The cell membrane is a selectively permeable membrane that controls movement of materials inside and out of the cell. It is a lipid bilayer with proteins embedded in between. The lipid bilayer is "composed of phospholipids, which have a hydrophilic head and two hydrophobic tails each." When exposed to water, the tails point in and the heads point out. There are transmembrane protein channels and transporters in the membrane. The transmembrane transporters pump materials across the membrane and the protein channels recognize and transport specific substances across the membrane. Integral membrane proteins are permanently attached to the membrane. They function as transporters, receptors, enzymes. They are permanently connected to the cell membrane with large sections embedded in the hydrophobic, or middle layer of the membrane. They acts as pathways for ions and molecules during active and passive transport. Channel proteins are important for facilitated diffusion to occur. These are embedded in the cell membrane and along with carrier proteins, they assist molecules like glucose, sodium and chloride ions to pass through the cell membrane which otherwise would be unable to pass through the lipid bilayer.