Random walks and cell size

For many years, it has been believed that diffusion is the principle motive force for distributing molecules within the cell. Yet, our current information about the cell makes this improbable. Furthermore, the argument that limitations responsible for the relative constancy of cell size—which seldom varies by more than a factor of 2, whereas organisms can vary in mass by up to 1024—are based on the limits of diffusion is questionable. This essay seeks to develop an alternative explanation based on transport of molecules along structural elements in the cytoplasm and nucleus. This mechanism can better account for cell size constancy, in light of modern biological knowledge of the complex microstructure of the cell, than simple diffusion. BioEssays 22:1018–1023, 2000. © 2000 John Wiley & Sons, Inc.