The endoplasmic reticulum (ER) is a complex network of membranes which is present throughout the cell. There are two types of ER – rough endoplasmic reticulum which has a sheet-like structure with ribosomes attached to the membrane surface; and smooth endoplasmic reticulum, which has a tubular structure and no ribosomes attached to the membrane surface. The ribosomes attached to the membrane surface of the rough ER carry out protein synthesis. In addition to this function, rough ER carries out a number of other important roles within a cell. These include providing an internal structural skeleton to support the cell’s shape, storage of the synthesised materials and minerals, forming an internal network through which materials can be transported and providing a large surface area on which chemical reactions can occur. The smooth ER has functions in several metabolic processes, including synthesis of lipids and steroids, metabolism of carbohydrates, regulation of calcium concentration, drug detoxification, attachment of receptors on cell membrane proteins and steroid metabolism.