Editing Retinal (section)

==Microbial rhodopsins==
{{Main article|Microbial rhodopsin}}
All-''trans''-retinal is also an essential component of [[microbial]] opsins such as [[bacteriorhodopsin]], [[channelrhodopsin]], and [[halorhodopsin]], which are important in [[bacteria]]l and [[archaea]]l [[anoxygenic photosynthesis]].  In these molecules, light causes the all-''trans''-retinal to become 13-''cis'' retinal, which then cycles back to all-''trans''-retinal in the dark state. These proteins are not evolutionarily related to animal opsins and are not GPCRs; the fact that they both use retinal is a result of [[convergent evolution]].<ref name=13CIS>{{cite journal |doi=10.1016/S1011-1344(02)00245-2 |pmid=11960728 |title=All-trans to 13-cis retinal isomerization in light-adapted bacteriorhodopsin at acidic pH |year=2002 |last1=Chen |first1=De-Liang |last2=Wang |first2=Guang-yu |last3=Xu |first3=Bing |last4=Hu |first4=Kun-Sheng |journal=Journal of Photochemistry and Photobiology B: Biology |volume=66 |issue=3 |pages=188–194|bibcode=2002JPPB...66..188C }}</ref>