The endoplasmic reticulum (ER) is an extensive, interconnected network of membranes found within the cytoplasm of eukaryotic cells. It is structurally divided into two functional components: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER). The RER is characterized by the presence of ribosomes attached to its surface, making it the primary site for the synthesis, folding, and modification of proteins that are destined for secretion, insertion into membranes, or delivery to other organelles. Conversely, the SER lacks ribosomes and is primarily responsible for lipid synthesis, including phospholipids and steroids, as well as detoxification processes and the storage and release of calcium ions. These interconnected processes allow the ER to play a central role in cellular homeostasis and the maintenance of cellular compartments.