What distinguishes smooth endoplasmic reticulum from rough endoplasmic reticulum?

The distinction between smooth endoplasmic reticulum (SER) and rough endoplasmic reticulum (RER) primarily lies in the presence of ribosomes on their surfaces. The smooth endoplasmic reticulum is characterized by the absence of ribosomes, which gives it a smooth appearance under a microscope. In contrast, the rough endoplasmic reticulum is studded with ribosomes, which are the sites of protein synthesis.

The smooth endoplasmic reticulum is involved in lipid synthesis, metabolism of carbohydrates, detoxification of drugs and poisons, and calcium ion storage, rather than protein synthesis. Since it lacks ribosomes, it does not partake in the same processes as the rough endoplasmic reticulum, which is specifically dedicated to synthesizing and folding proteins meant for secretion or for use in the cell's membrane systems.

The other options do not accurately define the differences between SER and RER. Thus, recognizing the absence of ribosomes as the defining characteristic is key in distinguishing the smooth endoplasmic reticulum from the rough endoplasmic reticulum.