Phospholipid arrangement in cell membranes.Phosphatidylcholine is the major component oflecithin. It is also a source forcholine in the synthesis of acetylcholine in cholinergic neurons.
Phospholipids are a key component of allcell membranes. They can formlipid bilayers because of theiramphiphilic characteristic. Ineukaryotes, cell membranes also contain another class of lipid,sterol, interspersed among the phospholipids. The combination provides fluidity in two dimensions combined with mechanical strength against rupture. Purified phospholipids are produced commercially and have found applications innanotechnology andmaterials science.[3]
The phospholipids areamphiphilic. The hydrophilic end usually contains a negatively charged phosphate group, and the hydrophobic end usually consists of two "tails" that are longfatty acid residues.[4]
In aqueous solutions, phospholipids are driven byhydrophobic interactions, which result in the fatty acid tails aggregating to minimize interactions with the water molecules. The result is often aphospholipid bilayer: a membrane that consists of two layers of oppositely oriented phospholipid molecules, with their heads exposed to the liquid on both sides, and with the tails directed into the membrane. That is the dominant structural motif of the membranes of allcells and of some other biological structures, such asvesicles orvirus coatings.[5][6]
Phospholipid bilayers are the main structural component of the cell membranes.
In biological membranes, the phospholipids often occur with other molecules (e.g.,proteins,glycolipids,sterols) in abilayer such as acell membrane.[7] Lipid bilayers occur when hydrophobic tails line up against one another, forming a membrane of hydrophilic heads on both sides facing the water.[8]
These specific properties allow phospholipids to play an important role in the cell membrane. Their movement can be described by thefluid 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.Sterols contribute to membrane fluidity by hindering the packing together of phospholipids. However, this model has now been superseded, as through the study oflipid polymorphism it is now known that the behaviour of lipids under physiological (and other) conditions is not simple.[citation needed]
Phospholipids have been widely used to prepare liposomal, ethosomal and other nanoformulations of topical, oral andparenteral drugs for differing reasons like improved bio-availability, reduced toxicity and increased permeability across membranes. Liposomes[9] are often composed ofphosphatidylcholine-enriched phospholipids and may also contain mixed phospholipid chains withsurfactant properties. The ethosomal formulation ofketoconazole using phospholipids is a promising option for transdermal delivery in fungal infections.[10] Advances in phospholipid research lead to exploring these biomolecules and their conformations using lipidomics[11].[citation needed]
Phospholipids are optically highlybirefringent, i.e. their refractive index is different along their axis as opposed to perpendicular to it. Measurement ofbirefringence can be achieved using cross polarisers in a microscope to obtain an image of e.g.vesicle walls or using techniques such asdual polarisation interferometry to quantify lipid order or disruption in supported bilayers.[citation needed]
There are no simple methods available for analysis of phospholipids, since the close range of polarity between different phospholipid species makes detection difficult. Oil chemists often use spectroscopy to determine total phosphorus abundance and then calculate approximate mass of phospholipids based on molecular weight of expected fatty acid species. Modern lipid profiling employs more absolute methods of analysis, withNMR spectroscopy, particularly31P-NMR,[13][14] whileHPLC-ELSD[15] provides relative values.
Phospholipid synthesis occurs in the cytosolic side ofER membrane[16] that is studded with proteins that act in synthesis (GPAT and LPAAT acyl transferases, phosphatase and choline phosphotransferase) and allocation (flippase and floppase). Eventually a vesicle will bud off from the ER containing phospholipids destined for the cytoplasmic cellular membrane on its exterior leaflet and phospholipids destined for the exoplasmic cellular membrane on its inner leaflet.[17][18]
Common sources of industrially produced phospholipids are soya, rapeseed, sunflower, chicken eggs, bovine milk, fish eggs etc. Phospholipids for gene delivery, such asdistearoylphosphatidylcholine anddioleoyl-3-trimethylammonium propane, are produced synthetically.[citation needed] Each source has a unique profile of individual phospholipid species, as well as fatty acids, and consequently differing applications in food, nutrition, pharmaceuticals, cosmetics, and drug delivery.[citation needed]
Phospholipids also intervene inprostaglandin signal pathways as the raw material used bylipase enzymes to produce the prostaglandin precursors. In plants they serve as the raw material to producejasmonic acid, aplant hormone similar in structure to prostaglandins that mediates defensive responses against pathogens.[citation needed]
Phospholipids can act asemulsifiers, enabling oils to form acolloid with water. Phospholipids are one of the components oflecithin, which is found in egg yolks, as well as being extracted fromsoybeans, and is used as afood additive in many products and can be purchased as adietary supplement. Lysolecithins are typically used for water–oil emulsions likemargarine, due to their higherHLB ratio.[citation needed]
