A ceramide is a lipid molecule composed of the amino acid sphingosine and a fatty acid. Ceramides exist in great concentrations in the plasma membrane of a cell and act as signaling molecules for a number of cellular functions. Ceramide may also have a role in certain pathological states, including cancer, diabetes, obesity, and inflammation.

All cells are covered with a semi-permeable membrane composed of a lipid bilayer, two rows of lipid molecules. Each lipid molecule has a hydrophilic protein head that faces the outside of the membrane and a tail composed of fatty acid. The cell membrane is designed to selectively allow molecules into or out of the cell and aids in a number of cellular functions. In a ceramide, the head is sphingosine.

Ceramide triggers cell functions including cell growth arrest, cell senescence, differentiation, proliferation, adhesion, cell migration, and apoptosis, or programmed cell death. It is believed to aid in suppressing the spread of cancer through some of these functions. A number of chemicals and other triggers associated with stress lead to the generation of ceramide in cells.

Ceramide is generated through three methods: sphingomyelin hydrolysis, de novo synthesis, and the salvage pathway. In sphingomyelin hydrolysis, sphingomyelin, a sphingolipid common in the cell membrane, is activated by the enzyme sphingomyelinase to undergo hydrolysis, in which the molecule breaks down through a reaction with water. Ceramide is produced as a result.

In de novo synthesis of ceramide, a series of enzymes create ceramide out of a number of simpler molecules. In de novo synthesis, ceramide is synthesized in the endoplasmic reticulum, then transferred to the Golgi apparatus. In the salvage pathway, complex sphingolipids are broken down to form sphingosine, which in turn is used to synthesize ceramide.

Ceramide is not yet fully understood, and the process by which it acts as a signaling molecule is unknown. It has been hypothesized that ceramide stabilizes lipid rafts, platforms made of smaller lipids, which can link extracellular signals to signals inside the cell.