Cells prioritize the regulation of cell mass density

A cell's global physical state is characterized by its volume and dry mass. The ratio of cell mass to volume defines the cell mass density (CMD), which is also a measure of macromolecular crowding and concentrations of all proteins. Using the fluorescence eXclusion method (FXm) and quantitative phase microscopy (QPM), we investigate CMD dynamics following sudden changes in media osmolarity. We find that while cell volume and mass exhibit complex behavior after osmotic shock, CMD follows a straightforward monotonic recovery over 48 hours. This recovery is cell cycle independent and depends on coordinated adjustment of protein synthesis and volume growth rates. Unexpectedly, the protein synthesis rate decreases when CMD increases. We observe that nucleoplasm-cytoplasm transport is CMD dependent, which contributes to negative regulatory feedback on CMD. The Na⁺/H⁺ exchanger helps regulate CMD by affecting both protein synthesis and volume change. Together, we reveal that cells have a robust control system that actively regulates CMD during environmental change.