Under the fluid mosaic model, the components of the cell membrane are mobile, and phospholipid mobility resolves into two geometrically distinct motions: lateral diffusion within a single leaflet, which is rapid and frequent because no polar head group must traverse the hydrophobic core, and transbilayer movement between leaflets, which is intrinsically very slow for that same reason. Because the slow motion must nevertheless occur, protein catalysts accelerate it with defined directional specificity — flippase moving lipids from the outer to the inner leaflet and floppase from inner to outer, both consuming ATP, while scramblase exchanges lipids bidirectionally without ATP. The concept belongs to cell biology and membrane biophysics, and it establishes the principle that membrane asymmetry is not a static structural fact but a kinetically maintained state whose direction is set by which catalysts are active.
Under the fluid mosaic model, the components of the cell membrane are mobile, and phospholipid mobility resolves into two geometrically distinct motions: lateral diffusion within a single leaflet, wh…