text: Bacteriorhodopsin belongs to the microbial rhodopsin family. Its homologs include the archaerhodopsins, the light-driven chloride pump halorhodopsin (for which the crystal structure is also known), and some directly light-activated channels such as channelrhodopsin.
  Bacteriorhodopsin is similar to vertebrate rhodopsins, the pigments that sense light in the retina. Rhodopsins also contain retinal; however, the functions of rhodopsin and bacteriorhodopsin are different, and there is limited similarity in their amino acid sequences. Both rhodopsin and bacteriorhodopsin belong to the 7TM receptor family of proteins, but rhodopsin is a G protein-coupled receptor and bacteriorhodopsin is not. In the first use of electron crystallography to obtain an atomic-level protein structure, the structure of bacteriorhodopsin was resolved in 1990. It was then used as a template to build models of G protein-coupled receptors before crystallographic structures were also available for these proteins. It has been excessively studied on both mica and glass substrates using Atomic force microscopy and Femtosecond crystallography.
  All other phototrophic systems in bacteria, algae, and plants use chlorophylls or bacteriochlorophylls rather than bacteriorhodopsin. These also produce a proton gradient, but in a quite different and more indirect way involving an electron transfer chain consisting of several other proteins. Furthermore, chlorophylls are aided in capturing light energy by other pigments known as "antennas"; these are not present in bacteriorhodopsin-based systems.  It is possible that phototrophy independently evolved at least twice, once in bacteria and once in archaea.