Fig. 3. The effect of membrane-permeable versus membrane-impermeable osmolytes on the internal volume of proteoliposomes. Calcein quenching was assayed in 100 mM KPi pH 7.0. The proteoliposomes were composed of 25 mol% DOPC, 50 mol% DOPE and 25 mol% DOPG. Iso-osmotic (1) and hyperosmotic conditions correspond to 190 and 390 mosmol/kg, respectively. To impose hyperosmotic conditions, 200 mM KCl (4), 270 mM glycerol (2) or 327 mM sucrose (3) were added to the sample after 1 min pre-incubation (indicated by an arrow).