sphingosine

Meaning: Sphingosine is an 18-carbon amino alcohol with a long unsaturated hydrocarbon tail. It is the backbone of sphingolipids, the way glycerol is the backbone of glycerophospholipids. It has a hydroxyl at C1, an amino group at C2 and a hydroxyl at C3. A fatty acid joins the C2 amino group by an amide bond to make ceramide. Attaching a head group to the C1 hydroxyl gives the rest: phosphocholine gives sphingomyelin, one sugar gives a cerebroside, and an oligosaccharide gives a ganglioside. The sphingosine chain and the fatty acid serve as the two nonpolar tails, so these lipids are amphipathic.

On the MCAT: A passage shows a lipid structure and asks you to classify it. An amide-linked fatty acid on an amino alcohol, with no glycerol, points to a sphingolipid. Sphingomyelin is abundant in the myelin sheath around axons, and ganglioside buildup in lysosomal storage disorders can be used to illustrate a missing degradative enzyme.

Common confusion: Sphingosine is not a fatty acid, and it is not a glycerol-based lipid. Its link to a fatty acid is an amide, whereas glycerolipids use ester bonds. Sphingomyelin has no glycerol but is still counted as a phospholipid because it contains a phosphate group.