Osmosis

<p>Osmosis  is the spontaneous net movement or <a href="/facts/Diffusion/KbAxroBa">diffusion</a> of <a href="/facts/Solvent/LoMPMz8S">solvent</a> molecules through a <a href="/facts/Selectively_permeable_membrane/YxENKzn5">selectively-permeable membrane</a> from a region of high <a href="/facts/Water_potential/ndU4I5Hz">water potential</a> (region of lower <a href="/facts/Solute/MGchhMQV">solute</a> concentration) to a region of low water potential (region of higher solute concentration), in the direction that tends to equalize the solute concentrations on the two sides. It may also be used to describe a physical process in which any solvent moves across a selectively permeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations. Osmosis can be made to do <a href="/facts/Work_(physics)/mbclFPBs">work</a>. <a href="/facts/Osmotic_pressure/6oWKTsXV">Osmotic pressure</a> is defined as the external <a href="/facts/Pressure/eyc37GRG">pressure</a> required to prevent net movement of solvent across the membrane.  Osmotic pressure is a <a href="/facts/Colligative_properties/8LEPNRpn">colligative property</a>, meaning that the osmotic pressure depends on the <a href="/facts/Molar_concentration/P2WL5b9q">molar concentration</a> of the solute but not on its identity.
</p><p>Osmosis is a vital process in <a href="/facts/Biology/1M1ee3pB">biological systems</a>, as <a href="/facts/Biological_membrane/ljBsfK4r">biological membranes</a> are semipermeable. In general, these membranes are impermeable to large and <a href="/facts/Chemical_polarity/KlIJ0zjB">polar</a> molecules, such as <a href="/facts/Ions/wrbwhF3z">ions</a>, <a href="/facts/Proteins/Jxmagu3E">proteins</a>, and <a href="/facts/Polysaccharides/lqXq2cJU">polysaccharides</a>, while being permeable to non-polar or <a href="/facts/Hydrophobic/Ifya0CXQ">hydrophobic</a> molecules like <a href="/facts/Lipids/TdQR87HX">lipids</a> as well as to small molecules like oxygen, carbon dioxide, nitrogen, and nitric oxide. Permeability depends on solubility, charge, or chemistry, as well as solute size. Water molecules travel through the plasma membrane, tonoplast membrane (vacuole) or organelle membranes by diffusing across the phospholipid bilayer via <a href="/facts/Aquaporin/d9pI3N4C">aquaporins</a> (small transmembrane proteins similar to those responsible for facilitated <a href="/facts/Diffusion/KbAxroBa">diffusion</a> and ion channels). Osmosis provides the primary means by which <a href="/facts/Water/0lkXnJPK">water</a> is transported into and out of <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a>. The <a href="/facts/Turgor_pressure/ptUbow92">turgor pressure</a> of a cell is largely maintained by osmosis across the cell membrane between the cell interior and its relatively hypotonic environment.
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Osmosis open-in-new

Osmosis