Phospholipids





Phospholipids 3340
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Phospholipids are an important class of biomolecules. Phospholipids are the fundamental building blocks of cellular membranes and are the major part of surfactant , the film that occupies the air/liquid interfaces in the lung. These molecules consist of a polar or charged head group and a pair of nonpolar fatty acid tails, connected via a glycerol linkage. This combination of polar and nonpolar segments is termed amphiphilic, and the word describes the tendency of these molecules to assemble at interfaces between polar and nonpolar phases.

The structure of the most common class of phospholipids, phosphoglycerides, is based on glycerol, a three-carbon alcohol with the formula CH 2 OH–CHOH–CH 2 OH. Two fatty acid chains, each typically having an even number of carbon atoms between 14 and 20, attach (via a dual esterification ) to the first and second carbons of the glycerol molecule, denoted as the sn1 and sn2 positions, respectively. The third hydroxyl group of glycerol, at position sn3, reacts with phosphoric acid to form phosphatidate. Common phospholipids, widely distributed in nature, are produced by further reaction of the phosphate group in phosphatidate with an alcohol, such as serine, ethanolamine, choline, glyercol, or inositol. The resulting lipids may be charged, for example, phosphatidyl serine (PS), phosphatidyl inositol (PI), and phosphatidyl glyercol (PG); or dipolar (having separate positively and negatively charged regions), for example, phosphatidyl choline (PC), and phosphatidyl ethanolamine (PE). The term "lecithin" refers to PC-type lipids. A typical phospholipid arrangement is the presence of a saturated fatty acid, such as palmitic or stearic acid, at the sn1 position, and an unsaturated or polyunsaturated fatty acid, such as oleic or arachodonic acid, at sn2 (see Figure 1 for the structure of a phosphoglyceride).

Another class of phospholipids is the sphingolipids. A sphingolipid molecule has the phosphatidyl-based headgroup structure described above, but (in contrast to a common phospholipid molecule) contains a single fatty acid

Figure 1. The structures of two phospholipids. Structure A represents a classic glycerophospholipid, POPC, and it is composed of choline, phosphate, glycerol, and two fatty acids. Structure B is an example of a sphingomyelin, and it is composed of choline, phosphate, sphingosine, and only one fatty acid.
Figure 1. The structures of two phospholipids. Structure A represents a classic glycerophospholipid, POPC, and it is composed of choline, phosphate, glycerol, and two fatty acids. Structure B is an example of a sphingomyelin, and it is composed of choline, phosphate, sphingosine, and only one fatty acid.

and a long-chain alcohol as its hydrophobic components. Additionally, the backbone of the sphingolipid is sphingosine, an amino alcohol (rather than glyercol). The structure of a representative sphingolipid, sphingomyelin, is also shown in Figure 1. Sphingolipids, occurring primarily in nervous tissue, are thought to form cholesterol-rich domains within lipid bilayer membranes that may be important to the functions of some membrane proteins.

Phospholipids have many functions in biological systems: as fuels, as membrane structural elements, as signaling agents, and as surfactants. For example, pulmonary surfactant is a mixture of lipids (primarily dipalmitoyl phosphatidyl choline [DPPC]) and proteins that controls the surface tension of the fluid lining of the inner lung (the site of gas exchange), allowing rapid expansion and compression of this lining during the breathing cycle. Phospholipids are the major lipid constituent in cell membranes, thus maintaining structural integrity between the cell and its environment and providing boundaries between compartments within the cell.

SEE ALSO Lipids ; Membrane ; Triglycerides .

Scott E. Feller

Ann T. S. Taylor

Bibliography

Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert (2002). Biochemistry, 5th edition. New York: W. H. Freeman.

Voet, Donald; Voet, Judith G.; and Pratt, Charlotte (1999). Fundamentals of Biochemistry. New York: Wiley.



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Aug 19, 2010 @ 1:01 am
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.[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 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.
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Sep 4, 2011 @ 10:10 am
STRUCTURE DETAILS OF PHOSPHOLIPIDS?
STRUCTURE DETAILS OF SPHINGOLIPIDS?
felix murumbutsa
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Mar 31, 2012 @ 8:08 am
please send me detailed properties of the various types of lipid molecules, e.g Phospholipid, Sphingolipid, Steroid, and Triglycerols . thanks in advance
shaheen
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Aug 31, 2012 @ 10:10 am
Phospholipids are unique properties in cell membrane because it not allow many macromolecules to enter or exit. It is boundary of cell and so packed and it allow and embedded some proteins which is fixed, on it for specific function.
Beside function in cell membrane, it act as surfactant in lungs and as important secondary messenger in cell signaling phenomena such as phosphoinsitol bi phosphate.
What you thing about its benefits as cosmetics, increased memory and role in bile (cholesterol & fat emulsification).? Shaheen P Khan
kerreen
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Oct 9, 2012 @ 4:16 pm
i need to know what the structure of a glycolipid looks like and what role does it play in cells,the same for phospholipids and triglycerides.

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