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Tonicity is a measure of the effective osmotic pressure gradient, as defined by the water potential of two solutions separated by a semipermeable membrane. In other words, tonicity is the relative concentration of solutions that determine the direction and extent of diffusion. 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 exert an effective 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.
There are three classifications of tonicity that one solution can have relative to another. The three are hypertonic, hypotonic, and isotonic.
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Hypertonic refers to a greater concentration of solutes. In biology, a hypertonic solution is one with a higher concentration of solutes outside the cell than inside the cell. When a cell is immersed into a hypertonic solution, the tendency is for water to flow out of the cell in order to balance the concentration of the solutes. Likewise, the cytosol of the cell is conversely categorized as hypotonic, opposite of the outer solution.
When plant cells are in a hypertonic solution, 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.
Hypotonic refers to a lesser concentration of solutes. In biology, a hypotonic solution has a lower concentration of solutes outside the cell than inside the cell. In an attempt to balance the concentrations of solutes inside and outside the cell, water will rush into the cell, and can cause it to burst (in the case of animal cells). When plant cells are in a hypotonic solution, the central vacuole takes on extra water and pushes the cell membrane against the cell wall. Due the rigidity of the cell wall, it pushes back preventing the cell from bursting. This is called turgor pressure.
An isotonic solution is one in which its effective osmole concentration is the same as the solute concentration of a cell. In this case the cell neither swells nor shrinks because there is no concentration gradient for water across the cell membrane. Water molecules diffuse through the plasma membrane in both directions, and as the rate of water diffusion is the same in each direction that cell will neither gain nor lose water. An ISO-osmolar solution can be hypotonic if the solute is able to penetrate the cell membrane. For example an ISO-osmolar urea solution is hypotonic to red blood cells causing their lysis. This is due to urea entering the cell down its concentration gradient followed by water. For example, the osmolarity of normal saline, 9 grams NaCl dissolved in water to a total volume of one litre, is a close approximation to the osmolarity of NaCl in blood (about 290 mOsm/L). Thus, normal saline is almost isotonic to blood plasma. Neither sodium nor chloride ions can freely pass through the plasma membrane, unlike urea.