Tonicity
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 presure, 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.
Classification
There are three classifications of tonicity that one solution can have relative to another. The three are hypertonic, hypotonic, and isotonic.
Hypertonicity
A hypertonic solution contains a greater concentration of impermeable solutes than the solution on the other side of the membrane.[1]
If a cell is placed in a hypertonic solution, there will be a net movement of water out of the cell until the concentration of impermeable solutes in the cell equals that of the hypertonic solution.
Hypotonicity
A hypotonic solution contains a smaller concentration of impermeable solutes than the solution on the other side of the membrane.[1]
If a cell is placed in a hypotonic solution, there will be a net movement of water into the cell until the concentration of impermeable solutes in the cell equals that of the hypotonic solution.
Isotonicity
An isotonic solution contains an equal concentration of impermeable solutes as the solution on the other side of the membrane.[1]
If a cell is placed in an isotonic solution, there will be no net movement of water in or out of the cell because the concentration of impermeable solutes in the cell equals that of the external environment.
Isotonic saline solution for medical applications has a sodium chloride concentration of 9g per litre of water or 0.9%(w/v).
Effect on cells
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In EukaryotAnimal cell animal cell being in a hypertonic environment results in crenation where the shape of the cell becomes distorted and wrinkled as water leaves the cell. Some organisms have evolved intricate methods of circumventing hypertonicity; for example, saltwater is hypertonic to the fish that live in it. Since they 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. This process is called osmoregulation.
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. The terms isotonic, hypotonic and hypertonic cannot be accurately used in plant cells however as the pressure potential exerted by the cell wall affects the equilibrium point significantly.
Animal cells will swell until they burst in a hypotonic environment, which is called Cytolysis. Fresh water fish, like the salt water fish, have adapted to their environment, and constantly have to urinate 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 intramuscular suspensions a slightly hypertonic solution is preferred in order to absorb water from the surrounding tissues and to increase the dissolution and absorption of the drug.