Membrane lipids form the structural foundation of biological membranes, providing a selective barrier that defines cellular and organellar boundaries. The amphipathic nature of membrane lipids drives their spontaneous assembly into bilayers in aqueous environments.
The Lipid Bilayer
The lipid bilayer is a two-dimensional sheet of amphipathic lipids oriented with their hydrophobic tails facing inward and their hydrophilic head groups facing the aqueous environment on both sides. This arrangement is thermodynamically stable and self-sealing. The bilayer is about 5 nm thick and acts as a permeability barrier to ions and polar molecules while allowing free diffusion of small nonpolar molecules.
Phospholipid Diversity
Glycerophospholipids are the most abundant membrane lipids. Phosphatidylcholine is the major phospholipid in most mammalian membranes, typically concentrated on the outer leaflet. Phosphatidylethanolamine and phosphatidylserine are enriched in the inner leaflet. Phosphatidylinositol is a minor component but serves as a precursor for important signaling molecules. Cardiolipin is found primarily in the inner mitochondrial membrane.
Sphingolipids are enriched in the outer leaflet of the plasma membrane, where they form microdomains called lipid rafts. Sphingomyelin is the most abundant sphingolipid in mammalian cells, while glycosphingolipids such as gangliosides are abundant in neuronal membranes and function in cell recognition.
Cholesterol in Membranes
Cholesterol intercalates between phospholipids in the bilayer, with its hydroxyl group positioned near the ester carbonyl groups of phospholipids. Cholesterol modulates membrane fluidity by filling gaps between unsaturated fatty acid chains, reducing the mobility of nearby hydrocarbon chains. At high temperatures, cholesterol reduces fluidity by restricting phospholipid motion, while at low temperatures, it prevents tight packing and maintains fluidity. This buffering effect is essential for maintaining membrane function across temperature variations.
Membrane Fluidity
Membrane fluidity is determined by lipid composition and temperature. Unsaturated fatty acids with cis double bonds introduce kinks that prevent tight packing, increasing fluidity. Shorter fatty acid chains also increase fluidity. Cholesterol modulates fluidity as described above. Cells regulate membrane fluidity by adjusting their fatty acid composition through desaturase enzymes, a process called homeoviscous adaptation.
Lipid Asymmetry
The two leaflets of the biological membrane have distinct lipid compositions. The outer leaflet is enriched in phosphatidylcholine and sphingomyelin, while the inner leaflet contains most of the phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol. This asymmetry is maintained by ATP-dependent flippases that translocate specific phospholipids to the inner leaflet. The loss of asymmetry, particularly the exposure of phosphatidylserine on the outer leaflet, is an early signal of apoptosis and triggers phagocytic clearance.
Lipid Rafts
Lipid rafts are ordered microdomains in the plasma membrane enriched in cholesterol, sphingolipids, and specific proteins. Sphingolipids with long, saturated acyl chains pack tightly with cholesterol to form a liquid-ordered phase that is more ordered than the surrounding liquid-disordered phase. Rafts concentrate signaling proteins such as GPI-anchored proteins, receptor tyrosine kinases, and Src-family kinases. These protein structure elements function as platforms for signal transduction, membrane trafficking, and cell adhesion.
