Nonlinear electric response of the diffuse double layer to an abrupt charge displacement inside a biological membrane

•Abrupt, intramolecular charge displacement inside a biological membrane.•Electric response of the diffuse double layer.•Primary light-induced electric signals of bacteriorhodopsin.•Bipolar voltage decay as a consequence of capacity change of diffuse double layer.•Possible physiological consequences are discussed. In order to elucidate the old, still unsolved problem of how the diffuse electric double layer responds to an abrupt, intramolecular charge displacement inside a biological membrane, we investigated the fastest components of the light-induced electric signals of bacteriorhodopsin and its mutants, in numerous ionic and buffer solutions. The obtained data for temperature and solute concentration dependence were interpreted as a consequence of changes in the capacity of the diffuse double layer surrounding the purple membrane. The possible physiological consequences of this so far not demonstrated phenomenon are discussed.