bilayers

A bilayer is a structure formed from two layers of molecules arranged so that their hydrophobic tails face inward and their hydrophilic heads face outward toward water. The most familiar example is a lipid bilayer that constitutes biological membranes, separating the interior of cells and organelles from their surroundings.

In biological systems, lipid bilayers are primarily composed of amphiphilic phospholipids, with cholesterol, glycolipids, and membrane proteins embedded within or associated with the surface. The two leaflets can differ in composition, a feature known as membrane asymmetry, and the overall thickness of a typical bilayer is around 4 to 5 nanometers. The hydrophobic core provides a strong barrier to most ions and polar solutes, while specific proteins create channels and transporters that regulate movement across the membrane. The bilayer is fluid, allowing lateral diffusion of lipids and proteins, which supports membrane flexibility and the function of membrane-associated processes.

Biological roles of bilayers include forming the plasma membrane and internal membranes of organelles, creating compartments, supporting signaling cascades, energy transduction in mitochondria and chloroplasts, and mediating vesicle trafficking, fusion, and endocytosis. The physical state of bilayers is influenced by temperature, lipid composition, and cholesterol content, which modulate fluidity, thickness, and permeability.

Beyond biology, bilayers also appear in materials science as synthetic or artificial membranes. These include liposomes and planar or polymer-supported bilayers used as model systems for studying membrane function, drug delivery, and biosensing. In inorganic contexts, the term can refer to two-layered materials such as bilayer graphene or oxide bilayers, which form the basis of various electronic, catalytic, and protective applications.