What distinguishes the smooth endoplasmic reticulum from the rough endoplasmic reticulum?

• A. Smooth ER has fixed ribosomes
• B. Smooth ER has no fixed ribosomes
• C. Smooth ER is the site of protein synthesis on ribosomes
• D. Smooth ER stores DNA

Answer: (B)

The key distinction is whether ribosomes are attached to the membrane. Rough endoplasmic reticulum is studded with fixed ribosomes on its cytosolic surface, giving it a rough appearance and making it the main site for synthesizing proteins that are to be secreted, inserted into membranes, or sent to lysosomes. Smooth endoplasmic reticulum, on the other hand, lacks these attached ribosomes, giving it a smooth appearance and slots in for lipid synthesis, detoxification processes, carbohydrate metabolism, and calcium storage in some cells. So, the defining feature is that the smooth form has no fixed ribosomes, which is why that statement best distinguishes it. The other options misplace protein synthesis on ribosomes (that's the rough ER) or suggest functions like DNA storage (not a role of the ER).