The fluid mosaic model describes the molecular organization of a typical cell membrane consisting of phospholipids and proteins. Phospholipids consist of hydrophobic nonpolar tails and hydrophilic polar heads. They interact with each other by nonpolar tails, whereas polar heads of phospholipids interact with water. These interactions result in the formation of a lipid bilayer that is characterized by fluidity and elasticity. Proteins are embedded in the lipid bilayer and interact with phospholipid hydrophobic tails via hydrophobic residues. Some proteins cross the bilipid membrane, whereas other proteins are anchored in one of the two layers of membranes.