Changing a few residues can change the function of homologous proteins. The chloride and proton affinity in the inward chloride-pumping halorhodopsin (HR) and outward proton-pumping bacteriorhodopsin (BR) are compared using classical electrostatic simulations. BR binds and releases protons from acidic residues that have been removed from HR. In the states where these acids are ionized in BR, HR binds a chloride. In the states where these acids bind a proton in BR, HR releases the chloride. Thus, BR uses static anions and mobile protons, whereas HR uses mobile ions to maintain the same charge states. The use of mobile ions makes HR more sensitive to external conditions.