Mitochondria are microscopic membrane organelles that provide the cell with energy. Therefore, they are called energy stations (accumulator) of cells.

Mitochondria are absent in the cells of the simplest organisms, bacteria, entameba, which live without the use of oxygen. Some green algae, trypanosomes contain one large mitochondria, and the cells of the heart muscle, brain have from 100 to 1000 of these organelles.

Structural features

Mitochondria are two-membrane organelles, they have outer and inner shells, an intermembrane space between them, and a matrix.

outer membrane . It is smooth, has no folds, delimits the internal contents from the cytoplasm. Its width is 7nm, it contains lipids and proteins. An important role is played by porin, a protein that forms channels in the outer membrane. They provide ion and molecular exchange.

intermembrane space. The size of the intermembrane space is about 20 nm. The substance that fills it is similar in composition to the cytoplasm, with the exception of large molecules that can penetrate here only by active transport.

Inner membrane. It is built mainly from protein, only a third is allocated to lipid substances. A large number of proteins are transport, since the inner membrane is devoid of freely passable pores. It forms many outgrowths - cristae, which look like flattened ridges. Oxidation of organic compounds to CO 2 in mitochondria occurs on the membranes of the cristae. This process is oxygen-dependent and is carried out under the action of ATP synthetase. The released energy is stored as ATP molecules and used as needed.

Matrix – internal environment mitochondria, has a granular homogeneous structure. AT electron microscope you can see granules and threads in balls that lie freely between the cristae. The matrix contains a semi-autonomous protein synthesis system - DNA, all types of RNA, ribosomes are located here. But still most of Protein comes from the nucleus, which is why mitochondria are called semi-autonomous organelles.

Cell location and division

chondriome is a group of mitochondria that are concentrated in one cell. They are located differently in the cytoplasm, which depends on the specialization of cells. Placement in the cytoplasm also depends on the surrounding organelles and inclusions. In plant cells, they occupy the periphery, since mitochondria are moved to the shell by the central vacuole. In the cells of the renal epithelium, the membrane forms protrusions, between which there are mitochondria.

In stem cells, where energy is used evenly by all organelles, mitochondria are placed randomly. In specialized cells, they are mainly concentrated in places of the highest energy consumption. For example, in striated muscles they are located near the myofibrils. In spermatozoa, they spirally cover the axis of the flagellum, since a lot of energy is needed to set it in motion and move the spermatozoon. Protozoa, which move by cilia, also contain a large number of mitochondria at their base.

Division. Mitochondria are capable of independent reproduction, having their own genome. Organelles divide by constriction or septa. The formation of new mitochondria in different cells differs in frequency, for example, in the liver tissue, they are replaced every 10 days.

Functions in a cell

1. The main function of mitochondria is the formation of ATP molecules.
2. Deposition of calcium ions.
3. Participation in the exchange of water.
4. Synthesis of precursors of steroid hormones.

Molecular biology is the science that studies the role of mitochondria in metabolism. They also convert pyruvate to acetyl coenzyme A, beta-oxidation fatty acids.

Similarities between mitochondria and chloroplasts

Common properties for mitochondria and chloroplasts are primarily due to the presence of a double membrane.

Signs of similarity also lie in the ability to independently synthesize protein. These organelles have their own DNA, RNA, ribosomes.

Both mitochondria and chloroplasts can divide by constriction.

They are also united by the ability to produce energy, mitochondria are more specialized in this function, but chloroplasts also form ATP molecules during photosynthetic processes. So, plant cells have fewer mitochondria than animals, because chloroplasts perform part of the functions for them.

Let's briefly describe the similarities and differences:

• They are double membrane organelles;
• the inner membrane forms protrusions: cristae are characteristic of mitochondria, thylakoids are characteristic of chloroplasts;
• have their own genome;
• capable of synthesizing proteins and energy.

These organelles differ in their functions: mitochondria are designed to synthesize energy, cellular respiration takes place here, chloroplasts are needed by plant cells for photosynthesis.

Mitochondria - energy converters and its suppliers to ensure cellular functions - occupy a significant part of the cytoplasm of cells and are concentrated in places of high consumption of ATP (for example, in the epithelium of the tubules of the kidney 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.An organelle functions on average for 10 days, mitochondria are renewed by dividing.

Morphology of mitochondria

Mitochondria often have the shape of a cylinder with a diameter of 0.2-1 microns and a length of up to 7 microns (about 2 microns on average). Mitochondria have two membranes - outer and inner; the latter forms cristae. Between the outer and inner membranes is the intermembrane space. The extramembrane volume of the mitochondria is the matrix.

∙ outer membrane permeable to many small molecules.

∙ intermembrane space. This is where the H + ions pumped out of the matrix accumulate, which creates 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 with succinate dehydrogenase (SDH).

∙ Matrix. The matrix contains all 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.

Mitochondrial Functions

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, and heat generation. The role of mitochondria in programmed (regulated) cell death is great.

∙ Thermal reproduction. The natural mechanism of uncoupling of oxidative phosphorylation functions in brown fat cells. In these cells, mitochondria have an atypical structure (their volume is reduced, matrix density is increased, intermembrane spaces are expanded) - condensed mitochondria. Such mitochondria can intensively capture water and swell in response to thyroxine, an increase in the concentration of Ca 2+ in the cytosol, while the uncoupling of oxidative phosphorylation is enhanced, and heat is released. These processes are provided by a special uncoupling protein thermogenin. Norepinephrine from the sympathetic division of the autonomic nervous system enhances the expression of the uncoupling protein and stimulates heat production.

∙ Apoptosis. Mitochondria play important role in regulated (programmed) cell death - apoptosis, releasing them into the cytosol factors 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 that activates caspases (representatives of the killer protease family).

What are mitochondria? If the answer to this question causes you difficulties, then our article is just for you. We will consider the structural features of these organelles in relation to their functions.

What are organelles

But first, let's remember what organelles are. They are called permanent cell structures. Mitochondria, ribosomes, plastids, lysosomes... All these are organelles. Like the cell itself, each such structure has overall plan buildings. Organelles consist of a surface apparatus and an internal content - a matrix. Each of them can be compared with the organs of living beings. Organelles also have their own characteristic features that determine their biological role.

Classification of cell structures

Organelles are grouped according to the structure of their surface apparatus. There are one-, two- and non-membrane permanent cell structures. The first group includes lysosomes, Golgi complex, endoplasmic reticulum, peroxisomes and different kinds vacuoles. The nucleus, mitochondria and plastids are two-membrane. And the ribosomes cell center and organelles of movement are completely devoid of surface apparatus.

Theory of symbiogenesis

What are mitochondria? For evolutionary doctrine it's not just cell structures. According to the symbiotic theory, mitochondria and chloroplasts are the result of prokaryotic metamorphosis. It is possible that mitochondria originated from aerobic bacteria, and plastids from photosynthetic bacteria. The proof of this theory is the fact that these structures have their own genetic apparatus, represented by a circular DNA molecule, a double membrane and ribosomes. There is also an assumption that later animal eukaryotic cells originated from mitochondria, and plant cells derived from chloroplasts.

Location in cells

Mitochondria are an integral part of the cells of the predominant part of plants, animals and fungi. They are absent only in anaerobic unicellular eukaryotes living in an oxygen-free environment.

Structure and biological role mitochondria long time remained a mystery. For the first time with the help of a microscope, Rudolf Kölliker managed to see them in 1850. In muscle cells, the scientist found numerous granules that looked like fluff in the light. Understanding the role of these amazing structures became possible thanks to the invention of Professor University of Pennsylvania Britton Chance. He designed a device that allowed him to see through the organelles. Thus, the structure was determined and the role of mitochondria in providing energy to cells and the body as a whole was proved.

Shape and size of mitochondria

General plan of the building

Consider what mitochondria are in terms of their structural features. They are double membrane organelles. Moreover, the outer one is smooth, and the inner one has outgrowths. Mitochondrial matrix is ​​represented by various enzymes, ribosomes, monomers organic matter, ions and clusters of circular DNA molecules. This composition makes it possible for the most important chemical reactions: loop tricarboxylic acids, urea, oxidative phosphorylation.

The value of the kinetoplast

mitochondrial membrane

Mitochondrial membranes are not identical in structure. The closed outer is smooth. It is formed by a bilayer of lipids with fragments of protein molecules. Its total thickness is 7 nm. This structure performs the functions of delimitation from the cytoplasm, as well as the relationship of the organelle with environment. The latter is possible due to the presence of the porin protein, which forms channels. Molecules move along them by means of active and passive transport.

Proteins form the chemical basis of the inner membrane. It forms numerous folds inside the organoid - cristae. These structures greatly increase the active surface of the organelle. Main Feature The structure of the inner membrane is completely impermeable to protons. It does not form channels for the penetration of ions from the outside. In some places, the outer and inner are in contact. Here is a special receptor protein. This is a kind of conductor. With its help, mitochondrial proteins that are encoded in the nucleus penetrate into the organelle. Between the membranes there is a space up to 20 nm thick. It contains various types of proteins that are essential components of the respiratory chain.

Mitochondrial Functions

The structure of the mitochondria is directly related to the functions performed. The main one is the synthesis of adenosine triphosphate (ATP). This is a macromolecule that will happen to be the main energy carrier in the cell. Its composition includes nitrogenous base adenine, monosaccharide ribose and three residues phosphoric acid. It is between the last elements that the main amount of energy is enclosed. When one of them breaks, it can release up to 60 kJ as much as possible. In general, a prokaryotic cell contains 1 billion ATP molecules. These structures are constantly in operation: the existence of each of them in an unchanged form does not last more than one minute. ATP molecules are constantly synthesized and broken down, providing the body with energy at the moment when it is needed.

For this reason, mitochondria are called "energy stations". It is in them that the oxidation of organic substances occurs under the action of enzymes. The energy that is produced in this process is stored and stored in the form of ATP. For example, during the oxidation of 1 g of carbohydrates, 36 macromolecules of this substance are formed.

The structure of mitochondria allows them to perform another function. Due to their semi-autonomy, they are an additional carrier hereditary information. Scientists have found that the DNA of the organelles themselves cannot function on their own. The fact is that they do not contain all the proteins necessary for their work, therefore they borrow them from the hereditary material of the nuclear apparatus.

So, in our article we examined what mitochondria are. These are two-membrane cellular structures, in the matrix of which a number of complex chemical processes. The result of the work of mitochondria is the synthesis of ATP - a compound that provides the body necessary quantity energy.

Mitochondria.

Mitochondria- an organelle consisting of two membranes with a thickness of about 0.5 microns.

Energy station of the cell; the main function is the oxidation of organic compounds and the use of the energy released during their decay in the synthesis of ATP molecules (a universal source of energy for all biochemical processes).

In their structure, they are cylindrical organelles found in a eukaryotic cell in quantities from several hundred to 1-2 thousand and occupying 10-20% of its internal volume. The size (from 1 to 70 μm) and shape of mitochondria also vary greatly. At the same time, the width of these parts of the cell is relatively constant (0.5–1 µm). Able to change shape. Depending on which parts of the cell at each particular moment there is an increased consumption of energy, mitochondria are able to move through the cytoplasm to the zones of the highest energy consumption, using the structures of the cell frame of the eukaryotic cell for movement.

Beauty mitochondria in 3D view)

An alternative to many disparate small mitochondria, functioning independently of each other and supplying small areas of the cytoplasm with ATP, is the existence of long and branched mitochondria, each of which can provide energy for distant parts of the cell. a variant of such an extended system can also be an ordered spatial association of many mitochondria (chondria or mitochondrion), which ensures their cooperative work.

This type of chondriome is especially complex in muscles, where groups of giant branched mitochondria are connected to each other using intermitochondrial contacts (MMK). The latter are formed by outer mitochondrial membranes tightly adjacent to each other, as a result of which the intermembrane space in this zone has an increased electron density (many negatively charged particles). MMCs are especially abundant in cardiac muscle cells, where they bind multiple individual mitochondria into a coherent working cooperative system.

Structure.

outer membrane.

The outer membrane of the mitochondria is about 7 nm thick, does not form invaginations and folds, and is closed on itself. the outer membrane accounts for about 7% of the surface area of ​​all membranes of cell organelles. The main function is to separate the mitochondria from the cytoplasm. The outer membrane of the mitochondria consists of a double fatty layer (as in the cell membrane) and proteins penetrating it. Proteins and fats in equal proportions by weight.
plays a special role porin - channel-forming protein.
It forms holes in the outer membrane with a diameter of 2-3 nm, through which they can penetrate small molecules and ions. Large molecules can only cross the outer membrane through active transport across mitochondrial membrane transport proteins. The outer mitochondrial membrane can interact with the endoplasmic reticulum membrane; it plays an important role in the transport of lipids and calcium ions.

inner membrane.

The inner membrane forms numerous ridge-like folds - cristae,
significantly increasing its surface area and, for example, in liver cells is about a third of all cell membranes. feature composition of the inner membrane of mitochondria is the presence in it cardiolopin - special complex fat, containing four fatty acids at once and making the membrane absolutely impermeable to protons (positively charged particles).

Another feature of the inner membrane of mitochondria is a very high content of proteins (up to 70% by weight), represented by transport proteins, enzymes of the respiratory chain, as well as large enzyme complexes producing ATP. The inner membrane of the mitochondria, unlike the outer one, does not have special openings for the transport of small molecules and ions; on it, on the side facing the matrix, there are special ATP-producing enzyme molecules, consisting of a head, a leg and a base. When protons pass through them, atf is created.
At the base of the particles, filling the entire thickness of the membrane, are the components of the respiratory chain. the outer and inner membranes touch in some places, there is a special receptor protein that promotes the transport of mitochondrial proteins encoded in the nucleus to the mitochondrial matrix.

Matrix.

Matrix- the space limited by an internal membrane. In the matrix (pink substance) of mitochondria there are enzyme systems for the oxidation of pyruvate of fatty acids, as well as enzymes such as tricarboxylic acids (cell respiration cycle). In addition, mitochondrial DNA, RNA and the mitochondrion's own protein-synthesizing apparatus are also located here.

pyruvates (salts of pyruvic acid)- important chemical compounds in biochemistry. They are the end product of glucose metabolism in the process of its breakdown.

Mitochondrial DNA.

A few differences from nuclear DNA:

Mitochondrial DNA is circular, unlike nuclear DNA, which is packed into chromosomes.

- between different evolutionary variants of mitochondrial DNA of the same species, the exchange of similar regions is impossible.

And so the whole molecule changes only by slowly mutating over millennia.

- code mutations in mitochondrial DNA can occur independently of nuclear DNA.

Mutation of the DNA nuclear code occurs mainly during cell division, but mitochondria divide independently of the cell, and can receive code mutations separately from nuclear DNA.

- the very structure of mitochondrial DNA is simplified, because many of the constituent processes of reading DNA have been lost.

- transport RNAs have the same structure. but mitochondrial RNAs are involved only in the synthesis of mitochondrial proteins.

Having its own genetic apparatus, the mitochondrion also has its own protein-synthesizing system, a feature of which in the cells of animals and fungi are very small ribosomes.

Functions.

Energy generation.

The main function of mitochondria is the synthesis of ATP, the universal form of chemical energy in any living cell.

This molecule can be formed in two ways:

- by reactions in which the energy released at certain oxidative stages of fermentation is stored in the form of ATP.

- thanks to the energy released during the oxidation of organic substances in the process cellular respiration.

Mitochondria implement both of these pathways, the first of which is characteristic of initial processes oxidation and occurs in the matrix, and the second completes the processes of energy production and is associated with mitochondrial cristae.
At the same time, the originality of mitochondria as energy-forming organelles of a eukaryotic cell determines precisely the second way of generating ATP, called "chemiosmotic conjugation."
In general, the entire process of energy production in mitochondria can be divided into four main stages, the first two of which occur in the matrix, and the last two - on the mitochondrial cristae:

1) The transformation of pyruvate (the end product of glucose breakdown) and fatty acids from the cytoplasm into mitochondria into acetyl-coa;

acetyl coa- an important compound in metabolism, used in many biochemical reactions. his main function- deliver carbon atoms (c) with an acetyl group (ch3 co) to the cellular respiration cycle so that they are oxidized with energy release.

cellular respiration - totality biochemical reactions occurring in the cells of living organisms, during which carbohydrates, fats and amino acids are oxidized to carbon dioxide and water.

2) Oxidation of acetyl-coa in the cycle of cellular respiration, leading to the formation of nadn;

NADH– coenzyme, performs the function of a carrier of electrons and hydrogen, which it receives from oxidized substances.

3) Transfer of electrons from nadn to oxygen along the respiratory chain;

4) The formation of ATP as a result of the activity of the membrane ATP-creating complex.

ATP synthase.

ATP synthetase– station for the production of ATP molecules.

In structural and functional terms, ATP synthetase consists of two large fragments, denoted by the symbols F1 and F0. The first of them (conjugation factor F1) is turned towards the mitochondrial matrix and noticeably protrudes from the membrane in the form of a spherical formation 8 nm high and 10 nm wide. It consists of nine subunits represented by five types of proteins. The polypeptide chains of three α subunits and the same number of β subunits are packed into protein globules similar in structure, which together form the (αβ)3 hexamer, which looks like a slightly flattened ball.

Subunit is a structural and functional component of any particle
Polypeptides - organic compounds containing from 6 to 80-90 amino acid residues.
Globule is the state of macromolecules in which the vibration of units is small.
Hexamer- a compound containing 6 subunits.

Like densely packed orange slices, the successive α and β subunits form a structure characterized by symmetry around a rotation angle of 120°. At the center of this hexamer is the γ subunit, which is formed by two extended polypeptide chains and resembles a slightly deformed curved rod about 9 nm long. Wherein Bottom part the γ subunit protrudes from the ball by 3 nm towards the F0 membrane complex. Also inside the hexamer is the minor subunit ε associated with γ. The last (ninth) subunit is denoted by the symbol δ and is located on outside F1.

minor- single subunit.

The membrane part of ATP synthetase is a water-repellent protein complex penetrating the membrane through and having two half-channels inside for the passage of hydrogen protons. In total, the F0 complex contains one protein subunit of the type a, two copies of the subunit b, as well as 9 to 12 copies of the small subunit c. Subunit a (molecular mass 20 kDa) is completely immersed in the membrane, where it forms six α-helical sections crossing it. Subunit b(molecular weight 30 kDa) contains only one relatively short α-helical region immersed in the membrane, while the rest of it noticeably protrudes from the membrane towards F1 and is fixed to the δ subunit located on its surface. Each of the 9-12 copies of the subunit c(molecular weight 6-11 kDa) is a relatively small protein of two water-repellent α-helices connected to each other by a short water-attractive loop oriented towards F1, and together they form a single ensemble, having the shape of a cylinder immersed in the membrane . The γ subunit protruding from the F1 complex towards F0 is just immersed inside this cylinder and is quite strongly hooked to it.
Thus, two groups of protein subunits can be distinguished in the ATPase molecule, which can be likened to two parts of a motor: a rotor and a stator.

"Stator" is immobile relative to the membrane and includes a spherical hexamer (αβ)3 located on its surface and a δ subunit, as well as subunits a and b membrane complex F0.

Movable relative to this design "rotor" consists of γ and ε subunits, which, protruding noticeably from the (αβ)3 complex, are connected to a ring of subunits immersed in the membrane c.

The ability to synthesize ATP is a property of a single complex F0F1, combined with the transfer of hydrogen protons through F0 to F1, in the latter of which the reaction centers are located that convert ADP and phosphate into an ATP molecule. The driving force for the work of ATP synthetase is the proton (positively charged) potential created on the inner membrane of mitochondria as a result of the operation of the electron (negatively charged) transport chain.
The force that drives the “rotor” of ATP synthetase occurs when a potential difference is reached between the outer and inner sides membrane > 220 10−3 Volt and is provided by the flow of protons flowing through a special channel in F0, located at the boundary between the subunits a and c. In this case, the proton transfer path includes the following structural elements:

1) Two “semi-channels” located on different axes, the first of which ensures the flow of protons from the intermembrane space to essential functional groups F0, and the other ensures their release into the mitochondrial matrix;

2) Ring of subunits c, each of which contains a protonated carboxyl group (COOH) in its central part, capable of adding H+ from the intermembrane space and donating them through the corresponding proton channels. As a result of periodic displacements of subunits with, due to the flow of protons through the proton channel, the γ subunit is rotated, immersed in the ring of subunits with.

Thus, the unifying activity of ATP synthetase is directly related to the rotation of its "rotor", in which the rotation of the γ subunit causes a simultaneous change in the conformation of all three unifying β subunits, which ultimately ensures the operation of the enzyme. Moreover, in the case of the formation of ATP, the "rotor" rotates clockwise at a speed of four revolutions per second, and the rotation itself occurs in exact jumps of 120 °, each of which is accompanied by the formation of one ATP molecule.
The work of ATP synthetase is associated with mechanical movements her separate parts, which made it possible to attribute this process to special type phenomena called "rotational catalysis". Similar to electricity in the motor winding drives the rotor relative to the stator, the directed transfer of protons through ATP synthetase causes the rotation of individual subunits of the F1 conjugation factor relative to other subunits of the enzyme complex, as a result of which this unique energy-producing device performs chemical work- synthesizes ATP molecules. Subsequently, ATP enters the cytoplasm of the cell, where it is spent on a wide variety of energy-dependent processes. Such a transfer is carried out by a special ATP/ADP-translocase enzyme built into the mitochondrial membrane.

ADP-translocase- a protein penetrating the inner membrane that exchanges newly synthesized ATP for cytoplasmic ADP, which guarantees the safety of the fund inside the mitochondria.

Mitochondria and heredity.

Mitochondrial DNA is inherited almost exclusively through the maternal line. Each mitochondrion has several sections of DNA nucleotides that are identical in all mitochondria (that is, there are many copies of mitochondrial DNA in the cell), which is very important for mitochondria that are unable to repair DNA from damage (observed high frequency mutations). Mutations in mitochondrial DNA are the cause of a number of hereditary human diseases.

3d model

Discovery

With English voice acting

A little about cell respiration and mitochondria in a foreign language

Building structure

Lecture number 6.

Number of hours: 2

MITOCHONDRIA AND PLASTIDS

1.

2. Plastids, structure, varieties, functions

3.

Mitochondria and plastids are two-membrane organelles of eukaryotic cells. Mitochondria are found in all animal and plant cells. Plastids are characteristic of plant cells that carry out photosynthetic processes. These organelles have a similar structural plan and some general properties. However, in terms of basic metabolic processes, they differ significantly from each other.

1. Mitochondria, structure, functional value

general characteristics mitochondria. Mitochondria (Greek “mitos” - thread, “chondrion” - grain, granule) are round, oval or rod-shaped two-membrane organelles with a diameter of about 0.2-1 microns and a length of up to 7-10 microns. These organellescan be detected using light microscopy, since they are of sufficient size and high density. Peculiarities internal structure they can only be studied with an electron microscope.Mitochondria were discovered in 1894 by R. Altman, who gave them the name "bioblasts".The term "mitochondria" was introduced by K. Benda in 1897. Mitochondria are present practically in all eukaryotic cells. In anaerobic organisms ( intestinal amoeba etc.) mitochondria are absent. Numbermitochondria in a cell ranges from 1 to 100 thousand.and depends on the type, functional activity and age of the cell. So in plant cells mitochondria are smaller than in animals; and inthere are more young cells than old ones.The life cycle of mitochondria is several days. In a cell, mitochondria usually accumulate near areas of the cytoplasm where there is a need for ATP. For example, in the heart muscle, mitochondria are located near the myofibrils, while in sperm cells they form a spiral sheath around the axis of the flagellum.

Ultramicroscopic structure of mitochondria. Mitochondria are bounded by two membranes, each about 7 nm thick. The outer membrane is separated from the inner one by an intermembrane space about 10–20 nm wide. The outer membrane is smooth, and the inner one forms folds - cristae (Latin “crista” - crest, outgrowth), increasing its surface. The number of cristae is not the same in mitochondria different cells. They can be from several tens to several hundreds. There are especially many cristae in the mitochondria of actively functioning cells, for example, muscle cells. The cristae contain chains of electron transport and associated ADP phosphorylation (oxidative phosphorylation). Inner space The mitochondria are filled with a homogeneous substance called the matrix. Mitochondrial cristae usually do not completely block the mitochondrial cavity. Therefore, the matrix throughout is continuous. The matrix contains circular DNA molecules, mitochondrial ribosomes, and there are deposits of calcium and magnesium salts. On mitochondrial DNA, RNA molecules of various types are synthesized, ribosomes are involved in the synthesis of a number of mitochondrial proteins. The small size of mitochondrial DNA does not allow encoding the synthesis of all mitochondrial proteins. Therefore, the synthesis of most mitochondrial proteins is under nuclear control and is carried out in the cytoplasm of the cell. Without these proteins, the growth and functioning of mitochondria is impossible. Mitochondrial DNA encodes structural proteins, responsible for the correct integration of individual functional components in mitochondrial membranes.

Reproduction of mitochondria. Mitochondria reproduce by constriction or fragmentation of large mitochondria into smaller ones. The mitochondria formed in this way can grow and divide again.

Mitochondrial functions. The main function of mitochondria is the synthesis of ATP. This process occurs as a result of the oxidation of organic substrates and ADP phosphorylation. The first stage of this process occurs in the cytoplasm under anaerobic conditions. Since the main substrate is glucose, the process is called glycolysis. On the this stage the substrate undergoes enzymatic cleavage to pyruvic acid with the simultaneous synthesis of a small amount of ATP. The second step occurs in the mitochondria and requires the presence of oxygen. At this stage, further oxidation of pyruvic acid occurs with the release of CO 2 and the transfer of electrons to acceptors. These reactions are carried out with the help of a number of enzymes of the tricarboxylic acid cycle, which are localized in the mitochondrial matrix. The electrons released during the oxidation process in the Krebs cycle are transferred to the respiratory chain (electron transport chain). In the respiratory chain, they combine with molecular oxygen to form water molecules. As a result, energy is released in small portions, which is stored in the form of ATP. The complete oxidation of one molecule of glucose with the formation of carbon dioxide and water provides energy for the recharging of 38 ATP molecules (2 molecules in the cytoplasm and 36 in mitochondria).

Mitochondrial analogues in bacteria. Bacteria do not have mitochondria. Instead, they have electron transport chains localized in the cell membrane.

2. Plastids, structure, varieties, functions. The problem of the origin of plastids

Plastids (from Greek. plastides- creating, forming) are two-membrane organelles characteristic of photosynthetic eukaryotic organisms.There are three main types of plastids: chloroplasts, chromoplasts and leukoplasts. The totality of plastids in a cell is called plastidoma. Plastids are interconnected by a single origin in ontogenesis from proplastids of meristematic cells.Each of these types, under certain conditions, can pass one into another. Like mitochondria, plastids contain their own DNA molecules. Therefore, they are also able to reproduce independently of cell division.

Chloroplasts(from the Greek. "chloros" - green, "plastos» - fashioned)are plastids in which photosynthesis takes place.

General characteristics of chloroplasts. Chloroplasts are green organelles 5-10 µm long and 2-4 µm wide. Green algae have giant chloroplasts (chromatophores), reaching a length of 50 microns. Chloroplasts in higher plants have biconvex or ellipsoid shape. The number of chloroplasts in a cell can vary from one (some green algae) to a thousand (shag). ATIn a cell of higher plants, on average, there are 15-50 chloroplasts.Usually chloroplasts are evenly distributed throughout the cytoplasm of the cell, but sometimes they are grouped near the nucleus or cell wall. Apparently, it depends on external influences (light intensity).

Ultramicroscopic structure of chloroplasts. Chloroplasts are separated from the cytoplasm by two membranes, each of which is about 7 nm thick. Between the membranes there is an intermembrane space with a diameter of about 20-30 nm. The outer membrane is smooth, the inner one has a folded structure. Between the folds are thylakoids, having the form of disks. Thylakoids form stacks like a column of coins, called grains. Mthe grana are connected to each other by other thylakoids ( lamellas, frets). The number of thylakoids in one facet varies from a few to 50 or more. In turn, in the chloroplast of higher plants there are about 50 grains (40-60), arranged in a checkerboard pattern. This arrangement ensures maximum illumination of each grain. In the center of the grana is chlorophyll surrounded by a layer of protein; then there is a layer of lipoids, again protein and chlorophyll. Chlorophyll has a complex chemical structure and exists in several modifications ( a, b, c, d ). Higher plants and algae contain x as the main pigment.lorophyll a with the formula C 55 H 72 O 5 N 4 M g . Contains additional chlorophyll b (higher plants, green algae), chlorophyll c (brown and diatoms), chlorophyll d (red algae).The formation of chlorophyll occurs only in the presence of light and iron, which plays the role of a catalyst.The chloroplast matrix is ​​a colorless homogeneous substance that fills the space between the thylakoids.In the matrix areenzymes" dark phase photosynthesis, DNA, RNA, ribosomes.In addition, primary deposition of starch in the form of starch grains occurs in the matrix.

Properties of chloroplasts:

· semi-autonomous (they have their own protein-synthesizing apparatus, but most of the genetic information is in the nucleus);

· the ability to move independently (go away from direct sunlight);

· the ability to reproduce independently.

reproduction of chloroplasts. Chloroplasts develop from proplastids, which are able to replicate by dividing. In higher plants, division of mature chloroplasts also occurs, but is extremely rare. With aging of leaves and stems, ripening of fruits, chloroplasts lose their green color, turning into chromoplasts.

Functions of chloroplasts. The main function of chloroplasts is photosynthesis. In addition to photosynthesis, chloroplasts carry out the synthesis of ATP from ADP (phosphorylation), the synthesis of lipids, starch, and proteins. Chloroplasts also synthesize enzymes that provide light phase photosynthesis.

Chromoplasts(from Greek chromatos - color, paint and " plastos "- fashioned)are colored plastids. Their color is due to the presence of the following pigments: carotene (orange-yellow), lycopene (red) and xanthophyll (yellow). Chromoplasts are especially numerous in the cells of flower petals and fruit membranes. Most chromoplasts are found in fruits and fading flowers and leaves. Chromoplasts can develop from chloroplasts, which lose chlorophyll and accumulate carotenoids. This happens when many fruits ripen: having filled with ripe juice, they turn yellow, turn pink or redden.The main function of chromoplasts is to provide color for flowers, fruits, and seeds.

Unlike leukoplasts and especially chloroplasts, the inner membrane of chloroplasts does not form thylakoids (or forms single ones). Chromoplasts are the final result of the development of plastids (chloroplasts and plastids turn into chromoplasts).

Leucoplasts(from Greek leucos - white, plastos - fashioned, created). These are colorless plastids.rounded, ovoid, spindle-shaped. Are situated in underground parts plants, seeds, epidermis, stem core. especially rich leukoplasts of potato tubers.The inner shell forms a few thylakoids. In light, chloroplasts form from chloroplasts.Leukoplasts in which secondary starch is synthesized and accumulated are called amyloplasts, oils - Eilaloplasts, proteins - proteoplasts. The main function of leukoplasts is the accumulation of nutrients.

3. The problem of the origin of mitochondria and plastids. Relative autonomy

There are two main theories of the origin of mitochondria and plastids. These are the theories of direct filiation and successive endosymbioses. According to the theory of direct filiation, mitochondria and plastids were formed by compartmentalization of the cell itself. Photosynthetic eukaryotes evolved from photosynthetic prokaryotes. In the resulting autotrophic eukaryotic cells, mitochondria were formed by intracellular differentiation. As a result of the loss of plastids, animals and fungi originated from autotrophs.

The most substantiated is the theory of successive endosymbioses. According to this theory, the emergence of a eukaryotic cell went through several stages of symbiosis with other cells. At the first stage, cells of the type of anaerobic heterotrophic bacteria included free-living aerobic bacteria transformed into mitochondria. In parallel, in the host cell, the prokaryotic genophore is formed into a nucleus isolated from the cytoplasm. In this way, the first eukaryotic cell arose, which was heterotrophic. Eukaryotic cells that arose through repeated symbioses included blue-green algae, which led to the appearance of chloroplast-type structures in them. Thus, mitochondria already existed in heterotrophic eukaryotic cells when the latter acquired plastids as a result of symbiosis. Later, as a result natural selection mitochondria and chloroplasts have lost some of their genetic material and turned into structures with limited autonomy.

Evidence for the endosymbiotic theory:

1. similarity of structure and energy processes in bacteria and mitochondria, on the one hand, and in blue-green algae and chloroplasts, on the other hand.

2. Mitochondria and plastids have their ownspecific system of protein synthesis (DNA, RNA, ribosomes). The specificity of this system lies in its autonomy and sharp difference from that in the cell.

3. The DNA of mitochondria and plastids issmall cyclic or linear moleculewhich differs from the DNA of the nucleus and in its characteristics approaches the DNA of prokaryotic cells.DNA synthesis of mitochondria and plastids is notdependent on nuclear DNA synthesis.

4. In mitochondria and chloroplasts there are m-RNA, t-RNA, r-RNA. The ribosomes and rRNA of these organelles differ sharply from those in the cytoplasm. In particular, mitochondrial and chloroplast ribosomes, unlike cytoplasmic ribosomes, are sensitive to the antibiotic chloramphenicol, which suppresses protein synthesis in prokaryotic cells.

5. The increase in the number of mitochondria occurs through the growth and division of the original mitochondria. The increase in the number of chloroplasts occurs through changes in proplastids, which, in turn, multiply by division.

This theory explains well the preservation of the remains of replication systems in mitochondria and plastids and makes it possible to construct a consistent phylogeny from prokaryotes to eukaryotes.

Relative autonomy of chloroplasts and plastids. In some respects, mitochondria and chloroplasts behave like autonomous organisms. For example, these structures are formed only from the original mitochondria and chloroplasts. This was demonstrated in experiments on plant cells, in which the formation of chloroplasts was inhibited by the antibiotic streptomycin, and on yeast cells, where the formation of mitochondria was inhibited by other drugs. After such influences, the cells never restored the missing organelles. The reason is that mitochondria and chloroplasts contain a certain amount of their own genetic material (DNA) that codes for part of their structure. If this DNA is lost, which is what happens when organelle formation is suppressed, then the structure cannot be recreated. Both types of organelles have their own protein-synthesizing system (ribosomes and transfer RNAs), which is somewhat different from the main protein-synthesizing system of the cell; it is known, for example, that the protein-synthesizing system of organelles can be suppressed by antibiotics, while they do not affect the main system. Organelle DNA is responsible for the bulk of extrachromosomal, or cytoplasmic, inheritance. Extrachromosomal heredity does not obey Mendelian laws, since during cell division, organelle DNA is transmitted to daughter cells in a different way than chromosomes. The study of mutations that occur in the DNA of organelles and the DNA of chromosomes has shown that organelle DNA is responsible for only a small part of the structure of organelles; most of their proteins are encoded in genes located on chromosomes. The relative autonomy of mitochondria and plastids is considered as one of the proofs of their symbiotic origin.