Svedberg unit

The Svedberg unit (Symbol S) is a measure of the sedimentation rate of a particle when centrifuged. More precisely, it is a measure of time and is equal to the value of 100 femtoseconds (10^-13 seconds). The sedimentation rate indicates the size of the molecule, with the larger molecules having a larger sedimentation coefficient. It is dependent on the particle's volume, shape and molecular mass. If a particle is heavier with a more compact shape, its Svedberg value will be greater than lighter particles with a less compact shape.

It is named after Theodor Svedberg (1884-1971), a Swedish chemist awarded the Nobel Prize in 1926 for the discovery of the ultracentrifuge.

The Use of the Svedberg Unit in Ribosomes

The ribosomes in prokaryotes are differentiated by their values of sedimentation rate, and this is how they are named. The 70S ribosome, which is key in protein translation, is made up of 50S and 30S ribosomes. These subunits have sedimentation times of 70 x 10^-13, 50 x 10^-13 and 30 x 10^-13 seconds respectively. The 70S ribosome is the largest, and so will sediment fastest, whereas 30S will sediment more slowly as it is the smallest subunit.