Halobacteria and people have in common a type of membrane protein (retinal proteins) that contains the molecule retinal and is activated by sun light. In Halobacteria the proteins serve the bacterium's energy needs as well as steer the bacterium into areas that are lit appropriately. In people, other vertebrates and also invertebrates, the protein provides the light receptors for vision. In Halobacteria one retinal protein, bacteriorhodopsin, forms a two-dimensional crystal in the bacterial cell wall. Relying on observations from crystallography and electron microscopy, computer modeling as [reported = purple membrane] recently could reconstruct the entire purple membrane, combining protein, lipids, and water. Here, for the first time, a fully functional biological system is known precisely, i.e., every atom type and position being identified. Now the mechanism underlying an integral biological function, in this case light absorption leading to the pumping of protons across a membrane, can be deduced from physical principles. More can be found on our website.