Phase transitions and molecular motion in the cell.

The cytoplasm exhibits all of the signature characteristics of a gel. The thesis put forth here is that the cytoplasm's gel-like character is central to the generation of biological movement. In artificial gels, a common vehicle for generating movement is the polymer-gel phase-transition. By undergoing phase-transition, gels produce motion of both solvent and solutes. It is argued that cells do the same. Three examples are given: the secretory system, the muscle contraction system and the biological streaming system. In each case it is shown that the characteristic motions may be created as proteins and water undergo transition from an expanded, hydrated state to a contracted, dehydrated state--or the reverse. These changes shift solutes and solvent in a characteristic way that depends on the respective organelle's structure. Phase-transitions are simple, powerful mechanisms that may be responsible for many, if not all, biological motions.