Phospholipid

A phospholipid is a lipid built much like a fat, but with one fatty acid replaced by a phosphate group. In the most common type, a glycerol backbone carries two fatty acid chains, and its third carbon holds a phosphate that is itself linked to a small charged or polar group such as choline, serine or ethanolamine. Others, such as sphingomyelin, are built on a different backbone called sphingosine.

The key feature of a phospholipid is that it is amphipathic: the phosphate head is hydrophilic and attracted to water, while the two fatty acid tails are hydrophobic and avoid it. In water, phospholipids therefore arrange themselves on their own into sheets two molecules thick, with heads facing the water on both sides and tails hidden in the middle. This phospholipid bilayer is the basic structure of the plasma membrane and of the membranes around organelles. Kinks in unsaturated tails, together with cholesterol, keep the membrane fluid, so its proteins and lipids can drift sideways.

Phospholipids do more than form barriers. Enzymes cut some of them to release signaling molecules inside cells, and the lungs secrete a phospholipid-rich surfactant that stops the tiny air sacs from collapsing. In the lab and in medicine, phospholipids are used to make liposomes, artificial vesicles that can carry drugs.