Which cellular structure is a membranous network in the cytoplasm studded with ribosomes for protein synthesis?

• A. Smooth endoplasmic reticulum
• B. Centrioles
• C. Rough endoplasmic reticulum
• D. Nucleolus

Answer: (C)

Rough endoplasmic reticulum is the membranous network in the cytoplasm whose outer surface is studded with ribosomes, giving it a rough appearance. These ribosomes synthesize proteins that are destined for secretion, incorporation into the plasma membrane, or delivery to lysosomes, with the newly made proteins entering the ER lumen for folding and modification before they move to the Golgi apparatus. This contrasts with smooth endoplasmic reticulum, which lacks ribosomes and handles lipid synthesis and detoxification. Centrioles are part of the centrosome and organize spindle fibers during cell division, not a ribosome-bearing membrane system. The nucleolus resides inside the nucleus and is the site of ribosomal RNA synthesis and ribosome assembly, not a cytoplasmic membranous network.