Changes in membrane ionic conductance, but not changes in slip, can account for the non-linear dependence of the electrochemical proton gradient upon the electron-transport rate in chromatophores.

Decrease in the rate of cyclic electron transport (JE) measured from the absorbance changes associated with reaction centre bacteriochlorophyll led to a less than proportionate decrease in the membrane potential (delta psi) measured by electrochromism. In principle this result can be explained either by a delta psi-dependent slip in the H+/e- coupling ratio (nE) or by a delta psi-dependent change in the membrane ionic conductance. Simultaneous measurement of the membrane ionic current (JDIS) did not reveal any significant changes in the H+/e- ratio (JDIS/JE) and showed that conductance changes (JDIS/delta psi) account quantitatively for the curved dependence of delta psi on JE. Simultaneous recordings of JDIS and the extravesicular pH from cresol-red absorbance changes, suggest that protons are the main current-carrying species across the chromatophore membrane at high values of delta psi in the presence and absence of Fo-ATPase inhibitor. At reduced delta psi the flux of other ions outweighs the hydrogen ion current.