Cell membrane and proteins, illustration. The cell membrane (yellow) separates the inner constituents of a cell from the extracellular space. The membrane is composed of a lipid bilayer, a two-layered sheet with the hydrophobic tails pointing toward its centre and hydrophilic heads exposed to the intracellular and extracellular space. Many different transmembrane proteins (pink) transmit important molecules and cellular signals across the cell membrane.