Which organelle is the site of protein synthesis and has ribosomes on its surface?

• A. Rough endoplasmic reticulum
• B. Ribosome
• C. Golgi apparatus
• D. Lysosome

Answer: (A)

Proteins destined for secretion, insertion into membranes, or lysosomal targeting are made by ribosomes that sit on the rough endoplasmic reticulum. The rough ER is the part of the endoplasmic reticulum studded with ribosomes on its cytosolic surface, giving it a rough appearance and enabling immediate entry of new polypeptides into the ER lumen or membrane. This setup allows those proteins to be processed and packaged as they move toward the Golgi apparatus for further sorting. In contrast, a ribosome by itself is the site of protein synthesis but isn’t a membrane-bound organelle with ribosomes on its surface. The Golgi apparatus processes and ships proteins, and lysosomes contain digestive enzymes rather than sites of protein synthesis. The combination of protein synthesis with ribosomes attached to a membrane-bound organelle points to the rough endoplasmic reticulum.