The endoplasmic reticulum (ER) is a type of organelle in the cells of eukaryotic organisms that forms an interconnected network of flattened, membrane-enclosed sacs or tubes known as cisternae. The endoplasm is the inner core of the cytoplasm and the membranes of the ER are continuous with the outer membrane of the nuclear envelope. There are two types of endoplasmic reticulum, rough and smooth. The outer (cytosolic) face of the rough endoplasmic reticulum is studded with ribosomes that are the sites of protein synthesis, whereas active smooth endoplasmic reticulum lacks ribosomes and functions in lipid metabolism and biosynthesis, carbohydrate metabolism, etc., and is especially abundant in mammalian liver and gonad cells. Liver is the place where lipid biosynthesis takes place.