The primary role of the endoplasmic reticulum (ER) in cells is to synthesize proteins and lipids. The ER is divided into two types: rough ER and smooth ER. Rough ER is studded with ribosomes, which play a crucial role in protein synthesis, while smooth ER is involved in the synthesis of lipids and detoxification processes. Additionally, the ER serves as a transportation network, facilitating the movement of materials within the cell. Overall, it plays a vital part in maintaining cellular homeostasis and function.