Tonicity
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Tonicity measures the ability of a solution to exert an osmotic pressure upon the membrane. Osmolality and osmolarity measure concentration of the solutes independently of their ability to cross the membrane. Hence, they do not measure the degree of osmotic pressure. Tonicity is the concentration of only the solutes that cannot cross the membrane, as only these solutes exert an osmotic pressure upon that membrane. Permeant solutes do not affect tonicity; impermeant solutes do affect it. One solution in relation to another might be hypertonic, or hypotonic, or isotonic (see below). In biology, the relative tonicity of a solution is defined in reference to that of the cytosol tonicity.
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[edit] Hypertonicity
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Look up hypertonic in Wiktionary, the free dictionary. |
A hypertonic solution contains a greater concentration of the impermeable solutes than cytosol. The resulting osmotic pressure causes a net movement of water out of the cell.
[edit] Hypotonicity
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Look up hypotonic in Wiktionary, the free dictionary. |
A hypotonic solution contains a lower concentration of impermeable solutes than cytosol. The resulting osmotic pressure causes a net movement of water into the cell.
[edit] Isotonicity
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Look up isotonic in Wiktionary, the free dictionary. |
A cell is surrounded by an environment that has the same concentration of solute as the cell. The water and permeable solutes will always be flowing, but the net movement of the water and permeable solutes is zero.
An isotonic solution is a solution in which the concentration of solutes is essentially equal to that of cytosol of the cell placed in that solution. There is no net osmotic pressure on a membrane placed between 2 isotonic solutions.
Isotonic saline solution for medical applications has a sodium chloride concentration of 9g per litre of water or 0.9%(w/v).
[edit] Effect on cells
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. 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.
