Cells maintain their content separate and distinct from the external environment by a semifluid lipid bilayer that prevents the free exchange of most biological molecules and acts as a barrier between the inside and the outside of a cell. Prokaryotic cells contain only one such membrane, the plasma membrane that delineates the border of the cell. Eukaryotic cells, in addition to the plasma membrane, contain a multitude of internal membranes that define distinct intracellular organelles. The role of the plasma membrane and the internal membranes is analogous—all prevent the free exchange of molecules between the inside and the outside of the enclosed compartment, thus allowing the generation and maintenance of specialized microenvironments within each membrane-enclosed space.