Smooth endoplasmic reticulum: definition, characteristics and functions

The endoplasmic reticulum is an organ of the cell made up of interconnected membranes. These membranes extend with those in the center of the cell, the cell nucleus.

There are two types of endoplasmic reticulum: one, said to be rough, the membranes form flattened cisterns with spherical structures called ribosomes, and the other called smooth, which is organized with membranes forming tubules without these ribosomes.

In this article let’s talk about smooth endoplasmic reticulum, And what are its parts and functions.

    What is Smooth Endoplasmic Reticulum?

    This cellular organelle is made up of a set of interconnected tubular and membranous structures that extend into the cisterns of its counterpart, the rough endoplasmic reticulum.

    The smooth endoplasmic reticulum it differs from its rough counterpart in that it does not have ribosomes attached to its membranes. It is for this reason that it receives the denomination of smooth. Due to this lack of ribosomes, the proteins present in this organelle have their origin in the rough endoplasmic reticulum.

    This body it performs essential functions for the survival of the cell and the organism in which it is locatedParticipates in the synthesis of lipids, detoxification of alcohol and other harmful substances, regulates blood levels of carbohydrates and also serves as a store of essential minerals like calcium.

    functions

    This organelle performs a number of important functions for the cell and for the proper maintenance of the body.

    1. Lipid synthesis

    In the smooth endoplasmic reticulum most of the fats that make up cell membranes are synthesized, Such as cholesterol and glycerophospholipids.

    It should be noted that more than the synthesis, what is achieved in this organelle is the coupling of lipids, the raw material comes from other organelles. For example, to make glycerophospholipids, the smooth endoplasmic reticulum requires fatty acids, which originate from the cytosol, i.e. from the internal environment of the cell.

    The transport of these lipids towards the membranes which will form can be done by vesicles or with transport proteins.

    One of the substances synthesized in the smooth endoplasmic reticulum is cholesterol. This substance acquires great importance because it is the component of the lipid double layer of the cell. This substance is transported from this organelle to the plasma wall by soluble protein transporters.

    In the smooth endoplasmic reticulum, in addition to cholesterol to form cell membranes, triacylglycerols are formed, lipids very present in fat cells, which store fat for thermal insulation and as a food reserve.

    too much is involved in the synthesis of certain hormones, Such as steroid hormones, bile acids and the lipid part of lipoproteins.

    2. Detoxification

    Detoxification is the process by which substances, such as drugs and metabolites produced in reactions that take place in the body, they are transformed into water-soluble compoundsThat is, they can be dissolved in water and excreted in urine.

    This process is possible thanks to the presence of oxygenase enzymes, located in the membranes of this organelle, just like the family of cytochrome P450 proteins. This family of enzymes acquires great importance in the metabolism of drugs.

    Detoxification is carried out primarily in liver cells, called hepatocytes. These cells are characterized by a well-developed smooth endoplasmic reticulum.

    It should be noted that the absence of ribosomes in this type of network allows a greater surface of enzymatic action in proportion to the size of the reticle.

      3.dephosphorylation of glucose-6-phosphate

      Glucose is a sugar stored as glycogen, primarily in the liver. For this reason, this organ is responsible for introducing sugar into the bloodstream, through two hormones responsible for regulating this process: glucagon and insulin.

      Glycogen, when broken down, forms glucose-6-phosphate. This substance cannot leave the cell because it is unable to cross membranes.

      Through the action of glucose-6-phosphatase, an enzyme, glucose-6-phosphate is broken down, forming glucose which can be carried in the blood.

      4. Intracellular calcium reserve

      The smooth endoplasmic reticulum is responsible for the collection and storage of calcium that floats in the cytosol. This collection of this element is carried out by the action of calcium pumps located in the membrane of the reticulum.

      Calcium inside this organelle can be expelled en masse if the necessary signal is given to it, whether it comes from inside or outside the cell.

      This process takes place mainly in the muscle cells, Where the smooth endoplasmic reticulum is called the sarcoplasmic reticulum.

      If a motor neuron, i.e. a neuron involved in muscle movement, receives a nerve impulse, the reticulum causes the release of acetylcholine in the neuromuscular plate. This acetylcholine binds to receptors in the muscle cell, causing the membrane to desporalize.

      When depolarization occurs, calcium ions are released which have been stored in the sarcoplasmic reticulum and go to the cytosol. Thus, through this process, muscle contraction controlled by the nervous system occurs.

      When the nerve impulse is stopped, calcium ions are released into the cytosol and what was released previously is actively collected in the sarcoplasmic reticulum.

      Bibliographical references:

      • Anglès, AR, Zurek, N., Voeltz, GK (2009). Structure and function of the peripheral ER. Current Opinion in Cell Biology, 21 ,: 506-602.
      • Daleke DL (2007). Phospholipid flippases. The Journal of Biological Chemistry. 282, 821-825.
      • Nixon-Abell J, Obara, CJ, Weig VA, Li D., Legant WR, Xu CS, Pasolli HA, Harvey K., Hess HF, Betzig E., Blackstone C., Lippincott-Schwartz3 J. (2016). Increased spatiotemporal resolution reveals very dynamic dense tubular matrices in peripheral REs. Science. 354, 3928-2.

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