Animation of the fluid nature of a cell membrane. Cell membranes are composed of a phospholipid bilayer, that is, two layers of phospholipid molecules. Each phospholipid molecule has a hydrophobic lipid tail black and a hydrophilic phosphate group orange attached to a choline molecule containing a blue nitrogen atom. The hydrophobic tails are attracted to each other, and repelled by the aqueous cell environment, so form a thick double layer, with the phosphate_choline groups forming the outer and inner covering of the membrane. The cell membrane also includes other large complexes, including receptors for chemicals outside the cell, and channels that permit small molecules to pass between the interior and exterior of the cell.