The endoplasmic reticulum is a network of membranes surrounding flattened membranous sacs called cisternae.The rough endoplasmic reticulum is continuous with the outer membrane of the nuclear envelope and has ribosomes attached. . The rough ER is the site of protein synthesis.Newly formed proteins are packaged in vesicles and transported to other parts of the cell such as the Golgi apparatus.The smooth endoplasmic reticulum is the site of the synthesis of lipids and carbohydrates and is also used for storage.The smooth ER helps the body make and metabolise fats, cholesterol and steroid hormones.The SER is found mainly towards the cell membrane or plasma membrane of cell.Different types of cells contain amounts of SER depending on the activities of the cell.The endoplasmic reticulum is not found in red blood cells, or spermatozoa.