In biology, a hypertonic cell environment has a higher concentration of solutes than inside the animal or plant cell. The ability of a solution to change the shape or tone of cells by altering their internal water volume is called tonicity (tono = tension). In a hypertonic environment, osmotic pressure causes water to flow out of the cell. If enough water is removed in this way, the cytoplasm will have such a small concentration of water that the cell has difficulty functioning.
A cell that is in a hypertonic environment has a higher concentration of solute in the environment than in the inside of the cell, making the net flow of water out of the cell. A cell in a hypotonic environment has a lower concentration of solute in the environment than in the inside of the cell, making the net flow of water into the cell and eventually causing cell lysis.
In animal cells, being in a hypertonic environment results in crenation, where the shape of the cell becomes distorted and wrinkled as water leaves the cell.
In plant cells, the effect is more dramatic. The cell membrane pulls away from the cell wall, but the cell remains joined to the adjacent cells at points called plasmodesmata. Thus, the cell takes on the appearance of a pincushion, with the plasmodesmata almost ceasing to function because they have become so constricted. This condition is known as plasmolysis.
Saltwater is hypertonic to the fish that live in it. The fish cannot isolate themselves from osmotic water loss, because they need a large surface area in their gills for gas exchange. They respond by drinking large amounts of water, and excreting the salt through their kidneys. This process is called osmoregulation.
The opposite of a hypertonic environment is a hypotonic one, where the net movement of water is into the cell; the intermediate state is called isotonic, where there is no net movement of water. This does not mean, however, that water is not moving; it means that water is moving both ways but with equal velocity, which equals zero net change.
"...a term describing organisms with body fluids with a lower concentration of water and higher solute concentration than the external environment." Taken from Ecology by Manuel C. Molles Jr.
This means that if a cell is hyperosmotic, it absorbs water from the surroundings to dilute the higher solute concentration, thus making the cell isotonic to the environment.