Why does the Rough Endoplasmic Reticulum look 'rough' under a microscope ?

The Rough Endoplasmic Reticulum (RER) derives its name from its characteristic 'rough' or 'bumpy' appearance when viewed under an electron microscope [1]. This distinct morphology is caused by the presence of numerous ribosomes attached to its cytoplasmic surface. These ribosomes are the sites of protein synthesis, specifically for proteins destined for secretion or membrane integration. In contrast, the Smooth Endoplasmic Reticulum (SER) lacks these surface ribosomes, giving it a smooth appearance [1]. The ribosomes on the RER are not permanently fixed; they bind to the membrane via translocons when translating mRNA that contains a specific signal peptide [1]. This granular or studded appearance is a key structural feature that distinguishes the RER from other organelles and the SER [2].

Sources

1. [1] https://www.sciencedirect.com/topics/immunology-and-microbiology/rough-endoplasmic-reticulum
2. [2] https://www.ncbi.nlm.nih.gov/books/NBK563126/