Ans. The endoplasmic reticulum (ER) is involved in the synthesis, folding, and transport of proteins and lipids. It consists of a network of tubules and flattened sacs that are connected to the nuclear envelope. The ER can be rough or smooth, depending on whether it has ribosomes attached to its surface. The rough ER is involved in protein synthesis, while the smooth ER is involved in lipid metabolism and detoxification of drugs and toxins.