The cell membrane (also called the plasma membrane) envelops the cell and is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteins and lipids. The approximate composition is proteins, 55 percent; phospholipids, 25 percent; cholesterol, 13 percent; other lipids, 4 percent; and carbohydrates, 3 percent.

The Cell Membrane Lipid Barrier Impedes Penetration by Water-soluble Substances. Figure 1 shows the structure of the cell membrane. Its basic structure is a lipid bilayer, which is a thin, double-layered film of lipids—each layer only one molecule thick—that is continuous over the entire cell surface. Interspersed in this lipid film are large globular proteins.

The basic lipid bilayer is composed of three main types of lipids: phospholipids, sphingolipids, and cholesterol. Phospholipids are the most abundant of the cell mem brane lipids. One end of each phospholipid molecule is soluble in water; that is, it is hydrophilic. The other end is soluble only in fats; that is, it is hydrophobic. The phosphate end of the phospholipid is hydrophilic, and the fatty acid portion is hydrophobic.

Because the hydrophobic portions of the phospholipid molecules are repelled by water but are mutually attracted to one another, they have a natural tendency to attach to one another in the middle of the membrane, as shown in Figure 1. The hydrophilic phosphate portions then constitute the two surfaces of the complete cell mem brane, in contact with intracellular water on the inside of the membrane and extracellular water on the outside surface.

The lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances, such as ions, glucose, and urea. Conversely, fat-soluble sub stances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease.

Sphingolipids, derived from the amino alcohol sphingosine, also have hydrophobic and hydrophilic groups and are present in small amounts in the cell membranes, especially nerve cells. Complex sphingolipids in cell mem branes are thought to serve several functions, including protection from harmful environmental factors, signal transmission, and as adhesion sites for extracellular proteins.

The cholesterol molecules in the membrane are also lipids because their steroid nuclei are highly fat soluble. These molecules, in a sense, are dissolved in the bilayer of the membrane. They mainly help determine the degree of permeability (or impermeability) of the bilayer to water-soluble constituents of body fluids. Cholesterol controls much of the fluidity of the membrane as well.

Integral and Peripheral Cell Membrane Proteins. Figure 1 also shows globular masses floating in the lipid bilayer. These membrane proteins are mainly glycoproteins. There are two types of cell membrane proteins: integral proteins that protrude all the way through the membrane and peripheral proteins that are attached only to one surface of the membrane and do not penetrate all the way through.

Many of the integral proteins provide structural channels (or pores) through which water molecules and water-soluble substances, especially ions, can diffuse between the extracellular and intracellular fluids. These protein channels also have selective properties that allow preferential diffusion of some substances over others.

Other integral proteins act as carrier proteins for trans porting substances that otherwise could not penetrate the lipid bilayer. Sometimes these carrier proteins even transport substances in the direction opposite to their electrochemical gradients for diffusion, which is called “active transport.” Still others act as enzymes.

Integral membrane proteins can also serve as recep tors for water-soluble chemicals, such as peptide hor mones, that do not easily penetrate the cell membrane. Interaction of cell membrane receptors with specific ligands that bind to the receptor causes conformational changes in the receptor protein. This process, in turn, enzymatically activates the intracellular part of the protein or induces interactions between the receptor and proteins in the cytoplasm that act as second messengers, relaying the signal from the extracellular part of the receptor to the interior of the cell. In this way, integral proteins spanning the cell membrane provide a means of conveying information about the environment to the cell interior.

Peripheral protein molecules are often attached to the integral proteins. These peripheral proteins function almost entirely as enzymes or as controllers of transport of substances through the cell membrane “pores.

Fig1.Structure of the cell membrane, showing that it is composed mainly of a lipid bilayer of phospholipid molecules, but with large  numbers of protein molecules protruding through the layer. Also, carbohydrate moieties are attached to the protein molecules on the outside  of the membrane and to additional protein molecules on the inside. (Modified from Lodish HF, Rothman JE: The assembly of cell membranes. Sci Am 240:48, 1979. Copyright George V. Kevin.)

Membrane Carbohydrates—The Cell “Glycocalyx.” Membrane carbohydrates occur almost invariably in combination with proteins or lipids in the form of glycoproteins or glycolipids. In fact, most of the integral proteins are glycoproteins, and about one tenth of the membrane lipid molecules are glycolipids. The “glyco” portions of these molecules almost invariably protrude to the outside of the cell, dangling outward from the cell surface. Many other carbohydrate compounds, called proteoglycans—which are mainly carbohydrate substances bound to small protein cores—are loosely attached to the outer surface of the cell as well. Thus, the entire outside surface of the cell often has a loose carbohydrate coat called the glycocalyx.

The carbohydrate moieties attached to the outer surface of the cell have several important functions:

 1. Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negatively charged objects.

2. The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another.

3. Many of the carbohydrates act as receptor sub stances for binding hormones, such as insulin; when bound, this combination activates attached internal proteins that, in turn, activate a cascade of intracellular enzymes.

4. Some carbohydrate moieties enter into immune reactions.