Signal transduction
Rhodopsins found in prokaryotes and algae commonly contain an all-"trans" retinal isomer at the ground state that isomerizes to 13-"cis" upon light activation, also known as microbial-type chromophore. Examples are bacterial sensory rhodopsins, channelrhodopsin, bacteriorhodopsin, halorhodopsin, and proteorhodopsin. They act as light-gated ion channels and can be further distinguished by the type of ion they channel. Bacteriorhodopsin functions as a proton pump, whereas halorhodopsin act as a chloride pump. Their functions range from bacterial photosynthesis (bacteriorhodopsin) to driving phototaxis (channelrhodopsins in flagellates). Signal transduction in phototaxis involves depolarization of the cell membrane.