The bilayer membrane found around all cells is the barrier that separates interior of the cell with its extracellular matrix. It keeps ions and other molecules where they are needed at the moment. It also can prevent molecules from diffusing from one compartment to another. Bilayers are composed of phospholipids that have a hydrophilic phosphate head (water-loving) and a hydrophobic (repelled from the water) tail consisting fatty acid chains. Because of this structure, hydrophilic molecules, for example, cannot easily cross the hydrophobic bilayer core.