Cell membrane

The cell membrane, also called the plasma membrane or plasmalemma, is a selectively permeable lipid bilayer which comprises the outer layer of a cell. The cell membrane works between the machinery on the inside of the cell and the outside fluid that bathes the cells. The plasma membrane lets molecules and ions into the cell such as glucose, amino acids, and lipids. Water is another important molecule that is diffused across the membrane. The process of diffusion of water is called osmosis. It is a semi-fluid patchwork of molecules including proteins and phospholipids, some of which are constantly diffusing through the membrane, that assume an asymmetric fluid mosaic structure and convey mobility to the membrane. It controls 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. The larger the cell, the faster its output rate.

Functions

In animal cells the cell 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 mostly a mechanical support role rather than a role as a selective boundary. One of the key roles of the membrane is to maintain the cell potential. The functions of the cell membrane include, but are not limited to:

• Sorting what goes in and out of the cell.
• Anchoring of the cytoskeleton to provide shape to the cell
• Attaching to the extracellular matrix to help group cells together in the formation of tissues
• Transportation of particles by way of ion pumps, ion channels, and carrier proteins
• Containing receptors that allow chemical messages to pass between cells and systems
• Participation in enzyme activity important in such things as metabolism and immunity

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.

Structure

The cell membrane and the membranes surrounding inner cell organelles are phospholipid bilayers about 10 nm thick discernable only faintly with a transmission electron microscope.

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 polar phosphate group and the two tails are the two highly hydrophobic non-polar fatty acid chains of the phospholipid. The plasma membrane consists of 1/3 cholesterol and 2/3 phospholipids (65-80%) and sphingolipids (20-35%). The outer leaflet contains 5% glycolipids.

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.

The cell membrane contains a large amount of protein. Individual proteins may be adjacent to the membrane or may be embedded within it; such proteins are called peripheral and integral membrane proteins, respectively. Many integral 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.

See also

• Bacterial cell structure
• Gram-positive bacteria
• Gram-negative bacteria
• Elasticity of cell membranes

References

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