  Physical thermodynamic and kinetic, chemical and biological reasoning
restrict the spatial dimensions of living cells and confine them to between one
and one hundred micrometers. Cells should necessarily be macroscopic,
dissipative objects, resisting thermal fluctuations and providing sufficient
informational capacity. The upper limit of 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. Among other important
consequences the macroscopic dimensions of living cells seen as dissipative
systems) give rise to the irreversibility of biological processes,
demonstrating the arrow of time.
