When we take a look at biology textbooks (Campbell, Pearson etc), they will only mention the difference between eukaryotic and prokaryotic ribosome sedimentation rates. Why is sedimentation rate numerically important for ribosomes and not for other organelles like the mitochondria or the nucleus?
- Ultracentrifugation was necessary to identify and ribosomes, and characterize them on the basis of their size. The method does not allow an absolute measurement of molecular mass, but uses a relative quantitation as sedimentation coefficient, expressed in Svedberg Units (S).
- Nuclei and mitochondria were characterized by light microscopy and, being much larger than ribosomes, sediment at speeds which do not require an ultracentrifuge and would not be resolved in an ultracentrifuge.
- Nuclei and mitochondria vary in size, so their size is not a defining property for them. Ribosomes from one source all have the same size, which may differ from that of ribosomes from other sources in a definitive manner.
- Ribosomes are composed of smaller subunits, which are assembled from precursor particles of a different size. They occur in larger complexes on mRNA (polysomes). These forms of different but specific size can be separated and characterized by on the basis of size in the ultracentrifuge — i.e. by their sedimentation coefficient.
Amplification and examples
Prokaryotic monoribosomes (e.g. E. coli) have a sedimentation value of 70S, the cytoplasmic monoribosomes of eukaryotes (e.g. rat) 80S, and those of mammalian mitochondria between 50 and 60S. The values may differ for different eukaryotes and prokaryotes, but are constant for one species, reflecting the constant ribonucleoprotein composition of the particles.
All ribosomes have two subunits — a large subunit (50S and 60S for the prokaryotes and eukaryotes mentioned, respectively) and a small subunit (30S and 40S). The subunits are identified by ultracentrifugation, which is also used to separate them preparatively. Hence it is natural that their S values should be used to name them.
The formation of ribosomal subunits from RNA and protein involves sequential addition of proteins. The different sized intermediates at different stages can be differentiated by their sedimentation, and it is natural to refer to them by their S values.
Ultracentrifugation is also used to separate and characterize the ribosomal RNAs in a similar manner.
The Wikipedia entry on ribosomes corroborates some of the factual data I have presented.
The Svedberg unit is measurement used to characterize the ribosomal subunits. And it represents rate of sedimentation in centrifugation process instead of size.
But every organelle and sub-cellular material has its own sedimentation rate. On basis of this characteristic, differential centrifugation technique works.
In case of ribosomes, it may be used so that it becomes easy to differentiate between different subunits.