Evidence for lateral diffusion of proteins
The cell is gray. The green dots illustrate the presence of fluorescently labeled antibodies which have bound to proteins on the surface of the cell. The green dots move across the surface of the cell. The researcher can apply a tiny but intense laser beam which bleaches the flourescent labels in a small area on the surface of the cell. This photobleaching produces a patch of unlabeled cell surface that appears gray. With time, however, proteins with fluorescent labels that were not bleached diffuse back into the area which was bleached and the bleached proteins diffuse away. The bleached spot disappears and the cell is uniformly labeled again. The rate at proteins diffuse back into the bleached area is much slower than is illustrated in this animation. This type of experiment has been used to provide evidence that proteins are free to diffuse laterally across the plane of the membrane. Similar experiments can be with fluorescently labeled lipids. In the case of lipids, the lateral diffusion rates are much greater.