How crowded is the prokaryotic cytoplasm?

• A bacterial cell in vivo is modeled as a two-phase cytosol–cytogel system.
• The sol is regarded as a dilute solution and the gel as highly crowded.
• A large number of calculations is given for coccoid geometry.
• Results show the cytogel extends the thickness of the cell envelope by 20–70 nm.
• New insights into the physiology and structure of a bacterial cell are discussed.

We consider biomacromolecular crowding within the cytoplasm of prokaryotic cells as a two-phase system of ‘supercrowded’ cytogel and ‘dilute’ cytosol; we simplify and quantify this model for a coccoid cell over a wide range of biomacromolecular crowding. The key result shows that the supercrowded cytogel extends the vectorial character of the plasma membrane deeper into the cytoplasm by about 20–70 nm. We discuss useful physiological insights that this model gives into the functioning of a prokaryotic cell on the micrometer scale.