The complicated combination of semiflexible filaments and binding proteins that form the structural matrix within a cell presents a formidable challenge to the scientific investigator. Many of the key interactions, such as the details of the FLNa binding to actin polymers, are still subjects of active research. The fraction of actin molecules polymerized into structural fibers varies from cell to cell, and the mean can range from 35 percent to 80 percent depending on the state of stress, the degree of cell-cell attachment, and time. As will be described in the section on actin dynamics, the actin filaments polymerize and depolymerize continuously, so that the whole internal structure is replaced within a time scale that is tens of minutes to hours. To make the situation even more interesting, thermal fluctuations of the filaments could contribute substantially to the apparent cell stiffness. Yet just the simplicity of the randomness tempts one to find models that can at least scale the behavior of the mixture and arrive at working conclusions regarding the structural rigidity of the cell when exposed to external forces.

It is attractive to look at two bodies of literature for examples from other fields. The first is polymer physics, where a body of literature is presented on various thermally driven models. These works are well summarized in the book by Boal (2002). Included in these models are floppy chains, semiflexible chains, and welded chains. A second field is the study of porous solids, which range from structured...