Which organelle is the site of protein synthesis and is studded with ribosomes?

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

Answer: (D)

Proteins destined for secretion or membranes are synthesized on the rough endoplasmic reticulum, the organelle whose surface is studded with ribosomes. Those ribosomes attach to the ER and translate proteins directly into the ER lumen or insert them into the membrane, starting their processing in this compartment. The rough ER thus combines the site of synthesis with a membrane-bound environment suitable for quality control and initial modification, before proteins move on to the Golgi for further processing and packaging.

Ribosomes themselves are the molecular machines that build proteins, but they are not an organelle with a surrounding membrane. The Golgi apparatus processes and sorts proteins after synthesis. Lysosomes house enzymes for degradation, not synthesis.