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 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. This opening chapter to Part I concentrates on the mechanical properties of individual filaments, such as their bending or stretching resistance; 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 3) or perhaps extending though the three-dimensional volume of the cell (Chapter 4).

Filaments in the cell

Our discussion of cellular ropes and rods begins with a look at their molecular composition and linear dimensions. All of the ropes are linear polymers, in the sense that they are constructed from individual monomeric units to form 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 Appendix B).