibly of non-diffusible polyelectrolytes within the chloroplast. Varying degrees of ionization of such groups would lead to changes in the amount of counter-ions within the structure, with concomitant changes in volume. The effect of many divalent cations (Mn 2+, Zn 2+, Cu 2+,Co 2+ and Ni 2+ as chloride salts) suggests that these cations may bind to fixed groups and induce contractions similar to those obtained by lowering the pH. 3. 3.The internal pH of chloroplasts was measured by using the distribution of a weak electrolyte (5,5′-dimethyloxazolidine-2,4-dione). It was found in general that (a) a difference in pH exists across the chloroplast thylakoid membrane with the internal pH higher than the external pH when the latter is below pH 7.1 and (b) the internal pH tends to remain near pH 7.1 as the external pH decreased to near 6.3 A pH gradient as large as 1.0 pH unit could also be induced when the external sucrose concentration was increased to 0.5–0.8 M, from 0.2 M sucrose.