Cell membranes maintain distinct inner and outer leaflets despite being fluids in which components diffuse rapidly. What accounts for this stable asymmetry?

C. Transverse movement of lipids across the bilayer is exceptionally rare

Lateral diffusion happens millions of times per second, but transverse flipping across the hydrophobic core requires enzymatic catalysis. Carbohydrates and the cytoskeleton localise specific components, but intrinsic bilayer dynamics preserve overall leaflet asymmetry.