tonicity

Effect of different solutions on blood cells

Plant cell under different environments

Tonicity is a measure of the osmotic pressure of two solutions separated by a semipermeable membrane. It is commonly used when describing the response of cells immersed in an external solution. Like osmotic pressure, tonicity is influenced only by solutes that cannot cross the membrane, as only these exert an osmotic pressure. Solutes able to freely cross the membrane do not affect tonicity because they will always be in equal concentrations on both sides of the membrane.

Osmotic pressure is the pressure that must be applied to a solution to prevent the inward flow of water across a semipermeable membrane

Classification

There are three classifications of tonicity that one solution can have relative to another. The three are hypertonic, hypotonic, and isotonic.

Hypertonicity

Look up hypertonic in Wiktionary, the free dictionary.

A hypertonic solution contains a greater concentration of impermeable solutes than the solution on the other side of the membrane.^[1]

Hypotonicity

A hypotonic solution contains a lesser concentration of impermeable solutes than the solution on the other side of the membrane.^[1]

Isotonicity

Isotonicity is the term used when there is an equal concentration of solutes.

Effect on cells

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In eukaryotic animal cells, a hypertonic environment forces water to leave the cell so that the shape of the cell becomes distorted and wrinkled, a state known as crenation. In plant cells, the effect is more dramatic. The flexible cell membrane pulls away from the rigid cell wall, but remains joined to the cell wall at points called plasmodesmata. The cell takes on the appearance of a pincushion, and the plasmodesmata almost cease to function because they become constricted — a condition known as plasmolysis. In plant cells the terms isotonic, hypotonic and hypertonic cannot strictly be used accurately because the pressure exerted by the cell wall significantly affects the osmotic equilibrium point.

Some organisms have evolved intricate methods of circumventing hypertonicity. For example, saltwater is hypertonic to the fish that live in it. They need a large surface area in their gills in contact with seawater for gas exchange, thus they lose water osmotically to the sea from gill cells. They respond to the loss by drinking large amounts of saltwater, and actively excreting the excess salt. This process is called osmoregulation.

In a hypotonic environment, animal cells will swell until they burst, a process known as cytolysis. Fresh water fish urinate constantly to prevent cytolysis. Plant cells tend to resist bursting, due to the reinforcement of their cell wall, which provides effective osmolarity or osmolality.

In some cases of suspensions intended for intramuscular injection, a slightly hypertonic solution is preferred in order to increase the dissolution and absorption of the drug by absorbing water from the surrounding tissues.

See also

• Osmosis
• Osmole (unit)

Notes

• Osmosis
• Osmole (unit)