The organelle responsible for detoxifying harmful substances in a cell is the smooth endoplasmic reticulum (SER). This membranous organelle is involved in lipid synthesis as well as the metabolism of carbohydrates and detoxification of drugs and poisons. The smooth endoplasmic reticulum lacks ribosomes, distinguishing it from its rough counterpart, and plays a crucial role in maintaining cellular health. By breaking down toxins, the SER helps to protect the cell and the overall organism from potential damage caused by various harmful chemicals.