The smooth endoplasmic reticulum (ER) is the part of the ER that does not have ribosomes attached to the cytoplasmic side of the membrane. It forms a tubular structure, distinct from the rough ER that is evenly distributed throughout the cytoplasm.
Since it has no bound ribosomes, the smooth ER cannot synthesize proteins; it mainly produces lipids and steroid hormones. The ratio of smooth ER to rough ER varies depending on the function of the cell. Most cells have only small patches of smooth ER. This is where transport vesicles bud off on the way to the Golgi apparatus. Muscle cells contract by the specific release of calcium ions that are stored in the smooth ER. Hepatocytes, the most abundant liver cells, have large amounts of smooth ER that allow the breakdown of glycogen granules to glucose. The smooth ER in cells helps detoxify some organic chemicals that are products of natural metabolism by converting them into safer water-soluble compounds. The smooth ER in the liver can double its surface area in a few days to assist with the metabolism of ethanol when too much alcohol or too many barbiturates are consumed. Once levels of the drug decrease, lysosomes perform autophagocytosis to return the smooth ER to normal size.