ABSTRACT
Binding and diffusion are central to the biochemical reactions that are the engine of cellular life. However, many of the assumptions that are hidden behind our use of these concepts seem to be at odds with the way cells really work. Crowding of spheres is one instructive way to understand excluded-volume effects and the depletion forces they engender. However, these ideas fall short as we try to more closely approximate structures and conditions within living organisms. Although formation of aligned filament bundles can greatly enhance the strength and mechanical properties beyond that of individual filaments, a shortcoming of this organizational motif is that it is extremely anisotropic. In addition to the important role of crowding in the three-dimensional setting of the cellular interior, crowding is also a fact of life in the cell membrane. Not only does crowding alter equilibrium properties, it also can significantly impact a variety of dynamical processes within cells.
