Endoplasmic Reticulum and Endoplasmic Stem Cells
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Synthesis of Lipids in Smooth Endoplasmic Reticulum
In eukaryotic cells, the endoplasmic reticulum (ER) is an organelle where membrane-bound proteins and secretory products are synthesized. It is also where lipids and nucleic acids are made. The ER is a complex network of flattened sacs and tubules that are continuous with the nuclear membrane. The ER functions as an intercellular quality control gate by sensing unfolded or misfolded proteins and activating protein folding pathways such as the Unfolded Protein Response (UPR). Moreover, it responds to cellular stress by inducing gene expression through its IRE1/XBP pathway. The ER is also involved in lipid metabolism and energy control.
The ER can be divided into two parts: the rough endoplasmic reticulum and the smooth endoplasmic reticulum (sER). Rough ER has ribosomes attached to it that are making proteins to be inserted into the cell’s membrane or secreted out of it. Smooth ER does not have ribosomes attached to it and synthesizes lipids, phospholipids, steroids, and hormones. It is also important in carbohydrate metabolism, lipid metabolism, and detoxification of metabolic wastes and alcohol.
Synthesis of Hormones in Smooth Endoplasmic Reticulum
The smooth endoplasmic reticulum (SER) is different from the rough endoplasmic reticulum (RER). The main difference is that it does not contain ribosomes, which are protein-synthesizing particles that give the RER its “rough” appearance. In contrast, SER contains enzymes that synthesize and concentrate substances needed by the cell. It also helps regulate the intracellular concentration of calcium ions. In muscle cells, for example, a special form of ER called the sarcoplasmic reticulum stores and releases calcium ions in response to hormone signals that stimulate or inhibit muscle contraction.
The sER is also involved in several processes that do not involve protein production. For example, it synthesizes lipids, steroid hormones and phospholipids that make up the membranes of all cell structures. It is also involved in carbohydrate metabolism and drug detoxification. Cells that produce steroid hormones, such as Leydig cells in the testes and follicular cells of the ovary, have abundant sER. It is also found in liver cells that handle harmful chemicals.
Synthesis of Proteins in Smooth Endoplasmic Reticulum
The endoplasmic reticulum is involved in many processes that allow the rest of the cell to function properly. This extensive organelle is a network of tightly-connected cisternae and sacs filled with lipids, proteins and other substances.
It is made up of two interconnected sub-compartments, the rough endoplasmic reticulum and the smooth endoplasmic reticulum. The rough ER has on its surface membrane ribosomes, and these are responsible for making protein that is needed by the rest of the cell.
In contrast, the smooth ER does not have ribosomes and is involved in synthesis of lipids and other substances. It is also involved in the synthesis of hormones and steroids. Cells that secrete these hormones, such as Leydig cells in the testis and follicular cells in the ovary, are rich in smooth ER. Hepatocytes in the liver are also abundant in smooth ER.
The ER is also a major site for the synthesis of cholesterol and glycoproteins. It is also a site for protein folding and quality control. Finally, the ER contains enzymes that can detoxify toxic hydrophobic substances (such as phenobarbital) and metabolize them to less-toxic forms.
Synthesis of Nucleic Acids in Smooth Endoplasmic Reticulum
The ribonucleotide precursors of nucleic acid chains are synthesized by either an energetically expensive de novo route from carbon dioxide, amino acids, and ribose sugars or by the much less costly salvage pathway. The ribonucleotide pools are recycled by the cell and the synthesis of nucleic acid molecules is controlled by feedback control.
Proteins destined for the secretory pathway are secreted from the rough endoplasmic reticulum in vesicles. The vesicles are tagged with the protein’s signal sequence and are targeted by clathrin to bind to plasma membranes, where they bud off as integral membrane proteins.
The ER is responsible for making the major class of phospholipids, such as phosphatidylcholine (lecithin). Phospholipid synthesis occurs in three steps. First, acyl transferases add two fatty acids to glycerol 3-phosphate, producing phosphatidic acid. Next, phosphatidic acid is linked to choline to produce phosphatidylcholine. The lipid bilayers of cells contain phosphatidylcholine, cholesterol, and other lipids. The ER is also responsible for the metabolism of carbohydrates, and in muscle cells it is involved in sequestering calcium.
