View source for Svedberg

{{Short description|Non-SI unit for sedimentation coefficients}}
{{For|the surname|Svedberg (surname)}}
[[File:Beckman-Coulter ultracentrifuge XL-100K -01.jpg|thumb|250px|A laboratory ultracentrifuge.]]

In [[chemistry]], a '''Svedberg unit''' or '''svedberg''' (symbol '''S''', sometimes '''Sv'''{{Efn|There is an [[SI]] unit with the same Sv abbreviation, the [[sievert]].}}) is a non-[[International System of Units|SI]] [[Metric_units#Time|metric unit]] for [[sedimentation coefficient]]s. The Svedberg unit offers a measure of a [[Particle (ecology)|particle]]'s size indirectly based on its [[sedimentation]] rate under acceleration (i.e. how fast a particle of given size and shape [[Settling|settles]] out of [[Suspension (chemistry)|suspension]]).<ref name="foster"/> The svedberg is a measure of time, defined as exactly 10<sup>−13</sup> [[second]]s (100&nbsp;[[femtosecond|fs]]).

For biological [[macromolecule]]s <!-- ribosomes are not organelles -->like [[ribosome]]s, the sedimentation rate is typically measured as the rate of travel in a [[centrifuge]] tube subjected to high [[g-force]].<ref name="foster"/> 

The svedberg (S) is distinct from the SI unit [[sievert]] or the non-SI unit [[sverdrup]], which also use the symbol Sv, and to the SI unit [[Siemens (unit)|Siemens]] which uses the symbol S too.

==Naming==
The unit is named after the [[Sweden|Swedish]] [[chemist]] [[Theodor Svedberg]] (1884–1971), winner of the 1926 [[Nobel Prize]]<ref>{{Cite web|url=https://www.nobelprize.org/nobel_prizes/chemistry/laureates/1926/|title = The Nobel Prize in Chemistry 1926}}</ref> in [[chemistry]] for his work on disperse systems, [[Colloid|colloids]] and his invention of the [[ultracentrifuge]].<ref>{{Cite web |last=Koehler |first=Christopher S. W. |date=2003 |title=Developing the Ultracentrifuge |url=https://pubsapp.acs.org/subscribe/archive/tcaw/12/i02/pdf/203chronicles.pdf? |archive-url=https://web.archive.org/web/20231002182851/https://pubsapp.acs.org/subscribe/archive/tcaw/12/i02/pdf/203chronicles.pdf |archive-date=2 October 2023 |access-date=}}</ref>

==Factors==
The Svedberg coefficient is a nonlinear function.<ref name="foster"/> A particle's mass, density, and shape will determine its S value. The S value depends on the frictional forces retarding its movement, which, in turn, are related to the average cross-sectional area of the particle.<ref name="foster">{{cite book |last1=Slonczewski |first1=Joan |first2=John Watkins |last2=Foster |title=Microbiology: An Evolving Science |location=New York |publisher=W.W. Norton |year=2009 |isbn=9780393978575 }}</ref>

The [[sedimentation coefficient]] is the ratio of the speed of a substance in a centrifuge to its acceleration in comparable units. A substance with a sedimentation coefficient of 26S ({{val|26|e=-13|u=s}}) will travel at 26 [[micrometre|micrometers]] per second ({{val|26|e=-6|u=m/s}}) under the influence of an acceleration of 1 million gravities (10<sup>7</sup>&nbsp;m/s<sup>2</sup>).{{efn|1= 1 G=9.8&nbsp;m/s<sup>2</sup>, i.e. approx 10&nbsp;m/s<sup>2</sup>; A million G = 10<sup>6</sup> x 10&nbsp;m/s<sup>2</sup> = 10<sup>7</sup>&nbsp;m/s<sup>2</sup>}} Centrifugal acceleration is given as ''rω''{{i sup|2}}; where ''r'' is the radial distance from the rotation axis and ''ω'' is the [[angular velocity]] in radians per second.

Bigger particles tend to sediment faster and so have higher Svedberg values.

Svedberg units are not directly additive since they represent a rate of sedimentation, not weight.<ref name="foster"/>

==Use==
In centrifugation of small biochemical species, a convention has developed in which sedimentation coefficients are expressed in the Svedberg units.

* The svedberg is the most important measure used to distinguish [[ribosome]]s. Ribosomes are composed of two complex subunits, each including [[rRNA]] and [[protein]] components. In [[prokaryotes]] (including bacteria), the subunits are named [[30S]] and [[50S]] for their "size" in Svedberg units. These subunits are made up of three forms of [[rRNA]]: [[16S ribosomal RNA|16S]], [[23S ribosomal RNA|23S]], and [[5S ribosomal RNA|5S]] and [[ribosomal protein]]s.<ref name="foster" />

* For bacterial ribosomes, ultracentrifugation yields intact ribosomes ([[70S]]) as well as separated ribosomal subunits, the large subunit ([[50S]]) and the small subunit ([[30S]]). Within cells, ribosomes normally exist as a mixture of joined and separate subunits. The largest particles (whole ribosomes) sediment near the bottom of the tube, whereas the smaller particles (separated 50S and 30S subunits) appear in upper fractions.<ref name="foster" />

==See also==
* [[Sedimentation coefficient]]
* [[Differential centrifugation]]

==Footnotes==
{{notelist|1}}

==References==
{{Reflist}}

==External links==
* [http://nobelprize.org/educational_games/medicine/dna/a/translation/svedberg_unit.html Svedberg unit] - nobelprize.org

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[[Category:Units of time]]
[[Category:Non-SI metric units]]