lipid bilayer

A lipid bilayer gives rise to the structure of a cell membrane, in which two layers of phospholipids spontaneously align so that the hydrophilic head groups are exposed to water, while the hydrophobic tails (consisting of fatty acids) are pointed toward the center of the membrane. This establishes a boundary separating the contents of the cell from its surroundings, or an organelle within a cell from the general intracellular environment.