When a plant wilts, it slumps and becomes a soft mess. Wilting occurs when water leaves the cells of the plant and disturbs its internal pressure. This is an example of one of the three types of tonicity. A solution can have three categories of tonicity relative to another solution: hypertonic, hypotonic, and isotonic. Along with osmosis, tonicity is integral for biological systems to operate properly. Each type of tonicity describes how osmosis will occur between different solutions.
To understand tonicity, it is important to first understand osmosis. The net movement of water across a semipermeable membrane from an area with a lower solute concentration to an area with a higher solute concentration is osmosis. A solute can be any material or cell that a solvent dissolves. Together, they form a solution. In the human body, solutes can be sugar, urea, potassium, or a variety of other materials. Osmosis is the primary means through which water enters and exits cells.
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