The lipid bilayer is a thin polar membrane made of two layers of lipid molecules.These membranes are flat sheets that form a continuous barrier around all cells.The cell membranes of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus and other sub-cellular structures.rokaryotes have only one lipid bilayer-the cell membrane (also known as the plasma membrane). Many prokaryotes also have a cell wall, but the cell wall is composed of proteins or long chain carbohydrates, not lipids.