Bacteriorhodopsin (bR) is the sole protein in the purple membrane of Halobacterium salinarium, which functions as proton pump, charge separation and photochromism. The chromophore retinal is covalently attached to Lys 216 via a protonated Schiff base. Upon illumination, the all-trans to 13-cis isomerization of the retinal results in deprotonation of the Schiff base followed by alterations in protonatable groups within bacteriorhodopsin. The changed force field induces changes, even in the tertiary structure, which facilitate and warrant the vectorial proton translocation.