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<title>Tonicity</title>

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</style><div role="note" class="hatnote navigation-not-searchable">"Hypotonic" and "Hypertonic" redirect here. For the physical diseases, see <a href="Hypotonia" title="Hypotonia">Hypotonia</a> and <a href="Hypertonia" title="Hypertonia">Hypertonia</a>.</div>

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<p>In <a href="Chemical_biology" title="Chemical biology">chemical biology</a>, <b>tonicity</b> is a measure of the effective <a href="Osmotic_pressure" title="Osmotic pressure">osmotic pressure</a> gradient; the <a href="Water_potential" title="Water potential">water potential</a> of two <a href="Solution_(chemistry)" title="Solution (chemistry)">solutions</a> separated by a <a href="Semipermeable_membrane" title="Semipermeable membrane">partially-permeable</a> cell <a href="Cell_membrane" title="Cell membrane">membrane</a>. Tonicity depends on the relative <a href="Concentration" title="Concentration">concentration</a> of selective membrane-impermeable <a href="Solutes" class="mw-redirect" title="Solutes">solutes</a> across a cell membrane which determines the direction and extent of osmotic <a href="Flux" title="Flux">flux</a>. It is commonly used when describing the swelling-versus-shrinking response of <a href="Cell_(biology)" title="Cell (biology)">cells</a> immersed in an external solution.

</p><p>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 equilibrate with equal concentrations on both sides of the membrane without net solvent movement. It is also a factor affecting <a href="Imbibition" title="Imbibition">imbibition</a>.

</p><p>There are three classifications of tonicity that one solution can have relative to another: <i>hypertonic</i>, <i>hypotonic</i>, and <i>isotonic</i>.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> A hypotonic solution example is distilled water.

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<div class="mw-heading mw-heading2"><h2 id="Hypertonic_solution">Hypertonic solution</h2></div>

<p>A hypertonic solution has a greater concentration of non-permeating <a href="Solute" class="mw-redirect" title="Solute">solutes</a> than another solution.<sup id="cite_ref-Biodiction_2-0" class="reference"><a href="#cite_note-Biodiction-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> In biology, the tonicity of a solution usually refers to its solute concentration relative to that of another solution on the opposite side of a <a href="Cell_membrane" title="Cell membrane">cell membrane</a>; a solution outside of a cell is called hypertonic if it has a greater concentration of solutes than the <a href="Cytosol" title="Cytosol">cytosol</a> inside the cell. When a cell is immersed in a hypertonic solution, osmotic pressure tends to force water to flow out of the cell in order to balance the concentrations of the solutes on either side of the cell membrane. The cytosol is conversely categorized as hypotonic, opposite of the outer solution.<sup id="cite_ref-med_lib_3-0" class="reference"><a href="#cite_note-med_lib-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-cureus_4-0" class="reference"><a href="#cite_note-cureus-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>

</p><p>When plant cells are in a hypertonic solution, the flexible cell membrane pulls away from the rigid <a href="Cell_wall" title="Cell wall">cell wall</a>, but remains joined to the cell wall at points called <a href="Plasmodesmata" class="mw-redirect" title="Plasmodesmata">plasmodesmata</a>. The cells often take on the appearance of a <a href="Pincushion" title="Pincushion">pincushion</a>, and the plasmodesmata almost cease to function because they become constricted, a condition known as <a href="Plasmolysis" title="Plasmolysis">plasmolysis</a>. 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.<sup id="cite_ref-Cell_Bio_5-0" class="reference"><a href="#cite_note-Cell_Bio-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>

</p><p>Some organisms have evolved intricate methods of circumventing hypertonicity. For example, <a href="Seawater" title="Seawater">saltwater</a> is hypertonic to the <a href="Fish" title="Fish">fish</a> that live in it. Because the fish need a large surface area in their <a href="Gill" title="Gill">gills</a> in contact with seawater for <a href="Gas_exchange" title="Gas exchange">gas exchange</a>, they lose water osmotically to the sea from gill cells. They respond to the loss by drinking large amounts of saltwater, and actively <a href="Excretion" title="Excretion">excreting</a> the excess salt.<sup id="cite_ref-Soult_6-0" class="reference"><a href="#cite_note-Soult-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> This process is called <a href="Osmoregulation" title="Osmoregulation">osmoregulation</a>.<sup id="cite_ref-ortiz_7-0" class="reference"><a href="#cite_note-ortiz-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Hypotonic_solution">Hypotonic solution</h2></div>

<p>A hypotonic solution has a lower concentration of solutes than another solution. In biology, a solution outside of a cell is called hypotonic if it has a lower concentration of solutes relative to the <a href="Cytosol" title="Cytosol">cytosol</a>. Due to <a href="Osmotic_pressure" title="Osmotic pressure">osmotic pressure</a>, water diffuses into the cell, and the cell often appears <a href="Turgid" class="mw-redirect" title="Turgid">turgid</a>, or bloated.

For cells without a <a href="Cell_wall" title="Cell wall">cell wall</a> such as animal cells, if the gradient is large enough, the uptake of excess water can produce enough pressure to induce <a href="Cytolysis" title="Cytolysis">cytolysis</a>, or <a href="Lysing" class="mw-redirect" title="Lysing">rupturing</a> of the cell.

When plant cells are in a hypotonic solution, the central <a href="Vacuole" title="Vacuole">vacuole</a> takes on extra water and pushes the cell membrane against the cell wall. Due to the rigidity of the cell wall, it pushes back, preventing the cell from bursting. This is called <a href="Turgor_pressure" title="Turgor pressure">turgor pressure</a>.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Isotonicity">Isotonicity</h2></div>

<p>A solution is isotonic when its effective <a href="Osmole_(unit)" class="mw-redirect" title="Osmole (unit)">osmole</a> concentration is the same as that of another solution. In biology, the solutions on either side of a cell membrane are isotonic if the concentration of solutes outside the cell is equal to the concentration of solutes inside the cell. In this case the cell neither swells nor shrinks because there is no concentration gradient to induce the diffusion of large amounts of water across the cell membrane. Water molecules freely diffuse through the plasma membrane in both directions, and as the rate of water diffusion is the same in each direction, the cell will neither gain nor lose water.

</p><p>An iso-osmolar solution can be hypotonic if the solute is able to penetrate the cell membrane. For example, an iso-osmolar <a href="Urea" title="Urea">urea</a> solution is hypotonic to red blood cells, causing their <a href="Lysis" title="Lysis">lysis</a>. This is due to urea entering the cell down its concentration gradient, followed by water. The osmolarity of <a href="Saline_(medicine)" title="Saline (medicine)">normal saline</a>, 9 grams <a href="Sodium_chloride" title="Sodium chloride">NaCl</a> dissolved in water to a total volume of one liter, is a close approximation to the osmolarity of NaCl in blood (about 290 <a href="Osmole_(unit)" class="mw-redirect" title="Osmole (unit)">mOsm</a>/<a href="Litre" title="Litre">L</a>). Thus, normal saline is almost isotonic to blood plasma. Neither sodium nor chloride ions can freely pass through the plasma membrane, unlike <a href="Urea" title="Urea">urea</a>.

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<ul><li><a href="Osmotic_concentration" title="Osmotic concentration">Osmotic concentration</a></li>

<li><a href="Osmosis" title="Osmosis">Osmosis</a></li>

<li><a href="Salinity" title="Salinity">Salinity</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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