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This pressure, turgidity, is caused by the osmotic flow of water from area of low solute concentration outside of the cell into the cell's vacuole, which has a higher solute concentration. Healthy plant cells are turgid and plants rely on turgidity to maintain rigidity. In contrast, this phenomenon is not observed in animal cells which have no cell walls to prevent them from being [cytolysis|burst] by the flow of water into the cell and must either [contractile vacuole|continually pump out water] or live in an isotonic solution where there is no osmotic pressure.
A physical phenomenon known as osmosis causes water to flow from an area of low solute concentration to an area of high solute concentration until the two areas have an equal ratio of solute to water. Normally, the solute diffuses toward equilibrium as well; however, all cells are surrounded by a lipid bilayer cell membrane which permits the flow of water in and out of the cell but restricts the flow of solute under many circumstances. As a result, when a cell is placed in a hypotonic solution, water rushes into the membrane, increasing the cell's volume.
Eventually, the cell's membrane is enlarged such that it pushes against the cell's rigid wall. At this point the cell is said to be turgid. In an isotonic solution, water flows into the cell at the same rate it flows out. The pressure pushing the cell's membrane against its wall is reduced and the cell is said to be 'flaccid'. When a cell is placed in a hypertonic solution water actually flows out of the cell into the surrounding solution. This, plasmolysis, causes the membrane to recede from the wall and is responsible for wilting in plant cells.