What are ribosomes called that are attached to the endoplasmic reticulum?

Ribosomes that are attached to the endoplasmic reticulum are referred to as bound ribosomes. These structures play a critical role in protein synthesis, particularly for proteins that are destined for secretion, incorporation into the cell membrane, or for use in lysosomes. When ribosomes are bound to the rough endoplasmic reticulum (RER), they translate mRNA into polypeptides that enter the RER lumen, allowing these proteins to be properly folded and modified.

This connection between bound ribosomes and the endoplasmic reticulum highlights the compartmentalization within eukaryotic cells, where specific organelles are responsible for distinct cellular functions. In contrast, free ribosomes float freely within the cytosol and typically synthesize proteins that function within the cytosol itself.

The terminology used to describe ribosomes reflects their location and association with the cellular structures that facilitate the synthesis and processing of proteins. Hence, the correct term for ribosomes attached to the endoplasmic reticulum is bound ribosomes.