See handout for various products made during fermentations. Light can provide energy by two fundamentally different mechanisms. Light-activated proton extrusion. Light-activated electron transport with secondary proton extrusion. The first mechanism is found in some members of the Archaea, namely the Halobacterium sp.
Protein is also somewhat similar to rhodopsin of the eye.
In the dark, the retinal re-isomerizes back to original configuration and bind a proton from the cytoplasmic side of the membrane. In this cycle, light-drive proton extrusion from cytoplasm to periplasm results. Bacteriorhodopsin and ATP synthase can be artificially inserted into phospholipid vesicles. Such vesicles were used by Racker and Stoekenius to provide strong support for the chemiosmotic coupling hypothesis.