Osmosis

Osmosis is the movement of water across a semipermeable membrane, such as a cell membrane, from an area of lower dissolved solute concentration to an area of higher dissolved solute concentration.

The dissolved solutes cannot cross this semipermeable membrane on their own and remain separated, which establishes a concentration gradient across the membrane.

Imagine one liter of a solution. The total number of dissolved particles in it is its osmolarity. As the number of solute particles increases, osmolarity also increases. The same idea applies to biological cells. Osmolarity determines osmotic pressure, the minimum pressure required to prevent net water movement across a semipermeable membrane.

When the solute concentration inside the cell is higher than the concentration outside, an osmotic gradient forms, with relatively more water outside the cell.

As a result, water slowly moves from the region of lower osmolarity to the region of higher osmolarity. Small channel proteins called aquaporins help water move across the membrane much faster. This movement of water continues until osmotic equilibrium is reached.