What is the key difference between bilayers and liposomes?
Distinction between bilayer organization and vesicular structures illustrates supramolecular assembly hierarchy. Phospholipid bilayer fundamental two-dimensional fluid with two opposed leaflets hydrophobic tails interior hydrocarbon slab three to four nanometer low dielectric excluding water, hydrophilic headgroups hydrated outward facing aqueous phases intra and extracellular, forms flexible sheet capable of undulations bending modulus about ten to twenty kBT, can exist as planar black lipid membrane across aperture separating two aqueous compartments or supported bilayer on mica for AFM. Liposome synthetic spherical vesicle composed of one or more concentric closed bilayers each enclosing discrete aqueous compartment distinct internal volume chemically isolated from bulk external medium, generated by hydrating dried lipid film mechanical agitation ultrasound extrusion hundred nanometer filter or detergent removal methods. Classification small unilamellar thirty to fifty nanometers, large unilamellar hundred to thousand nanometers, multilamellar onion-like many lamellae. Applications drug delivery doxorubicin liposomal, gene delivery, membrane protein reconstitution functional assays, permeability studies, vaccine adjuvants. Key difference therefore liposomes enclose aqueous core while planar bilayer sheet does not although both share bilayer building block.
Ref: Bangham et al., Liposomes as Models of Biological Membranes, J Mol Biol 1965.