Assembly of Biological Macromolecules

Abstract

The study of animal tissues is complex because water, ions, cells, macro-molecules, tissues, and organs exist in equilibrium. From a structural point of view, biological tissues contain highly ordered arrays of macromolecules. One might wonder why biological structures need to be made up of highly ordered arrays of proteins, polysaccharides, and lipids. The reason is that individual polymer molecules cannot sustain the weight of gravity without rearranging. For example, if your skin were made of just collagen molecules without being cross-linked into crystalline fibers, it would sag. This is because individual molecules, in a similar manner to water molecules, can move around or diffuse. In the case of water molecules, a container is needed to shape them. In the case of tissues, the molecules need to assemble into ordered structures and be cross-linked for the shape of the tissue to be maintained. In some cases, assemblies of macro-molecules are purposely not cross-linked so that shape can be changed quickly. For example, cytoskeletal actin filaments are rapidly assembled and disassembled to allow for changes in cell shape. In this example, cross-links prevent rapid shape changes; however, actin filaments by themselves are rigid enough to maintain cell shape at any one instant. In contrast, collagen fibers in the skin must be cross-linked to form force-bearing units to prevent tearing when skin is stretched. This comparison is used to underscore the complexity of biological tissue structure and its relationship to physical properties. In some cases, rigidity is sacrificed for structural flexibility. In other cases, structural flexibility is sacrificed for permanence.