Membrane

All living creatures are made of cells. One cellular component, the membrane, plays a crucial role in almost all cellular activities. The primary function of all cell membranes is to act as barriers between the intracellular and extracellular environments, and as sites for diverse biochemical activities. The cell itself is encapsulated by its own membrane, the plasma membrane. Although the composition of membranes varies, in general, lipid molecules make up approximately 40 percent of their dry weight; proteins, approximately 60 percent. The lipids and proteins are held together by noncovalent interactions.

Among several possible stable arrangements of protein and lipid molecules in membranes, the bilayer model, first described over seventy years ago, characterizes most biological membranes. An important feature of this model is that the hydrophilic groups of the lipid molecules are oriented toward the surfaces of the bilayer, and the hydrophobic groups toward the interior. In 1972 Jonathan Singer and Garth Nicolson postulated a unified theory of membrane structure called the fluid-mosaic model. They proposed that the matrix, or continuous part, of membrane structure is a fluid bilayer, and that globular amphiphilic proteins are embedded in a single monolayer, with some proteins spanning the thickness of both monolayers. Both proteins and lipids are mobile and, thus, the membrane can be viewed as a twodimensional solution of proteins in lipids.

The major class of lipids in plasma membranes is phospholipids. Phospholipids consist of a glycerol backbone and two fatty acids joined by ester linkage to the first two carbons of glycerol, and a phosphate group joined to the third. Different groups can be esterified to the phosphate, and these groups define the different classes of phospholipids. In addition, the fatty acids have varying chain lengths and degrees of unsaturation. The presence of the nonpolar acyl chain regions and the polar head groups gives the phospholipid molecules their amphipathic character, which allows them to assume the bilayer arrangements of membranes. In addition to phospholipids, two other kinds of lipids are found in the membranes of animal cells: glycolipids and cholesterol. Glycolipids usually make up only a small fraction of the lipids in the membrane but have been shown to possess many biological functions, one of which is their capacity to function as recognition sites. Cholesterol is an important component of plasma membranes and has been shown to play a key role in the control of membrane fluidity.

Several membrane functions are believed to be largely mediated by proteins. Membrane proteins have been put into two general categories: peripheral and integral. Peripheral proteins (or extrinsic proteins) are those that do not penetrate the bilayer to any significant degree and are associated with it by virtue of noncovalent interactions (ionic interactions and hydrogen bonds ) between membrane surfaces and protein surfaces. Integral proteins (or intrinsic proteins), in contrast, possess hydrophobic surfaces that readily penetrate the lipid bilayer, as well as other surfaces that prefer contact with aqueous medium. These proteins can either insert into the membrane or extend all the way across it and expose themselves to the aqueous solutions on both sides.

One of the main functions of the plasma membrane is to separate cytoplasm from extracellular surroundings. In fact, membranes are highly selective permeability barriers, as they contain specific channels and pumps that enable the transport of substances across membranes. These transport systems to a large degree regulate the molecular and ionic composition of intracellular media. Membranes also control the flow of information between cells, and between cells and their extracellular environments, and they contain specific receptors that make membranes sensible to external stimuli. In addition, some membranes conduct and pass on signals that can be chemical or electrical, as in the transmission of nerve impulses. Thus, membranes play a central role in signal transduction processes and in biological communication.

RECOGNITION SITES

Glycolipids and glycoproteins can act as recognition sites in a variety of processes involving recognition between cell types or recognition of cellular structures by other molecules. Recognition events are important in normal cell growth, fertilization, transformation of cells, and other processes.

see also Cholesterol; Lipids; Phospholipids; Transmembrane Protein.

Michèle Auger

Bibliography

Garrett, Reginald H., and Grisham, Charles M. (2002). Principles of Biochemistry with a Human Focus. Fort Worth, TX: Harcourt College Publishers.

Harrison, Roger, and Lunt, George G. (1980). Biological Membranes: Their Structure and Function. New York: Wiley.

Singer, S. Jonathan, and Nicolson, Garth L. (1972). "The Fluid Mosaic Model of the Structure of Cell Membranes." Nature 175:720–731.

Membrane

A cell membrane (plasma membrane) is a structure that is composed mainly of lipid molecules. The membrane, whose two outer surfaces are hydrophilic (water-loving) and whose interior is hydrophobic (water-hating) surrounds both eukaryotic cells (whose genetic material is enclosed in a more specialized nuclear membrane) and prokaryotic cells (whose genetic material is dispersed in the cells’ interior). Membranes provide the separation between the interior of a cell and the external environment, and can exert some control over the environment inside a cell.

In eukaryotic cells, in addition to the nucleus, other structures such as the mitochondria, endoplasmic reticulum, and Golgi bodies are also bounded by membranes.

A membrane helps control a cells’ internal environment because it can allow the passage of some molecules while impeding the passage of other molecules; in other words, membranes are semipermeable.

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 hydrophilic portion of a phospholipid is the phosphate group; these face outwards. The more lipidlike tail of a phospholipid is the hydrophobic region, and this portion orients to the membranes’ interior. 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 can function to selectively allow certain 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).

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.

Membrane

A cell membrane or plasma membrane is a thin barrier that separates a cell from its surroundings. It also keeps the cell's cytoplasm and organelles on the inside. Membranes are selectively permeable, meaning that some things can pass through the membrane and some cannot.

All cells have a cell membrane that is a thin but double layer of molecules that surrounds it. This membrane acts as a barrier and helps protect the cell while controlling the movement of substances in and out of

the cell. The membrane also allows a cell to maintain a constant internal environment, despite changes in its external environment. It is able to do this because of the semipermeable nature of its layers that regulate the passage of all substances going through it. The membrane is able to keep things out that it does not want or need while allowing in what it must have. A membrane surrounds not only the entire cell, but each organelle or specialized structure inside the cell also has a membrane around it. For both the cell and its organelles, the membrane is a place of constant activity. Although this membrane is extremely thin, it is very strong and can heal itself if broken. Examined very closely, a membrane is like a mesh bag whose little, square holes are small and strong enough to hold a dozen oranges or five pounds of potatoes, but which will also let water flow out or in completely. The bag is semipermeable, since it keeps certain-size things in (oranges) but let things of another size (water molecules) pass through. Most cell membranes are permeable to oxygen and water but not to large organic molecules like proteins.

One way that substances move through a membrane is by a process called passive transport. Passive transport involves no use of energy on the part of the cell, since certain substances are able to move freely in or out of it. Diffusion and osmosis are forms of passive transport since the cell does not need to use any energy to move substances. In diffusion, molecules of a substance spread themselves out more evenly from an area of high concentration. Substances like carbon dioxide, salts, and oxygen move in and out of cells this way. In osmosis, water molecules move from an area where they are crowded and cross a membrane to where they are less crowded. This process stops of its own accord when the solutions on either side of the membrane are at equal strength.

Active transport is a different way that a cell moves molecules in or out through its membrane, and it involves the use of energy. Active transport occurs when a cell wants to bring in more of a substance than will enter via passive transport. This happens when a plant's nearly-full root cells want to store even more minerals, and must move them from an area where they are less crowded to one where they are more crowded (inside the cell). In order to pack in more molecules, the cell uses carrier molecules that literally carry the desired molecule to a membrane slot into which it fits and then forces it into the cell. This process requires that the cell uses its own energy. Cell membranes are much more than walls or barriers that hold a cell together and keep it separate from its environment, since membranes control the movement of substances into and out of a cell.

[See alsoCell; Diffusion; Osmosis ]

Membrane

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).

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.

See also Nucleus, cellular; Osmosis (cellular).

mem·brane / ˈmemˌbrān/ • n. Anat. & Zool. a pliable sheetlike structure acting as a boundary, lining, or partition in an organism. ∎  a thin pliable sheet or skin of various kinds: the concrete should include a membrane to prevent water seepage. ∎ Biol. a microscopic double layer of lipids and proteins that bounds cells and organelles and forms structures within cells.DERIVATIVES: mem·bra·na·ceous / ˌmembrəˈnāshəs/ adj.mem·bra·ne·ous / memˈbrānēəs/ adj.mem·bra·nous / ˈmembrənəs; memˈbrānəs/ adj.