55S
Ribosomes - Organelles like mitochondria and chloroplasts which have an independent protein synthesis machinery also contain ribosomes.
These ribosomes were formerly thought to be of the 70S prokaryote type. The sedimentation coefficient of mitochondrial ribosomes has, however, been shown to be about 55S, with a large subunit of 40S and a small subunit of 308.
The large subunit contains 16.17S and 5S RNA and the small subunit 12-13S RNA.
