A membrane-bound protein manufactured by the halophilic (‘salt-loving’) archaeans, such as Halobacterium salinarum, that exploits light energy to make ATP without the involvement of chlorophyll-like pigments. 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 α‎-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.