Osmosis

Osmosis is the net movement of water across a selectively permeable membrane in response to a difference in solute concentration across that membrane. When the membrane allows water to cross more readily than the relevant solute, water tends to move toward the side with the higher concentration of solute that is effectively restricted by the membrane .^1,2

Osmosis is a passive process, meaning that the movement of water itself does not require direct cellular energy expenditure. Water molecules move continuously in both directions across a membrane, but when an osmotic gradient exists, there is net water movement in one direction. Net movement continues until the driving forces are balanced or another force, such as hydrostatic pressure, counteracts the osmotic gradient .^1,2

The pressure required to prevent net osmotic water movement is called osmotic pressure. It depends on the concentration of dissolved particles and, across a particular membrane, on how effectively those particles are restricted from crossing it. A solute that crosses a membrane readily produces little sustained osmotic effect across that membrane, whereas an effectively impermeant solute can maintain a gradient that drives water movement .^1,2

Osmosis is related to diffusion but is not identical to it. Diffusion broadly describes the net movement of particles down a concentration gradient, whereas osmosis specifically concerns water movement across a selectively permeable barrier. In biological systems, osmosis helps determine how water is distributed between fluid compartments and whether cells swell, shrink, or maintain their volume .¹