patch. As in capping, the particle moves toward the back of the cell. Proposed mechanisms "Flow" Abercrombie thought that the carbon particle is a marker on the cell surface and its behaviour reflects what the surface is doing. This led him to propose that, as a cell moves, membrane from internal stores is added at the front of the cell—enabling the cell to extend forward—and retrieved toward the rear of the cell. This process of exocytosis at the front of the cell and endocytosis elsewhere has been modified by Bretscher. He and Hopkins showed that the specific membrane endocytosed by coated pits on motile cells is returned by exocytosis to the cell surface at the leading edge. The spatial difference between the sites of exocytosis (at the front) and endocytosis (everywhere on the surface) leads to a flow of the matrix of the plasma membrane—lipids—from the front toward the rear. Large objects, such as patches, would be swept along with this flow, whereas non-crosslinked small molecules would be able to diffuse by Brownian motion against the flow and so evade being swept backward. Hence, in this theory, the need for crosslinking. Bretscher proposed that on stationary cells exocytosis is random—and therefore a major difference between motile and nonmotile cells. "Cytoskeleton" An alternative view is that the patches are moved to the rear of the cell by direct attachment to the actin cytoskeleton. The molecular mechanism for how this could be achieved is unclear, since, when glycolipids or GPI-bound proteins (in the outer monolayer of the cell's surface bilayer) are crosslinked, they cap, just like any surface protein. As these molecules cannot themselves interact directly with the cytoplasmic actin cytoskeleton, this scheme seems unlikely. "Rake" A third scheme, by de Petris, suggests that a motile cell is continuously raking its surface from front to back: Any aggregates (but not uncrosslinked molecules) caught in the teeth of the rake are moved to the back of the cell. In this scheme, the nature of the tines of the rake are not specified but could, for example, be surface integrins that often act as the feet of the cell to attach it to the substrate. The force required to rake the surface could be provided by the actin cytoskeleton. "Surf" A fourth scheme, by Hewitt, suggests that motile cells have rearward waves on their surfaces: patches, but not single molecules, become entrained in these