|This article relies largely or entirely upon a single source. (December 2013)|
A biological membrane or biomembrane is an enclosing or separating membrane that acts as a selectively permeable barrier within living things. Biological membranes, in the form of cell membranes, often consist of a phospholipid bilayer with embedded, integral and peripheral proteins used in communication and transportation of chemicals and ions. Bulk lipid in membrane provides a fluid matrix for proteins to rotate and laterally diffuse for physiological functioning. Proteins are adapted to high membrane fluidity environment of lipid bilayer with the presence of an annular lipid shell, consisting of lipid molecules bound tightly to surface of integral membrane proteins. The cellular membranes should not be confused with isolating tissues formed by layers of cells, such as mucous membranes and basement membranes.
Biological molecules are amphiphilic or amphipathic, or are simultaneously hydrophobic and hydrophilic.  The phospholipid bilayer contains charged hydrophilic headgroups, which interact with polar water. The lipids also contain hydrophobic tails, which meet with the hydrophobic tails of the complementary layer. The hydrophobic tails are usually fatty acids that differ in lengths.  The interactions of lipids, especially the hydrophobic tails, determine the lipid bilayer physical properties such as fluidity.
The phospholipid bilayer is formed due to formation of aggregate in aqueous solutions. Aggregating is caused by the hydrophobic effect, where hydrophobic ends are kept away from water and hydrophilic ends are in contact with it. This creates a favorable molecular arrangement by reducing unfavorable contact between hydrophobic tails and water and increasing hydrogen bonding between the hydrophilic heads and water. This aggregation of nonpolar tails increases the system entropy by reducing the surface area of the nonpolar tails and, thereby decreasing the interactions between the non polar tails and water. Less water is allowed to interact with the hydrophobic ends and, therefore, hydrogen bonding between hydrophilic heads and water is increased. The increase in hydrogen bonding increases the entropy of the system, creating a spontaneous process. Aggregation of non polar substances in water is, therefore, entropically driven and spontaneously occurring.  The aggregation formed due to the hydrophobic effect is partially responsible for the shape of biological membranes. 
Membranes in cells 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, chemicals that can be toxic to the cell, and the cell membrane separates a cell from its surrounding medium. Peroxisomes are one form of vacuole found in the cell that contain by-products of chemical reactions within the cell. 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 in doing so. Selective permeability is essential for effective separation of a cell or organelle from its surroundings. Biological membranes also have certain mechanical or elastic properties that allow them to change shape and move as required.
Particles that are required for cellular function but are unable to diffuse freely across a membrane enter through a membrane transport protein or are taken in by means of endocytosis, where the membrane allows for a vacuole to join onto it and push its contents into the cell. Many types of specialized plasma membranes can separate cell from external environment: apical, basolateral, presynaptic and postsynaptic ones, membranes of flagella, cilia, microvillus, filopodia and lamellipodia, the sarcolemma of muscle cells, as well as specialized myelin and dendritic spine membranes of neurons. Plasma membranes can also form different types of "supramembrane" structures such as caveola, postsynaptic density, podosome, invadopodium, desmosome, hemidesmosome, focal adhesion, and cell junctions. These types of membranes differ in lipid and protein composition.
Distinct types of membranes also create intracellular organelles: endosome; smooth and rough endoplasmic reticulum; sarcoplasmic reticulum; Golgi apparatus; lysosome; mitochondrion (inner and outer membranes); nucleus (inner and outer membranes); peroxisome; vacuole; cytoplasmic granules; cell vesicles (phagosome, autophagosome, clathrin-coated vesicles, COPI-coated and COPII-coated vesicles) and secretory vesicles (including synaptosome, acrosomes, melanosomes, and chromaffin granules). Different types of biological membranes have diverse lipid and protein compositions. The content of membranes defines their physical and biological properties. Some components of membranes play a key role in medicine, such as the efflux pumps that pump drugs out of a cell.
The inner leaflet of the phospholipid bilayer is constantly in motion because of rotations around the bonds of lipid tails. Hydrophobic tails of a bilayer bend and lock together. However, because of hydrogen bonding with water, the hydrophilic head groups exhibit less movement as their rotation and mobility are constrained. This results in increasing viscosity of the lipid bilayer closer to the hydrophilic heads. 
Below a transition temperature, a lipid bilayer loses fluidity when the highly mobile lipids exhibits less movement. The lipid bilayer under this certain temperature becomes a gel-like solid that is thicker than at the state above the temperature. The transition temperature depends on such components of the lipid bilayer as the hydrocarbon chain length and the saturation of its fatty acids. Temperature-dependence fluidity constitutes an important physiological attribute for bacteria and cold-blooded organisms. These organisms maintain a constant fluidity by modifying membrane lipid fatty acid composition in accordance with differing temperatures. 
The components of bilayers are distributed unequally between the two surfaces to create asymmetry between the outer and inner surfaces of the phospholipid bilayer. The asymmetric nature between the leaflets of membranes was determined by using phospholipase, which does not pass through membranes and, thus, only rests on the outer leaflet of cells. This asymmetric organization is important for cell functions such as cell signaling.  The asymmetry of the biological membrane reflects the different functions of the two leaflets of the membrane.
Phospholipid bilayers contain different proteins. These membrane proteins have various functions and characteristics and catalyze different chemical reactions. Integral proteins hold strong association with the lipid bilayer and cannot easily become detached. They will dissociate only with chemical treatment that breaks the membrane. Integral proteins are often associated with the inner membrane of the bilayer. Peripheral proteins are unlike integral proteins in that they hold weak interactions with the surface of the bilayer and can easily become dissociated from the membrane. Membrane proteins are located on only one face of a membrane and create membrane asymmetry.
Membranes contain sugar-containing lipid molecules known as glycolipids. In the bilayer, the sugar groups of glycolipids are exposed at the cell surface, where they can form hydrogen bonds. Glycolipids provide the most extreme example of asymmetry in the lipid bilayer.  Glycolipids perform a vast number of functions in the biological membrane, including cell recognition and cell-cell adhesion.
- Membrane lipids
- Membrane protein
- Annular lipid shell
- Lipid bilayer
- Membrane fluidity
- Membrane biology
- Membrane models
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- Nickels, Jonathan D.; Smith, Jeremy C.; Cheng, Xiaolin. "Lateral organization, bilayer asymmetry, and inter-leaflet coupling of biological membranes". Chemistry and Physics of Lipids. doi:10.1016/j.chemphyslip.2015.07.012.