The behaviors of biological cells are constrained by the laws
of physics. And, cells are mostly protein. We develop simple quantitative
arguments for how some cell behaviors arise from the physical properties of
proteins and proteomes. For example, cells are highly sensitive to
temperature, reflecting a possible underlying basis in the thermal
stabilities of proteins. We also consider other properties of proteins
inside cells: their densities and their diffusion and folding and synthesis
rates, to rationalize the limits on cell replication speeds.