Endoplasmic reticulum (ER) is like a manufacturing plant, like a factory, where proteins and lipids are made. This is also where things are packaged into boxed and sent off to different places. In the cell the smooth ER is a network of membrane bound bodies which lack ribosomes (the molecules used in protein synthesis) and its primary function is to modify, encapsulate and transport newly synthesized proteins and lipids which will be secreted or remain in the cytoplasm as membrane bound vesicles. The smooth ER can also be compared to a highway, or a protein and lipid highway, if you will. It is sometimes called the transitional ER because it contains exit sites from which transport vesicles carrying these proteins and lipids bud off for transport to the Golgi apparatus. It is usually prominent in cells that specialize in lipid metabolism and synthesis.