What happens in mitochondria. Why do mitochondria have their own genes?

Mitochondria - microscopic two-membrane semi-autonomous general-purpose organelles that provide the cell with energy, obtained through oxidation processes and stored in the form phosphate bonds of ATP. Mitochondria are also involved in steroid biosynthesis, fatty acid oxidation, and nucleic acid synthesis. Present in all eukaryotic cells. There are no mitochondria in prokaryotic cells, their function is performed by mesosomes - the invagination of the outer cytoplasmic membrane into the cell.

Mitochondria can have elliptical, spherical, rod-shaped, filamentous, and other shapes that can change over time. The number of mitochondria in cells that perform various functions varies widely - from 50 to 500-5000 in the most active cells. There are more of them where synthetic processes are intensive (liver) or energy costs are high (muscle cells). In liver cells (hepatocytes), their number is 800. And the volume they occupy is approximately 20% of the volume of the cytoplasm. The size of mitochondria is from 0.2 to 1-2 microns in diameter and from 2 to 5-7 (10) microns in length. At the optical level, mitochondria are detected in the cytoplasm special methods and have the appearance of small grains and threads (which led to their name - from the Greek mitos - thread and chondros - grain).

In the cytoplasm, mitochondria can be located diffusely, but usually they concentrated in areas of maximum energy consumption, for example, near ion pumps, contractile elements (myofibrils), organelles of movement (sperm axonemes, cilia), components of a synthetic apparatus (ER cisterns). According to one hypothesis, all the mitochondria of a cell are connected to each other and form a three-dimensional network.

Mitochondria surrounded two membranes - outer and inner, divided intermembrane space, and contain mitochondrial matrix, into which the folds of the inner membrane face - cristae.

Outer mitochondrial membrane smooth, on chemical composition similar to the outer cytoplasmic membrane and has a high permeability for molecules weighing up to 10 kilodaltons, penetrating from the cytosol into the intermembrane space. In its composition, it is similar to the plasmalemma, 25% are proteins, 75% are lipids. Lipids include cholesterol. The outer membranea contains many specialized molecules transport proteins(For example, porins), which form wide hydrophilic channels and provide its high permeability, and also do not a large number of enzyme systems. On it are receptors recognition proteins that are carried across both mitochondrial membranes at special points of their contact - adhesion zones.

The inner membrane has outgrowths inside- ridges or cristae that divide the mitochondrial matrix into compartments. The cristae increase the surface area of ​​the inner membrane. Thus, the inner mitochondrial membrane is larger than the outer one. The cristae are located perpendicular or longitudinal to the length of the mitochondria. The cristae may be vesicular, tubular, or lamellar in shape.

The chemical composition of the inner membrane of mitochondria is similar to the membranes of prokaryotes (for example, it contains a special lipid - cardiodipin and lacks cholesterol). In the inner mitochondrial membrane, proteins predominate, making up 75%. Three types of proteins are built into the inner membrane (a) proteins of the electron transport chain (respiratory chain) - NAD "H-dehydrogenase and FAD" H dehydrogenase - and other transport proteins,(b) mushroom bodies of ATP synthase(whose heads are turned towards the matrix) and (c) part of the Krebs cycle enzymes (succinate dehydrogenase). The inner mitochondrial membrane is characterized by extremely low permeability, the transport of substances is carried out through contact sites. Low inner membrane permeability to small ions due to high phospholipid content

Mitochondria - semi-autonomous cell organelles, tk. contain their own DNA, a semi-autonomous system of replication, transcription and their own protein-synthesizing apparatus - a semi-autonomous translation system (70S-type ribosomes and t-RNA). Due to this, mitochondria synthesize some of their own proteins. Mitochondria can divide independently of cell division. If all mitochondria are removed from the cell, then new ones will not appear in it. According to the theory of endosymbiosis, mitochondria originated from aerobic prokaryotic cells that entered the host cell, but were not digested, entered the path of deep symbiosis and gradually, having lost their autonomy, turned into mitochondria.

Mitochondria - semi-autonomous organelles, which is expressed by the following features:

1) the presence of its own genetic material (DNA strands), which allows for protein synthesis, and also allows you to independently divide, regardless of the cell;

2) the presence of a double membrane;

3) plastids and mitochondria are capable of synthesizing ATP (for chloroplasts, the energy source is light; in mitochondria, ATP is formed as a result of the oxidation of organic substances).

Mitochondrial functions:

1) Energy- ATP synthesis (hence these organelles got the name "energy stations of the cell"):

During aerobic respiration, oxidative phosphorylation occurs on the cristae (the formation of ATP from ADP and inorganic phosphate due to the energy released during oxidation organic matter) and the transfer of electrons along the electron transport chain. On the inner membrane of the mitochondria are enzymes involved in cellular respiration;

2) participation in biosynthesis many compounds (some amino acids, steroids (steroidogenesis) are synthesized in mitochondria, some of their own proteins are synthesized), as well as the accumulation of ions (Ca 2+), glycoproteins, proteins, lipids;

3) oxidation fatty acids;

4) genetic- synthesis of nucleic acids (there are processes of replication and transcription). Mitochondrial DNA provides cytoplasmic inheritance.

ATP

ATP was discovered in 1929 by the German chemist Lohmann. In 1935, Vladimir Engelhardt drew attention to the fact that muscle contractions are impossible without the presence of ATP. In the period from 1939 to 1941, Nobel Prize winner Fritz Lipmann proved that ATP is the main source of energy for the metabolic reaction, and coined the term "energy-rich phosphate bonds." Cardinal changes in the study of the action of ATP on the body occurred in the mid-70s, when the presence of specific receptors on the outer surface of cell membranes that are sensitive to the ATP molecule was discovered. Since then, the trigger (regulatory) effect of ATP on various body functions has been intensively studied.

Adenosine triphosphoric acid ( ATP, adenine triphosphoric acid) - a nucleotide that plays an extremely important role in the exchange of energy and substances in organisms; First of all, the compound is known as a universal source of energy for all biochemical processes occurring in living systems.

Chemically, ATP is the triphosphate ester of adenosine, which is a derivative of adenine and ribose.

The purine nitrogenous base - adenine - is connected by a β-N-glycosidic bond to the 5 "carbon of ribose, to which three phosphoric acid molecules are sequentially attached, denoted respectively by the letters: α, β and γ.

ATP refers to the so-called macroergic compounds, that is, to chemical compounds containing bonds, during the hydrolysis of which a significant amount of energy is released. Hydrolysis of the phosphoester bonds of the ATP molecule, accompanied by the elimination of 1 or 2 phosphoric acid residues, leads to the release, according to various sources, from 40 to 60 kJ/mol.

ATP + H 2 O → ADP + H 3 PO 4 + energy

ATP + H 2 O → AMP + H 4 P 2 O 7 + energy

The released energy is used in a variety of processes that require energy.

functions

1) The main one is energy. ATP serves as a direct source of energy for many energy-intensive biochemical and physiological processes.

2) synthesis of nucleic acids.

3) regulation of many biochemical processes. ATP, joining the regulatory centers of enzymes, enhances or suppresses their activity.

a direct precursor of the synthesis of cycloadenosine monophosphate - a secondary mediator of the transmission of a hormonal signal into the cell.

mediator in synapses

synthesis paths:

In the body, ATP is synthesized from ADP using the energy of oxidizing substances:

ADP + H 3 PO 4 + energy→ ATP + H 2 O.

Phosphorylation of ADP is possible in two ways: substrate phosphorylation and oxidative phosphorylation. The bulk of ATP is formed on membranes in mitochondria by oxidative phosphorylation by the enzyme H-dependent ATP synthetase. Substrate phosphorylation of ADP does not require the participation of membranes; it occurs in the process of glycolysis or by transferring a phosphate group from other macroergic compounds.

The reactions of ADP phosphorylation and the subsequent use of ATP as an energy source form a cyclic process that is the essence of energy metabolism.

In the body, ATP is one of the most frequently updated substances. During the day, one ATP molecule goes through an average of 2000-3000 resynthesis cycles (the human body synthesizes about 40 kg per day), that is, there is practically no ATP reserve in the body, and for normal life it is necessary to constantly synthesize new ATP molecules.

An important role in the life of each cell is played by special structures - mitochondria. The structure of mitochondria allows the organelle to work in a semi-autonomous mode.

general characteristics

Mitochondria were discovered in 1850. However, it became possible to understand the structure and functional purpose of mitochondria only in 1948.

Due to their rather large size, the organelles are clearly visible in a light microscope. Maximum length- 10 µm, diameter does not exceed 1 µm.

Mitochondria are present in all eukaryotic cells. These are two-membrane organelles, usually bean-shaped. There are also mitochondria of spherical, filamentous, spiral shape.

The number of mitochondria can vary considerably. For example, there are about a thousand of them in liver cells, and 300 thousand in oocytes. plant cells contain fewer mitochondria than animals.

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Rice. 1. Finding mitochondria in the cell.

Mitochondria are plastic. They change shape and move to the active centers of the cell. Usually there are more mitochondria in those cells and parts of the cytoplasm where the need for ATP is higher.

Structure

Each mitochondrion is separated from the cytoplasm by two membranes. The outer membrane is smooth. The structure of the inner membrane is more complex. It forms numerous folds - cristae, which increase the functional surface. Between the two membranes is a space of 10-20 nm filled with enzymes. Inside the organelle is a matrix - a gel-like substance.

Rice. 2. Internal structure mitochondria.

The table “Structure and functions of mitochondria” describes in detail the components of the organelle.

The main function of mitochondria is the generation of cell energy in the form of ATP molecules due to the reaction of oxidative phosphorylation - cellular respiration.

In addition to mitochondria, plant cells contain additional semi-autonomous organelles - plastids.
Depending on the functional purpose, there are three types of plastids:

• chromoplasts - accumulate and store pigments (carotenes) of different shades, giving color to plant flowers;
• leucoplasts - store nutrients, for example, starch, in the form of grains and granules;
• chloroplasts - the most important organelles containing the green pigment (chlorophyll), which gives color to plants, and carries out photosynthesis.

Rice. 3. Plastids.

What have we learned?

Considered the structural features of mitochondria - two-membrane organelles that carry out cellular respiration. The outer membrane consists of proteins and lipids and transports substances. The inner membrane forms folds - cristae, on which hydrogen is oxidized. Crista is surrounded by a matrix - a gel-like substance in which part of the reactions of cellular respiration takes place. The matrix contains mitochondrial DNA and RNA.

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Mitochondria- This double membrane organoid eukaryotic cell, the main function of which ATP synthesis- 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 intensive, there are more of them. The size of mitochondria and their shape also vary (rounded, elongated, spiral, cup-shaped, etc.). More often they have a rounded elongated shape, up to 1 micrometer in diameter and up to 10 microns long. They can move in the cell with the current of the cytoplasm or remain in one position. They move to places where energy generation 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. A feature of the function of mitochondria is that not only oxygen-free oxidation reactions take place in them, but also the oxygen stage of energy metabolism.

In other words, the function of mitochondria is an active participation in cellular respiration, which includes many reactions of oxidation of organic substances, the transfer of hydrogen protons and electrons, which go with the release of energy, which is accumulated in ATP.

Mitochondrial enzymes

Enzymes translocases The inner mitochondrial membrane actively transports ADP and ATP.

In the structure of cristae, elementary particles are distinguished, consisting of a head, a leg and a base. On heads made of enzyme ATPase synthesis of ATP takes place. ATPase provides conjugation of ADP phosphorylation with the reactions of the respiratory chain.

Components of the respiratory chain are at the base elementary particles in the depth of the membrane.

The matrix contains most Krebs cycle enzymes and fatty acid oxidation.

As a result of the activity of the electrotransport respiratory chain, hydrogen ions enter it from the matrix and are released on the outer side 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 leads to the appearance of a pH gradient.

The energy to maintain the gradient is supplied by the transfer of electrons through the respiratory chain. Otherwise, hydrogen ions would diffuse back.

The energy of the pH gradient is used to synthesize ATP from ADP:

ADP + F \u003d ATP + H 2 O (the reaction is reversible)

The resulting water is enzymatically removed. This, along with other factors, makes it easier for the reaction to flow from left to right.

Structure. The surface apparatus of mitochondria consists of two membranes - outer and inner. outer membrane smooth, it separates the mitochondria from the hyaloplasm. Under it is a folded inner membrane, which forms Christie(combs). On both sides of the cristae, small mushroom-shaped bodies called oxysomes, or ATP-somes. They contain enzymes involved in oxidative phosphorylation (attachment of phosphate residues to ADP to form ATP). The number of cristae in mitochondria is related to the energy needs of the cell, in particular, in muscle cells, mitochondria contain a very large number of cristae. With increased function, mitochondrial cells become more oval or elongated, and the number of cristae increases.

Mitochondria have their own genome, their 70S-type ribosomes differ from those of the cytoplasm. Mitochondrial DNA predominantly has a cyclic form (plasmids), encodes all three types of its own RNA, and provides information for the synthesis of some mitochondrial proteins (about 9%). Thus, mitochondria can be considered semi-autonomous organelles. Mitochondria are self-replicating (able to reproduce) organelles. Mitochondrial renewal occurs throughout the entire cell cycle. For example, in liver cells, they are replaced by new ones after almost 10 days. The most likely way of reproduction of mitochondria is considered to be their separation: a constriction appears in the middle of the mitochondria or a partition appears, after which the organelles break up into two new mitochondria. Mitochondria are formed from promitochondria - round bodies up to 50 nm in diameter with a double membrane.

Functions . Mitochondria are involved in the energy processes of the cell, they contain enzymes associated with the formation of energy and cellular respiration. In other words, the mitochondrion is a kind of biochemical mini-factory that converts energy organic compounds for the applied energy of ATP. In mitochondria, the energy process begins in the matrix, where pyruvic acid is broken down in the Krebs cycle. During this process, hydrogen atoms are released and transported by the respiratory chain. The energy that is released in this case is used in several parts of the respiratory chain to carry out the phosphorylation reaction - the synthesis of ATP, that is, the addition of a phosphate group to ADP. It occurs on the inner membrane of the mitochondria. So, energy function mitochondria integrates with: a) oxidation of organic compounds that occurs in the matrix, due to which mitochondria are called respiratory center of cells b) ATP synthesis, carried out on the cristae, due to which mitochondria are called energy stations of cells. In addition, mitochondria are involved in the regulation of water metabolism, the deposition of calcium ions, the production of steroid hormone precursors, metabolism (for example, mitochondria in liver cells contain enzymes that allow them to neutralize ammonia) and others.

BIOLOGY + Mitochondrial diseases are a group of hereditary diseases associated with mitochondrial defects that lead to disruption of cellular respiration. They are transmitted through the female line to children of both sexes, since the egg has a larger volume of cytoplasm and, accordingly, passes on to descendants and large quantity mitochondria. Mitochondrial DNA, unlike nuclear DNA, is not protected by histone proteins, and the repair mechanisms inherited from ancestral bacteria are imperfect. Therefore, mutations in mitochondrial DNA accumulate 10-20 times faster than in nuclear DNA, which leads to mitochondrial diseases. In modern medicine, about 50 of them are now known. For example, chronic fatigue syndrome, migraine, Barth's syndrome, Pearson's syndrome and many others.

MITOCHONDRIA (mitochondria; Greek, mitos thread + chondrion grain) - organelles present in the cytoplasm of animal and plant cells. M. take part in the processes of respiration and oxidative phosphorylation, produce the energy necessary for the functioning of the cell, thus representing its "power stations".

The term "mitochondria" was proposed in 1894 by S. Benda. In the mid 30s. 20th century for the first time it was succeeded to allocate M. from cells of a liver that allowed to investigate these structures biochemical, methods. In 1948, G. Hogeboom received definitive evidence that M. are indeed centers of cellular respiration. Significant progress in the study of these organelles was made in the 60-70s. in connection with the use of electron microscopy and molecular biology methods.

M.'s shape varies from almost round to strongly elongated, having the form of threads (Fig. 1). Their size ranges from 0.1 to 7 microns. The amount of M. in a cell depends on the type of tissue and the functional state of the organism. So, in spermatozoa, the number of M. is small - approx. 20 (per cell), mammalian renal tubule epithelial cells contain up to 300 of them each, and 500,000 mitochondria were found in the giant amoeba (Chaos chaos). In one rat liver cell, approx. 3000 M., however, in the process of starvation of the animal, the number of M. can be reduced to 700. Usually M. are distributed quite evenly in the cytoplasm, however, in the cells of certain tissues, M. can be constantly localized in areas that are especially in need of energy. For example, in a skeletal muscle M. are often in contact with contractile sites of myofibrils, forming the correct three-dimensional structures. In spermatozoa, M. form a spiral case around the axial thread of the tail, which is probably associated with the ability to use the ATP energy synthesized in M. for tail movements. In M.'s axons, they are concentrated near synaptic endings, where the process of transmission of nerve impulses occurs, accompanied by energy consumption. In cells of an epithelium of renal tubules M. are connected with protrusions of a basal cellular membrane. This is due to the need for a constant and intensive supply of energy to the process of active transfer of water and substances dissolved in it, which occurs in the kidneys.

Electron-microscopically it is established that M. contains two membranes - external and internal. Thickness of each membrane approx. 6 nm, the distance between them is 6-8 nm. The outer membrane is smooth, the inner one forms complex outgrowths (cristae) protruding into the mitochondrial cavity (Fig. 2). M.'s internal space bears the name of a matrix. The membranes are a film of compactly packed molecules of proteins and lipids, while the matrix is ​​like a gel and contains soluble proteins, phosphates and other chemicals. connections. Usually the matrix looks homogeneous, only in nek-ry cases it is possible to find thin threads, tubes and granules containing calcium and magnesium ions in it.

Of the structural features of the inner membrane, it is necessary to note the presence in it of spherical particles of approx. 8-10 nm across, sitting on a short stalk and sometimes protruding into the matrix. These particles were discovered in 1962 by H. Fernandez-Moran. They consist of a protein with ATPase activity, designated F1. The protein is attached to the inner membrane only from the side facing the matrix. F1 particles are located at a distance of 10 nm from each other, and each M. contains 10 4 -10 5 such particles.

The cristae and internal membranes of M. contain most of the respiratory enzymes (see), respiratory enzymes are organized into compact ensembles distributed at regular intervals in M.'s cristae at a distance of 20 nm from each other.

M. of almost all types of animal and plant cells are built according to a single principle, however, deviations in details are possible. So, cristae can be located not only across the long axis of the organoid, but also longitudinally, for example, in the M. of the synaptic zone of the axon. In some cases, cristae may branch. In M. of the simplest organisms, nek-ry insects and in cells of a glomerular zone of adrenal glands cristae have the form of tubules. The number of cristae varies; so, in M. of liver cells and germ cells, there are very few cristae and they are short, while the matrix is ​​abundant; in M. of muscle cells, cristae are numerous, and there is little matrix. There is an opinion that the number of cristae correlates with the oxidative activity of M.

In the inner membrane of M., three processes are carried out in parallel: the oxidation of the substrate of the Krebs cycle (see Tricarboxylic acid cycle), the transfer of electrons released during this, and the accumulation of energy through the formation of high-energy bonds of adenosine triphosphate (see Adenosine phosphoric acids). The main function of M. is the conjugation of ATP synthesis (from ADP and inorganic phosphorus) and the aerobic oxidation process (see Biological oxidation). The energy accumulated in ATP molecules can be transformed into mechanical (in muscles), electrical ( nervous system), osmotic (kidneys), etc. The processes of aerobic respiration (see Biological oxidation) and the oxidative phosphorylation associated with it (see) are the main functions of M. In addition, oxidation can occur in the outer membrane of M. fatty to-t, phospholipids and some other compounds.

In 1963, Nass and Nass (M. Nass, S. Nass) found that M. contains DNA (one or more molecules). All mitochondrial DNA from animal cells studied so far consist of covalently closed rings dia. OK. 5 nm. In plants, mitochondrial DNA is much longer and is not always ring-shaped. Mitochondrial DNA differs from nuclear DNA in many ways. DNA replication occurs through the usual mechanism, but does not coincide in time with the replication of nuclear DNA. The amount of genetic information contained in a mitochondrial DNA molecule is apparently not enough to encode all the proteins and enzymes contained in M. Mitochondrial genes encode mainly structural membrane proteins and proteins involved in mitochondrial morphogenesis. M. have their own transport RNA and synthetases, contain all the components necessary for protein synthesis; their ribosomes are smaller than cytoplasmic ones and more similar to bacterial ribosomes.

M.'s life expectancy is rather small. So, the renewal time of half of the amount of M. is 9.6-10.2 days for the liver, and 12.4 days for the kidney. Replenishment of M.'s population occurs, as a rule, from preexisting (maternal) M. by their division or budding.

It has long been suggested that in the process of evolution M. probably arose by endosymbiosis of primitive nucleated cells with bacterium-like organisms. There is a large amount of evidence for this: the presence of its own DNA, more similar to the DNA of bacteria than to the DNA of the cell nucleus; presence in M. of ribosomes; synthesis of DNA-dependent RNA; sensitivity of mitochondrial proteins to the antibacterial drug - chloramphenicol; similarity with bacteria in the implementation of the respiratory chain; morfol., biochemical, and fiziol, differences between the inner and outer membrane. According to the symbiotic theory the host cell is considered as an anaerobic organism, a source of energy for to-rogo is the glycolysis (flowing in cytoplasm). In the "symbiont", the Krebs cycle and the respiratory chain are realized; it is capable of respiration and oxidative phosphorylation (see).

M. are very labile intracellular organoids, earlier than others reacting to emergence of any patol, conditions. Changes in the number of M. in a cell (or rather, in their populations) or changes in their structure are possible. Eg, during fasting, the action of ionizing radiation, the number of M. decreases. Structural changes usually consist of swelling of the entire organoid, matrix enlightenment, destruction of cristae, and violation of the integrity of the outer membrane.

Swelling is accompanied by a significant change in the volume of M. In particular, with myocardial ischemia, the volume of M. increases 10 times or more. There are two types of swelling: in one case, it is associated with a change in the osmotic pressure inside the cell, in other cases, with changes in cellular respiration associated with enzymatic reactions and primary functional disorders that cause changes in water metabolism. In addition to swelling, vacuolization of M can occur.

Regardless of the reasons causing patol, the state (hypoxia, hyperfunction, intoxication), M.'s changes are quite stereotyped and nonspecific.

Such changes in the structure and function of M. are observed, to-rye, apparently, became the cause of the disease. In 1962, R. Luft described a case of "mitochondrial disease". A patient with a sharply increased metabolic rate (with normal thyroid function) underwent a puncture skeletal muscle and an increased number of M. was found, as well as a violation of the structure of the cristae. Defective mitochondria in liver cells were also observed in severe thyrotoxicosis. Grapes (J. Vinograd) et al. (from 1937 to 1969) found that in patients with certain forms of leukemia, mitochondrial DNA from leukocytes was markedly different from normal. They were open rings or groups of linked rings. The frequency of these abnormal forms decreased as a result of chemotherapy.

Bibliography: Gause G. G. Mitochondrial DNA, M., 1977, bibliogr.; D e P o-bertis E., Novinsky V. and C and e with F. Biology of the cell, trans. from English, M., 1973; Ozernyuk N. D. Growth and reproduction of mitochondria, M., 1978, bibliogr.; Polikar A. and Bessie M. Elements of cell pathology, trans. from French, Moscow, 1970; RudinD. and Wilkie D. Mitochondrial biogenesis, trans. from English, M., 1970, bibliography; Serov V. V. and Spiders V. S. Ultrastructural pathology, M., 1975; S e r R. Cytoplasmic genes and organelles, trans. from English, M., 1975.

T. A. Zaletaeva.