Tonicity

Tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a semipermeable cell membrane. It is commonly used when describing the response of cells immersed in an external solution. Unlike osmotic pressure, tonicity is influenced only by solutes that cannot cross the membrane, as only these can cause water to move in or out of the cell.

Types of Tonicity

There are three classifications of tonicity that one can find in living organisms and cells: hypertonic, hypotonic, and isotonic.

Hypertonic

A hypertonic solution has a greater concentration of solutes than another solution. In biology, the tonicity of a solution usually refers to its solute concentration relative to that of another solution on the opposite side of a cell membrane; a solution outside of a cell is said to be hypertonic if it has a greater concentration of solutes than the cytosol inside the cell.

Hypotonic

A hypotonic solution has a lower concentration of solutes than another solution. In biology, a solution outside of a cell is called hypotonic if it has a lower concentration of solutes relative to the cytosol inside the cell.

Isotonic

An isotonic solution has an equal concentration of solutes as another solution. In biology, a solution is said to be isotonic when the concentration of solutes in the solution is the same as the concentration of solutes inside the cell.

Effects on Cells

The tonicity of a solution has a direct impact on cells. The tonicity of a solution can cause a cell to shrivel, make it swell, or keep it the same.

See Also

• Osmosis
• Cell membrane
• Solute
• Solution

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