Which organelle is primarily involved in protein synthesis for secretion and is often studded with ribosomes?

• A. Mitochondrion
• B. Golgi apparatus
• C. Rough ER
• D. Lysosome

Answer: (D)

Protein synthesis for secretion happens on ribosomes that are attached to the rough endoplasmic reticulum. The rough ER provides a membrane-bound surface where the growing peptide chains enter the ER lumen as they are made, allowing them to fold and receive initial modifications. From there, these secreted proteins are packaged into vesicles and shipped to the Golgi for further processing and eventual secretion outside the cell. This combination of ribosome binding and a controlled entry pathway into the ER is why the rough ER is the primary site for making proteins that will be secreted.

Mitochondria generate ATP and aren’t the main site of secreted protein synthesis; the Golgi apparatus is involved in modifying and sorting proteins after synthesis, not making them from scratch; lysosomes house enzymes for digestion rather than producing secreted proteins.