What is osmosis?

Osmosis is a special case of diffusion involving only the solvent, usually water, moving through a semipermeable membrane that lets water pass but blocks larger dissolved particles. Water flows from the side with more free water molecules (lower solute concentration) to the side with less free water (higher solute concentration) until equilibrium is reached or an opposing pressure balances the flow. This osmotic pressure is vital to living cells: plant roots absorb water from soil by osmosis, and red blood cells swell or shrink depending on the concentration of the surrounding fluid. In plants, osmosis keeps cells turgid and rigid, which is why a wilted plant perks up after watering. Kidneys, food preservation with salt or sugar, and the way freshwater fish regulate their bodies all rely on osmotic principles.