A theory is presented for a phase transition between resting and depolarized states of a protoplasmic droplet of Nitella induced by variation of Ca2+ and Na+ concentrations. In the theory, a membrane formed on a surface of the droplet by lipids and proteins is regarded as a kind of adsorbed bilayer. Effects of bridging among adsorbed molecules through Ca2+ are taken into account, besides an electrostatic interaction between polar groups of adsorbed molecules and monovalent and divalent cations. It is shown that an excitable membrane is stabilized only in a certain range of Ca2+ and Na+ concentrations, where a phase separation through the Ca2+-bridging occurs between Ca2+-rich membranous and Ca2+-depleted domains. This kind of phase separation is mainly subject to a competitive effect between monovalent cations and Ca2+ at lower Ca2+ concentrations but an ionic strength at higher Ca2+ concentrations. The present results can explain fairly well quantitative features of existing observed data.
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