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The Endoplasmic
Reticulum
The endoplasmic reticulum (ER) is continuous with the nuclear envelope, and appears as a complex, folded mass. The endoplasmic reticulum creates an extensive network of membranes within the eukaryotic cell. The ER is a membranous bag of separate compartments, or sacs, which are connected by membranous tubules. The inside of this membranous bag is called the ER lumen, which is the space between the nuclear envelope and the endoplasmic reticulum. The ER lumen is impenetrable by the cytosol, and the endoplasmic membrane controls the movements in and out of the lumen. The lumen itself provides the cell separation from new synthesized molecules that belong both inside and outside of the cytosol (Brum & Mckane, 1989). Anatomically, this elaborate system contains folded stacks of membrane that makes up more than half of the total membrane in a cell (British Society for Cell Biology, n.d.; Reece et al., 2011, p. 104). “Endoplasmic means ‘within the cytoplasm’ and reticulum meaning ‘little net’” (Reece et al., 2011, p. 104). Hence, the endoplasmic reticulum functions as a circulation system within the cytoplasm of the cell. The endoplasmic reticulum creates pathways that channel materials to different locations within the cell, and functions to export materials produced by the cell during secretion (Brum & Mckane, 2011). Therefore without the ER, cells could die from the accumulation of high concentration of wastes. There are two distinct regions of the endoplasmic reticulum: smooth ER and rough ER. They each differ in structure and function.
The endoplasmic reticulum (ER) is continuous with the nuclear envelope, and appears as a complex, folded mass. The endoplasmic reticulum creates an extensive network of membranes within the eukaryotic cell. The ER is a membranous bag of separate compartments, or sacs, which are connected by membranous tubules. The inside of this membranous bag is called the ER lumen, which is the space between the nuclear envelope and the endoplasmic reticulum. The ER lumen is impenetrable by the cytosol, and the endoplasmic membrane controls the movements in and out of the lumen. The lumen itself provides the cell separation from new synthesized molecules that belong both inside and outside of the cytosol (Brum & Mckane, 1989). Anatomically, this elaborate system contains folded stacks of membrane that makes up more than half of the total membrane in a cell (British Society for Cell Biology, n.d.; Reece et al., 2011, p. 104). “Endoplasmic means ‘within the cytoplasm’ and reticulum meaning ‘little net’” (Reece et al., 2011, p. 104). Hence, the endoplasmic reticulum functions as a circulation system within the cytoplasm of the cell. The endoplasmic reticulum creates pathways that channel materials to different locations within the cell, and functions to export materials produced by the cell during secretion (Brum & Mckane, 2011). Therefore without the ER, cells could die from the accumulation of high concentration of wastes. There are two distinct regions of the endoplasmic reticulum: smooth ER and rough ER. They each differ in structure and function.
Smooth endoplasmic reticulum. The smooth ER derived its name from the lack of ribosomes on its outer surface, which can be detected through an electron micrograph. This organelle is prominently located in skeletal cells, kidney cells, and steroid-producing endocrine cells (Bucknell University, n.d.). The smooth ER has a tubular structure and functions in diverse metabolic processes. These processes include the “synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium” (Reece et al., 2011, p. 104). In order to carry out these processes, the smooth ER acts on enzymes. The smooth endoplasmic reticulum produces enzymes to synthesize lipids such as oils, phospholipids, and steroids. The steroids that are produced are secreted by the adrenal glands to become sex hormones. This directly correlates with the high concentration of smooth ER in steroid producing endocrine cells such as in the testes and ovaries. In addition, enzymes can help detoxify drugs or poisons by metabolizing the enzymes of the smooth ER to flush out barbiturates or drug molecules in the body. The smooth ER functions as storage for calcium by pumping calcium ions in the muscle cell from the cytosol into the ER lumen, triggering the contraction of a muscle cell (Reese et al., 2011). The smooth ER an extremely vital part of the endomembrane system and carries out many important functions. Rough endoplasmic reticulum. The rough ER derived its name from the studded ribosomes on its outer membrane, also detectable by an electron micrograph. Not only does the ribosomes give the rough ER its name, but also its function. Ribosomes are the site for protein synthesis. Hence, the ribosomes on the rough endoplasmic reticulum are membrane bound, and control the assembly of proteins. The rough ER is responsible for protein synthesis by manufacturing, processing, and preparing proteins for transport. In addition, the rough ER acts as a membrane factory for the cell. By adding membrane proteins and phospholipids to its own membrane, the rough ER is able to expand and bud off portions to be transported (Brum & Mckane, 1989; Reece et al., 2011). In order to do this, the endoplasmic reticulum captures proteins from the cytosol and releases them into the ER lumen. Secretory proteins are then produced by the rough ER, and wrapped into the membrane. Once the secretory proteins form, they create pockets that later break into transport vesicles. These vesicles are then transported to the Golgi apparatus. |