Smooth endoplasmic reticulum: definition, characteristics and functions
The endoplasmic reticulum is an organ of the cell formed by membranes connected to each other. These membranes continue with those in the center of the cell, the cell nucleus.
There are two types of endoplasmic reticulum: one, called rough, whose 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 we will talk about the smooth endoplasmic reticulum , and what are its parts and functions.
What is the smooth endoplasmic reticulum?
This cellular organelle consists of a set of tubular and membranous structures that are connected to each other, which are continued in the cisterns of its counterpart, the endoplasmic rugged reticulum.
The smooth endoplasmic reticulum differs from its roughened counterpart in that it does not have ribosomes attached to its membranes . This is why it is called smooth. Due to this lack of ribosomes, the proteins present in this organelle have their origin in the rugged endoplasmic reticulum.
This organelle performs very necessary functions for the survival of the cell and the organism in which it is found , participating in the synthesis of lipids, detoxification of alcohol and other harmful substances, regulates the levels of carbohydrates in the blood and also serves as a reserve of essential minerals such as calcium.
This organism performs a number of important functions for the cell and for the good maintenance of the organism.
1. Lipid synthesis
The smooth endoplasmic reticulum synthesizes most of the fats that make up the cell membranes , such as cholesterol and glycerophospholipids.
It should be noted that more than synthesis what is carried out in this organelle is the assembly of lipids, whose raw material comes from other organelles. For example, to manufacture glycerophospholipids, the smooth endoplasmic reticulum requires fatty acids, which come from the cytosol, that is, the internal environment of the cell.
The transport of these lipids to the membranes they will constitute can be done by vesicles or with transport proteins.
One of the substances that are synthesized in the smooth endoplasmic reticulum is cholesterol . This substance takes on great importance since it is the component of the double lipid layer of the cell. This substance is transported from this organelle to the plasma wall by soluble protein transporters.
In the smooth endoplasmic reticulum, besides cholesterol to form the cell membranes, triacylglycerols are formed, lipids that are very present in the adipose cells, which store fat for thermal insulation and as a food reserve.
Also is involved in the synthesis of some hormones , such as steroid hormones, bile acids and the lipid part of lipoproteins.
Detoxification is understood as the process in which substances, such as drugs and metabolites produced in the reactions that take place within the organism, are transformed into water-soluble compounds , that is, they can be dissolved in water and excreted through the urine.
This process is possible thanks to the presence of some oxygen enzymes, which are located in the membranes of this organelle, such as the family of proteins of the cytochrome P450. This family of enzymes acquires great importance in the metabolism of drugs.
The detoxification takes place mainly in the cells of the liver, which are called hepatocytes. These cells are characterized by a highly developed smooth endoplasmic reticulum.
It should be noted that the absence of ribosomes in this type of reticulum allows a greater surface area of enzymatic action in proportion to the size of the reticulum.
3. Defosphorylation of glucose-6-phosphate
Glucose is a sugar that is stored as glycogen, mostly in the liver. For this reason, this organ is in charge of introducing sugar into the bloodstream, by means of two hormones that are in charge of regulating this process: glucagon and insulin .
Glycogen, when degraded, forms glucose-6-phosphate. This substance cannot leave the cell, because it is unable to pass through the membranes.
Through the action of glucose-6-phosphatase, an enzyme, glucose-6-phosphate is degraded, forming glucose which can be carried into the blood.
4. Intracellular calcium reserve
The smooth endoplasmic reticulum is responsible for collecting and storing the calcium that is floating in the cytosol . This collection of this element is carried out by means of the action of calcium pumps that are found in the membrane of the reticulum.
The calcium found inside this organism can be expelled massively to the outside if the necessary signal is given, whether it originates from inside or outside the cell.
This process takes place mostly in 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 acetylcholine to be released into the neuromuscular plate. This acetylcholine binds to muscle cell receptors, causing the membrane to become deporalized.
When depolarization occurs, calcium ions that were stored in the sarcoplasmic reticulum are released and end up in the cytosol. Thus, through this process, the muscle contraction ordered by the nervous system occurs.
When the nerve impulse is stopped, calcium ions are no longer released into the cytosol and the previously released calcium ions are actively collected towards the sarcoplasmic reticulum.
- English, A. R., Zurek, N., Voeltz, G. K. (2009). Peripheral ER structure and function. Current opinion in cell biology, 21, :506-602.
- Daleke D. L . (2007). Phospholipid Flippases. The journal of biological chemistry. 282, 821-825.
- Nixon-Abell J, Obara, C. J., Weig V. A., Li D., Legant W. R., Xu C. S., Pasolli H. A., Harvey K., Hess H. F. Betzig E., Blackstone C., Lippincott-Schwartz3 J.( 2016). Increased spatiotemporal resolution reveals highly dynamic dense tubular matrices in the peripheral ER. Science. 354, 3928-2.