What distinguishes the Rough Endoplasmic Reticulum (RER) from the Smooth Endoplasmic Reticulum?

The distinguishing feature of the Rough Endoplasmic Reticulum (RER) is the presence of ribosomes on its cytoplasmic surface, which gives it a "rough" appearance compared to the Smooth Endoplasmic Reticulum (SER). These ribosomes are sites of protein synthesis, specifically for proteins that are to be secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle.

The Smooth Endoplasmic Reticulum, on the other hand, lacks ribosomes and is primarily involved in the synthesis of lipids, detoxification of drugs and poisons, and calcium ion storage. The functional differences between the RER and SER are a result of their structural characteristics, notably the presence of ribosomes on the RER, which is key for its role in producing proteins.