Where are ribosomes that synthesize secretory and membrane proteins located?

• A. Free in the cytosol
• B. On the cytosolic surface of the rough endoplasmic reticulum
• C. Inside the mitochondrial matrix
• D. In the lumen of the Golgi apparatus

Answer: (B)

Ribosomes that synthesize secretory and membrane proteins are attached to the cytosolic surface of the rough endoplasmic reticulum. The nascent polypeptide carries a signal peptide that is recognized by a signal recognition particle, which pauses translation and targets the ribosome–polypeptide complex to the ER membrane. There, translation resumes and the growing chain is threaded into the ER lumen or inserted into the ER membrane through a translocon. This co-translational targeting into the ER sets these proteins on the secretory pathway or as membrane components, after which they traffic through the Golgi and to their final destinations.

Free ribosomes in the cytosol make cytosolic proteins (and some nuclear or organelle-targeted proteins after post-translational import); ribosomes inside mitochondria synthesize mitochondrial-encoded proteins; and the Golgi lumen does not host translation.