The chemical potential of water () provides an essential thermodynamic characterization of the environment of living organisms, and it is of equal significance as the temperature. For cells, is conventionally expressed in terms of the osmotic pressure (π_osm). We have previously suggested that the main contribution to the intracellular π_osm of the bacterium E. coli is from soluble negatively-charged proteins and their counter-ions. Here, we expand on this analysis by examining how evolutionary divergent cell types cope with the challenge of maintaining π_osm within viable values. Complex organisms, like mammals, maintain constant internal π_osm ≈ 0.285 osmol, matching that of 0.154 M NaCl. For bacteria it appears that optimal growth conditions are found for similar or slightly higher π_osm (0.25-0.4 osmol), despite that they represent a much earlier stage in evolution. We argue that this value reflects a general adaptation for optimising metabolic function under crowded intracellular conditions. Environmental π_osm that differ from this optimum require therefore special measures, as exemplified with gram-positive and gram-negative bacteria. To handle such situations, their membrane encapsulations allow for a compensating turgor pressure that can take both positive and negative values, where positive pressures allow increased frequency of metabolic events through increased intracellular protein concentrations. A remarkable exception to the rule of 0.25-0.4 osmol, is found for halophilic archaea with internal π_osm ≈ 15 osmol. The internal organization of these archaea differs in that they utilize a repulsive electrostatic mechanism operating only in the ionic-liquid regime to avoid aggregation, and that they stand out from other organisms by having no turgor pressure.

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1	Medicine	10	11	199
2	Natural Sciences	8	8	139
3	Biochemistry	5	5	102
4	Engineering	2	2	38
5	Allergy	1	1	22

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