Permeability of pea chloroplasts to alcohols and aldoses as measured by reflection coefficients

Abstract

1. 1. Intact chloroplasts were isolated in 2 min from Pisum sativum and then placed in external solutions varying in composition and in osmotic pressure. The relative osmotic changes in chloroplast volume caused by the impermeable solute sucrose compared with equal concentrations of various alcohols and aldoses were determined. From such osmotic responses the reflection coefficients ( σ x ) of the latter compounds were calculated using a version of the Boyle Van 't Hoff relation derived from irreversible thermodynamics. 2. 2. The reflection coefficients of pea chloroplasts for alcohols became progressively larger as hydroxymethyl groups were added to the molecule, viz. methanol ( σ x = 0.00 ), ethylene glycol (0.40), glycerol (0.63), meso-erythritol (0.90), and adonitol (1.00). This increase in σ x (decrease in permeability) with increasing molecular size parallels the decreasing lipid-water partition coefficients of the solutes. The reflection coefficient of d-mannitol was 1.01, of d-sorbitol was 1.02, and of meso-inositol was 1.00, suggesting that these six-carbon polyhydroxy alcohols are impermeant and hence would be suitable osmotica in which to suspend chloroplasts. 3. 3. Chloroplasts are more permeable to certain aldopentoses, viz. d-xylose ( σ x = 0.43 ), d-lyxose (0.47) and l-arabinose (0.53), than they are to the optical isomers of the same compounds, i.e. l-xylose (0.87), l-lyxose (0.93), and d-arabinose (0.85). Although all of these compounds predominantly occur in a chair form of the pyranose ring, the first three are in the C1 conformation, while the latter three are in the 1C conformation. The greater permeability of the C1 aldopentoses was even more apparent for d-ribose, which had a reflection coefficient of 0.00. 4. 4. Based on osmotic responses from 14 to 50 mM, the aldohexoses tended to have higher reflection coefficients for the limiting membranes of pea chloroplasts than did the aldopentoses: α- d-glucose ( σ x = 0.98 ), β- d-glucose (0.99), 2-deoxy- d-glucose (1.00), d-galactose (0.92), 2-deoxy- d-galactose (0.91), 6-deoxy- d-galactose (0.89), d-mannose (0.85), l-mannose (1.00), and 6-deoxy- l-mannose (1.00). 5. 5. Based on the correlation between the reflection coefficient and the particular sugar stereoisomer, it is proposed that carriers in the limiting membranes of pea chloroplasts are specific for aldopentoses in the C1 form of the chair conformation of the pyranose ring and, furthermore, are particularly efficient in shuttling d-ribose into pea chloroplasts. At concentrations above 50 mM, the apparent reflection coefficient of the C1 aldopentoses increased, suggesting that such carriers were becoming saturated. Moreover, the osmotic responses when two sugars were present together indicated that d-xylose, d-lyxose, l-arabinose, and d-ribose all competed for the same carrier.