National Institutes of General Medical Sciences (NIH) Homepage: Science & Education Dec. 4, 2002

The Surface Membrane, Versatile Gatekeeper

In the beginning," wrote biologist Gerald Weissmann, "there must have been a membrane! Whatever flash of lightning there was that organized purines, pyrimidines, and amino acids into macromolecules capable of reproducing themselves, it would not have yielded cells but for the organizational trick afforded by the design of a membrane wrapping." Weissmann imagines these primitive membranes forming bubbles in which the first macromolecules were enclosed and protected from dissipation in the salty primordial seas.

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The surface membrane of a cell consists of a double layer of lipid molecules interspersed with protein molecules. Surface membranes play many roles, including keeping the cell intact and allowing appropriate substances to enter and leave the cell. 

A cell's outer membrane is often thought of as a boundary that distinguishes the living cell from its surroundings. And, indeed, surface membranes are crucial in keeping cells intact. Moreover, the internal membranes that wrap around many organelles in eukaryotic cells separate the cytoplasm into discrete regions, somewhat like the walls that form rooms in a house. These inner membranes enable the cell to perform many otherwise incompatible biochemical activities simultaneously, thereby greatly increasing the cell's efficiency.

Yet despite its barrier functions, the cell membrane--which is often less than 0.01 micrometer thick--is not impassive. Rather, it is exquisitely sensitive to its surroundings and selectively allows certain substances to enter and leave the cell while barring others. It takes in nutrients and excretes wastes. It sends and receives chemical and electrical messages, including signals for the cell to manufacture proteins or to divide. In multicellular organisms, it joins with other cells to form tissues. 

These myriad abilities are due to the membrane's composition. Although surface membranes differ in their precise composition depending on the cell's type, and although a membrane's configuration changes from moment to moment, all membranes are composed of two basic kinds of molecules--proteins and lipids (fats).

In 1972, S. Jonathan Singer and Garth Nicolson of the University of California, San Diego proposed a model to describe the relationship of proteins and lipids in an idealized membrane. They compared the proteins to "icebergs floating in a sea of lipids," and suggested that some of the proteins are folded so that the "tips" poke above and below the plane of the membrane, while the middle of the protein is embedded in the membrane itself.

Although such tripartite proteins were unknown at the time Singer and Nicolson proposed their model, they have since been shown to play important roles in a number of biological processes, including those that involve the transport of molecules into the cell. Many other membrane proteins that are attached either to the inner or outer face of the surface membrane have also been studied in detail in the years since Singer and Nicolson proposed their so-called fluid-mosaic model.

The lipids that make up the bulk of a cell's surface membrane fall into three classes: phospholipids, steroids (primarily cholesterol), and glycolipids. About half of the molecules in an average membrane are phospholipids. Each phospholipid molecule has a water-seeking phosphate "head" and two flexible, water-avoiding lipid "tails." In a surface membrane, phospholipids spontaneously arrange themselves into a double layer with the phosphate heads touching the watery interior and exterior of the cell, and the lipid tails buried in the middle of the layer.

Cholesterol is a rigid molecule that helps stabilize the membranes of animal cells. It is manufactured within the cell (in the endoplasmic reticulum) and is also brought into the cell from the blood. Cholesterol is present only in animal cells; plant cells are stiffened by a very rigid cell wall composed mainly of cellulose.

Glycolipids are composed of a sugar ("glyco" is derived from the Greek word for sweet) and a lipid portion, and make up about 5 percent of the lipid population. A person's blood group (O, A, B, or AB) is determined by the particular kind of glycolipids present on the surface of his or her red blood cells.