Membrane cellular organelles
Last reviewed: 23.04.2024
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Cell Organelles
Organelles (organellae) are mandatory microstructures for all cells performing certain vital functions. There are membrane and non-membrane organelles. Membrane organelles, separated from the surrounding membrane by the hyaloplasm, include the endoplasmic reticulum, the inner reticular apparatus (Golgi complex), lysosomes, peroxisomes, mitochondria.
Membrane cell organelles
All membrane organelles are constructed from elementary membranes, the principle of organization of which is similar to the structure of cytolemmas. Cytofiziologicheskie processes are associated with the constant adhesion, fusion and separation of membranes, while sticking and unification of only topologically identical monolayers of membranes is possible. Thus, the outer layer of any membrane of the organelle facing the hyaloplasm is identical to the inner layer of the cytolemma, and the inner layer facing the organelle is similar to the outer layer of the cytolemma.
The endoplasmic reticulum (reticulum endoplasmaticum) is a single continuous structure formed by a system of cisterns, tubes and flattened sacs. Electron micrographs distinguish granular (rough, granular) and non-grained (smooth, agranular) endoplasmic reticulum. The outer side of the granular network is covered with ribosomes, the ungrain is devoid of ribosomes. The granular endoplasmic reticulum synthesizes (on ribosomes) and transports proteins. A non-natural network synthesizes lipids and carbohydrates and participates in their metabolism [for example, steroid hormones in the adrenal cortex and Leydig cells (testicular cells) of the testes; glycogen - in the liver cells]. One of the most important functions of the endoplasmic reticulum is the synthesis of membrane proteins and lipids for all cellular organelles.
The internal reticular apparatus, or the apparatus of the reticularis internus, is a collection of saccules, vesicles, cisterns, tubules, plates bounded by a biological membrane. Elements of the Golgi complex are interconnected by narrow channels. In the structures of the Golgi complex, the synthesis and accumulation of polysaccharides, protein-carbohydrate complexes, which are derived from cells, occur. So secretory granules are formed. The Golgi complex is present in all human cells, except for erythrocytes and horny scales of the epidermis. In most cells, the Golgi complex is located around or near the nucleus, in exocrine cells - above the nucleus, in the apical part of the cell. The internal convex surface of the Golgi complex structures faces the endoplasmic reticulum, and the outer, concave, surface of the Golgi complex faces the cytoplasm.
Membranes of the Golgi complex are formed by a granular endoplasmic reticulum and are transported by transport vesicles. From the outside of the Golgi complex, secretory vesicles are constantly budding, and the membranes of its cisterns are constantly updated. Secretory vesicles provide a membrane material for the cell membrane and glycocalyx. Thus, the plasma membrane is renewed.
Lysosomes (lysosomae) are vesicles 0.2-0.5 microns in diameter, containing about 50 species of various hydrolytic enzymes (protease, lipase, phospholipase, nuclease, glycosidase, phosphatase). Lysosomal enzymes are synthesized on the ribosomes of the granular endoplasmic reticulum, from where they are transported by transport vesicles to the Golgi complex. From the vesicles of the Golgi complex, primary lysosomes are budded. The acidic medium is maintained in lysosomes, its pH ranges from 3.5 to 5.0. Membranes of lysosomes are resistant to the enzymes contained in them and protect the cytoplasm from their action. Violation of the permeability of the lysosomal membrane leads to the activation of enzymes and severe damage to the cell until its death.
In secondary (mature) lysosomes (phagolysosomes), biopolymers are digested to monomers. The latter are transported through the lysosomal membrane into the cell's hyaloplasm. Undigested substances remain in the lysosome, as a result of which the lysosome is converted into a so-called residual body of high electron density.
Peroxysomes (peroxysomae) are vesicles with a diameter of 0.3 to 1.5 microns. They contain oxidative enzymes that destroy hydrogen peroxide. Peroxisomes are involved in the cleavage of amino acids, the exchange of lipids, including cholesterol, purines, in the neutralization of many toxic substances. It is believed that peroxisome membranes are formed by budding from an ungrain endoplasmic reticulum, and the enzymes are synthesized by polyribosomes.
Mitochondria ("mitochondrii"), which are "cell power stations," participate in processes of cellular respiration and conversion of energy into forms available for use by the cell. Their main functions are the oxidation of organic substances and the synthesis of adenosine triphosphate (ATP). Mitochondria have the appearance of rounded, elongated or rod-shaped structures 0.5-1.0 μm long and 0.2-1.0 μm wide. The number, size and location of mitochondria depend on the function of the cell, its energy requirements. Many large mitochondria in cardiomyocytes, muscle fibers of the diaphragm. They are located in groups between myofibrils, surrounded by glycogen granules and elements of an ungrain endoplasmic reticulum. Mitochondria are organelles with double membranes (each thickness is about 7 nm). Between the outer and inner mitochondrial membranes there is an intermembrane space 10-20 nm in width. The inner membrane forms numerous folds, or cristae. Usually, the cristae are oriented across the long axis of the mitochondria and do not reach the opposite side of the mitochondrial membrane. Thanks to the crystals, the area of the inner membrane sharply increases. Thus, the surface of the crista of one mitochondria of the hepatocyte is about 16 μm. Within the mitochondria, between the cristae, is a fine-grained matrix, in which granules with a diameter of about 15 nm (mitochondrial ribosomes) and thin filaments, which are molecules of deoxyribonucleic acid (DNA), are visible.
Synthesis of ATP in mitochondria is preceded by the initial stages occurring in the hyaloplasm. In it (in the absence of oxygen), the sugars are oxidized to pyruvate (pyruvic acid). Simultaneously, a small amount of ATP is synthesized. The main synthesis of ATP occurs on the membranes of cristae in mitochondria involving oxygen (aerobic oxidation) and enzymes present in the matrix. With this oxidation, energy is generated for the functions of the cell, and carbon dioxide (CO 2 ) and water (H 2 O) are also released. In mitochondria, molecules of information, transport and ribosomal nucleic acids (RNA) are synthesized on the DNA's own molecules.
In the matrix of mitochondria there are also ribosomes up to 15 nm in size. However, mitochondrial nucleic acids and ribosomes differ from similar structures of this cell. Thus, mitochondria have their own system, which is necessary for the synthesis of proteins and for self-reproduction. The increase in the number of mitochondria in a cell occurs by dividing it into smaller parts that grow, increase in size, and are able to divide again.
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