What distinguishes rough endoplasmic reticulum from smooth endoplasmic reticulum?

The rough endoplasmic reticulum (RER) is characterized by the presence of ribosomes attached to its cytoplasmic surface, which gives it a "rough" appearance under a microscope. These ribosomes play a crucial role in protein synthesis, as they translate mRNA into polypeptide chains that enter the RER for folding and modifications before being transported to their final destinations within or outside of the cell.

In contrast, the smooth endoplasmic reticulum (SER) lacks ribosomes and is involved in different functions such as lipid synthesis, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions. The distinction in ribosome presence thus distinctly differentiates the rough endoplasmic reticulum from its smooth counterpart, highlighting their specialized roles in cellular function and metabolism.

The other options do not directly address the key distinguishing feature of the two types of endoplasmic reticulum, making the identification of ribosomes the fundamental aspect that sets them apart.