Bacteriorhodopsin is a proton pump protein, containing the chromophore retinal. Light energy catalyses the isomerisation of retinal, inducing a conformational change (straining) in the bacteriorhodopsin protein. This allows D85 to release a proton to the exterior of the cell. The release of a H+ changes the pKa of D96, releasing a proton to reprotonate D85. This causes the pKa of D96 to change again, allowing it to take up a proton from the cytoplasm, returning the retinal to its resting state.
This pumping cycle results in the net movement of protons out of the cell, against the concentration gradient. The gradient can then be used for ATP synthesis or other active processes requiring a proton gradient.
During this pumping cycle, the bacteriorhodopsin changes colour during the different stages. This makes it easy to follow the pumping cycle, using a microscope and camera. The slow pumping process further helps here, removing the requirement for highly specialised equipment.