The endoplasmic reticulum (ER) has a vital role in cells, functioning primarily in the synthesis of proteins and lipids. The rough ER, studded with ribosomes, is responsible for producing and folding proteins destined for export or for use within the cell. Meanwhile, the smooth ER synthesizes lipids and detoxifies substances. Additionally, the ER plays a role in calcium storage and regulation, which is essential for various cellular functions. Overall, the ER is critical for maintaining cellular homeostasis and functionality.