Phospholipids

Phospholipids are a class of lipids that contain two fatty acid chains, a glycerol backbone, and a phosphate-containing head group. They are amphipathic, with hydrophobic tails and a hydrophilic head, which drives their spontaneous organization into lipid bilayers in aqueous environments. In glycerophospholipids, fatty acids are esterified to the first and second carbons of glycerol and a phosphate-linked head group occupies the third carbon. Sphingolipids such as sphingomyelin are related phospholipids that use sphingosine as the backbone instead of glycerol.

Common phospholipids include phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and cardiolipin. They are major components of cellular membranes, forming the bilayer that acts as a permeability barrier and a platform for membrane proteins. The head groups influence surface charge, curvature, and protein interactions, contributing to membrane asymmetry between the inner and outer leaflets. For example, phosphatidylserine is typically enriched on the cytosolic side and becomes exposed on the outer surface during apoptosis.

Biological roles of phospholipids extend beyond membrane structure. They provide substrates for enzymes such as phospholipases that release fatty acids and head-group molecules involved in signaling. Phosphatidylinositol phosphates act as key second messengers in signaling cascades. Phospholipids are synthesized mainly in the endoplasmic reticulum and mitochondria, then remodeled by acyltransferases and redistributed by flippases and scramblases to maintain membrane composition and function. They are essential for membrane trafficking, organelle integrity, and lipoprotein assembly.