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. In simple language, biologists call it an 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 during 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 can be large and branched.
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 small granules create mitochondrial ribosomes.
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.
Structure and functions of the plant cell nucleus.
Core- an essential part of a eukaryotic cell. This is the place of storage and reproduction of hereditary information. The nucleus also serves as the control center for metabolism and almost all processes occurring in the cell. Most often, cells have only one nucleus, rarely two or more. Its shape is most often spherical or ellipsoidal. In young, especially meristematic, cells it occupies a central position, but later it usually moves to the membrane, pushed aside by the growing vacuole. On the outside, the nucleus is covered with a double membrane - a nuclear membrane, permeated with pores (nuclear pores are dynamic formations, they can open and close; in this way the exchange between the nucleus and the cytoplasm can be regulated) at the edges of which the outer membrane passes into the inner one. The outer nuclear membrane connects to the membrane channels of the ER. Ribosomes are located on it. The inner membrane may develop invaginations.
The internal contents of the nucleus are karyoplasm with chromatin, nucleoli, and ribosomes immersed in it. Karyoplasm (nucleoplasm) is a jelly-like solution that fills the space between the nuclear structures (chromatin and nucleoli). It contains ions, nucleotides, enzymes.
Chromatin is a despiralized form of chromosome existence. In a despiralized state, chromatin is found in the nucleus of a nondividing cell. Chromatin and chromosomes interchange into each other. In terms of chemical organization, both chromatin and chromosomes do not differ. The chemical basis is deoxyribonucleoprotein - a complex of DNA with proteins. With the help of proteins, multi-level packaging of DNA molecules occurs, while chromatin acquires a compact shape.
The nucleolus, usually spherical in shape (one or more), is not surrounded by a membrane, contains fibrillar protein threads and RNA. Nucleoli are not permanent formations; they disappear at the beginning of cell division and are restored after its completion. Nucleoli are present only in non-dividing cells. In the nucleoli, ribosomes are formed and nuclear proteins are synthesized. The nucleoli themselves are formed in areas of secondary chromosome constrictions (nucleolar organizers).
The nucleus is an essential part of a eukaryotic cell. The core diameter ranges from 5 to 20 microns. The main function of the nucleus is to store genetic material in the form of DNA and transfer it to daughter cells during cell division. In addition, the nucleus controls protein synthesis and controls all vital processes of the cell. (in a plant cell the nucleus was described by R. Brown in 1831, in an animal cell by T. Schwann in 1838).
The chemical composition of the nucleus is represented mainly by nucleic acids and proteins.
Structure and functions of mitochondria.
Mitochondria or chondriosomes are the “power” stations of the cell; most of the respiration reactions are localized in them (aerobic phase). In mitochondria, respiration energy is accumulated in adenosine triphosphate (ATP). The energy stored in ATP serves as the main source for the physiological activities of the cell. Mitochondria usually have an elongated rod-shaped shape with a length of 4-7 µm and a diameter of 0.5-2 µm. The number of mitochondria in a cell can vary from 500 to 1000 and depends on the role of this organ in energy metabolism processes.
The chemical composition of mitochondria varies somewhat. These are mainly protein-lipid organelles. The protein content in them is 60-65%, with structural and enzymatic proteins contained in approximately equal proportions, as well as about 30% lipids. It is very important that mitochondria contain nucleic acids: RNA - 1% and DNA -0.5%. Mitochondria contain not only DNA, but also the entire protein synthesis system, including ribosomes.
Mitochondria are surrounded by a double membrane. The thickness of the membranes is 6-10 nm. Mitochondria membranes are 70% protein. Membrane phospholipids are represented by phosphatidylcholine, phosphatidylethanolamine, as well as specific phospholipids, for example, cardiolipin. Mitochondrial membranes do not allow H+ to pass through and serve as a barrier to their transport.
Between the membranes is the fluid-filled perimitochondrial space. The internal space of mitochondria is filled with a matrix in the form of a gelatinous semi-liquid mass. The enzymes of the Krebs cycle are concentrated in the matrix. The inner membrane gives rise to outgrowths - cristae in the form of plates and tubes; they divide the internal space of mitochondria into separate compartments. The respiratory chain (electron transport chain) is localized in the inner membrane.
Structure. The surface apparatus of mitochondria consists of two membranes - outer and inner. Outer membrane smooth, it separates the mitochondria from the hyaloplasm. Beneath it is a folded inner membrane, which forms Christie(ridges). On both sides of the cristae, small mushroom-shaped bodies called oxysomes, or ATP-somami. They contain enzymes involved in oxidative phosphorylation (the addition 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 cell function, mitochondria become more oval or elongated, and the number of cristae increases.
Mitochondria have their own genome, their 70S type ribosomes differ from the ribosomes of the cytoplasm. Mitochondrial DNA predominantly has a cyclic form (plasmids), encodes all three types of its own RNA and supplies information for the synthesis of some mitochondrial proteins (about 9%). So, mitochondria can be considered semi-autonomous organelles. Mitochondria are self-replicating (capable of reproduction) organelles. Mitochondrial renewal occurs throughout the cell cycle. For example, in liver cells they are replaced by new ones after almost 10 days. The most likely way of reproducing mitochondria is considered to be their division: a constriction appears in the middle of the mitochondria or a septum appears, after which the organelles split into two new mitochondria. Mitochondria are formed with promitochondria - round bodies with a diameter of up to 50 nm with a double membrane.
Functions . Mitochondria are involved in the energy processes of the cell; they contain enzymes associated with energy production and cellular respiration. In other words, the mitochondrion is a kind of biochemical mini-factory that converts the energy of organic compounds into the applied energy of ATP. In mitochondria, the energy process begins in the matrix, where the breakdown of pyruvic acid occurs 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. This occurs on the inner membrane of mitochondria. So, energy function mitochondria integrates with: a) the oxidation of organic compounds that occurs in the matrix, due to which mitochondria are called respiratory center of cells b) ATP synthesis is carried out on cristae, due to which mitochondria are called energy stations of cells. In addition, mitochondria take part 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 impaired 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 a larger number of mitochondria to its descendants. Mitochondrial DNA, unlike nuclear DNA, is not protected by histone proteins, and the repair mechanisms inherited from ancestral bacteria are imperfect. Therefore, mutations accumulate in mitochondrial DNA 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 syndrome, Pearson syndrome and many others.
Mitochondria (from Greek μίτος (mitos) - thread and χονδρίον (chondrion) - granule) is a cellular two-membrane organelle that contains its own genetic material, mitochondrial. They are found as spherical or tubular cell structures in almost all eukaryotes, but not in prokaryotes.
Mitochondria are organelles that regenerate the high-energy molecule adenosine triphosphate through the respiratory chain. In addition to this oxidative phosphorylation, they perform other important tasks, e.g. participate in the formation of iron and sulfur clusters. The structure and functions of such organelles are discussed in detail below.
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General information
There are especially many mitochondria in areas with high energy consumption. These include muscle, nerve, sensory cells and oocytes. In the cellular structures of the heart muscle, the volume fraction of these organelles reaches 36%. They have a diameter of about 0.5-1.5 microns and a variety of shapes, from spheres to complex threads. Their number is adjusted taking into account the energy needs of the cell.
Eukaryotic cells that lose their mitochondria can't restore them. There are also eukaryotes without them, for example, some protozoa. The number of these organelles per cell unit is usually from 1000 to 2000 with a volume fraction of 25%. But these values can vary greatly depending on the type of cell structure and organism. There are about four to five of them in a mature sperm cell, and several hundred thousand in a mature egg.
Mitochondria are transmitted through the plasma of the egg only from the mother, which was the reason for the study of maternal lines. It has now been established that also through sperm, some male organelles are imported into the plasma of the fertilized egg (zygote). They will probably be resolved fairly quickly. However, there are several cases where doctors were able to prove that the child's mitochondria were from the paternal line. Diseases caused by mutations in mitochondrial genes are inherited only from the mother.
Interesting! The popular scientific term "powerhouse of the cell" was coined in 1957 by Philip Sikiewitz.
Mitochondria structure diagram
Let us consider the structural features of these important structures. They are formed as a result of a combination of several elements. The shell of these organelles consists of an outer and inner membrane; they, in turn, consist of phospholipid bilayers and proteins. Both shells differ in their properties. Between them there are five different compartments: the outer membrane, the intermembrane space (the space between two membranes), the inner membrane, the crista and the matrix (the space inside the inner membrane), in general - the internal structures of the organelle.
In illustrations in textbooks, the mitochondrion primarily looks like a separate bean-shaped organelle. Is it really? No, they form tubular mitochondrial network, which can pass through and change the entire cellular unit. Mitochondria in a cell are capable of combining (by fusion) and re-dividing (by fission).
Note! In yeast, about two mitochondrial fusions occur in one minute. Therefore, it is impossible to accurately determine the current number of mitochondria in cells.
Outer membrane
The outer shell surrounds the entire organelle and includes channels of protein complexes that allow the exchange of molecules and ions between the mitochondrion and the cytosol. Large molecules cannot pass through the membrane.
The outer one, which spans the entire organelle and is not folded, has a phospholipid to protein weight ratio of 1:1 and is thus similar to the eukaryotic plasma membrane. It contains many integral proteins, porins. Porins form channels that allow free diffusion of molecules with a mass of up to 5000 daltons through the membrane. Larger proteins can invade when a signal sequence at the N-terminus binds to the large subunit of the transloxase protein, from which they then actively move along the membrane envelope.
If cracks occur in the outer membrane, proteins from the intermembrane space can escape into the cytosol, which can lead to cell death. The outer membrane can fuse with the endoplasmic reticulum membrane and then form a structure called MAM (mitochondrion-associated ER). It is important for signaling between the ER and the mitochondrion, which is also necessary for transport.
Intermembrane space
The area is a gap between the outer and inner membranes. Since the external one allows the free penetration of small molecules, their concentration, such as ions and sugars, in the intermembrane space is identical to the concentrations in the cytosol. However, large proteins require the transmission of a specific signal sequence, so that protein composition differs between the intermembrane space and the cytosol. Thus, the protein that is retained in the intermembrane space is cytochrome.
Inner membrane
The inner mitochondrial membrane contains proteins with four types of functions:
- Proteins – carry out oxidation reactions of the respiratory chain.
- Adenosine triphosphate synthase, which produces ATP in the matrix.
- Specific transport proteins that regulate the passage of metabolites between the matrix and the cytoplasm.
- Protein import systems.
The internal one has, in particular, a double phospholipid, cardiolipin, replaced by four fatty acids. Cardiolipin is commonly found in mitochondrial membranes and bacterial plasma membranes. It is mainly present in the human body in areas of high metabolic activity or high energy activity, such as contractile cardiomyocytes, in the myocardium.
Attention! The inner membrane contains more than 150 different polypeptides, about 1/8 of all mitochondrial proteins. As a result, the lipid concentration is lower than that of the outer bilayer and its permeability is lower.
Divided into numerous cristae, they expand the outer region of the inner mitochondrial membrane, increasing its ability to produce ATP.
In a typical liver mitochondria, for example, the outer region, particularly the cristae, is approximately five times the area of the outer membrane. Energy stations of cells that have higher ATP requirements, e.g. muscle cells contain more cristae, than a typical liver mitochondria.
The inner shell encloses the matrix, the inner fluid of the mitochondria. It corresponds to the cytosol of bacteria and contains mitochondrial DNA, citrate cycle enzymes and their own mitochondrial ribosomes, which are different from the ribosomes in the cytosol (but also from bacteria). The intermembrane space contains enzymes that can phosphorylate nucleotides by consuming ATP.
Functions
- Important degradation pathways: the citrate cycle, for which pyruvate is introduced from the cytosol into the matrix. Pyruvate is then decarboxylated by pyruvate dehydrogenase to acetyl coenzyme A. Another source of acetyl coenzyme A is the degradation of fatty acids (β-oxidation), which occurs in animal cells in mitochondria, but in plant cells only in glyoxysomes and peroxisomes. To this end, acyl-coenzyme A is transferred from the cytosol by binding to carnitine across the inner mitochondrial membrane and converted to acetyl-coenzyme A. From this, most of the reducing equivalents in the Krebs cycle (also known as the Krebs cycle or tricarboxylic acid cycle), which are then converted to ATP in the oxidative chain .
- Oxidative chain. An electrochemical gradient has been established between the intermembrane space and the mitochondrial matrix, which serves to produce ATP using ATP synthase, through the processes of electron transfer and proton accumulation. The electrons and protons needed to create the gradient are obtained by oxidative degradation from nutrients(such as glucose) absorbed by the body. Glycolysis initially occurs in the cytoplasm.
- Apoptosis (programmed cell death)
- Calcium storage: Through the ability to absorb calcium ions and then release them, mitochondria interfere with cell homeostasis.
- Synthesis of iron-sulfur clusters required, among other things, by many enzymes of the respiratory chain. This function is now considered an essential function of mitochondria, i.e. as this is the reason why almost all cells rely on energy stations for survival.
Matrix
This is a space included in the inner mitochondrial membrane. Contains about two-thirds of the total protein. Plays a crucial role in the production of ATP through ATP synthase, included in the inner membrane. Contains a highly concentrated mixture of hundreds of different enzymes (mainly involved in the degradation of fatty acids and pyruvate), mitochondria-specific ribosomes, messenger RNA and several copies of the DNA of the mitochondrial genome.
These organelles have their own genome, as well as the enzymatic equipment necessary for carrying out its own protein biosynthesis.
Mitochondria What is Mitochondria and its functions
Structure and functioning of mitochondria
Conclusion
Thus, mitochondria are called cellular power plants that produce energy and occupy a leading place in the life and survival of an individual cell in particular and a living organism in general. Mitochondria are an integral part of a living cell, including plant cells, which have not yet been fully studied. There are especially many mitochondria in those cells that require more energy.
Mitochondria - energy converters and energy suppliers to ensure cellular functions - occupy a significant part of the cytoplasm of cells and are concentrated in areas of high ATP consumption (for example, in the epithelium of the kidney tubules they are located near the plasma membrane (providing reabsorption), and in neurons - in synapses (providing electrogenesis and secretion). The number of mitochondria in a cell is measured in hundreds. Mitochondria have their own genome. The organelle functions on average 10 days, mitochondria are renewed by dividing them.
Morphology of mitochondria
Mitochondria most often have the shape of a cylinder with a diameter of 0.2-1 microns and a length of up to 7 microns (on average about 2 microns). Mitochondria have two membranes - outer and inner; the latter forms cristae. Between the outer and inner membranes there is an intermembrane space. The extramembrane volume of the mitochondrion is the matrix.
∙ Outer membrane permeable to many small molecules.
∙ Intermembrane space. H+ ions pumped out of the matrix accumulate here, creating a proton concentration gradient on both sides of the inner membrane.
∙ Inner membrane selectively permeable; contains transport systems for the transfer of substances (ATP, ADP, P 1, pyruvate, succinate, α-ketoglurate, malate, citrate, cytidine triphosphate, GTP, diphosphates) in both directions and electron transport chain complexes associated with oxidative phosphorylation enzymes, as well as succinate dehydrogenase (SDH).
∙ Matrix. The matrix contains all the enzymes of the Krebs cycle (except SDH), enzymes of β-oxidation of fatty acids and some enzymes of other systems. The matrix contains granules with Mg 2+ and Ca 2+.
∙ Cytochemical markers of mitochondria– cytochrome oxidase and SDH.
Functions of mitochondria
Mitochondria perform many functions in the cell: oxidation in the Krebs cycle, electron transport, chemiosmotic coupling, ADP phosphorylation, coupling of oxidation and phosphorylation, the function of controlling intracellular calcium concentration, protein synthesis, heat generation. The role of mitochondria in programmed (regulated) cell death is great.
∙ Thermal reproduction. A natural mechanism for uncoupling oxidative phosphorylation operates in brown fat cells. In these cells, mitochondria have an atypical structure (their volume is reduced, the density of the matrix is increased, the intermembrane spaces are expanded) - condensed mitochondria. Such mitochondria can intensively take up water and swell in response to thyroxine, an increase in the concentration of Ca 2+ in the cytosol, while the uncoupling of oxidative phosphorylation increases, and heat is released. These processes are ensured by a special uncoupling protein called thermogenin. Norepinephrine from the sympathetic division of the autonomic nervous system increases the expression of the uncoupling protein and stimulates heat production.
∙ Apoptosis. Mitochondria play an important role in regulated (programmed) cell death - apoptosis, releasing factors into the cytosol that increase the likelihood of cell death. One of them is cytochrome C, a protein that transfers electrons between protein complexes in the inner membrane of mitochondria. Released from mitochondria, cytochrome C is included in the apoptosome, which activates caspases (representatives of the killer protease family).
