Phospholipid









Phospholipids are a class of lipids and are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from sphingosine instead of glycerol. The first phospholipid identified as such in biological tissues was lecithin, or phosphatidylcholine, in the egg yolk, by Theodore Nicolas Gobley, a French chemist and pharmacist, in 1847. The structure of the a phospholipid molecule consists of hydrophobic tails and hydrophilic heads, it also consists of cholesterol molecules which are found in-between the spaces of the phospholipid.

Amphipathic character
The 'head' is hydrophilic (attracted to water), while the hydrophobic 'tails' are repelled by water and are forced to aggregate. The hydrophillic head contains the negatively charged phosphate group, and may contain other polar groups. The hydrophobic tail usually consists of long fatty acid hydrocarbon chains. When placed in water, phospholipids form a variety of structures depending on the specific properties of the phospholipid. These specific properties allow phospholipids to play an important role in the phospholipid bilayer. In biological systems, the phospholipids often occur with other molecules (e.g., proteins, glycolipids, cholesterol) in a bilayer such as a cell membrane. Lipid bilayers occur when hydrophobic tails line up against one another, forming a membrane hydrophilic heads on both sides facing the water.

Such movement can be described by the Fluid Mosaic Model, that 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. Cholesterol contributes to membrane fluidity by hindering the packing together of phospholipids. However, this model has now been superseded, as through the study of lipid polymorphism it is now known that the behaviour of lipids under physiological (and other) conditions is not simple.

Diacylglyceride structures

 * See: Glycerophospholipid


 * Phosphatidic acid (phosphatidate) (PA)
 * Phosphatidylethanolamine (cephalin) (PE)
 * Phosphatidylcholine (lecithin) (PC)
 * Phosphatidylserine (PS)
 * Phosphoinositides:
 * Phosphatidylinositol (PI)
 * Phosphatidylinositol phosphate (PIP)
 * Phosphatidylinositol bisphosphate (PIP2) and
 * Phosphatidylinositol triphosphate (PIP3).

Phosphosphingolipids

 * Ceramide phosphorylcholine (Sphingomyelin) (SPH)
 * Ceramide phosphorylethanolamine (Sphingomyelin) (Cer-PE)
 * Ceramide phosphorylglycerol

Simulations
Computational simulations of phospholipids are often performed using molecular dynamics with force fields such as GROMOS, CHARMM, or AMBER.

Characterization
Phospholipids are optically highly birefringent, i.e. their refractive index is different along their axis as opposed to perpendicular to it. Measurement of birefringence can be achieved using cross polarisers in a microscope to obtain an image of e.g. vesicle walls or using techniques such as dual polarisation interferometry to quantify lipid order or disruption in supported bilayers.

Phospholipid synthesis
Phospholipid synthesis occurs in the cytosol adjacent to ER membrane 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.

In signal transduction
Some types of phospholipid can be split to produce products that function as second messengers in signal transduction. Examples include phosphatidylinositol (4,5)-bisphosphate (PIP2), that can be split by the enzyme Phospholipase C into inositol triphosphate (IP3) and diacylglycerol (DAG), which both carry out the functions of the Gq type of G protein in response to various stimuli and intervene in various processes from long term depression in neurons to leukocyte signal pathways started by chemokine receptors.

Phospholipids also intervene in prostaglandin signal pathways as the raw material used by lipase enzymes to produce the prostaglandin precursors. In plants they serve as the raw material to produce Jasmonic acid, a plant hormone similar in structure to prostaglandins that mediates defensive responses against pathogens.

Food technology
Phospholipids can also act as an emulsifier, enabling oils to dissolve in water. Phospholipids called lecithin are extracted out of cooking oil and then used as food additives in many things such as bread and can also be purchased separately in a health food store.

Phospholipid derivatives

 * See table below for an extensive list.


 * Natural phospholipid derivates:
 * egg PC, egg PG, soy PC, hydrogenated soy PC, sphingomyelin as natural phospholipids.
 * Synthetic phospholipid derivates:
 * Phosphatidic acid (DMPA, DPPA, DSPA)
 * Phosphatidylcholine (DDPC, DLPC, DMPC, DPPC, DSPC, DOPC, POPC, DEPC)
 * Phosphatidylglycerol (DMPG, DPPG, DSPG, POPG)
 * Phosphatidylethanolamine (DMPE, DPPE, DSPE DOPE)
 * Phosphatidylserine (DOPS)
 * PEG phospholipid (mPEG-phospholipid, polyglycerin-phospholipid, funcitionalized-phospholipid, terminal activated-phospholipid)