The endoplasmic reticulum (ER) plays a critical role in protein processing within eukaryotic cells. It consists of a network of membranes where newly synthesized polypeptides are folded and modified. The rough ER, studded with ribosomes, is specifically involved in the synthesis of proteins destined for secretion or for use in membranes. Post-translational modifications, such as glycosylation, occur within this organelle, ensuring proteins acquire the necessary structure for their function. Additionally, the smooth ER is involved in lipid synthesis and detoxification processes, illustrating the ER's essential roles in cellular function and maintenance.