- The photoresponses of planar bimolecular lipid membranes (BLM) modified with liposomes containing bacteriorhodopsin (BR) were investigated in the presence of Ca 2+ cations. Short-circuit currents during switch-on and switch-off of the light were measured within a time scale exceeding the BR photocycle, and the dependence of the currents on external field and membrane conductance was studied. A comparison of the experimental data with computed values indicates that the photoresponses observed are due not only to BR contained in the proteoliposomes adhered to the BLM, as generally believed, but also to BR in BLM itself. The transfer of BR to BLM may result from the complete fusion of liposomes with the planar membrane. The existence of such a transfer is indicative of the possibility of obtaining the most convenient model system for BR studies: a planar bimolecular lipid membrane containing incorporated BR.