The endoplasmic reticulum is a cellular organelle composed of interconnected membranes (Rubio, 2019). These membranes extend from the cell nucleus, which is the center of the cell. Two types of endoplasmic reticulum can be identified: rough and smooth. The rough endoplasmic reticulum is characterized by its membranes that form flattened cisternae, on which spherical structures called ribosomes are found. On the other hand, according to Rubio (2019), the smooth endoplasmic reticulum is organized into membranes that form tubules but lacks ribosomes on its surface.
What is the Smooth Endoplasmic Reticulum?
The smooth endoplasmic reticulum is an essential organelle in eukaryotic cells, characterized by its smooth appearance, as it does not have ribosomes attached to its membrane (Fernández, 2024). This organelle consists of an interconnected set of tubular and membranous structures that extend into the cisternae of the rough endoplasmic reticulum (Rubio, 2019). Unlike the rough endoplasmic reticulum, the smooth endoplasmic reticulum lacks ribosomes in its membranes, which explains its "smooth" designation (Fernández, 2024). Due to this absence of ribosomes, Rubio (2019) mentions that the proteins present in this organelle come from the rough endoplasmic reticulum.
This organelle performs vital functions for the survival of both the cell and the organism in which it resides (Rubio, 2019). It plays a crucial role in processes such as lipid synthesis, detoxification of alcohol and other harmful substances, and regulation of blood glucose levels. Additionally, according to Rubio (2019), the smooth endoplasmic reticulum acts as a reservoir for essential minerals, including calcium.
Functions
Lipid Synthesis
This organelle is responsible for synthesizing most of the fats that make up cellular membranes, including cholesterol and glycerophospholipids (Megías et al., 2023). It is important to note that, rather than simply synthesizing, the smooth endoplasmic reticulum assembles lipids using raw materials from other organelles (Rubio, 2019). For example, fatty acids from the cytosol are required for the production of glycerophospholipids. According to Rubio (2019), lipids are transported to the membranes they will form, either via vesicles or through carrier proteins.
One of the products synthesized in the smooth endoplasmic reticulum is cholesterol, an essential component of the cell's lipid bilayer (Rubio, 2019). This cholesterol is transported from the organelle to the plasma membrane via soluble protein carriers. In addition to cholesterol, the smooth endoplasmic reticulum also produces triacylglycerols, which are lipids found in adipose cells that store fat for thermal insulation and as a food reserve. Furthermore, according to Rubio (2019), it is involved in synthesizing various hormones, including steroid hormones, bile acids, and the lipid fraction of lipoproteins.
Detoxification
The detoxification process refers to the transformation of substances, such as drugs and metabolites generated in internal reactions of the organism, into water-soluble compounds that can be excreted through urine (Porto & Gardey, 2014). This conversion is made possible by the presence of oxygenase enzymes located in the membranes of the smooth endoplasmic reticulum, such as the cytochrome P450 family of proteins (Rubio, 2019). This enzyme family plays a crucial role in drug metabolism. Detoxification occurs primarily in liver cells, known as hepatocytes, which are characterized by having a highly developed smooth endoplasmic reticulum. According to Rubio (2019), it is relevant to mention that the absence of ribosomes in this type of reticulum allows for a larger enzymatic action surface relative to the organelle's size.
Dephosphorylation of Glucose-6-Phosphate
Glucose, a type of sugar, is stored as glycogen, mainly in the liver (Rubio, 2019). Therefore, this organ is responsible for introducing sugar into the bloodstream through two hormones that regulate this process: glucagon and insulin (Megías et al., 2023). When glycogen is degraded, it produces glucose-6-phosphate (Rubio, 2019). This substance cannot leave the cell, as it cannot cross membranes. According to Rubio (2019), glucose-6-phosphatase, an enzyme, converts glucose-6-phosphate into glucose, which can be transported into the blood.
Intracellular Calcium Reservoir
The smooth endoplasmic reticulum is also responsible for collecting and storing calcium present in the cytosol (Rubio, 2019). This process is carried out by calcium pumps located in the membrane. The calcium stored inside the organelle can be massively released when the corresponding signal is received, either from inside or outside the cell (Rubio, 2019). This mechanism is particularly important in muscle cells, where the smooth endoplasmic reticulum is known as the sarcoplasmic reticulum (Fernández, 2024). When a motor neuron responsible for muscle movement receives a nerve impulse, the reticulum triggers the release of acetylcholine at the neuromuscular junction (Rubio, 2019). Acetylcholine binds to receptors on the muscle cell, causing membrane depolarization. As a result of this depolarization, calcium ions stored in the sarcoplasmic reticulum are released into the cytosol. Thus, according to Rubio (2019), muscle contraction ordered by the nervous system occurs.
Structure
The smooth endoplasmic reticulum is distributed throughout the cell's cytoplasm and is made up of a network of interconnected membranous tubules (Fernández, 2024). Unlike the rough endoplasmic reticulum, the smooth endoplasmic reticulum lacks ribosomes on its membrane. Its structure varies depending on the cell type and may have cisternae, which are expansions or dilations in its form. Additionally, in the rough endoplasmic reticulum, there are small areas of smooth endoplasmic reticulum that serve as exit points for vesicles that bud off from the rough endoplasmic reticulum. According to Fernández (2024), these areas are known as the transitional endoplasmic reticulum.
Differences Between Smooth and Rough Endoplasmic Reticulum
The smooth and rough endoplasmic reticulum are two components of the endoplasmic reticulum that perform distinct functions within the cell (Fernández, 2024). The main difference in their appearance lies in the presence of ribosomes. The smooth endoplasmic reticulum lacks ribosomes on its membrane, giving it a smooth appearance. In contrast, the rough endoplasmic reticulum has ribosomes attached to its outer membrane, giving it a rough appearance. In terms of function, the smooth endoplasmic reticulum is involved in lipid synthesis, carbohydrate metabolism, calcium storage, and detoxification of toxic substances. On the other hand, according to Fernández (2024), the rough endoplasmic reticulum is responsible for protein synthesis and their subsequent modification.
In terms of location, the smooth endoplasmic reticulum is distributed throughout the cytoplasm, while the rough endoplasmic reticulum is located closer to the nuclear membrane (Fernández, 2024). Additionally, the structure of the smooth endoplasmic reticulum is tubular, consisting of a network of interconnected membranous tubules. In contrast, the rough endoplasmic reticulum has a more complex structure due to the presence of ribosomes on its outer surface. Finally, in terms of abundance, the smooth endoplasmic reticulum is predominant in cells that require high lipid synthesis and metabolism, such as liver cells responsible for detoxification. On the other hand, according to Fernández (2024), the rough endoplasmic reticulum is more abundant in cells specialized in protein production, such as pancreatic cells responsible for producing digestive enzymes.
References
Fernández, K. C. (2024, marzo 15). Retículo endoplasmático liso: qué es y función. ecologiaverde.com. https://www.ecologiaverde.com/reticulo-endoplasmatico-liso-que-es-y-funcion-4840.html
Megías, M., Molist, P., & Pombal, M. Á. (2023). Retículo Endoplasmático. Atlas de Histología Vegetal y Animal. https://mmegias.webs.uvigo.es/5-celulas/5-reticulo.php
Porto, J. P., & Gardey, A. (2014, mayo 5). Retículo endoplasmático liso. Definición.de; Definicion.de. https://definicion.de/reticulo-endoplasmatico-liso/
Rubio, N. M. (2019, agosto 9). Retículo endoplasmático liso: definición, características y funciones. pymOrganization. https://psicologiaymente.com/salud/reticulo-endoplasmatico-liso
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