Smooth Endoplasmic Reticulum
What Is Smooth Endoplasmic Reticulum
Smooth Endoplasmic Reticulum (SER) is a mesh-like network of membrane-bound tube-like structures lying near the outer membrane of the nucleus. It is a sub-compartment of endoplasmic reticulum found in all eukaryotic cells, including plant and animal cells. SER is associated with various cellular functions, such as lipid synthesis, carbohydrate metabolism, regulation of calcium concentration, and drug detoxification.
Smooth endoplasmic reticulum is made up of small tube-like structures called tubules. These tubules sometimes divide into branches and give a network or reticulate-like appearance. Unlike rough endoplasmic reticulum (RER), it lacks ribosomes on its outer membrane, thus named ‘smooth’. The interior of the SER is called the lumen, which remains enclosed by a phospholipid membrane.
The function of the SER may vary, depending on the cell type. For instance, in some endocrine glands it synthesizes lipids, whereas in liver it helps in the detoxification or removal of several drugs from the body. Some other functions of SER would include regulation of calcium concentration in the muscle cells, and carbohydrate metabolism in liver and kidneys.
Let us discuss these functions of SER in details:
1. Lipid synthesis: The smooth endoplasmic reticulum is the primary site for lipid synthesis, especially at its membrane contact sites (MCS). MCS are the areas where membranes of SER make close contact with other cytoplasmic organelles, such as mitochondria, Golgi apparatus, lysosomes, peroxisomes, and plasma membrane, and allow the transfer of substances. The enzymes present in these sites are activated by signals from outside and inside the cell. Different types of lipids formed by SER are phospholipids, ceramides, sterols, and steroids. The lipids thus synthesized form a major portion of the lipid bilayer of the cell membrane. They also play a vital role as a structural element of the cell.
2. Drug detoxification: A specialized group of enzymes, cytochrome P450s present on the smooth endoplasmic reticulum membrane, help detoxify drugs and poisons, especially in liver cells. These enzymes act by adding a hydroxyl group to drug molecules, making them more soluble and easier to flush from the body.
3. Regulation of calcium concentration: In muscle cells, a specialized SER called the sarcoplasmic reticulum (SR) is responsible for the storage and regulation of calcium ions that are required to trigger the coordinated contractions of the muscle cells.
The SR takes up the calcium ions from the cytoplasm and releases them when the muscle cell is triggered by a nervous stimulus, resulting in muscle contraction. In this manner, the sarcoplasmic reticulum helps regulate calcium ion concentrations in the cytoplasm of skeletal muscle cells.
Except SR, calcium ions released from the smooth endoplasmic reticulum stimulate other responses, such as the secretion of vesicles carrying newly synthesized proteins.
4. Carbohydrate metabolism: Smooth endoplasmic reticulum produces an enzyme called glucose 6-phosphatase which can hydrolyze glucose 6-phosphate into glucose during gluconeogenesis.
Gluconeogenesis is the process by which glucose is synthesized from non-carbohydrate precursors like pyruvate, oxaloacetate, succinate, and lactate. In most tissues, free glucose is not generated. So, the final compound of this process is glucose 6-phosphate, which cannot diffuse out of the cell. Thus, it is stored inside the cell.
In this case, glucose-6-phosphate gets transported to the lumen of the smooth endoplasmic reticulum, where it finally gets converted into glucose by the action of the glucose-6-phosphatase.
Article was last reviewed on Friday, November 26, 2021