The lipid molecules in cell membrane are spontaneously arrange themselves so that non-
polar, hydrophobic fatty acid tail regions are shielded from the surrounding polar fluid, causing
the hydrophilic head region to associate with the cytosolic and extracellular faces of the resulting
bilayer. The proteins usually span from one side of the phospholipids bilayer to the other
(integral proteins), but can also sit on one of the surfaces (peripheral proteins). The peripheral
proteins can slide around the membrane very quickly and collide with each other, but can never
flip from one side to the other. The extracellular surface of the cell membrane is decorated
7