Fosfolipiidide

Fosfolipiidide (phospholipids) are a class of lipids that form a major component of cell membranes. They are amphipathic: a hydrophilic head containing a phosphate group and hydrophobic fatty acid tails. Most phospholipids are glycerophospholipids with a glycerol backbone bearing two fatty acyl chains and a phosphate-containing head group. Common head groups include choline, ethanolamine, serine, and inositol, yielding phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol; cardiolipin is a distinctive dimeric phospholipid found in mitochondrial membranes, and sphingomyelin is a related phospholipid with a sphingosine backbone. Some species also contain ether-linked or plasmalogen forms.

Functionally, phospholipids form the structural matrix of membranes, contributing to barrier function, membrane fluidity, thickness, and curvature. They also participate in signaling and membrane trafficking; for example, phosphatidylinositol 4,5-bisphosphate (PIP2) is hydrolyzed by phospholipase C to generate the second messengers IP3 and DAG. Externalization of phosphatidylserine serves as a signal in apoptosis.

Synthesis and turnover occur mainly in the endoplasmic reticulum, followed by remodeling of acyl chains by enzymes in the Lands’ cycle. Degradation is mediated by phospholipases A, C, and D among other enzymes. Abnormal phospholipid metabolism is implicated in metabolic, cardiovascular, and neurodegenerative diseases.