AbstractThe plasma membranes of many animal cells can be disrupted into small sealed vesicles that can be purified centrifugally and utilized for studies on membrane transport. The vesicles behave as micro‐osmometers. However, the presence of charges fixed at the internal and external surfaces of the membrane walls produce pH levels at these surfaces that deviate considerably from bulk pH. Transverse symmetry of charge distribution further leads to transverse asymmetry of surface pH. Finally, charges fixed at the internal membrane surface produce significant Donnan osmotic effects that depend upon membrane composition and ionic environmen
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