Phospholipid

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Phospholipid

Polar group of the molecule, highlighted in red.
The U indicates the uncharged hydrophobic portion of the molecule, highlighted in blue.

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. 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. They are a type of molecule. They form a lipid bilayer within a cell membrane.

[edit] Amphipathic character

The 'head' of a phospholipid is hydrophilic (attracted to water) whereas 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.^[1] Lipid bilayers occur when hydrophobic tails line up against one another, forming a membrane with hydrophilic heads on both sides facing the water.

This 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 However, this model has now been superseded, as through the st of lipid polymorphism it is now known that the behaviour of lipids under physiological (and other) conditions is not simple.

Also, the hydrophilic and hydrophobic ends allow the phospholipids to transfer and be associated with water. It is able to be resistant and associate with it at the same time. This happens because only one end of the molecule is soluble in water. Thus phospholipids act as an emulsifier enabling oils to disolve freely 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.

[edit] 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.^[2]

[edit] 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 into inositol triphosphate (IP₃) and diacylglycerol (DAG), which both carry out the functions of the G_q type of G protein in response to various stimuli.

this is known as the second messenger system where the G-protein(guanosine nucleo tide binding proteins)activate adenosine monophosphate phosphatase cycle (c-amp),to carry out intercellular functions.

[edit] References

1. ^ Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0-13-250882-6. http://www.phschool.com/el_marketing.html. 
2. ^ Lodish, Harvey; Berk, Krieger, Kaiser, Scott, Bretsher, Ploegh, Matsuaira (2008). Molecular Cell Biology. W.H. Freeman and Company. ISBN 0716776014. http://bcs.whfreeman.com/lodish6e/default.asp?s=&n=&i=&v=&o=&ns=0&uid=0&rau=0. 

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