Tonicity is the measure of difference in concentrations of two solutions, taking only the non-penetrating solutes into account. It helps to determine the osmotic pressure gradient and therefore the direction in which the water would flow. If a cell is placed in a hypertonic solution, i.e. a solution with more non-penetrating solutes than the cell, water will flow from the cell into the solution, thus causing the cell to shrink and eventually shrivel. If it is placed inside a hypertonic solution, with low concentration of non-penetrating solutes, water will flow from the solution into the cell, causing it to increase in size and eventually burst.
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Osmolarity, also known as osmotic concentration, is the measure of the solute concentration of a solution. It can also be regarded as the osmotic pressure of a solution.
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