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Aqueous compartments enclosed by lipid bilayer membranes; liposomes are also known as lipid vesicles. Phospholipid molecules consist of an elongated nonpolar (hydrophobic) structure with a polar (hydrophilic) structure at one end. When dispersed in water, they spontaneously form bilayer membranes, also called lamellae, which are composed of two monolayer layer sheets of lipid molecules with their nonpolar (hydrophobic) surfaces facing each other and their polar (hydrophilic) surfaces facing the aqueous medium. The membranes enclose a portion of the aqueous phase much like the cell membrane which encloses the cell; in fact, the bilayer membrane is essentially a cell membrane without its protein components.
Liposomes are often used to study the characteristics of the lipid bilayer. Properties of liposomes have been characterized by a variety of techniques: molecular organization by x-ray diffraction, nuclear magnetic resonance, electron paramagnetic resonance, and Raman spectroscopy; melting behavior (that is, crystal to liquid-crystal transition) by calorimetry; net electric surface charge by microelectrophoresis; size by light scattering and electron microscopy. See Lipid
Liposomes have numerous uses as biochemical and biophysical tools: (1) as vehicles for the delivery of both water- and of oil-soluble materials to the cell; (2) as immunological adjuvants; (3) as substrates for the study of membrane properties such as rotational or translational diffusion in the plane of the membrane; and (4) as intermediates in the construction of bilayers large enough for the study of electrical properties of membranes. See Cell membranes