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[[Image:Semipermeable membrane.png|thumb|250px|Scheme of semipermeable membrane during [[hemodialysis]], where red is blood, blue is the dialysing fluid, and yellow is the membrane.]]
A '''semipermeable membrane''', also termed a '''selectively-permeable membrane''', a '''partially-permeable membrane''' or a '''differentially-permeable membrane''', is a membrane that will allow certain [[molecule]]s or [[ion]]s to pass through it by [[diffusion]] and occasionally specialized "[[facilitated diffusion]]." The rate of passage depends on the [[pressure]], [[concentration]], and [[temperature]] of the molecules or [[solute]]s on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on solute size, [[solubility]], properties, or chemistry. An example of a semi-permeable membrane is a [[lipid bilayer]], on which is based the [[plasma membrane]] that surrounds all [[cell (biology)|biological cell]]s. Many natural and synthetic materials thicker than a membrane are also semipermeable.  An example of this is the thin film on the inside of an [[Egg (biology)|egg]].

One example of a semipermeable membrane is a [[phospholipid bilayer]], a group of [[phospholipids]] (consisting of a phosphate head and 2 fatty-acid tails) arranged into a double-layer, with the hydrophilic phosphate heads exposed to the water content outside and within the cell and the hydrophobic fatty-acid tails hidden in the inside. The phospholipid bilayer is the most permeable to small, uncharged solutes. Protein channels float through the phospholipids, and, collectively, this model is known as the [[fluid mosaic model]].

In the process of [[reverse osmosis]], [[Thin film composite membrane|Thin film composite membranes]] (TFC or TFM) are used. Thin film composite membranes are semipermeable membranes manufactured principally for use in [[water purification]] or [[desalination]] systems. They also have use in chemical applications such as batteries and fuel cells. In essence, a TFC material is a molecular sieve constructed in the form of a film from two or more layered materials.

Membranes used in reverse osmosis are, in general, made out of [[polyimide]], chosen primarily for its permeability to water and relative impermeability to various dissolved impurities including salt ions and other small molecules that cannot be filtered. Another example of a semipermeable membrane is [[dialysis tubing]].

== See also ==
* [[ADME]]
* [[Barrer]]
* [[Osmosis]]
* [[Reverse osmosis]]

==External links==
*[http://www.iupac.org/publications/pac/1996/pdf/6807x1479.pdf Membrane terminology. International Union of Pure and Applied Chemistry recommendations.]

[[Category:Diffusion]]
[[Category:Filters]]
[[Category:Membrane biology]]
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[[cs:Polopropustná membrána]]
[[da:Semipermeabel membran]]
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[[es:Membrana semipermeable]]
[[fr:Membrane hémiperméable]]
[[he:ממברנה חדירה למחצה]]
[[ja:半透膜]]
[[pl:Membrana półprzepuszczalna]]
[[zh:半透膜]]

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