The fluid mosaic model was first proposed by S. J. Singer and G. L. Nicolson in 1972 to describe the structure of cell membranes (Singer and Nicolson 1972). In this now-accepted theory about cell structure, phospholipid molecules, each with one hydrophobic, and one hydrophilic end, make up most of the membrane. The hydrophilic heads form the inner and outer surfaces the membrane and the hydrophobic tails, which are repelled by the water within and outside the cell, are sandwiched in between (see figure). This is known as the lipid bilayer.
This arrangement is fluid (see video at right), not solid, because the various functional macromolecules embedded in the phospholipid matrix can move about the surface of the cell. Because of this fluidity such membranes are often called plasma membranes (one meaning of the word plasma is a complex fluid).
The model is called mosaic because it proposes that the membrane is made up of many different parts, including proteins, carbohydrates, and lipids (see figure). This composite structure allows the membrane to perform multiple functions. For example, certain embedded proteins may act as channels allowing particular molecules to pass through the membrane. Others may serve as labels allowing recognition of the cell. Still others may act as sensors that detect various features of the ambient environment of the cell.