Phospholipids are a cornerstone subclass of complex lipids characterized by a glycerol backbone esterified to two fatty acids and a phosphorylated headgroup. This amphipathic architecture is essential for biological membrane assembly. As the predominant lipid species in eukaryotic plasma membranes (40–60 mol%), phospholipids regulate selective permeability, membrane fluidity, and the formation of signaling platforms.
Structural Diversity
Phospholipids consist of a diacylglycerol (DAG) scaffold with an sn-1 chain (typically saturated, 16:0/18:0) and an sn-2 chain (commonly unsaturated, 18:1/20:4) linked via ester bonds. A phosphate group at the sn-3 position carries polar headgroups such as choline (PC), ethanolamine (PE), serine (PS), or inositol (PI).
Specialized variants include plasmalogens, which contain a vinyl-ether bond at sn-1 providing oxidative resistance, and cardiolipins, which possess four acyl chains and are enriched in mitochondrial membranes. The packing parameter (V/aLc ≈ 0.5–1) determines whether phospholipids assemble into bilayers, micelles, or non-lamellar hexagonal phases depending on chain unsaturation and headgroup charge.
Biophysical Properties
Hydrophilic headgroups (e.g., phosphocholine zwitterions, pKa ≈ 1/13) orient toward aqueous environments, while hydrophobic fatty acyl tails (12–24 carbons) pack inward, forming self-sealing bilayers approximately 4 nm thick. Membrane phase behavior ranges from gel states (Lβ, Tm > 25°C) to liquid-crystalline phases (Lα, Tm < 15°C), with cholesterol serving as a key modulator of transition temperatures.
Phospholipids exhibit rapid lateral diffusion (~1 μm²/s), whereas transbilayer flip-flop occurs extremely slowly (half-times of days) unless catalyzed by specialized enzymes such as flippases.
Biosynthesis Pathways
Phospholipid biosynthesis proceeds through both de novo and remodeling pathways. In the CDP-DAG branch, CTP reacts with phosphatidic acid (PA) to form CDP-DAG, which subsequently combines with headgroup alcohols such as choline or ethanolamine through the Kennedy pathway.
Additional remodeling occurs via the Lands cycle, mediated by lysophospholipid acyltransferases. The endoplasmic reticulum primarily synthesizes PC and PE, while trafficking proteins coordinate lipid distribution to the plasma membrane. Cardiolipin biosynthesis occurs in mitochondria through cardiolipin synthase, underscoring its role in mitochondrial membrane integrity and bioenergetics.
