phospholipid

Phospholipids are a class of lipids that contain a glycerol backbone, two fatty acid tails, and a phosphate-containing head group. The molecule is amphipathic: the hydrophobic fatty acyl chains face inward away from water, while the polar head group is exposed to aqueous environments. This arrangement drives the spontaneous formation of lipid bilayers, the fundamental structure of cell membranes. The most common phospholipids in eukaryotic membranes are phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol; cardiolipin is abundant in mitochondrial membranes. Variations in tail length and saturation influence membrane fluidity and curvature.

In membranes phospholipids arrange into bilayers that form barriers and surfaces for proteins. They contribute to membrane curvature and microdomain formation and serve as precursors for signaling molecules. Phosphatidylinositol-derived PIP2 and its metabolites participate in signaling cascades. Phospholipids are also carried in the bloodstream bound to lipoproteins and are integral to lipoprotein surfaces.

Biosynthesis typically occurs in the endoplasmic reticulum. The Kennedy pathway generates phosphatidylcholine and phosphatidylethanolamine from choline or ethanolamine. Phosphatidylserine is formed and decarboxylated to phosphatidylethanolamine; phosphatidylinositol is synthesized via CDP-diacylglycerol routes. Phospholipids are degraded by phospholipases A1, A2, C, and D, producing lyso-phospholipids, DAG, IP3, or choline-containing products, and are recycled for membrane maintenance.

Dietary sources include eggs, soy, and other plant and animal foods; phospholipids are also present in plasma lipoproteins. Abnormal phospholipid metabolism is linked to various disorders, including liver disease and certain inherited conditions affecting lipid transport.