Bacteriorhodopsin is a membrane protein that captures radiation energy which is used to create a proton gradient. It can be found mainly in Haloarchea. Bacteriorhodopsin is a membrane protein usually found in two-dimensional crystalline patches known as purple membrane. The repeating element of the hexagonal lattice is composed of three identical protein chains, each rotated by 120° relative to the others. Each chain has seven transmembrane alpha helices and contains one molecule of retinal buried deep within. It is the retinal molecule that changes its conformation when absorbing a photon, resulting in a conformational change of the bacteriorhodopsin promoting the proton pumping action. It can be considered the simplest photosynthetic system known. All other phototrophic systems (bacteria, algae, chloroplasts) use chlorophylls instead of bacteriorhodopsin to generate proton gradients.