plural: smooth endoplasmic reticula
(cell biology) (A part of) endoplasmic reticulum that is tubular in form (rather than sheet-like) and lacks ribosomes, and is associated with lipid synthesis, carbohydrate metabolism, calcium concentration, drug detoxification, and attachment of receptors on cell membrane proteins
Endoplasmic reticulum is one of the most prominent organelles of a eukaryotic cell. The endoplasmic reticulum (ER) is an organelle that occurs as interconnected network of flattened sacs or tubules (called cisternae) in the cytoplasm. The membranes of the ER are connected to the outer nuclear envelope. They may also extend into the cell membrane. The ER is one of the three components of the GERL system, in which the Golgi apparatus and the lysosomes are the other components. It is found in different cell types. However, it is absent in red blood cells and spermatozoa. There are two kinds of ER: the rER, or the rough endoplasmic reticulum, and the sER, or the smooth endoplasmic reticulum.
The sER is seen connected to the nuclear envelope, and consists of tubules and vesicles that branch forming a network to increase surface area for the action or storage of key enzymes. The sER, as opposed to the rough endoplasmic reticulum, does not have ribosomes on its surface, thus the name smooth.
A specialized type of SER occurs in muscle cells where calcium ions are stored. It releases calcium ions during muscle contraction and absorb them during relaxation. The regulation of calcium ion level has to be regulated. Too much calcium inside the cell could lead to calcification and hardening of intracellular structures, and eventually to cell death. In muscle cells, this smooth endoplasmic reticulum is referred to as sarcoplasmic reticulum.
The major functions of sER include lipid synthesis, carbohydrate metabolism, regulation of intracellular calcium concentration, drug detoxification, and attachment of receptors on cell membrane proteins. In carbohydrate metabolism, the sER contains the enzyme glucose-6-phosphatase that converts glucose-6-phosphate to glucose, a step in gluconeogenesis. In drug detoxification, the sER is capable of detoxifying lipophilic drugs. Thus, it can be seen abundant in hepatocytes. Other specialized cells with abundant sER are cells of sebaceous glands and gonad cells (e.g. testes and ovaries). At the cellular level, sER is involved in the intracellular transport of materials. For instance, it transports the products of the rER to other cell parts like Golgi apparatus.
Common biological reactions
Lipid synthesis and transport
sER is the major site of lipid synthesis, particularly at the membrane contact sites (MCS). MCS are areas where ER membranes make close contact with other cytoplasmic organelles, such as Golgi, mitochondria, lysosomes, peroxisomes, endosomes, chloroplasts, and plasma membrane, and allow the transfer of substances. In particular, contact sites between ER and mitochondria allow the synthesis of phospholipids.
- agranular endoplasmic reticulum
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