The structural elements of the cell can be broadly classified as filaments or sheets, where by the term filament, we mean a string-like object whose length is much greater than its width. Some filaments, such as DNA, function as mechanically independent units, but most structural filaments in the cell are linked to form two- or three-dimensional networks. As seen on the cellular length scale of a micron, individual filaments may be relatively straight or highly convoluted, reflecting, in part, their resistance to bending. Part I of this book concentrates on the mechanical properties of biofilaments: Chapter 3 covers the bending and stretching of simple filaments while Chapter 4 explores the structure and torsion resistance of complex filaments. The two chapters making up the remainder of Part I consider how filaments are knitted together to form networks, perhaps closely associated with a membrane as a two-dimensional web (Chapter 5) or perhaps extending though the three-dimensional volume of the cell (Chapter 6).

Polymers and simple biofilaments

At the molecular level, the cell’s ropes and rods are composed of linear polymers, individual monomeric units that are linked together as an unbranched chain. The monomers need not be identical, and may themselves be constructed of more elementary chemical units. For example, the monomeric unit of DNA and RNA is a troika of phosphate, sugar and organic base, with the phosphate and sugar units alternating along the backbone of the polymer (see Chapter 4 and Appendix B). However, the monomers are not completely identical because the base may vary from one monomer to the next. The double helix of DNA contains two such sugar–base–phosphate strands, with a length along the helix of 0.34 nm per pair of organic bases, and a corresponding molecular mass per unit length of about 1.9 kDa/nm (Saenger, 1984).