The function of the endoplasmic reticulum in a cell is multifaceted, involving the synthesis, folding, modification, and transport of proteins and lipids. The rough ER is studded with ribosomes, facilitating protein synthesis, while the smooth ER is involved in lipid metabolism and detoxification processes. Additionally, the endoplasmic reticulum acts as a storage site for calcium ions and plays a role in the overall cellular organizing and processing of biomolecules.