Membrane organelles of the cell

Cell organelles

Organelles (organellae) are obligatory microstructures for all cells, performing certain vital functions. A distinction is made between membrane and non-membrane organelles. Membrane organelles, separated from the surrounding hyaloplasm by membranes, include the endoplasmic reticulum, the internal mesh apparatus (Golgi complex), lysosomes, peroxisomes, and mitochondria.

Membrane organelles of the cell

All membrane organelles are built from elementary membranes, the principle of organization of which is similar to the structure of cytolemmas. Cytophysiological processes are associated with constant adhesion, fusion and separation of membranes, while adhesion and unification of only topologically identical membrane monolayers are possible. Thus, the outer layer of any organelle membrane facing the hyaloplasm is identical to the inner layer of the cytolemma, and the inner layer facing the cavity of 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, tubules, and flattened sacs. Electron micrographs distinguish between granular (rough, granular) and non-granular (smooth, agranular) endoplasmic reticulum. The outer side of the granular reticulum is covered with ribosomes, while the non-granular side lacks ribosomes. The granular endoplasmic reticulum synthesizes (on ribosomes) and transports proteins. The non-granular reticulum synthesizes lipids and carbohydrates and participates in their metabolism [for example, steroid hormones in the adrenal cortex and Leydig cells (sustenocytes) of the testes; glycogen in 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 Golgi complex (apparatus reticularis internus), is a collection of sacs, vesicles, cisterns, tubes, and plates bounded by a biological membrane. The elements of the Golgi complex are connected to each other by narrow channels. The structures of the Golgi complex are where polysaccharides, protein-carbohydrate complexes, are synthesized and accumulated and are excreted from the cells. This is how secretory granules are formed. The Golgi complex is present in all human cells except 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 inner convex surface of the Golgi complex structures faces the endoplasmic reticulum, and the outer, concave surface faces the cytoplasm.

The membranes of the Golgi complex are formed by the granular endoplasmic reticulum and are transported by transport vesicles. Secretory vesicles constantly bud off from the outer side of the Golgi complex, and the membranes of its cisterns are constantly renewed. Secretory vesicles supply membrane material for the cell membrane and glycocalyx. This ensures the renewal of the plasma membrane.

Lysosomes (lysosomae) are vesicles with a diameter of 0.2-0.5 μm, containing about 50 types of various hydrolytic enzymes (proteases, lipases, phospholipases, nucleases, glycosidases, phosphatases). Lysosomal enzymes are synthesized on the ribosomes of the granular endoplasmic reticulum, from where they are transferred by transport vesicles to the Golgi complex. Primary lysosomes bud off from the vesicles of the Golgi complex. An acidic environment is maintained in lysosomes, its pH fluctuates from 3.5 to 5.0. The 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, including its death.

In secondary (mature) lysosomes (phagolysosomes), biopolymers are digested into monomers. The latter are transported through the lysosomal membrane into the cell hyaloplasm. Undigested substances remain in the lysosome, as a result of which the lysosome is transformed into a so-called residual body of high electron density.

Peroxisomes (peroxysomae) are vesicles with a diameter of 0.3 to 1.5 µm. They contain oxidative enzymes that destroy hydrogen peroxide. Peroxisomes participate in the breakdown of amino acids, the metabolism of lipids, including cholesterol, purines, and the detoxification of many toxic substances. It is believed that peroxisome membranes are formed by budding from the non-granular endoplasmic reticulum, and the enzymes are synthesized by polyribosomes.

Mitochondria (mitochondrii), which are the "energy stations of the cell", participate in the processes of cellular respiration and the 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 are round, 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 needs. There are many large mitochondria in cardiomyocytes, muscle fibers of the diaphragm. They are located in groups between myofibrils, surrounded by glycogen granules and elements of the non-granular endoplasmic reticulum. Mitochondria are organelles with double membranes (each about 7 nm thick). Between the outer and inner mitochondrial membranes there is an intermembrane space 10-20 nm wide. The inner membrane forms numerous folds, or cristae. Usually, the cristae are oriented across the long axis of the mitochondrion and do not reach the opposite side of the mitochondrial membrane. Thanks to the cristae, the area of the inner membrane increases dramatically. Thus, the surface of the cristae of one mitochondrion of a hepatocyte is about 16 μm. Inside the mitochondrion, between the cristae, there is a fine-grained matrix in which granules with a diameter of about 15 nm (mitochondrial ribosomes) and thin threads representing molecules of deoxyribonucleic acid (DNA) are visible.

The synthesis of ATP in mitochondria is preceded by initial stages occurring in the hyaloplasm. In it (in the absence of oxygen), sugars are oxidized to pyruvate (pyruvic acid). At the same time, a small amount of ATP is synthesized. The main synthesis of ATP occurs on the membranes of the cristae in the mitochondria with the participation of oxygen (aerobic oxidation) and enzymes present in the matrix. During this oxidation, energy is formed for cell functions, and carbon dioxide (CO ₂ ) and water (H ₂ O) are released. In the mitochondria, molecules of information, transport and ribosomal nucleic acids (RNA) are synthesized on their own DNA molecules.

The mitochondrial matrix also contains 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 necessary for protein synthesis and self-reproduction. The increase in the number of mitochondria in a cell occurs through its division into smaller parts that grow, increase in size and are able to divide again.

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