Overview of type I microbial rhodopsins. Bacteriorhodopsin and proteorhodopsins pump protons from the cytoplasm to the extracellular medium, whereas halorhodopsins pump chloride into the cytoplasm; all three change the membrane voltage in the same direction. In sensory rhodopsins, the internal proton movement is converted to conformational changes that drive signaling cascades. Algal channelrhodopsins flux cations across the membrane in both directions but always along the electrochemical gradient of the transported ions. In sensory and channelrhodopsins, proton translocation within the protein is required for efficient photocycle progression, but these protons are not necessarily exchanged between the intra- and extracellular space.