During our group quiz today on The Cell, one of the questions was related to the size of Ribosomes in Prokaryotic and Eukaryotic cells. My friends and I joked around saying aloud “40S..40S..Yea man that is it.” Obviously it wasn’t one of the options but we were having fun! And by fun I mean trying to not give anyone the answer who didn’t know! Our little group knew that the size of Ribosome for Prokaryotic cells were slightly smaller than those of Eukaryotic cells; 70S and 80S respectfully. This had me thinking, I know that the non-SI Unit “S” represents the Svedberg unit but what does that really represent and what does that mean to me as a Biochemistry student? The entire day onwards it had me thinking that in all the comparisons between Prokaryotic and Eukaryotic cells that “S” stood out like a SUCKER!
What is a Ribosome?
Ribosomes are complex molecular structures that serve as the site for Protein Synthesis.
What is “S” in 70S/80S?
“S” represents the Svedberg unit, a non-SI unit for sedimentation rate. This is the rate at which particles of a given size and given shape travel to the bottom of a tube under centrifugal force.This reflects the rate at which a molecule sediments under the centrifugal force of a centrifuge, where sediment refer to the break down into smaller particles.
What does this mean to me as a Biochemistry student?
It simply represents the size of the subunits that make the Ribosome structure. Quoting from Nobelprize.org;
“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 depend 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.”
So now I understand what is the Svedberg unit and I know the guy who discovered it to ……..Swedish chemist Theodor Svedberg (1884–1971).
I wonder if he liked cheese. O_O
Reference: Nobelprize.org The official site of the Nobel Prize. 2013. “The Svedberg Unit (S)” Assessed January 30th, 2013. http://www.nobelprize.org/educational/medicine/dna/a/translation/svedberg_unit.html