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Cell membranes or plasma membranes surround cells, separating the cytoplasm and organelles on the inside from the extracellular fluid on the outside. Several cell organelles (mitochondria, endoplasmic reticulum, and Golgi bodies) are also bounded by membranes. The membrane allows a cell or organelle to maintain a constant internal environment, usually one that is quite different from the medium surrounding it. This is accomplished by the semipermeable nature of the membrane that regulates the passage of all substances going through it.

The detailed chemical composition of a membrane varies, depending on its location and the functions it performs. However, all membranes do have the same basic structure. The majority of the membrane is composed of two layers of phospholipid molecules lined up side by side with their fatty acid "tails" facing inward. The outer edges of the membrane are hydrophilic (soluble in water), while the interior area is hydrophobic (insoluble in water). Because of this dual chemical nature of the phospholipid bilayer, the entire membrane surface is permeable to gases (such as oxygen and carbon dioxide), to small, uncharged polar molecules (such as water and ammonia), and to nonpolar molecules (such as lipids). However, the membrane is impermeable to charged molecules (such as ions and proteins) and to larger, uncharged polar molecules.

Embedded within and spanning the phospholipid bilayer are various transport proteins that serve as "gates," selectively allowing charged molecules and ions and larger molecules to pass through the membrane. These transport proteins channel molecules by a variety of methods, including facilitated diffusion (movement with the concentration gradient, using no ATP energy) and active transport (movement against the concentration gradient, using ATP energy).

Color enhanced scanning transmission electron micrograph of cell membranes of adjoining cells separated by an intercellular cleft. Photograph by Don Fawcett. Photo Researchers, Inc. Reproduced by permission.

The plasma membrane that forms the boundary of a cell has several other molecules in addition to the basic membrane structure. These include integral proteins, cholesterol, glycoproteins, and glycolipids. The phospholipid bilayer with its biochemical inclusions is known as the fluid mosaic model of membrane structure. Some membrane proteins serve as receptors for hormones, transferring the signal to the interior of the cell (via G proteins) without allowing the "messenger" molecule to enter, thus protecting the integrity of the cell. Other carbohydrate molecules attached to the exterior of the plasma membrane act as "markers," identifying the cell as a particular type.

Cystic fibrosis—a fatal, hereditary disease characterized by a heavy mucus buildup in the lungs—is caused by a defective plasma membrane protein. In persons with cystic fibrosis this transport protein, known as the sodium-potassium pump, abnormally transports sodium ions across the membrane without carrying the chloride ions that usually accompany them. Research is currently underway to correct through genetic engineering the faulty gene that codes for the plasma membrane protein.

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