Editing Differential centrifugation (section)

==Ultracentrifugation==
The lysed sample is now ready for centrifugation in an [[ultracentrifuge]]. An ultracentrifuge consists of a refrigerated, low-pressure chamber containing a rotor which is driven by an electrical motor capable of high speed rotation. Samples are placed in tubes within or attached to the rotor. Rotational speed may reach up to 100,000 rpm for floor model, 150,000 rpm for bench-top model (Beckman Optima Max-XP or Sorvall MTX150 or himac CS150NX), creating centrifugal speed forces of 800,000g to 1,000,000g. This force causes [[sedimentation]] of macromolecules, and can even cause non-uniform distributions of small molecules.<ref>{{cite journal |last1=Taylor |first1=Douglas D. |last2=Shah |first2=Sahil |title=Methods of isolating extracellular vesicles impact down-stream analyses of their cargoes |journal=Methods |date=1 October 2015 |volume=87 |pages=3–10 |doi=10.1016/j.ymeth.2015.02.019|pmid=25766927 }}</ref>

Since different fragments of a cell have different sizes and densities, each fragment will settle into a pellet with different minimum centrifugal forces. Thus, separation of the sample into different layers can be done by first centrifuging the original lysate under weak forces, removing the pellet, then exposing the subsequent supernatants to sequentially greater centrifugal fields. Each time a portion of different density is sedimented to the bottom of the container and extracted, and repeated application produces a rank of layers which includes different parts of the original sample. Additional steps can be taken to further refine each of the obtained pellets.

Sedimentation depends on mass, shape, and [[partial specific volume]] of a macromolecule, as well as solvent density, rotor size and rate of rotation. The sedimentation velocity can be monitored during the experiment to calculate [[molecular weight]].
Values of [[sedimentation coefficient]] (S) can be calculated. Large values of S (faster sedimentation rate) correspond to larger molecular weight. Dense particle sediments more rapidly. Elongated proteins have larger frictional coefficients, and sediment more slowly to ensure accuracy.
<ref>{{cite book |last1=Vance |first1=Dennis E. |last2=Vance |first2=J. E. |title=Biochemistry of Lipids, Lipoproteins and Membranes |publisher=Elsevier |isbn=978-0-08-086092-3 |chapter-url=https://books.google.com/books?id=EGMuwXqDbkcC&q=%22Structure%2C+assembly+and+secretion+of+lipoproteins%22&pg=PA473 |language=en |chapter=Structure, assembly and secretion of lipoproteins|date=6 August 1996 }}</ref>