Before launching into our concluding chapter, we catch our breath and survey the results obtained so far. In the opening chapter of the text, we identified the important structural elements of the cell as soft filaments and sheets, their flexibility arising from their nanometer thickness compared to their micron length. The mechanical properties of filaments in isolation (Chapter 2) and as components of networks (Chapters 3 and 4) were established in Part I, where we saw how the elasticity of soft filaments has its roots in both energy and entropy. The bending resistance of a biofilament may be sufficiently small that the filament noticeably changes shape in response to thermal fluctuations in its energy. Entropy is consequently reduced when a fluctuating filament is forced to straighten out at non-zero temperature and, of course, energy is increased when the intermolecular separation is driven away from its equilibrium value by tension; both of these effects contribute to the stretching resistance of a filament. The two chapters in Part II uncovered similar generic properties for flexible membranes: the energy required to bend a thin membrane may not be extravagant on the thermal energy scale kBT, so that a biomembrane may display gentle undulations at ambient temperatures. Under lateral tension, energy increases with intermolecular separation and entropy is reduced when the undulations are suppressed; again, both of these effects lead to area compression resistance.

These two categories of materials – ropes and sheets – are used to construct cells in Part III.