The primary function of the cell's endoplasmic reticulum (ER) is to synthesize and process proteins and lipids. The ER plays a vital role in the production of proteins destined for export outside the cell or for use in cell membranes. It is divided into the rough ER, which is studded with ribosomes and involved in protein synthesis, and the smooth ER, which is involved in lipid synthesis and detoxification processes. Overall, the ER is crucial for maintaining cellular function and homeostasis.