How does the rough endoplasmic reticulum (Rough ER) differ from the smooth ER?

The rough endoplasmic reticulum (Rough ER) is primarily involved in the synthesis of proteins, particularly those that are to be secreted from the cell, integrated into cellular membranes, or sent to organelles such as the lysosomes. This occurs because the Rough ER is studded with ribosomes on its cytoplasmic surface, giving it a "rough" appearance. These ribosomes translate mRNA into polypeptide chains, which are then folded and modified within the lumen of the Rough ER.

In contrast, the smooth endoplasmic reticulum (Smooth ER) is not associated with ribosomes and does not participate in protein synthesis. Instead, it focuses on lipid synthesis, detoxification of harmful metabolic byproducts, and storage of ions like calcium. The distinction in function between the Rough ER and Smooth ER is fundamentally tied to their structural differences—specifically, the presence of ribosomes on the Rough ER.

Therefore, the statement that the Rough ER synthesizes proteins while the Smooth ER synthesizes lipids accurately captures the essential distinction between these two types of endoplasmic reticulum, highlighting their separate but complementary roles in cellular metabolism.