PHOSPHOLIPID

also called Phosphatide, any member of a large class of fatlike, phosphorus-containing substances that play important structural and metabolic roles in living cells. The phospholipids, with the sphingolipids, the glycolipids, and the lipoproteins, are called complex lipids, as distinguished from the simple lipids (fats and waxes) and from other fat-soluble cell components, mostly isoprenoids and steroids. The term phosphoglyceride is used by some as a synonym for phospholipid and by others to denote a subgroup of phospholipids. In general, phospholipids are composed of a phosphate group, two alcohols, and one or two fatty acids. On one end of the molecule are the phosphate group and one alcohol; this end is polar, i.e., has an electric charge, and is attracted to water (hydrophilic). The other end, which consists of the fatty acids, is neutral; it is hydrophobic and water-insoluble but is fat-soluble. This amphipathic nature (containing both hydrophobic and hydrophilic groups) makes phospholipids important in membranes; they form a two-layer structure, called the lipid bilayer, with the polar head facing out on each surface to interact with water, and with the neutral "tails" driven inward and pointing toward one another. The lipid bilayer is the structural basis of all cell membranes and is nearly impermeable to ions and most polar molecules. Proteins embedded in the phospholipid matrix transport many substances through the membrane. Lecithin (q.v.; phosphatidyl choline) and the cephalins (phosphatidyl ethanolamine and phosphatidyl serine) are groups of phospholipids of widespread occurrence in plants and animals; lecithin is the most abundant, but is rare in microorganisms. Other phospholipids include plasmalogens, present in brain and heart and apparently of limited occurrence in nonanimal tissues; phosphoinositides, present in brain; and cardiolipin, initially isolated from heart.