1. 1.|Comparable liposome preparations were obtained from lipids differing in degree of unsaturation and cholesterol content.\ud \ud 2. 2.|An exchange between alkali ions and protons through the bilayers was induced by replacing the alkali ions on the one side of the outer lipid membrane by impermeable choline ions.\ud \ud 3. 3.|The equilibration of the gradient was measured directly with ion selective electrodes or, indirectly as a volume change following a net uptake of osmotic material.\ud \ud 4. 4.|Individual permeabilities of H+ and alkali ions were measured by making the membrane freely permeable to the counter ion using specific ion carriers; valinomycin for K+ and FCCP (p-trifluoromethoxycarbonylcyanide phenylhydrazone) for H+.\ud \ud 5. 5.|Lipid bilayers with different degrees of unsaturation or cholesterol content all demonstrated a very limited permeability to protons.\ud \ud 6. 6.|With increasing unsaturation, a significant increase in K+ permeability could be measured.\ud \ud 7. 7.|The increase in Na+ permeability with increasing unsaturation was much less than for K+. This indicates a pertinent discrimination in permeability of K+ with respect to Na+ and H+ in the very unsaturated membranes
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