In a phospholipid bilayer, the polar head of each layer points outwards, while the long nonpolar lipid chains of each layer points towards the middle. A phospholipid bilayer membrane is a specific type of lipid bilayer where all the polar heads contain a phosphate group.
Lipid bilayers are an extremely common type of polar membrane that can be found in almost any cell. They consist of two layers of lipid molecules. The lipid molecules have a polar head, often phosphate, and two long nonpolar chains.
Because of water inside the cell, the heads of the inner layer point towards the cell. Additionally, because of water surrounding the cell, the polar heads of the outer layer point outwards.