a broad class of organic products found in living systems. Most are insoluble in water but soluble in nonpolar solvents. The definition excludes the mineral oils and other petroleum products obtained from fossil material. Major classes of lipids include the fatty acids
, the glycerol-derived lipids (including the fats and oils
and the phospholipids
), the sphingosine-derived lipids (including the ceramides, cerebrosides, gangliosides, and sphingomyelins), the steroids
and their derivatives, the terpenes and their derivatives, certain aromatic compounds, and long-chain alcohols and waxes
. In living organisms lipids serve as the basis of cell
membranes and as a form of fuel storage. Often lipids are found conjugated with proteins or carbohydrates, and the resulting substances are known as lipoproteins
and lipopolysaccharides. The fat-soluble vitamins
can be classified as lipids. Liposomes
are spherical vesicles formed by mixing lipids with water or water solutions. They have found applications in the oral administration of some drugs (e.g., insulin and some cancer drugs), since they retain their integrity until they are broken down by the lipases
in the stomach and small intestine.
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