Although the cell membrane is the functional barrier between the inside and the outside of a cell, many structures are produced by cells and secreted to the outside of the cell membrane, where they play essential roles in protecting, supporting, or attaching cells to each other. Because they are outside the cell membrane, these structures are said to be extracellular. The peptidoglycan cell wall of bacteria is an example of an extracellular structure (see Figure 5.4). In eukaryotes, other extracellular structures—the cell walls of plants and the extracellular matrices found between the cells of animals—play similar roles. Each of these structures is made up of two main components: a prominent fibrous macromolecule and a gel-like medium in which the fibers are embedded.