The cell structure that serves as a boundary from its environment is the cell membrane. The cell membrane is a thin layer that surrounds the cell and regulates the movement of materials in and out of the cell. It is made up of a phospholipid bilayer, which is a double layer of phospholipids, with the hydrophilic (water-loving) heads facing outward and the hydrophobic (water-hating) tails facing inward. This arrangement creates a barrier that is impermeable to most molecules, including water. The cell membrane also contains membrane proteins, which are proteins that are embedded in the membrane and help to transport materials across the membrane.