Hypotonic
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In biology, a hypotonic solution has the lower osmotic pressure of two fluids and also describes a cell environment with a lower concentration of solutes than the cytoplasm of the cell. Given a cell placed in a hypotonic environment, osmosis causes a net flow of water into the cell, causing swelling and expansion. This swelling can cause the cell to burst.
Solutions and cell environments are also described, in terms of osmotic pressure, as being either hypotonic, hypertonic or isotonic.
[edit] Living in a Hypotonic Environment
Plants thrive in hypotonic environments. Their cells have rigid cell walls that prevents bursting, or lysis. The pressure of the cytoplasm against the cell wall keeps the plant from wilting and losing its shape. This pressure is called turgor pressure or osmotic pressure. On the other hand, cells without cell walls will swell and, if the environment is sufficiently hypotonic, burst (lyse) and die (referred to as cytolysis).
Some protists (such as Paramecium) counteract this with the use of contractile vacuoles that pump water rapidly out of the cell. Other organisms actually eject solutes from the cell in order to lower the concentration gradient of the solute in the cell and hopefully create an isotonic environment.