Comparison between the fractions of volume and radioactivity of vesicles. The fraction of volume of vesicles as a percentage of the total reaction volume (mean ± SEM; n = 3) and that of radioactivity in the vesicles as a percentage of total radioactivity (n = 6) determined 2.5 h after initiation of the experiment are plotted as an open bar and a closed bar, respectively. At the beginning of experiment, the vesicles were free of substrate, whereas the bathing solution contained 2 mM radioactive Arg⁺.