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 CH2OH–CHOH–CH2OH. 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
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
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.
Phospholipids are complex lipids made up of fatty acids, alcohols, and phosphate. They are extremely important components of living cells, with both structural and metabolic roles. They are the chief constituents of most biological membranes.
At one end of a phospholipid molecule is a phosphate group linked to an alcohol. This is a polar part of the molecule—it has an electric charge and is water-soluble (hydrophilic). At the other end of the molecule are fatty acids, which are non-polar, hydrophobic , fat soluble, and water insoluble.
Because of the dual nature of the phospholipid molecules, with a water-soluble group attached to a water-insoluble group in the same molecule, they are called amphipathic or polar lipids. The amphipathic nature of phospholipids make them ideal components of biological membranes, where they form a lipid bilayer with the polar region of each layer facing out to interact with water, and the non-polar fatty acid "tail" portions pointing inward toward each other in the interior of the bilayer. The lipid bilayer structure of cell membranes makes them nearly impermeable to polar molecules such as ions, but proteins embedded in the membrane are able to carry many substances through that they could not otherwise pass.
Phosphoglycerides, considered by some as synonymous for phospholipids, are structurally related to 3-phosphoglyceraldehyde (PGA), an intermediate in the catabolic metabolism of glucose. Phosphoglycerides differ from phospholipids because they contain an alcohol rather than an aldehyde group on the 1-carbon. Fatty acids are attached by an ester linkage to one or both of the free hydroxyl (-OH) groups of the glyceride on carbons 1 and 2. Except in phosphatidic acid, the simplest of all phosphoglycerides, the phosphate attached to the 3-carbon of the glyceride is also linked to another alcohol. The nature of this alcohol varies considerably.
See also Bacteremic; Bacterial growth and division; Bacterial membranes and cell wall; Bacterial surface layers; Bacterial ultrastructure; Biochemistry; Cell membrane transport; Membrane fluidity
Cell membranes are a double layer of phospholipids with the fatty acid side‐chains on the inside and the water‐soluble compound esterified to the phosphate interacts with water. This is why phospholipids can be used to emulsify oils and fats in water and are commonly used in food manufacture as emulsifiers.
From the energy point of view they can be regarded as being equivalent to simple fats (triacylglycerols); they also provide a dietary source of choline and inositol, neither of which is a dietary essential.