Ribosomes

Ribosomes are particles of ribonucleoprotein, each built from a large and a small subunit, and their sizes are expressed in Svedberg units (S), which measure the rate of sedimentation in a centrifuge. As pointed out in NCERT Class 11, Chapter 8 (Cell: The Unit of Life), the eukaryotic cytoplasmic ribosome is of the 80S type, formed by the association of a 60S large subunit and a 40S small subunit. The figure of 100S is wrong because the Svedberg coefficient depends on mass, shape and density together, so the values of the two subunits do not add up arithmetically; the assembled particle sediments at 80S, not at the sum 100S. The value 70S describes prokaryotic ribosomes made of 50S and 30S subunits, which is not the case here. The 55S figure refers to the smaller ribosomes inside mitochondria and chloroplasts of certain organisms, again not the cytoplasmic eukaryotic ribosome. The reason the values are non-additive is that sedimentation rate rises with mass but falls with a less compact shape, and when the two subunits join, the assembled particle becomes more compact relative to its combined mass. This is the same principle that makes the prokaryotic 50S and 30S subunits combine into a 70S, not an 80S, ribosome. A numerical sanity check reinforces the concept: since S values are non-additive, the only consistent answer for a 60S plus 40S combination is the experimentally observed 80S ribosome.