The Svedberg Unit (S)

As ribosomal particles were first isolated from cell lysates by ultracentrifugation, the ribosomes and their sub-particles were named according to their sedimentation characteristics during centrifugation. The sedimentation properties of a particle depends on its molecular size and geometrical shape. The sedimentation characteristics also depend on the physical properties of the solution through which the particle is sedimenting.

The two eukaryotic ribosomal subunits have sedimentation coefficients of 40 x 10^-13 and 60 x 10^-13. As one Svedberg (S) unit is 10^-13, the two ribosomal subunits are referred to as the 40S and the 60S ribosomal subunits.

The molecular mass of the 40S and 60S particles are 1.5 and 3.0 million g/mol, respectively. Thus, the complete ribosome has a mass of approximately 4.5 million g/mol. The complete ribosome is referred to as the 80S ribosome.