Osmosis

Osmosis is the movement of water through a membrane that lets water pass but holds back some dissolved substances. Water moves toward the side with the higher concentration of solutes, which is the side with relatively less free water, so osmosis can be thought of as water diffusing down its own concentration gradient. It continues until the concentrations even out or until pressure building up on one side balances the flow.

Biologists describe a cell's surroundings by their tonicity. In a hypotonic solution, water enters the cell: animal cells such as red blood cells swell and may burst, while plant cells become firm with turgor pressure, held in by their cell walls. In a hypertonic solution, water leaves the cell, so animal cells shrivel and plant cells undergo plasmolysis, which makes the plant wilt. In an isotonic solution, there is no net movement of water.

Water often crosses membranes through channel proteins called aquaporins. Organisms control osmosis in many ways: the kidneys regulate the body's water balance, and freshwater protists use contractile vacuoles to pump out excess water.