Phospholipid



Phospholipids are a class of lipids formed from four components: fatty acids, a negatively-charged phosphate group, nitrogen containing alcohol and a backbone. Phospholipids with a glycerol backbone are known as glycerophospholipids or phosphoglycerides. There is only one type of phospholipid with a sphingosine backbone; sphingomyelin. Phospholipids are a major component of all biological membranes, along with glycolipids and cholesterol.

Phosphoglycerides
In phosphoglycerides, the carboxyl group of each fatty acid is esterified to the hydroxyl groups on carbon-1 and carbon-2 of the glycerol molecule. The phosphate group is attached to carbon-3 by an ester link. This molecule, known as a phosphatidate, is present in small quantities in membranes, but is also a precursor for the other phosphoglycerides.

Phosphatidyl choline
Phosphatidyl choline is the major component of lecithin. It is also a source for choline in the synthesis of acetylcholine in cholinergic neurons.

Phosphatidyl ethanolamine
Phosphatidyl ethanolamine is the major component of cephalin.

Synthesis
In phosphoglyceride synthesis, phosphatidates must be activated first. Phospholipids can be formed from an activated diacylglycerol or an activated alcohol. Phosphatidyl serine and phosphatidyl inositol are formed from a phosphoester linkage between the hydroxyl of an alcohol (serine or inositol) and cytidine diphosphodiacylglycerol (CDP-diacylglycerol).

In 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 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 serine. The amino terminal is acylated with a by a long-chain acyl CoA to yield 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 nonpolar tails are hydrophobic (not attracted to water). When placed in water, phospholipids form a bilayer, where the hydrophobic tails line up against each other, forming a membrane with hydrophilic heads on both sides extending out into 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, very flexible, and has fluid properties, in which embedded proteins and phospholipid molecules are constantly moving laterally across the membrane because of the forces generated by their vibrations. Such movement can be described by the Fluid Mosaic Model, which describes the membrane as a "mosaic" of lipid molecules that act as a solvent for all the substances and proteins within it, so proteins and lipid molecules are then free to diffuse laterally through the lipid matrix and migrate over the membrane.