Now, what about proteins? Well, unlike these phospholipids, which basically move at a relatively uniform rate of 1 μm squared per second, proteins basically vary in the rate of movement. Some are immobile, they move very slowly, while others basically move very quickly and so are mobile. Now, what determines the mobility of these proteins? Well, basically, it's the functionality of that protein is the protein attached onto some other component, onto some other structure. Now, for instance, if we look at Rhodopsin, which is basically a photopigment that is found in the retina cells of our body, rhodopsin's functionality depends on its ability to move quickly along the membrane.