Purple membrane fragments isolated from the cell membrane of the photosynthetic bacteria Halobacterium halobium S.9 strain are incorporated into egg yolk phosphatidylcholine liposomes. Purple membrane contains crystalline patches of a retinal protein called bacteriorhodopsin. Upon illumination, bacteriorhodopsin undergoes a reversible photoreaction in which a proton is released on one side of the membrane and a proton is bound on the other side, thus resulting in an electro-chemical gradient across the membrane. The net rate of proton pumping is the combination of the rates of photoreaction and of simple diffusion of protons across the lipid membrane to compensate for the concentration difference between the two sides of the membrane. A mechanistic model is also proposed for the photoreaction which includes activation, proton dissociation, translocation and association reactions. Activation and translocation of bacteriorhodopsin are considered to be fast, but proton dissociation and association steps are considered to be slow. The resultant rate expression is compared with light on and light off experimental data. The model is in accordance with experimental data for initial pH values around 7.