Domains of Life; Prokaryotic vs. Eukaryotic Cells

ID #2171

Are ribosomes considered organelles since they are part of the rough endoplasmic reticulum (RER), or are they not, because the subunits are synthesized in the cell?

Ribosomes are not organelles and this is very important so it is a good thing that you asked. They are not membrane-enclosed, instead they are macromolecules made of both RNA and proteins. If you ever have trouble remembering, just recall that there are ribosomes found in bacteria even though bacteria do not contain organelles. 

Moreover, not all ribosomes are located at the ER in eukaryotes (remember there is no ER in bacteria). Many of the ribosomes are free-floating in the cytoplasm. The ribosomes that are at the ER are only there temporarily. They are not technically part of the RER.

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