Compare rough and smooth endoplasmic reticulum in terms of ribosome association and primary functions.

• A. Rough ER lacks ribosomes and specializes in lipid synthesis.
• B. Rough ER has ribosomes on its cytosolic surface and synthesizes secretory and membrane proteins.
• C. Rough ER is the site of DNA replication.
• D. Rough ER stores calcium exclusively.

Answer: (B)

Ribosome association with the endoplasmic reticulum determines its primary protein production role. Rough endoplasmic reticulum is studded with ribosomes on its cytosolic surface, and these ribosomes translate proteins that are destined for secretion or insertion as membrane proteins. The growing polypeptide is threaded into the ER lumen or integrated into the membrane through the translocon, where it can fold, be quality-checked, and begin initial glycosylation. In contrast, smooth endoplasmic reticulum lacks ribosomes and focuses on lipid synthesis and metabolism, detoxification, and calcium storage. So the description that rough ER has ribosomes on its surface and synthesizes secretory and membrane proteins best captures its defining role.