What distinguishes Smooth ER from Rough ER?

The distinguishing feature between Smooth Endoplasmic Reticulum (Smooth ER) and Rough Endoplasmic Reticulum (Rough ER) is the presence of ribosomes. Rough ER is studded with ribosomes on its cytoplasmic surface, which gives it a "rough" appearance under the microscope. These ribosomes are the sites of protein synthesis, which is critical for producing proteins that are either secreted from the cell or sent to the cell's membrane or other organelles.

In contrast, Smooth ER lacks ribosomes and is primarily involved in lipid synthesis, metabolism, and detoxification processes. While Smooth ER does play a role in various metabolic activities, it does not contribute directly to protein synthesis like Rough ER does.

This fundamental difference in ribosome presence is what typically defines and differentiates the two types of endoplasmic reticulum within the cell.

Other potential attributes that could be discussed include the location within the cell or the specific functions both types of ER perform, but the primary visual and functional distinction is indeed the presence of ribosomes on Rough ER.