Singer and Nicholson first proposed the fluid mosaic model in 1972 to describe the structure of the cell membrane. According to this model, the membrane is a fluid lipid bilayer composed of two layers of phospholipids. Phospholipids contain a hydrophilic head and two hydrophobic fatty acid tails. They spontaneously form a lipid bilayer with their hydrophilic heads facing outward and their hydrophobic tails facing inward. In the bilayer, proteins are randomly distributed, forming a mosaic-like structure. Proteins that are loosely attached to the membrane are classified as peripheral proteins, while proteins that penetrate the lipid bilayer are known as integral proteins. Membrane proteins can move laterally and form complexes with other proteins unless they are restricted by other molecules inside or outside of the membrane. More recently, scientists have found that cell membranes also contain sphingolipids and cholesterol-rich microdomains known as lipid rafts. Lipid rafts play a significant role in cell functions such as signal transduction, cell adhesion, and membrane trafficking.