Figure 1. Possible types of membrane flow.

A–E: View of the motile keratocyte cell's lamellipodium from the side. Shaded ellipsoid is the cell body. A: Growth of actin network (green) inside the lamellipodium pushes the leading edge forward, resisted by the membrane tension at the front (yellow arrows,T_f). At the rear, membrane tension (yellow arrows, T_r) pushes forward the disassembling actin networks. Besides the breaking actin network, breaking adhesions (orange rectangle) also resist rear retraction. Myosin (dark red dumbbells) powered contraction assists rear retraction. The membrane flows forward in the lab coordinate system (blue arrows) on the ventral and dorsal surfaces. Adhesions (blue rectangles) linked to the stationary actin network resist this flow at the ventral surface; transmembrane proteins (light blue ovals) resist this flow at the dorsal surface. B–E: blue (red) arrows show the membrane flow in the lab (moving cell) coordinate systems, respectively. B: One possibility is that the membrane flow is the same at the ventral and dorsal surfaces. In this case, these flows' rates are the same as the cell speed in the lab coordinate system, and the flows are zero in the cell frame. C: Example of tank-tread flow. D: In this case the membrane is transported from the rear to the front solely through the intracellular transport of membrane vesicles. The membrane flow is zero in the lab coordinate system and directed to the rear in the cell frame. E: Hypothesis about how the membrane flow can be the same on the ventral and dorsal surfaces for any different protein concentrations at these surfaces. This can be explained if the membrane flow across the leading edge (black) is obstructed. In this case, the membrane tension at the rear of the dorsal and ventral surfaces is the same, but rear-to-front gradients of tension are different along the ventral and dorsal surfaces because the same ventral and dorsal flows are resisted by different protein concentrations at these surfaces. Thus, tensions at the fronts of the ventral and dorsal surfaces are different. F: View of the motile keratocyte cell's lamellipodium from above. Shaded ellipsoid is the cell body. Insert: Cartoon of membrane flow around solid circular obstacles (proteins). Proteins attach to the cytoskeleton and/or the external environment, and so stay stationary (in the lab frame of reference). Thus, as the cell moves, the membrane is forced to flow around the proteins.