thumb|185px|Phospholipidthumb|185px|Polar group of the molecule, highlighted in red.
The U indicates the uncharged hydrophobic portion of the molecule, highlighted in blue.thumb|185px|[[Phosphatidyl choline is the major component of
lecithin. It is also a source for
choline in the synthesis of acetylcholine in cholinergic neurons.]]
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.
Amphipathic character
The 'head' of a phospholipid is
hydrophilic (attracted to
water), while the
hydrophobic 'tails' repel water. 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 with hydrophilic heads on both sides facing the water.
This type of membrane is partially permeable, capable of elastic movement, and has
fluid properties, in which embedded proteins (integral or peripheral proteins) and phospholipid molecules are able to move laterally. 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 However, this model has now been superseded, as through the study of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.
Types of phospholipid
Diacylglyceride structures
See:
GlycerophospholipidPhosphatidic acid (phosphatidate)
Phosphatidylethanolamine (cephalin)
Phosphatidylcholine (lecithin)
PhosphatidylserinePhosphoinositides:
Phosphatidylinositol,
phosphatidylinositol phosphate,
phosphatidylinositol bisphosphate and
phosphatidylinositol triphosphate.
Other structures
Sphingomyelin Simulations
Modeling of phospholipids is usually made by the method of
molecular dynamics in
force fields such as
GROMACS,
CHARMM and their modifications.
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 (PIP
2), that can be split into
inositol triphosphate (IP
3) and
diacylglycerol (DAG), which both carry out the
functions of the Gq type of G protein in response to various stimuli.
Food technology
Phospholipids can also act as an emulsifiers, 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.
See also
Lipids
