Figure 1.

Intracellular osmotic pressure sets cell size and prevents cell shrinkage fostered by extracellular osmotic pressure and cortical tension. (A) Sketch of the bleb-based cell swimming model. Plasma membrane and actomyosin cortex are mechanically linked by reversible Hookean elastic linkers that stochastically associate and dissociate in a force-independent and -dependent manner, respectively. Intracellular and extracellular spaces are viscoelastic materials with associated fluid viscosity ${\text{η}}_{\text{f}}$, polymer viscosity ${\text{η}}_{\text{p}}$ and polymer stress relaxation time ${\text{λ}}_{\text{p}}$. Osmotic effects are included. (B) Time-evolution of cell area for three different osmotic conditions. Intracellular osmotic pressure: ${\text{Π}}_{\text{in}}$, extracellular osmotic pressure: ${\text{Π}}_{\text{out}}$. (C) Cell shrinkage due to a reverse osmotic shock (${\text{Π}}_{\text{in}}=0$). Thus, inclusion of intracellular osmolytes is necessary to prevent cell collapse due to inward cortical tension and extracellular osmotic forces that tend to force water out of the cell.