A living cell is defined by its separation from the exterior medium. Lipids and proteins form this semi-permeable barrier. This boundary is important for the cell to protect itself against biotic or abiotic stresses, and to regulate exchanges with its environment. However, this membrane, eventhough fairly resistant mechanically, is prone to breaking under osmotic stress, which can lead to a damaging stretch of the membrane. In resting conditions, a cell actively maintains an osmotic pressure differential, through active regulation of the cytoskeleton, membrane trafficking and ionic channels. Those long term (minute-hours) processes have been studied in detail in the previous decades but cannot however account for the short term (second-minute) response to acute osmotic shocks, and thus the processes by which cells cope up with osmotic shock remains elusive. How do cells react immediately to osmotic stresses before their machinery tries to counteract it?