Ribosomes are found in both prokaryotes and eukaryotes. They are complexes of proteins and three (prokaryotes) or four (eukaryotes) rRNA molecules called subunits. The main function of ribosomes is as the site of mRNA translation; once the two subunits are joined by the mRNA from the nucleus, the ribosome translates mRNA into a specific sequence of amino acids.
There are two types of Ribosomes: free and bound. Ribosomes canbe found at various locations within the cell; but the location of ribosomes depends on the function of the cell:

Free ribosomes

  • Found in the cytosol
  • Are more abundant than bound ribosomes in cells retaining most of their manufactured protein

Bound ribosomes

  • Found bound to the exterior of the endoplasmic reticulum (ER)
  • More abundant than free ribosomes in cells that secrete their manufactured proteins.

Ribosomes are also located in mitochondria and chloroplasts of eukaryotic cells; they are always smaller than cytoplasmic ribosomes. Prokaryotic and eukaryotic ribosomes perform the same functions; however, eukaryotic ribosomes are much larger than prokaryotic ones and most of their proteins are different.


Related Organelles: Cytoplasm, Rough Endoplasmic Reticulum, Mitochondria, Chloroplast