The endoplasmic reticulum is a network of membranes surrounding tubules and flattened membranous sacs called cisternae.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 the 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.