Smooth Endoplasmic Reticulum
The smooth endoplasmic reticulum (ER) is a network of connected tubular, fatty membrane sacs called cisternae that have mostly round openings. In some types of cells, the smooth ER is connected with the nuclear envelope, but because the smooth ER doesn’t have the functional link that the rough ER has with the nucleus, it doesn’t much matter whether the smooth ER directly touches the nucleus or not. The smooth ER neighbors the rough ER and shares some of its functions, particularly its collaboration with the Golgi apparatus for protein transport. The smooth ER specializes, more so than the rough ER, in the synthesis of proteins and the storage of enzymes, partly because its focus is the cell’s metabolism.
Smooth ER synthesizes lipids (fatty, waxy molecules), phospholipids, and steroids (special chemical compounds). It carries out the metabolism of carbohydrates. In muscle cells, the smooth ER regulates calcium ion concentration – it stores calcium ions in its cisternae and when muscles need them, it sends them to the location in the cell where they are needed. Muscle cell smooth ER, given a special name sarcoplasmic reticulum, therefore, play a crucial role in the contraction of muscles. Sarcoplasmic reticulum is saturated with proteins. The presence of the smooth ER is somewhat accessory or existing on a needs basis – in cells where its functions are needed more, the smooth ER exists in abundance and may even have special functions that it doesn’t have in other cell types, yet, still, and in other cells, it is not present at all.
Smooth endoplasmic reticulum is a network of mostly round, tubular, lipid membrane sacs. These function mainly in the cell’s metabolism but in some cases, like the smooth ER in muscle cells, they can have special functionality. Like the rough ER, they also work with the Golgi apparatus to transport proteins throughout the cell as they are themselves very big manufacturers of proteins. Also like the rough ER, they also work to create and store enzymes. In metabolic processes, they specialize more than the rough ER. And they are one of the most accessory organelles – in abundance where they are needed or have special function and non-existent where they are not.