The endoplasmic reticulum is a cellular structure composed of interconnected membranes that extend to the nucleus of the cell (Rubio, 2019). According to Rubio (2019), two types of endoplasmic reticulum are distinguished: rough endoplasmic reticulum, characterized by membranes that form flattened cisternae and are associated with ribosomes, and smooth endoplasmic reticulum, whose membranes adopt a tubular arrangement without the presence of ribosomes.
What is the Rough Endoplasmic Reticulum?
The rough endoplasmic reticulum, also known as granular endoplasmic reticulum or ergastoplasm, is an organelle found only in eukaryotic cells (Rubio, 2019). Structurally, it consists of a network of channels, flattened sacs, and cisternae distributed throughout the cytoplasm. Inside, chains formed by peptides are introduced, leading to the production of complex proteins. These proteins are then transported to other parts of the cell, such as the Golgi apparatus and the smooth endoplasmic reticulum. Around the sacs, numerous ribosomes can be found, which are specialized vesicles containing proteins and other substances. These ribosomes give it its characteristic rough appearance under the microscope. The main function of this structure is protein synthesis, which is destined for various parts of the cell to perform multiple functions. Furthermore, according to Rubio (2019), it is responsible for controlling the structural and functional quality of the proteins it produces.
Functions
The main functions of the rough endoplasmic reticulum are highlighted by their relevance in cell survival and the synthesis of essential proteins for the proper functioning of the organism (Rubio, 2019). One of the most important functions is glycosylation, a post-translational modification process that proteins undergo in the secretory pathway (Cruz Fernández, 2024). In accordance with Cruz Fernández (2024), N-glycosylation, in particular, is crucial for cell survival, as it plays a key role in the biological activity and physicochemical properties of proteins.
Protein synthesis is another of its functions (Rubio, 2019). The synthesized proteins can serve various structural functions, act as hormones, enzymes, or transport substances. Depending on their destination, these proteins may remain inside the cell where they were synthesized or be transported outside. Most of the proteins that are part of cellular organelles originate from the ribosomes attached to this reticulum. The synthesis process begins when messenger RNA binds to the ribosomal subunits, initiating translation and producing a chain of amino acids called a signal peptide. Through interaction with an SRP (signal recognition particle) molecule, the resulting complex is directed to the reticulum, where the peptide chain is inserted and synthesis is completed. Once finished, Rubio (2019) mentions that the amino acid chain folds into its three-dimensional structure to become a functional protein.
The rough endoplasmic reticulum also has a fundamental quality control function, as it detects and corrects misfolded or defective proteins (Rubio, 2019). In this process, a glucosyltransferase enzyme adds glucose to the malformed protein, allowing a chaperone, such as calnexin, to identify it and return it to its original location for another attempt at correct folding. If correction is not possible, the protein is sent to the proteasome, where it is degraded into amino acids that can be reused. According to Rubio (2019), this mechanism ensures that only functional proteins reach maturity, while defective ones are eliminated or recycled, thus contributing to cellular hygiene.
Finally, the rough endoplasmic reticulum participates in the production of phospholipids and other components of cellular membranes (Rothschuh Osorio, 2024). These phospholipids, along with integral proteins, are transported to the Golgi apparatus for further processing and distribution. Moreover, in accordance with Rothschuh Osorio (2024), this organelle produces enzymes such as lipases, DNases, phosphatases, and hydrolases, which are essential for various metabolic functions.
Structure
The structure of the rough endoplasmic reticulum is characterized by a network of interconnected membranes that form flattened sacs and tubules, distributed throughout the cytoplasm (Cruz Fernández, 2024). These sacs are covered with ribosomes on their surface, giving the reticulum a rough appearance when viewed under the microscope. According to Cruz Fernández (2024), the membranes that make up this network are formed by a lipid bilayer that houses various proteins and enzymes, thus facilitating the functions performed by this organelle.
The ribosomes attached to the rough endoplasmic reticulum are arranged on the cytosolic side of the membrane, approximately 15 nanometers apart from each other (Cruz Fernández, 2024). These ribosomes bind to the surface of the reticulum via the large ribosomal subunit and are anchored through transmembrane glycoproteins, specifically ribophorin I and II. In accordance with Cruz Fernández (2024), this interaction is essential for the synthesis of proteins that will then be modified and transported to other areas of the cell.
The rough endoplasmic reticulum is also organized into cisternae and tubules (Cruz Fernández, 2024). Cisternae are flat and elongated structures, while tubules are narrow and cylindrical, all interconnected with each other. According to Cruz Fernández (2024), crucial processes such as the folding and modification of newly synthesized proteins occur in the lumen of the rough endoplasmic reticulum, which is the internal space delimited by the membranes, before they are sent to other parts of the cell.
The structure of the rough endoplasmic reticulum may vary depending on the type of cell it is found in, and it may even be referred to by different names (Rubio, 2019). In secretory cells, this organelle appears as numerous parallel chains or sacs that are very close to each other, allowing for the formation of vesicles necessary for the synthesis of substances. In the nervous system, this reticulum is called Nissl bodies and presents with more separated cisternae and numerous free ribosomes in the cytosol. Rubio (2019) mentions that, despite the presence of this organelle in some neurons, they synthesize few proteins.
Difference Between Rough and Smooth Endoplasmic Reticulum
The rough endoplasmic reticulum and the smooth endoplasmic reticulum are distinguished by various characteristics (Cruz Fernández, 2024). First, from a morphological perspective, the rough endoplasmic reticulum has a rough appearance under the electron microscope due to the presence of ribosomes attached to its outer surface. In contrast, according to Cruz Fernández (2024), the smooth endoplasmic reticulum lacks ribosomes, which gives it a smooth appearance when observed under the same type of microscope.
In terms of functions, the rough endoplasmic reticulum specializes in the synthesis and modification of proteins, a process essential for the production and transport of these molecules within the cell (Cruz Fernández, 2024). On the other hand, in accordance with Cruz Fernández (2024), the smooth endoplasmic reticulum is involved in a variety of significant functions, such as lipid synthesis, cellular detoxification, calcium storage, and regulation of carbohydrate metabolism.
Furthermore, its localization within cells varies according to its functions (Cruz Fernández, 2024). The rough endoplasmic reticulum is predominant in cells that produce and secrete large amounts of proteins, such as pancreatic cells and antibody-producing cells. In contrast, the smooth endoplasmic reticulum is found in greater quantities in liver and muscle cells, where it performs specialized metabolic functions. Finally, although both are composed of lipid membranes and proteins, the presence of ribosomes in the rough endoplasmic reticulum gives it a distinct structure and protein composition compared to the smooth endoplasmic reticulum. According to Cruz Fernández (2024), this reinforces the idea that, while they share structural characteristics, their roles within the cell are fundamentally different.
References
Cruz Fernández, K. (2024, abril 3). Retículo Endoplasmático Rugoso: Qué es y Función. Ecología Verde. https://www.ecologiaverde.com/reticulo-endoplasmatico-rugoso-que-es-y-funcion-4880.html
Rothschuh Osorio, U. (2024). Retículo Endoplasmático Rugoso: Qué es, Función, Estructura y Diferencia con el Liso. Bio Enciclopedia. https://www.bioenciclopedia.com/reticulo-endoplasmatico-rugoso-que-es-funcion-estructura-y-diferencia-con-el-liso-1001.html
Rubio, N. M. (2019, agosto 9). Retículo endoplasmático rugoso: definición, características y funciones. pymOrganization. https://psicologiaymente.com/miscelanea/reticulo-endoplasmatico-rugoso
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