Biological membrane

A biological membrane or biomembrane is a membrane which acts as a barrier within or around a cell. It is, almost invariably, a lipid bilayer (except for Archaea which have isoprene membranes), composed of a double layer of lipid-class molecules, specifically phospholipids, with occasional proteins intertwined, some of which function as channels.

Such membranes typically define enclosed spaces or compartments in which cells may maintain a chemical or biochemical environment that differs from the outside. For example, the membrane around peroxisomes shields the rest of the cell from peroxides, and the plasma membrane separates a cell from its surrounding medium. Most organelles are defined by such membranes, and are called membrane-bound organelles.

Probably the most important feature of a biomembrane is that it is a selectively permeable structure. This means that the size, charge and other chemical properties of the atoms and molecules attempting to cross it will determine whether they succeed to do so. Selective permeability is essential for effective separation of a cell or organelle from its surroundings.

If a particle is too large or otherwise unable to cross the membrane by itself, but is still needed by a cell, it could either go through one of the protein channels, or be taken in by means of endocytosis.

Types of biological membranes

 * Cell membrane
 * Mucous membrane
 * S-layer

Composition
The three major classes of membrane lipids are phospholipids, glycolipids, and cholesterol.



Phospholipids and glycolipids consist of two long, nonpolar (hydrophobic) hydrocarbon chains linked to a hydrophilic head group. In the phospholipids the head consist of phosphorylated either:
 * Glycerol (and hence the name phosphoglycerides given to this group of lipids).
 * Sphingosine (with only one member - sphingomyelin).

In the glycolipids the head contains of sphingosine with one or several sugar units attached to it. The hydrophobic chains belong either to:
 * two FAs - in the case of the phosphoglycerides.
 * one FA and the hydrocarbon tail of sphingosine - in the case of sphingomyelin and the glycolipids.

The FAs in phospho- and glycolipids usually contain an even number of carbon atoms, typically between 14 and 24. The 16- and 18-carbon FAs are the most common ones. FAs may be saturated or unsaturated, with the configuration of the double bonds nearly always cis. The length and the degree of unsaturation of FAs chains have a profound effect on membranes fluidity.

In phosphoglycerides, the hydroxyl groups at C-1 and C-2 of glycerol are esterified to the carboxyl groups of the FAs. The C-3 hydroxyl group is esterified to phosphoric acid. The resulting compound, called phosphatidate, is the simplest phosphoglycerate. Only small amounts of phosphatidate are present in membranes. However, it is a key intermediate in the biosynthesis of the other phosphoglycerides.

Sphingosine is an amino alcohol that contains long, unsaturates hydrocarbon chain. In sphingomyelin and glycolipids, the amino group of sphingosine is linked to a FAs by an amid bond. In sphingomyelin the primary hydroxyl group of sphingosine is esterified to phosphoryl choline. In glycolipids, the sugar component is attached to this group. The simplest glycolipid is cerebroside, in which there is only one sugar residue, either Glc or Gal. More complex glycolipids, such as gangliosides, contain a branched chain of as many as seven sugar residues.