Biological membrane

This article is about various membranes in living things. For the membranes surrounding cells, see cell membrane.

Cross section view of the structures that can be formed by phospholipids in aqueous solutions

A biological membrane or biomembrane is an enclosing or separating membrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content.^[1] The cellular membranes should not be confused with isolating tissues formed by layers of cells, such as mucous and basement membranes.

Function

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, and the cell 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 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.

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.

Diversity of biological membranes

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.

See also

• Bladder (disambiguation)
• S-layer
• Membrane lipids
• Biofilm
• Membrane protein
• Osmosis
• Mucous membrane
• Fetal membranes of amnion and chorion^[2]
• Basement membrane
• Serous membrane
• Tympanic membrane

References

1. ^ Mark L. Latash (2007). Neurophysiological basis of movement. Human Kinetics Publishers. ISBN 978-0-7360-6367-8. 
2. ^ UpToDate Patient Preview > Fetal membranes: Anatomy and biochemistry Author: Seth Guller, PhD. Retrieved on Mars 7, 2010

• von Heijne G, Rees D (August 2008). "Membranes: reading between the lines". Curr. Opin. Struct. Biol. 18 (4): 403–5. doi:10.1016/j.sbi.2008.06.003. PMID 18634876. http://linkinghub.elsevier.com/retrieve/pii/S0959-440X(08)00091-2. 

External links

• MeSH Membranes