7 - Cells, gels, and mechanics

Summary

ABSTRACT: The cell is known to be a gel. If so, then a logical approach to the understanding of cell function may be through an understanding of gel function. Great strides have been made recently in understanding the principles of gel dynamics. It has become clear that a central mechanism in biology is the polymer-gel phase-transition – a major structural change prompted by a subtle change of environment. Phase-transitions are capable of doing work, and such mechanisms could be responsible for much of the work of the cell. Here, we consider this approach. We set up a polymer-gel-based foundation for cell function, and explore the extent to which this foundation explains how the cell generates various types of mechanical motion.

Introduction

The cell is a network of biopolymers, including proteins, nucleic acids, and sugars, whose interaction with solvent (water) confers a gel-like consistency. This revelation is anything but new. Even before the classic book by Frey-Wyssling a half-century ago (Frey-Wyssling, 1953), the cytoplasm's gel-like consistency had been perfectly evident to any who ventured to crack open a raw egg. The “gel-sol” transition as a central biological mechanism is increasingly debated (Jones, 1999; Berry, et al., 2000), as are other consequences of the cytoplasm's gel-like consistency (Janmey, et al., 2001; Hochachka, 1999). Such phenomena are well studied by engineers, surface scientists, and polymer scientists, but the fruits of their understanding have made little headway into the biological arena.

Perhaps it is for this reason that virtually all cell biological mechanisms build on the notion of an aqueous solution – or, more specifically, on free diffusion of solutes in aqueous solution.