What is the sedimentation coefficient? Explain what the 'S' value signifies in 70S and 80S ribosomes.

The sedimentation coefficient (S) is a measure of a particle's sedimentation rate in a centrifuge. It is a non-additive value that depends on the particle's mass, shape, and density. The 'S' stands for Svedberg unit. In ribosomes, the 'S' value signifies the rate at which a ribosome (or its subunits) sediments during ultracentrifugation. 70S ribosomes are characteristic of prokaryotic cells, mitochondria, and chloroplasts, while 80S ribosomes are found in eukaryotic cells. The difference in S-values reflects differences in their size, mass, and composition.