A membrane­ bound protein of the halophilic (salt­resistant) bacterium Halobacterium halobium. When activated by light, it pumps protons out of the cell; this creates a concentration gradient, which enables ATP to be synthesized. Bacteriorhodopsin is composed of seven a-helix segments, which span the membrane and are joined to­gether by short amino-acid chains. It contains the prosthetic group retinal, which is also found in the pigment rhodopsin in the rod cells of verte­brates.