Cell membrane

From Encyclopedia Jr, free information reference for Kids

Illustration of a lipid bilayer

A cell membrane, plasma membrane or plasmalemma is a selectively permeable lipid bilayer coated by proteins which comprises the outer layer of a cell. The plasma membrane works between the machinery on the inside of the cell and the outside fluid that bathes the cells. The plasma membrane lets nutrients into the cell such as glucose, amino acids, and lipids. It is a semi-fluid patchwork of molecules, some of which are constantly diffusing through the membrane. This mobility is due to the flexible proteins and oily phospholipids that make up most of the membrane's chemical structure. Carbohydrates attached to the proteins and phospholipids form glycoproteins and glycolipids. It consists of, among other components, phospholipid and protein molecules which separate the cell interior from its surroundings within animal cells, and control the input and output of the cell through the use of receptor and cell adhesion proteins, which also play a role in cell behavior and the organization of cells within tissues. Plasma membranes is also known as an asymmetric fluid mosaic structure.

[edit] Functions

In animal cells the plasma membrane alone establishes a separation between interior and environment, whereas in fungi, bacteria, and plants an additional cell wall forms the outermost boundary. However the cell wall plays a mechanical support role rather than a role as a selective boundary. The plasma membrane is only about 10 nm thick and may be discerned only faintly with a transmission electron microscope. One of the key roles of the membrane is to maintain the cell potential.

Phospholipid molecules in the cell membrane are "fluid," in the sense that they are free to diffuse and exhibit rapid lateral diffusion. Lipid rafts and caveolae are examples of cholesterol-enriched microdomains in the cell membrane.

Many proteins are not free to diffuse. The cytoskeleton undergirds the cell membrane and provides anchoring points for integral membrane proteins. Anchoring restricts them to a particular cell face or surface – for example, the "apical" surface of epithelial cells that line the vertebrate gut – and limits how far they may diffuse within the bilayer. Rather than presenting always a formless and fluid contour, the plasma membrane surface of cells may show structure. Returning to the example of epithelial cells in the gut, the apical surfaces of many such cells are dense with involutions, all similar in size. The finger-like projections, called microvilli, increase cell surface area and facilitate the absorption of molecules from the outside. Synapses are another example of highly-structured membrane.

New material is incorporated into the membrane, or deleted from it, by a variety of mechanisms.

• Fusion of intracellular vesicles with the membrane not only excretes the contents of the vesicle, but also incorporates the vesicle membrane's components into the cell membrane. The membrane may form blebs that pinch off to become vesicles.
• If a membrane is continuous with a tubular structure made of membrane material, then material from the tube can be drawn into the membrane continuously.
• Although the concentration of membrane components in the aqueous phase is low (stable membrane components have low solubility in water), exchange of molecules with this small reservoir is possible.

In all cases, the mechanical tension in the membrane has an effect on the rate of exchange. In some cells, usually having a smooth shape, the membrane tension and area are interrelated by elastic and dynamical mechanical properties, and the time-dependent interrelation is sometimes called homeostasis, area regulation or tension regulation.

[edit] Structure

The outer cell membrane and the membranes surrounding inner cell organelles are phospholipid bilayers. To perform the function of the organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles for that cell or organelle. In the cell membrane, phospholipid molecules create a spherical three dimensional lipid bilayer shell around the cell. A phospholipid molecule is composed of a head and two tails. The circle, or head, is the negatively charged phosphate group and the two tails are the two highly hydrophobic fatty acid chains of the phospholipid.

[edit] List of Functions

• It attaches parts of the cytoskeleton to the cell membrane in order to provide shape.
• It attaches cells to an extra-cellular matrix in grouping cells together to form tissues.
• It transports molecules into and out of cells by such methods as ion pumps, channel proteins and carrier proteins.
• It acts as receptor for the various chemical messages that pass between cells such as nerve impulses and hormone activity.
• It takes part in enzyme activity which can be important in the metabolism or as part of the body's defense mechanism.

[edit] See also

• Bacterial cell structure
• Gram-positive bacteria
• Gram-negative bacteria

[edit] References

[edit] External links

• Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology
• Cell membrane protein extraction protocol
• Membrane homeostasis, tension regulation, mechanosensitive membrane exchange and membrane traffic
• 3D structures of proteins associated with plasma membrane of eukaryotic cells