What characterizes smooth endoplasmic reticulum?

Smooth endoplasmic reticulum (SER) is characterized by the absence of ribosomes on its cytoplasmic surface. This distinguishes it from rough endoplasmic reticulum (RER), which is studded with ribosomes and is primarily involved in protein synthesis. The lack of ribosomes on the smooth endoplasmic reticulum means that its primary functions are related to metabolism and the synthesis of lipids, such as phospholipids and cholesterol, as well as the detoxification of drugs and poisons in liver cells.

Additionally, smooth endoplasmic reticulum may contain various enzymes that facilitate these metabolic processes. However, the defining feature that characterizes the structure of the smooth endoplasmic reticulum with respect to ribosomes is its absence, which plays a critical role in differentiating its functions from that of the rough endoplasmic reticulum.