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Bacteriorhodopsin is the smallest autonomous light-harvesting protein, transforming the energy of light into that of transmembrane H+ gradient. Here a semi-quantitative model of its proton--pumping mechanism is developed, based on calculated energy levels of the protonation states of the groups along the proton transfer pathway in the key conformational states of the photocycle -- BR, M, N, and O. The model explains the directionality of proton pumping -- from the cytoplasmic to the extracellular side -- solely on the basis of a pattern of changing relative energies of protonation states during the photocycle. The model operates with collective states of protonation involving all key groups, rather than individual group's proton affinities pK traditionally used, which proves crucial for consistent description of non-equilibrium charge transfer. Predictions are made about several important structural aspects of the states along the cycle whose high resolution structures are not available.