Most cells of multicelled organisms are surrounded and organized by a nonliving, complex mixture of fibrous proteins and polysaccharides called extracellular matrix, or ECM. Secreted by the cells it surrounds, ECM supports and anchors cells, separates tissues, and functions in cell signaling.
Different types of cells secrete different kinds of ECM. The cell wall around the plasma membrane of plant cells is a type of ECM that is structurally different from the cell wall of bacteria and archaeans. Both types of wall protect, support, and impart shape to a cell. Both are also porous: Water and solutes easily cross it on the way to and from the plasma membrane. Cells could not live without exchanging these substances with their environment. Plant and animals secrete substances such as collagen, proteoglycans, lignin and fibronectin with their ECM. Cells send and receive ions, molecules, or signals through some junctions. ECM proteins bind to receptor proteins in the plasma membrane called integrins.