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Append& A: GLOSSARY OF TERMS Actin: a protein characteristic of muscle but also found in other types of cells. Actin may exist in a monomeric, globular form that polymerizes to form filaments. Ampullate gland: the silk-producing gland of spiders. Aragonite: a form of the mineral CaCO3 with an orthorhombic crystal structure. Articular cartilage: the tissue that covers bones within freely moving joints (hip, knee, shoulder, etc.~. Axon: a long, thin process of a nerve cell that extends away from the cell body (which contains the nucleus and most of the other organelles) and terminates in numerous small branches. Cancellous bone: the bone of sponge-like loading within the hard cortical bones. Carpus bone: a bone of the wrist or part of an animal fore-limb. Catch connective tissue: the ligaments that attach the calcite spines of sea urchins to muscle. Cellulose: a fibrous substance that makes up plant cell walls. Chitin: a structural polysaccharide found in the shell of crustaceans and insects and in the cell walls of fungi. Chitin is a linear polymer of N-acetylglucosamine. 123
124 Hierarchical Structures in Biology as a Guide for New Materials Technology Collagen: a family of filamentous proteins that are organized in a triple helix conformation and that provide tensile stiffness and strength to biological entities. Cytoskeleton: a submicroscopic intracellular complex of protein, which is believed to account for the mechanical properties of cytoplasm. Actin and tubulin are examples of cytoskeletal proteins. Diarthrodial joint: a freely moving joint of the body (hip, knee, shoulder, etc.~. Donnan osmotic pressure: the osmotic pressure in a solution of electrolytes that are freely permeable across a semipermeable membrane but are distributed unequally because of the pressure of a nondiffusible charged substance on one side of the membrane. Epitaxy: oriented growth of one crystalline substance on a different crystalline substrate. Fibril: an elemental unit of a collagen composed of a number of triple helical collagen molecules; a microscopic fiber. Ganglion: an aggregation of nerve cell bodies. Hyaluronic acid: a copolymer composed of glucuronic acid and M-acetylglucosamine and glucuronic units with a molecular weight in excess of 105. Haversian bone: the concentric arrangement of bone cells and inorganic material around a central, blood vessel~arrying canal. Canaculi connect with the central canal, with each other, and with bone cell-containing spaces. Hemicellulose: a polysaccharide similar to cellulose in that it is insoluble in water and hydrolyzable in acid. Hemicelluloses are probably of smaller molecular dimensions than celluloses and contain, in addition to glucose, xylose, galactose, uranic acid, and other sugars. Hydroxyspatite: a calcium phosphate that crystallizes into hexagonal platelets and that can be approximated by the formula 3Ca3(PA4~2 Ca(OH)2.
Appendix A 125 Integrin protein: a protein component of a lipid bilayer membrane that is so intimately associated with the membrane that it can only be removed by using detergents. Lignin: an organic polymer component in wood. Living polymer: a mixture of polymer molecules that contain active sites at which polymerization can continue when monomer is added to the mixture. Lumen: the internal cavity of a hollow organ or organelle. Mesoglea: the collagenous connective tissue in the body walls of sea anemones. Metathesis: a reaction in which chemical groups attached to carbon- carbon double bonds are exchanged. Microfibril: the substructure of a collagen fibril that is composed of associated tropocollagen units. Nacre: iridescent inner layer of various mollusk shells that consists chiefly of calcium carbonate deposited in thin overlapping sheets with some organic matter. Organelle: a specialized part of a cell, which performs functions analogous to those of the organs of many-celled animals. Proteoglycan: a "core" protein linked to many glycosaminoglycan chains, which composes a macromolecule exceeding 1 o6 daltons. Proteoglycan aggregates: proteoglycans linked to hyaluronate chains via a like protein. The molecular weight of this molecule often exceeds 107. Its high density of charge groups (carboxyl and sulfate) enable it to provide valuable function in biological tissues. 1
