Phospholipid

Components

They are built upon to a ethanolamine or an organic compound such as choline. The "head" with hydrophilic built, adopts a lipophilic characteristic in the polar phospholipid area. The "tails" however are non-polar and they adopt a characteristic of hydrophobic- (being repelled by water). In its simplest form, a phospholipid is composed of one glycerol bonded to two fatty acids and one phosphate group.

Types

Phosphoglycerides

In cell membranes, phosphoglycerides are the more common of the two phospholipids. In phosphoglycerides, the phosphatidate, is present in small quantities in membranes, but is also a precursor for the other phosphoglycerides.

In phosphoglyceride synthesis, diacylglycerol or an activated alcohol.

• Phosphatidyl serine and phosphatidyl inositol are formed from a inositol) and cytidine diphosphodiacylglycerol (CDP-diacylglycerol).

• In animals, plants and yeast the synthesis of phospatidyl ethanolamine, the alcohol is phosphorylated by ATP first, and subsequently reacts with cytidine triphosphate (CTP) to form the activated alcohol (CDP-ethanolamine). The alcohol then reacts with a diacylglycerol to form the final product. In bacteria, the serine moiety of phosphatidyl serine is decarboxylated to give phospatidyl ethanolamine.

• In mammals, phosphatidyl choline can be synthesized via two separate pathways; a series of reactions similar to phosphatidyl ethanolamine synthesis, and the methylation of phosphatidyl ethanolamine, which is catalyzed by phosphatidyl ethanolamine methyltransferase, an enzyme produced in the liver.

 

Sphingomyelin

The backbone of sphingomyelin is sphingosine, an amino alcohol formed from palmitate and ceramide. Subsequent substitution of the terminal hydroxyl group by phosphatidyl choline forms sphingomyelin.

Sphingomyelin is also present in all eukaryotic cell membranes, especially the plasma membrane, and is particularly concentrated in the nervous system because sphingomyelin is a major component of myelin, the fatty insulation wrapped around nerve cells by Schwann cells or oligodendrocytes. Multiple sclerosis is a disease characterised by deterioration of the myelin sheath, leading to impairment of nervous conduction.

Amphipathic character

Due to its polar nature, the head of a phospholipid is hydrophilic (attracted to water); the bilayers, in which the lipophilic tails line up against one another, forming a membrane with hydrophilic heads on both sides facing the water. This allows it to form liposomes spontaneously, or small lipid vesicles, which can then be used to transport materials into living organisms and study diffusion rates into or out of a cell membrane.

This membrane is partially permeable, capable of elastic movement, and has lipid polymorphism it is now known that the behaviour of lipids under physiological (and other) conditions is not simple.

See also

• Alkylphosphocholines
• Antiphospholipid syndrome
• Biochemistry
• Lipid
• Lipid bilayer (e.g., Cell membrane)
• Polymorphism (biophysics)

References

1. J.M.Berg, J.L. Tymoczko, and L. Stryer, Biochemistry. 5th ed. 2002, New York: W.H. Freeman. xxxviii, 974, [976] (various pages)

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Lipid bilayer - Phospholipids - Proteolipids - Sphingolipids - Sterols

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