Which statement best contrasts the rough endoplasmic reticulum with the smooth endoplasmic reticulum?

• A. Rough ER has ribosomes and synthesizes secreted and membrane proteins; smooth ER lacks ribosomes and synthesizes lipids/detoxification
• B. Rough ER lacks ribosomes and synthesizes lipids
• C. Smooth ER has ribosomes and synthesizes secreted proteins
• D. Rough ER participates in DNA replication

Answer: (A)

The difference in structure and function between the rough and smooth endoplasmic reticulum is what’s being tested. The rough ER is studded with ribosomes, making it the site where proteins destined for secretion or for membranes are synthesized and folded, then processed as they enter the ER lumen or become part of the ER membrane. The smooth ER lacks ribosomes and thus does not synthesize proteins; instead it focuses on lipid production for membranes and on detoxification processes, such as metabolizing drugs and toxins—especially in liver cells. So, saying that the rough ER has ribosomes and makes secreted and membrane proteins, while the smooth ER lacks ribosomes and makes lipids and handles detoxification, captures the key contrast. The other statements misstate ribosome presence or the primary roles of each ER type, or note functions (like DNA replication) that aren’t tied to ER activity.