Non-proton ion release in purple membrane.

Large conductivity changes have been measured during the photocycle of bacteriorhodopsin in purple membrane. These phenomena were explained as being due to the occurrence of large-scale non-proton ion release. Here we show that these conductivity changes do not appear if the purple membrane is immobilized. We propose an alternative hypothesis that explains the presence of conductivity change in suspensions and their absence in gels, as well as several related effects suggesting that the observed conductivity changes are due to alteration of the polarizability of purple membrane during the photocycle.