The two broad categories of structural components of the cell are filaments, the focus of Part I of this book, and sheets, which we treat in this chapter and its companion. In principle, two-dimensional sheets may display more complex mechanical behavior than one-dimensional filaments, including resistance to both out-of-plane bending and in-plane shear. As examples, the plasma membrane is a two-dimensional fluid having no resistance to in-plane shear, whereas the cell wall possesses shear rigidity as a result of its fixed internal cross-links. In this first chapter of Part II, we introduce the chemical composition of biomembranes and describe their mechanical properties, including elasticity and failure. We focus here on fluid membranes and defer a discussion of polymerized structures, such as the cell wall, to Chapter 6. The softness of biomembranes means that their thermally induced undulations are important, as will be explored in Chapters 6 and 8. More extensive reviews of biomembranes than those provided here can be found in Evans and Skalak (1980), Cevc and Marsh (1987) and Sackmann (1990).

Composition of biomembranes

The design principles of Chapter 1 argue that the cell's membranes should be very thin, perhaps even just a few molecules in thickness, if their purpose is solely to isolate the cell's contents. The membrane need not contribute to the cell's mechanical strength, as this attribute can be provided by the cytoskeleton or cell wall. Are there materials in nature that meet these design specifications?