Spatial scales of living cells and their energetic and informational capacity.

Physical (thermodynamic and kinetic), chemical, and biological reasoning restrict the spatial dimensions of living cells (prokaryotic and eukaryotic) and confine them to between 1 and 100 µm. Cells should necessarily be macroscopic, dissipative objects, resisting thermal fluctuations and providing sufficient informational capacity. The upper limit of the spatial dimensions of cells is supplied by their ability to withstand gravity and inertia forces under reasonable deformations. The upper limit of cell dimensions is also governed by the hierarchy of characteristic time scales, inherent for mass and heat transport. For micron-scaled cells, the "traffic time" (namely a typical time necessary for the migration of one enzyme to another) is on the order of magnitude of a millisecond, which coincides with the characteristic time scale of a single round of the catalytic enzyme cycle. The macroscopic dimensions of living cells (seen as dissipative systems) and the hierarchy of time scales of the mass transfer processes vs. those inherent for heat transport and viscous dissipation give rise to the irreversibility of biological processes.