Which cellular organelle is a membranous network in the cytoplasm with ribosomes that participates in protein synthesis?

• A. Smooth endoplasmic reticulum
• B. Cell nucleus
• C. Rough endoplasmic reticulum
• D. Centrioles

Answer: (C)

Rough endoplasmic reticulum is a membranous network in the cytoplasm that bears ribosomes on its surface, making it the main site where many proteins are synthesized and prepared for secretion, insertion into membranes, or delivery to lysosomes. The ribosomes translate mRNA into polypeptides that are threaded into the ER lumen or integrated into the ER membrane as they are made, allowing initial folding and processing (such as glycosylation) with the help of chaperone proteins. This combination of a ribosome-studded membrane network that handles protein synthesis sets it apart. In contrast, the smooth endoplasmic reticulum lacks ribosomes and focuses on lipid synthesis and detoxification; the nucleus is a separate, membrane-bound organelle housing genetic material; and centrioles are non-membranous microtubule-organizing structures.