Flip-flop (cell biology)

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In cell biology, a flip-flop is the “change of side” of a lipid molecule in a cell membrane .

Biomembranes consist of two lipid layers, with the water-repellent structures of the molecules ("tails") pointing inwards, the water-attracting ("heads") pointing outwards, towards the surface of the membrane ('inside' and 'outside' in relation to the membrane - comparable to a wall: 'outside' = surfaces, 'inside' = membrane body).

The two levels of the biomembrane behave like two-dimensional liquids . Although the lipid molecules can very freely within its own membrane level diffuse , but only very difficult to leave this plane. A change from one side of the bilayer to the other is extremely unlikely and happens spontaneously only about once a minute in a normal cell . As a result, the cell membrane has an “inside” and “outside” in relation to the cell (again the example of a 'wall' between two rooms, one side is wallpapered, the other is tiled; wallpaper and tiles remain on their side).

However, a flip-flop can be catalyzed by an enzyme called a flippase . In this way, the cell can u. a. actively determine their topology , i.e. the curvature of the membrane. It is also possible that the incorporation of new lipids happens unevenly at first, but later the flip-flops restore the balance.