What distinguishes rough endoplasmic reticulum from smooth endoplasmic reticulum?

The distinction between rough and smooth endoplasmic reticulum primarily lies in the presence of ribosomes on the surface of rough endoplasmic reticulum (Rough ER), which gives it a "rough" appearance under a microscope. These ribosomes play a crucial role in synthesizing proteins that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle.

In contrast, smooth endoplasmic reticulum (Smooth ER) lacks ribosomes and is primarily involved in lipid synthesis, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions. The presence of ribosomes is a key feature that differentiates the rough ER from the smooth ER, as this structural characteristic determines their distinct functions within the cell.

The other choices do not accurately define the relationship between rough and smooth endoplasmic reticulum. For instance, while rough ER is indeed involved in protein synthesis, smooth ER is typically associated with lipid production, not the rough ER. Additionally, both types of ER are found in eukaryotic cells, including plant cells, and neither is exclusive to prokaryotic cells.