Chapter 16: Cell Walls, Extracellular Matrix, & Cell Interactions

Cell Walls, Extracellular Matrix, & Cell Interactions begins by analyzing the rigid cell walls of bacteria, which are composed of peptidoglycan shells cross-linked by peptides, and explains how cytoskeletal proteins like FtsZ and MreB determine bacterial shape. The text contrasts these with eukaryotic cell walls, specifically the chitin-based walls of fungi and the cellulose-rich walls of plants, detailing how cellulose microfibrils are synthesized by transmembrane enzyme complexes and oriented by cortical microtubules to direct cell expansion. A significant portion of the chapter is dedicated to the animal extracellular matrix, a dynamic network of fibrous proteins and polysaccharides. It describes the structural diversity of collagens—from fibril-forming types in connective tissue to network-forming types in basal laminae—and the role of elastic fibers in tissue flexibility. The summary explains how glycosaminoglycans (GAGs) and proteoglycans form hydrated gels that resist compression, while adhesion proteins like fibronectin and laminin serve as cross-linkers that connect the matrix to the cell surface. Central to these interactions are integrins, the primary transmembrane receptors that link the ECM to the cytoskeleton at specialized sites known as focal adhesions (anchored to actin) and hemidesmosomes (anchored to intermediate filaments). The chapter further explores selective cell-cell adhesion mediated by selectins, the immunoglobulin (Ig) superfamily, integrins, and cadherins, emphasizing the role of calcium in these interactions. It details the architecture of stable junctions, including adherens junctions and desmosomes which mechanically link cells, and tight junctions which establish permeability barriers and cell polarity in epithelia. Finally, the chapter covers mechanisms of direct intercellular communication, comparing the structure and function of gap junctions (formed by connexins) in animal tissues with plasmodesmata in plant cells, both of which allow the regulated passage of ions and signaling molecules.