bacteriorhodopsin is a protein used by archaea, the most notable one being halobacteria. it acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. the resulting proton gradient is subsequently converted into chemical energy. == structure == thumb|right|300px|conformational change bacteriorhodopsin is an integral membrane protein usually found in two-dimensional crystalline patches known as "purple membrane", which can occupy up to nearly 50% of the surface area of the archaeal cell. the repeating element of the hexagonal lattice is composed of three identical protein chains, each rotated by 120 degrees relative to the others. each chain has seven transmembrane alpha helices and contains one molecule of retinal buried deep within, the typical structure for retinylidene proteins. == bacteriorhodopsin is a light-driven proton pump == it is the retinal molecule that changes its conformation when absorbing a photon, resulting in a conformational change of the surrounding protein and the proton pumping action. it is covalently linked to lys216 in the chromophore by schiff base action. after photoisomerization of the retinal molecule, asp85 becomes a proton acceptor of the donor proton from the retinal molecule. this releases a proton from a "holding site" into the extracellular side (ec) of the membrane. reprotonation of the retinal molecule by asp96 restores its original isomerized form. this results in a second proton being released to the ec side. asp85 releases its proton into the "holding site," where a new cycle may begin. alt text [[file:bacteriorhodopsin chemiosmosis.gif|thumb|220px|chemiosmotic coupling between the sun energy, bacteriorhodopsin and phosphorylation by atp synthase (chemical energy) during photosynthesis in halophilic archaea halobacterium salinarum (syn. h. halobium). the archaeal cell wall is omitted.{{cite book |title=bioen
7TM_receptors Integral_membrane_proteins Photosynthesis