Navigation Links
Lipid


Figure 1: Structure of a Lipid. Many lipids consists of a polar head group (P) and a nonpolar tail (U for unpolar). The lipid shown is a phospholipid (two tails). The image on the left is a zoomed version of the more schematic image on the right, which will be used from now on to represent lipids with one, two, or three chains.

The term lipid comprises a diverse range of molecules and to some extent is a catch-all for relatively water-insoluble or nonpolar compounds of biological origin, including waxes, fatty acids, fatty-acid derived phospholipids, sphingolipids, glycolipids and terpenoids, such as retinoids and steroids. Some lipids are linear aliphatic molecules, while others have ring structures. Some are aromatic, while others are not. Some are flexible, while others are rigid.

Most lipids have some polar character in addition to being largely nonpolar. Generally, the bulk of their structure is nonpolar or hydrophobic ("water-fearing"), meaning that it does not interact well with polar solvents like water. Another part of their structure is polar or hydrophilic ("water-loving") and will tend to associate with polar solvents like water. This makes them amphiphilic molecules (having both hydrophobic and hydrophilic portions). In the case of cholesterol, the polar group is a mere -OH (hydroxyl or alcohol). In the case of phospholipids, the polar groups are considerably larger and more polar, as described below.

Phospholipids or, more precisely, glycerophospholipids , are built on a glycerol core to which are linked two fatty acid-derived "tails" by ester linkages and one "head" group by a phosphate ester linkage. Fatty acids are unbranched hydrocarbon chains, connected by single bonds alone (saturated fatty acids) or by both single and double bonds (unsaturated fatty acids). The chains are usually 14-24 carbon groups long. The head groups of the phospholipids found in biological_membranes are phosphatidylcholine (lecithin), phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol, whose head group can be modified by the addition of one to three more phosphate groups. While phospholipids are the major component of biological membranes, other lipid components like sphingolipids and sterols (such as cholesterol in animal cell membranes) are also found in biological membranes.

In an aqueous milieu, the heads of lipids tend to face the polar, aqueous environment, while the hydrophobic tails tend to minimize their contact with water. The nonpolar tails of lipids (U) tend to cluster together, forming a lipid bilayer (1) or a micelle (2). The polar heads (P) face the aqueous environment. Micelles form when single-tailed amphiphilic lipids are placed in a polar milieu, while lipid bilayers form when two-tailed phospholipids are placed in a polar environment (Fig. 2). Micelles are "monolayer" spheres and can only reach a certain size, whereas bilayers can be considerably larger. They can also form tubules. Bilayers that fold back upon themselves form a hollow sphere, enclosing an a separate aqueous compartment, which is essentially the basis of cellular membranes.

Micelles and bilayers separate out from the polar milieu by a process known as the "hydrophobic effect." When dissolving a nonpolar substance in a polar environment, the polar molecules (i.e. water in an aqueous solution) become more ordered around the dissolved nonpolar substance, since the polar molecules cannot form hydrogen bonds to the nonpolar molecule. Therefore, in an aqueous environment, the polar water molecules form an ordered "clathrate" cage around the dissolved nonpolar molecule. However, when the nonpolar molecules separate out from the polar liquid, the entropy (state of disorder) of the polar molecules in the liquid increases. This is essentially a form of phase separation, similar to the spontaneous separation of oil and water into two separate phases when one puts them together.

Figure 2: Self-organization of lipids. A lipid bilayer is shown on the left and a micelle on the right.

The self-organisation depends on the concentration of the lipid present in solution. Below the critical micelle concentration the lipids form a single layer on the liquid surface and are dispersed in solution. At the first critical micelle concentration (CMC-I), the lipids organise in spherical micelles, at the second critical micelle concentration (CMC-II) into elongated pipes, and at the lamellar point (LM or CMC-III) into stacked lamellae of pipes. The CMC depends on the chemical composition, mainly on the ratio of the head area and the tail length.

Lipid bilayers form the foundation of all biological membranes.

See also

External links


'"/>


(Date:8/28/2014)... method for delivering compounds that could positively ... AIDS may be possible, thanks to researchers ... A semi-soft vaginal suppository made from the ... the antiviral drug Tenofovir provides a woman-initiated, ... spread of sexually transmitted infections during unprotected ...
(Date:8/28/2014)... than 25 million kilometres of new roads will be ... slice into Earth,s last wildernesses, where they bring an ... , Now, an ambitious study has created a ,global ... try to balance the competing demands of development and ... ,environmental-values, layer that estimates that natural importance of ecosystems ...
(Date:8/28/2014)... areas conserve biodiversity and more action is needed to ... researchers say. , Published in PLOS ONE , ... of Exeter, used meta-analysis - combining results from different ... of research into these areas, to determine whether they ... of Biological Sciences, said protecting an area from human ...
Breaking Biology News(10 mins):Sensory-tested drug-delivery vehicle could limit spread of HIV, AIDS 2Sensory-tested drug-delivery vehicle could limit spread of HIV, AIDS 3Study shows where on the planet new roads should and should not go 2Study shows where on the planet new roads should and should not go 3Protected areas proven to protect biodiversity 2
... you are a rice breeder and one day within a ... characteristics you have been looking for. You happily take your ... the spontaneous, beneficial event was due to inactivation of a ... many different strains grown in different parts of the world, ...
... biometric products and expanded markets, JERICHO, N.Y., ... and Oki Semiconductor Company, a division of Oki,America ... Industry Co, Ltd.,jointly announced that they are working ... significantly lowering bioMETRX,s overall build costs for,its expanding ...
... The song of passerine birds is a conspicuous and ... of male-male competition or mate attraction. At the level ... male quality. Paola Laiolo and colleagues at ... system of the Duponts lark in north-eastern Spain and ...
Cached Biology News:Less can be more, for plant breeders too 2Less can be more, for plant breeders too 3bioMETRX Incorporates OKI's Fingerprint Engine in its smartTOUCH Product Line 2bioMETRX Incorporates OKI's Fingerprint Engine in its smartTOUCH Product Line 3
Other biology definitionOther Tags