On the osmotic pressure of cells

The chemical potential of water (//h2o) provides an essential thermodynamic characterization of the environment of living organisms, and it is of equal significance as the temperature. For cells, //H,0 is conventionally expressed in terms of the osmotic pressure. We have previously suggested that the main contribution to the intracellular osmotic pressure of the bacterium E. coli is from soluble negatively-charged proteins and their counter-ions (1). Here, we expand on this analysis by examining how evolutionary divergent cell types cope with the challenge of maintaining the osmotic pressure within viable values. Complex organisms, like mammals, maintain constant internal osmotic pressure of around 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 osmotic pressures (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 osmotic pressures that differ from this optimum require therefore special measures, as exemplified with gram-positive and gram-negative bacteria. To handle such situations, their specific membrane encapsulations allow for a compensating turgor pressure that can take both positive and negative values. Moreover, the establishment of a positive turgor pressure allows 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 osmotic pressures around 15 osmol. Tlie 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.