What are the common features of membranes?

Sheet-like flexible structure, 30–100 Å (3–10 nm) thick • Main structure is composed of two leaflets of lipids (bilayer) • Form spontaneously in aqueous solution and are stabilized by noncovalent forces, especially hydrophobic effect • Protein molecules span the lipid bilayer • Asymmetric Fluid structures: two-dimensional solution of oriented lipids