Which organelle is studded with ribosomes and synthesizes proteins destined for secretion or membranes?

• A. Nucleus
• B. Rough ER
• C. Smooth ER
• D. Golgi apparatus

Answer: (B)

Proteins that are meant to be secreted or become part of membranes start their synthesis on ribosomes that are attached to the rough endoplasmic reticulum. The ribosomes give the rough ER its “rough” appearance, and as they translate the mRNA, the new polypeptides are threaded into the ER lumen or inserted into the ER membrane. Inside the ER, these proteins begin folding and may receive initial modifications before being packaged into vesicles and sent to the Golgi for further processing and sorting to their final locations.

The rough ER is distinguished from the smooth ER, which lacks ribosomes and focuses on lipid synthesis and detoxification rather than protein production. The Golgi apparatus, while essential for modifying and sorting proteins, is not the site where ribosome-based protein synthesis occurs.