The Cell Membrane and the Fluid Mosaic Model

Every cell, whether prokaryotic or eukaryotic, is bounded by a plasma membrane (also called the cell membrane) -- the thin, selectively permeable barrier that separates the living interior of the cell from its surroundings while still allowing the controlled exchange of materials across it. Understanding of the membrane's structure developed gradually over the twentieth century. As early as 1899, Overton proposed, on the basis of how easily different substances entered cells, that the membrane must be made largely of lipid material. In 1954, Danielli and Davson refined this idea into what became known as the "sandwich" or unit-membrane model, picturing the membrane as a central layer of lipid flanked on both sides by continuous layers of protein.

The model that is now widely accepted, however, is the fluid mosaic model, proposed in 1972 by S.J. Singer and G.L. Nicolson. According to this model, the plasma membrane is built around a bilayer of phospholipid molecules. Each phospholipid molecule has a "head" that is hydrophilic (water-attracting) and two "tails" that are hydrophobic (water-repelling); in the bilayer, the molecules arrange themselves so that the hydrophilic heads face outward toward the watery environments on either side of the membrane, while the hydrophobic tails point inward, away from water, toward one another. This arrangement is thermodynamically favourable and forms spontaneously, giving the membrane its basic two-layered structure.

Embedded within this lipid bilayer are membrane proteins, which the model divides into two broad categories. Integral proteins are partly or wholly embedded within the bilayer itself, and some of these -- called transmembrane proteins -- span its entire width, with portions exposed on both the inner and outer faces of the membrane; many of these proteins act as channels or carriers that allow specific substances to cross the membrane. Peripheral proteins, by contrast, are not embedded in the bilayer at all but are instead attached loosely to its inner or outer surface. Rather than being fixed in place, these proteins are scattered through the lipid bilayer in an irregular pattern, giving the membrane the "mosaic" appearance from which the model takes its name.

The word "fluid" in the model's name refers to the fact that the lipid molecules of the bilayer, and many of the embedded proteins, are not rigidly fixed in position but are free to drift laterally within their own layer of the membrane, rather like objects floating and slowly moving about on the surface of a pond -- a property described as the membrane being quasi-fluid. The degree of this fluidity depends on the membrane's exact lipid composition and on temperature; in animal cell membranes, cholesterol molecules interspersed among the phospholipids help regulate this fluidity, keeping the membrane from becoming either too rigid or too fluid across a range of temperatures. …