A biological cell, for example a plant cell, contains ions, sugars, amino acids, and other substances. In a hypotonic environment, water flows across the plasma membrane into the cell (since the concentration of water is lower inside the cell than outside), causing it to expand. The cell wall of a plant cell restricts the expansion, causing the cell to press against the wall. The resulting pressure is called turgor.
The osmotic pressure π of a dilute solution can be calculated using the formula
Note the similarity of the above formula to the ideal gas law.