A model for membrane potential and intracellular ion distribution

Most cells carry a negative electric charge. It produces a potential difference across the membrane, which regulates voltage-sensitive ion transport and ATP synthesis in mitochondria. The negative charge comes partly from an excess of negative ions in the cell interior (Donnan potential) and partly from ionized groups on the membrane (surface potential). In this work we propose some important modifications to the existing theory of membrane potential. First, we calculate the concentration profile of intracellular positive ions and derive a simple equation to assess the submembrane depletion of positive ions that gives rise to the Donnan potential. The extent of depletion varies with potential, which may provide a regulatory mechanism for ion pumps and channels. Next we consider the surface component of the potential and note that the standard Gouy-Chapman theory has been developed for planar membranes, whereas real cell membranes have a closed geometry. In this case, charges on the membrane surface are not expected to generate fields extending into the cell interior. This fact calls for reinterpretation of some theoretical points as well as experimental data. In particular, the experimentally demonstrated electrostatic attraction between cationic proteins and the negative membrane must now be explained without invoking intracellular fields, and we suggest a new mechanism that can account for this interaction.