What happens in mitochondria? Mitochondria
Mitochondria
Mitochondria are rod-shaped or oval-shaped structures (Greek. mitos- a thread, chondros- granule). They are found in all animal cells (excluding mature red blood cells): in higher plants, algae and protozoa. They are absent only in prokaryotic bacteria.
These organelles were first discovered and described at the end of the last century by Altman. Somewhat later, these structures were called mitochondria. In 1948, Hogeboom pointed out the importance of mitochondria as the center of cellular respiration, and in 1949, Kennedy and Lehninger established that a cycle of oxidative phosphorylation occurs in mitochondria. Thus, it was proven that mitochondria serve as a place for generating energy.
Mitochondria are visible in a conventional light microscope using special staining methods. In a phase contrast microscope and in a “dark field” they can be observed in living cells.
Structure, dimensions, shape mitochondria are very variable. This depends primarily on the functional state of the cells. For example, it has been established that in the motor neurons of flies that fly continuously for 2 hours, a huge number of spherical mitochondria appear, while in flies with glued wings the number of mitochondria is much smaller and they are rod-shaped (L. B. Levinson). In shape they can be thread-like, rod-shaped, rounded and dumbbell-shaped, even within the same cell.
Mitochondria are localized in the cell, as a rule, either in those areas where energy is consumed, or near accumulations of substrate (for example, lipid droplets), if any.
Strict orientation of mitochondria is found along the sperm flagella, in the striated muscle tissue, where they are located along the myofibrils, in the epithelium of the renal tubules they are localized in invaginations of the basement membrane, etc.
The number of mitochondria in cells has organ characteristics, for example, the liver cells of rats contain from 100 to 2500 mitochondria, and the cells of the collecting ducts of the kidney - 300, in the sperm of various animal species from 20 to 72, in the giant amoeba Chaos chaos their number reaches 500,000. The sizes of mitochondria range from 1 to 10 microns.
The ultramicroscopic structure of mitochondria is the same, regardless of their shape and size. They are covered with two lipoprotein membranes: outer and inner. Between them there is an intermembrane space.
Invaginations of the inner membrane that protrude into the body of mitochondria are called Christami. The arrangement of cristae in mitochondria can be transverse or longitudinal. The shape of the cristae can be simple or branched. Sometimes they form a complex network. In some cells, for example, in the cells of the zona glomerulosa of the adrenal gland, the cristae look like tubes. The number of cristae is directly proportional to the intensity of oxidative processes occurring in mitochondria. For example, in the mitochondria of cardiomyocytes there are several times more of them than in the mitochondria of hepatocytes. The space enclosed by the inner membrane constitutes the inner chamber of the mitochondria. In it, between the cristae, there is a mitochondrial matrix - a relatively electron-dense substance.
Inner membrane proteins are synthesized by mitoribosomes, and outer membrane proteins are synthesized by cytoribosomes.
“The outer membrane of mitochondria is similar in many respects to the membranes of the ER. It is poor in oxidative enzymes. There are few of them in the membrane space. But the inner membrane and mitochondrial matrix are literally saturated with them. Thus, enzymes of the Krebs cycle and fatty acid oxidation are concentrated in the mitochondrial matrix. In the inner The electron transport chain, phosphorylation enzymes (formation of ATP from ADP), and numerous transport systems are localized in the membrane.
In addition to protein and lipids, the composition of mitochondrial membranes includes RNA and DNA, the latter has genetic specificity and differs in its physicochemical properties from nuclear DNA.
Electron microscopic studies revealed that the surface of the outer membrane is covered with small spherical elementary particles. The inner membrane and cristae contain similar elementary particles on “legs,” the so-called mushroom bodies. They consist of three parts: a spherical head (diameter 90-100 A°), a cylindrical leg, 5 nm long and 3-4 nm wide, and a base measuring 4 by 11 nm. Mushroom body heads are associated with phosphorylation, and the heads are then found to contain an enzyme with ATP-ide activity.
In the intermembrane space there is a substance with a lower electron density than the matrix. It provides communication between membranes and supplies auxiliary coenzyme catalysts for enzymes located in both membranes.
It is now known that the outer membrane of mitochondria is highly permeable to substances with low molecular weight, in particular protein compounds. The inner membrane of mitochondria is selectively permeable. It is practically impermeable to anions (Cl -1, Br -1, SO 4 -2, HCO 3 -1, Sn +2, Mg +2 cations, a number of sugars and most amino acids, while Ca 2+, Mn 2+, phosphate , polycarboxylic acids easily penetrate through it. There is evidence of the presence in the inner membrane of several transporters specific to certain groups of penetrating anions and cations. Active transport of substances through membranes is carried out using the energy of the ATPase system or the electrical potential generated on the membrane as a result of work respiratory chain Even ATP synthesized in mitochondria can be released by a transporter (coupled transport).
The mitochondrial matrix is represented by a fine-grained electron-dense substance. It contains mitoribosomes, fibrillar structures consisting of DNA molecules and granules with a diameter of more than 200A ◦ formed by salts: Ca 3 (PO 4), Ba 3 (PO 4) 2, Mg 3 (PO 4). It is believed that the granules serve as a reservoir of Ca +2 and Mg +2 ions. Their number increases with changes in the permeability of mitochondrial membranes.
The presence of DNA in mitochondria ensures the participation of mitochondria in the synthesis of RNA and specific proteins, and also indicates the existence of cytoplasmic inheritance. Each mitochondria contains, depending on its size, one or several DNA molecules (from 2 to 10). The molecular weight of mitochondrial DNA is about (30-40) * 10 6 in protozoa, yeast, and fungi. In higher animals there are about (9–10) * 10 6.
Its length in yeast is approximately 5 microns, in plants - 30 microns. The amount of genetic information contained in mitochondrial DNA is small: it consists of 15-75 thousand base pairs, which can encode on average 25-125 protein chains with a molecular weight of about 40,000.
Mitochondrial DNA differs from nuclear DNA in a number of features: it has a higher synthesis rate (5-7 times), it is more resistant to the action of DNase, it is a double-circular molecule, contains more guanine and cytosine, is denatured at a higher temperature and is easier to restore. However, not all mitochondrial proteins are synthesized by the mitochondrial system. Thus, the synthesis of cytochrome C and other enzymes is provided by the information contained in the nucleus. Vitamins A, B2, B12, K, E, as well as glycogen are localized in the mitochondrial matrix.
Mitochondrial function consists in the formation of energy necessary for the life of cells. Various compounds can serve as a source of energy in the cell: proteins, fats, carbohydrates. However, the only substrate that is immediately included in energy processes is glucose.
Biological processes, as a result of which energy is generated in mitochondria, can be divided into 3 groups: Group I - oxidative reactions, including two phases: anaerobic (glycolysis) and aerobic. Group II - dephosphorylation, ATP breakdown and energy release. Group III - phosphorylation associated with the oxidation process.
The process of glucose oxidation initially occurs without the participation of oxygen (anaerobically or glycolytically) to pyruvic or lactic acid.
However, only a small amount of energy is released. Subsequently, these acids are involved in oxidation processes that occur with the participation of oxygen, i.e., they are aerobic. As a result of the oxidation process of pyruvic and lactic acid, called the Krebs cycle, carbon dioxide, water and a large number of energy.
The resulting energy is not released in the form of heat, which would lead to overheating of cells and death of the entire organism, but is accumulated in a form convenient for storage and transport in the form of adenosine triphosphoric acid (ATP). ATP synthesis occurs from ADP and phosphoric acid and is therefore called phosphorylation.
In healthy cells, phosphorylation is coupled with oxidation. In diseases, conjugation can become uncoupled, so the substrate is oxidized, but phosphorylation does not occur, and oxidation turns into heat, and the ATP content in cells decreases. As a result, the temperature rises and the functional activity of cells decreases.
So, the main function of mitochondria is to produce almost all the energy of the cell and the synthesis of components necessary for the activity of the organelle itself, enzymes of the “respiratory ensemble”, phospholipids and proteins occurs.
Another aspect of the activity of mitochondria is their participation in specific syntheses, for example, in the synthesis of steroid hormones and individual lipids. In the oocytes of various animals, accumulations of yolk form in the mitochondria, and they lose their basic system. Spent mitochondria can also accumulate excretion products.
In some cases (liver, kidneys), mitochondria are capable of accumulating harmful substances and poisons that enter the cell, isolating them from the main cytoplasm and partially blocking the harmful effects of these substances. Thus, mitochondria are capable of taking on the functions of other cell organelles when this is required to fully ensure a particular process under normal conditions or under extreme conditions.
Biogenesis of mitochondria. Mitochondria are renewable structures with a rather short life cycle (in rat liver cells, for example, the half-life of mitochondria covers about 10 days). Mitochondria are formed as a result of the growth and division of previous mitochondria. Their division can occur in three ways: constriction, budding of small sections and the emergence of daughter mitochondria inside the mother. The division (reproduction) of mitochondria is preceded by the reproduction of its own genetic system - mitochondrial DNA.
So, according to the views of most researchers, the formation of mitochondria occurs mainly through self-reproduction de novo.
Mitochondria are one of the most important components of any cell. They are also called chondriosomes. These are granular or thread-like organelles that are part of the cytoplasm of plants and animals. They are the producers of ATP molecules, which are so necessary for many processes in the cell.
What are mitochondria?
Mitochondria are the energy base of cells; their activity is based on the oxidation and use of energy released during the breakdown of ATP molecules. Biologists on in simple language it is called the energy production station for cells.
In 1850, mitochondria were identified as granules in muscles. Their number varied depending on growth conditions: they accumulate more in those cells where there is a high oxygen deficiency. This happens most often when physical activity. In such tissues, an acute lack of energy appears, which is replenished by mitochondria.
Appearance of the term and place in the theory of symbiogenesis
In 1897, Bend first introduced the concept of “mitochondrion” to designate a granular and filamentous structure in which they vary in shape and size: thickness is 0.6 µm, length - from 1 to 11 µm. In rare situations, mitochondria may be big size and a branched node.
The theory of symbiogenesis gives a clear idea of what mitochondria are and how they appeared in cells. It says that the chondriosome arose in the process of damage to bacterial cells, prokaryotes. Since they could not autonomously use oxygen to generate energy, this prevented them from fully developing, while progenotes could develop unhindered. During evolution, the connection between them made it possible for progenotes to transfer their genes to eukaryotes. Thanks to this progress, mitochondria are no longer independent organisms. Their gene pool cannot be fully realized, since it is partially blocked by enzymes that are present in any cell.
Where do they live?
Mitochondria are concentrated in those areas of the cytoplasm where the need for ATP appears. For example, in the muscle tissue of the heart they are located near the myofibrils, and in spermatozoa they form a protective camouflage around the axis of the cord. There they generate a lot of energy to make the “tail” spin. This is how the sperm moves towards the egg.
In cells, new mitochondria are formed by simple division of previous organelles. During it, all hereditary information is preserved.
Mitochondria: what they look like
The shape of the mitochondria resembles a cylinder. They are often found in eukaryotes, occupying from 10 to 21% of the cell volume. Their sizes and shapes vary greatly and can change depending on conditions, but the width is constant: 0.5-1 microns. The movements of chondriosomes depend on the places in the cell where energy is rapidly wasted. They move through the cytoplasm using cytoskeletal structures for movement.
A replacement for mitochondria of different sizes, which work separately from each other and supply energy to certain zones of the cytoplasm, are long and branched mitochondria. They are able to provide energy to areas of cells located far from each other. Such joint work of chondriosomes is observed not only in unicellular organisms, but also in multicellular ones. The most complex structure of chondriosomes is found in the muscles of the mammalian skeleton, where the largest branched chondriosomes are joined to each other using intermitochondrial contacts (IMCs).
They are narrow gaps between adjacent mitochondrial membranes. This space has a high electron density. MMKs are more common in cells where they bind together with working chondriosomes.
To better understand the issue, you need to briefly describe the significance of mitochondria, the structure and functions of these amazing organelles.
How are they built?
To understand what mitochondria are, you need to know their structure. This unusual source of energy is spherical in shape, but often elongated. Two membranes are located close to each other:
- external (smooth);
- internal, which forms leaf-shaped (cristae) and tubular (tubules) outgrowths.
Apart from the size and shape of the mitochondria, their structure and functions are the same. The chondriosome is delimited by two membranes measuring 6 nm. The outer membrane of the mitochondria resembles a container that protects them from the hyaloplasm. The inner membrane is separated from the outer membrane by a region 11-19 nm wide. A distinctive feature of the inner membrane is its ability to protrude into the mitochondria, taking the form of flattened ridges.
The internal cavity of the mitochondrion is filled with a matrix, which has a fine-grained structure, where threads and granules (15-20 nm) are sometimes found. Matrix threads create organelles, and granules small sizes- ribosomes, mitochondria.
At the first stage it takes place in the hyaloplasm. At this stage, the initial oxidation of substrates or glucose occurs to These procedures take place without oxygen - anaerobic oxidation. The next stage of energy production consists of aerobic oxidation and breakdown of ATP, this process occurs in the mitochondria of cells.
What do mitochondria do?
The main functions of this organelle are:
The presence of its own deoxyribonucleic acid in mitochondria once again confirms the symbiotic theory of the appearance of these organelles. Also, in addition to their main work, they are involved in the synthesis of hormones and amino acids.
Mitochondrial pathology
Mutations occurring in the mitochondrial genome lead to depressing consequences. The human carrier is DNA, which is passed on to offspring from parents, while the mitochondrial genome is passed on only from the mother. This fact is explained very simply: children receive the cytoplasm with chondriosomes enclosed in it along with the female egg; they are absent in sperm. Women with this disorder can pass on mitochondrial disease to their offspring, but a sick man cannot.
Under normal conditions, chondriosomes have the same copy of DNA - homoplasmy. Mutations can occur in the mitochondrial genome, and heteroplasmy occurs due to the coexistence of healthy and mutated cells.
Thanks to modern medicine, more than 200 diseases have been identified today, the cause of which was a mutation in mitochondrial DNA. Not in all cases, but mitochondrial diseases respond well to therapeutic maintenance and treatment.
So we figured out the question of what mitochondria are. Like all other organelles, they are very important for the cell. They indirectly take part in all processes that require energy.
There is a firmly established opinion that human endurance is associated with training the heart muscle, and what is needed for this long time perform low-intensity work.
In fact, everything is not so: endurance is inextricably linked with the mitochondria inside muscle fibers. Therefore, endurance training is nothing more than development maximum quantity mitochondria inside each muscle fiber.
And because Since the maximum number of mitochondria is limited by the space inside the muscle fiber, the development of endurance is limited by the number of muscles that are present in a particular person.
Briefly speaking: The more mitochondria a person has within specific muscle groups, the more endurance those specific muscle groups have.
And the most important: there is no general endurance. There is only local endurance of specific muscle groups.
Mitochondria. What it is
Mitochondria are special organelles (structures) inside cells human body, which are responsible for producing energy for muscle contractions. They are sometimes called the energy stations of the cell.
In this case, the process of energy production inside mitochondria occurs in the presence of oxygen. Oxygen makes the process of obtaining energy inside mitochondria as efficient as possible when compared to the process of obtaining energy without oxygen.
The fuel for energy production can be completely different substances: fat, glycogen, glucose, lactate, hydrogen ions.
Mitochondria and endurance. How does this happen
During muscle contraction, a residual product always appears. This is usually lactic acid, a chemical compound made of lactate and hydrogen ions.
As hydrogen ions accumulate inside the muscle fiber (muscle cell), they begin to interfere with the process of producing energy to contract the muscle fiber. And as soon as the concentration of hydrogen ions reaches a critical level, muscle contraction stops. AND this moment may indicate the maximum level of endurance of a particular muscle group.
Mitochondria have the ability to absorb hydrogen ions and process them internally.
This results in the following situation. If a large number of mitochondria are present inside the muscle fibers, then they are able to utilize and large quantity hydrogen ions. This means working a specific muscle longer without having to stop the effort.
Ideally, if there are enough mitochondria inside working muscle fibers to utilize the entire amount of hydrogen ions produced, then such a muscle fiber becomes almost tireless and is able to continue working as long as there is a sufficient amount nutrients for muscle contraction.
Example.
Almost every one of us is capable of walking at a fast pace for a long time, but quite soon we are forced to stop running at a fast pace. Why does this happen?
When walking fast, the so-called oxidative and intermediate muscle fibers. Oxidative muscle fibers are characterized by the maximum possible number of mitochondria, roughly speaking, there are 100% mitochondria there.
In intermediate muscle fibers there are noticeably fewer mitochondria, let it be 50% of the maximum number. As a result, hydrogen ions gradually begin to accumulate inside the intermediate muscle fibers, which should lead to the cessation of muscle fiber contraction.
But this does not happen due to the fact that hydrogen ions penetrate into the oxidative muscle fibers, where mitochondria easily cope with their utilization.
As a result, we are able to continue moving as long as there is enough glycogen in the body, as well as fat reserves inside the working oxidative muscle fibers. Then we will be forced to take a rest to replenish our energy reserves.
In the case of fast running, in addition to the mentioned oxidative and intermediate muscle fibers, the so-called. glycolytic muscle fibers, in which there are almost no mitochondria. Therefore, glycolytic muscle fibers are able to work only for a short time, but extremely intensely. This is how your running speed increases.
Then the total number of hydrogen ions becomes such that the entire number of mitochondria present there is no longer able to utilize them. There is a refusal to perform work of the proposed intensity.
But what would happen if all muscle groups had only oxidative muscle fibers inside them?
In this case, the muscle group with oxidative fibers becomes tireless. Her endurance becomes equal to infinity (provided there is a sufficient amount of nutrients - fats and glycogen).
We draw the following conclusion: For endurance training, the development of mitochondria within working muscle fibers is of primary importance. It is thanks to mitochondria that the endurance of muscle groups is achieved.
There is no general body endurance, because endurance (the ability to perform work of the proposed intensity) is associated with the presence of mitochondria in working muscles. The more mitochondria there are, the more endurance the muscles can show.
Mitochondria- This double membrane organelle eukaryotic cell, whose main function is ATP synthesis– a source of energy for the life of the cell.
The number of mitochondria in cells is not constant, on average from several units to several thousand. Where synthesis processes are intense, there are more of them. The size of mitochondria and their shape also varies (round, elongated, spiral, cup-shaped, etc.). More often they have a round, elongated shape, up to 1 micrometer in diameter and up to 10 microns in length. They can move in the cell with the flow of cytoplasm or remain in one position. They move to places where energy production is most needed.
It should be borne in mind that in cells ATP is synthesized not only in mitochondria, but also in the cytoplasm during glycolysis. However, the efficiency of these reactions is low. The peculiarity of the function of mitochondria is that not only oxygen-free oxidation reactions occur in them, but also the oxygen stage of energy metabolism.
In other words, the function of mitochondria is active participation in cellular respiration, which includes many oxidation reactions organic matter, the transfer of hydrogen protons and electrons, releasing energy that is accumulated in ATP.
Mitochondrial enzymes
Enzymes translocases The inner membrane of mitochondria carries out active transport of ADP and ATP.
In the structure of cristae, elementary particles are distinguished, consisting of a head, a stalk and a base. On heads consisting of enzyme ATPases, ATP synthesis occurs. ATPase ensures the coupling of ADP phosphorylation with reactions of the respiratory chain.
Components of the respiratory chain are at the base elementary particles in the thickness of the membrane.
The matrix contains most of Krebs cycle enzymes and fatty acid oxidation.
As a result of the activity of the electrical transport respiratory chain, hydrogen ions enter it from the matrix and are released on the outside of the inner membrane. This is carried out by certain membrane enzymes. The difference in the concentration of hydrogen ions on different sides of the membrane results in a pH gradient.
The energy to maintain the gradient is supplied by the transfer of electrons along the respiratory chain. Otherwise, hydrogen ions would diffuse back.
The energy from the pH gradient is used to synthesize ATP from ADP:
ADP + P = ATP + H 2 O (reaction is reversible)
The resulting water is removed enzymatically. This, along with other factors, facilitates the reaction from left to right.
Mitochondria are the “powerhouses” of eukaryotes, producing energy for cellular activity. These generate energy by converting it into forms that can be used by the cell. Located in, mitochondria serve as the “base” for cellular respiration. - a process that generates energy for cell activity. Mitochondria are also involved in other cellular processes such as growth and.
Distinctive characteristics
Mitochondria have a characteristic oblong or oval shape and are covered with a double membrane. They are found both in and in. The number of mitochondria within a cell varies depending on the type and function of the cell. Some cells, such as mature red blood cells, do not contain mitochondria at all. The absence of mitochondria and other organelles leaves room for the millions of hemoglobin molecules needed to transport oxygen throughout the body. On the other hand, muscle cells can contain thousands of mitochondria, which generate the energy needed for muscle activity. Mitochondria are also abundant in fat cells and liver cells.
Mitochondrial DNA
Mitochondria have their own DNA (mtDNA) and can synthesize their own proteins. mtDNA encodes proteins involved in electron transfer and oxidative phosphorylation that occur during cellular respiration. Oxidative phosphorylation in the mitochondrial matrix generates energy in the form of ATP. Proteins synthesized from mtDNA are also encoded to produce RNA molecules that transmit RNA and ribosomal RNA.
Mitochondrial DNA differs from the DNA found in , in that it does not possess the DNA repair mechanisms that help prevent mutations in nuclear DNA. As a result, mtDNA has a much higher mutation rate than nuclear DNA. Exposure to reactive oxygen produced by oxidative phosphorylation also damages mtDNA.
The structure of mitochondria
Mitochondria are surrounded by double . Each of these membranes is a phospholipid bilayer with embedded proteins. The outer membrane is smooth, but the inner membrane has many folds. These folds are called cristae. They increase the “productivity” of cellular respiration by increasing the available surface area.
Double membranes divide the mitochondrion into two distinct parts: the intermembrane space and the mitochondrial matrix. The intermembrane space is the narrow part between two membranes, while the mitochondrial matrix is the part enclosed within the membranes.
The mitochondrial matrix contains mtDNA, ribosomes and enzymes. Some of the stages of cellular respiration, including the cycle citric acid and oxidative phosphorylation occur in the matrix due to the high concentration of enzymes.
Mitochondria are semi-autonomous, as they are only partially dependent on the cell to replicate and grow. They have their own DNA, ribosomes, proteins and control over their synthesis. Like bacteria, mitochondria have circular DNA and replicate by a reproductive process called binary fission. Before replication, mitochondria fuse together in a process called fusion. This is necessary to maintain stability, since without it the mitochondria will shrink as they divide. Reduced mitochondria are not able to produce enough energy necessary for normal cell functioning.
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