Cell Coats, Walls, and Shells

Like bacteria, most cells of higher plants and animals are surrounded by extracellular materials. Plants have rigid walls rich in cellulose and other carbohydrate polymers. Outside surfaces of plant cells are covered with a cuticle containing layers of a polyester called cutin and of wax. Surfaces of animal cells are usually lined with carbohydrate molecules which are attached to specific surface proteins to form glycoproteins. Spaces between cells are filled with such “cementing substances” as pectins in plants and hyaluronic acid in animals. Insoluble proteins such as collagen and elastin surround connective tissue cells. Cells that lie on a surface (epithelial and endothelial cells) are often lined on one side with a thin, collagen-containing basement membrane. Inorganic deposits such as calcium phosphate (in bone), calcium carbonate (eggshells and spicules of sponges), and silicon dioxide (shells of diatoms) are laid down, often by cooperative action of several or many cells.