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 (ΔΨ) measured by electrochromism. In principle this result can be explained either by a ΔΨ‐dependent slip in the H+/e− coupling ratio (nE) or by a ΔΨ‐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/ΔΨ) account quantitatively for the curved dependence of ΔΨ 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 ΔΨ in the presence and absence of F0‐ATPase inhibitor. At reduced ΔΨ the flux of other ions outweighs the hydrogen ion current.