Which structure synthesizes proteins and is primarily found on the rough endoplasmic reticulum?

• A. Ribosome
• B. Lysosome
• C. Golgi
• D. Mitochondrion

Answer: (A)

Proteins are built by ribosomes. When ribosomes are attached to the rough endoplasmic reticulum, they synthesize proteins that are destined for secretion, insertion into membranes, or delivery to other organelles. The rough ER has ribosomes on its surface, giving it its rough appearance, and as a new polypeptide is made, it is threaded into the ER lumen where it can be folded and first modified before being packaged into vesicles for transport. In contrast, other organelles have different roles—lysosomes break down waste, the Golgi modifies and sorts proteins, and mitochondria generate ATP—so the presence of ribosomes on the rough ER specifically points to its role in protein synthesis for secreted or membrane-bound proteins.