Fig. 3.

Single-channel water permeability. Membrane conductance, G, was measured in a four-electrode configuration. The black line denotes Mg²⁺ dilution near the unmodified bilayer by osmotic flow. Increase in the number of reconstituted KcsA channels (8,300, 17,000, 25,000, 36,000, and 45,000 channels with a unitary conductance of 40 pS) resulted in increasing concentration polarizations (from dark gray to light gray), which allowed calculation of the corresponding P_f values (27, 34, 43, 51, and 59μm/s, respectively). (Inset) Four additional runs of the experiment and three similar experiments, in which 50 mM K⁺ were substituted for 25 mM Rb⁺, were analyzed. From the slope of the plot P_f versus G and the known single-channel conductances of 40 and 10 pS for K⁺ and Rb⁺, respectively, p_f was calculated (9) to be 4.8 × 10^–12 cm³·s^–1.