The main function of the smooth endoplasmic reticulum (SER) in a cell is the synthesis of lipids and the detoxification of metabolites. Unlike the rough endoplasmic reticulum, which is studded with ribosomes for protein synthesis, the surface of the SER is smooth. It plays a crucial role in producing phospholipids for cell membranes, synthesizing steroid hormones, and metabolizing carbohydrates. Additionally, the smooth ER helps in the detoxification of harmful substances, making it essential for maintaining cellular health and function.