Mitochondria. History of the discovery of ribosomes Location in the cell and division

Mitochondria- This double membrane organelle eukaryotic cell, whose main function is ATP synthesis– a source of energy for the life of the cell.

The number of mitochondria in cells is not constant, on average from several units to several thousand. Where synthesis processes are intense, there are more of them. The size of mitochondria and their shape also varies (round, elongated, spiral, cup-shaped, etc.). More often they have a round, elongated shape, up to 1 micrometer in diameter and up to 10 microns in length. They can move in the cell with the flow of cytoplasm or remain in one position. They move to places where energy production is most needed.

It should be borne in mind that in cells ATP is synthesized not only in mitochondria, but also in the cytoplasm during glycolysis. However, the efficiency of these reactions is low. The peculiarity of the function of mitochondria is that not only oxygen-free oxidation reactions occur in them, but also the oxygen stage of energy metabolism.

In other words, the function of mitochondria is to actively participate in cellular respiration, which includes many reactions of oxidation of organic substances, transfer of hydrogen protons and electrons, releasing energy that is accumulated in ATP.

Mitochondrial enzymes

Enzymes translocases The inner membrane of mitochondria carries out active transport of ADP and ATP.

In the structure of cristae, elementary particles are distinguished, consisting of a head, a stalk and a base. On heads consisting of enzyme ATPases, ATP synthesis occurs. ATPase ensures the coupling of ADP phosphorylation with reactions of the respiratory chain.

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

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

As a result of the activity of the electrical transport respiratory chain, hydrogen ions enter it from the matrix and are released on the outside of the inner membrane. This is carried out by certain membrane enzymes. The difference in the concentration of hydrogen ions on different sides of the membrane results in a pH gradient.

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

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

ADP + P = ATP + H 2 O (reaction is reversible)

The resulting water is removed enzymatically. This, along with other factors, facilitates the reaction from left to right.

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

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

Structural features

Mitochondria are double-membrane organelles; they have outer and inner membranes, an intermembrane space between them, and a matrix.

Outer membrane. It is smooth, has no folds, and separates the internal contents from the cytoplasm. Its width is 7 nm and 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 filling it is similar in composition to the cytoplasm, with the exception of large molecules that can penetrate here only through 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 proteins, since the inner membrane lacks freely passable pores. It forms many outgrowths - cristae, which look like flattened ridges. The 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 in the form of ATP molecules and is used as needed.

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

Cell location and division

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

In stem cells, where energy is used equally by all organelles, mitochondria are randomly distributed. In specialized cells, they are mainly concentrated in areas of greatest 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 sperm. Protozoa that move using cilia also contain large numbers of mitochondria at their base.

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

Functions in the cell

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

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

Similarities between mitochondria and chloroplasts

The common properties of mitochondria and chloroplasts are primarily due to the presence of a double membrane.

Signs of similarity also include the ability to independently synthesize protein. These organelles have their own DNA, RNA, and 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 produce ATP molecules during photosynthetic processes. Thus, plant cells have fewer mitochondria than animal cells, because chloroplasts partially perform the functions for them.

Let us briefly describe the similarities and differences:

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

These organelles differ in their functions: mitochondria are intended for energy synthesis, cellular respiration occurs here, chloroplasts are needed by plant cells for photosynthesis.

Ribosomes: structure and functions

Definition 1

Note 1

The main function of ribosomes is protein synthesis.

Ribosomal subunits are formed in the nucleolus and then enter the cytoplasm separately from each other through nuclear pores.

Their number in the cytoplasm depends on the synthetic activity of the cell and can range from hundreds to thousands per cell. The largest number of ribosomes can be found in cells that synthesize proteins. They are also found in the mitochondrial matrix and chloroplasts.

Ribosomes in various organisms, from bacteria to mammals, are characterized by a similar structure and composition, although prokaryotic cells have smaller ribosomes and are more numerous.

Each subunit consists of several types of rRNA molecules and dozens of types of proteins in approximately equal proportions.

The small and large subunits are found alone in the cytoplasm until they are involved in the process of protein biosynthesis. They combine with each other and the mRNA molecule when synthesis is necessary and break apart again when the process is completed.

The mRNA molecules that were synthesized in the nucleus enter the cytoplasm to the ribosomes. From the cytosol, tRNA molecules deliver amino acids to ribosomes, where proteins are synthesized with the participation of enzymes and ATP.

If several ribosomes bind to an mRNA molecule, they form polysomes, which contain from 5 to 70 ribosomes.

Plastids: chloroplasts

Plastids - organelles characteristic only of plant cells, absent in the cells of animals, fungi, bacteria and cyanobacteria.

Cells of higher plants contain 10-200 plastids. Their size ranges from 3 to 10 microns. Most are in the form of a biconvex lens, but sometimes they can be in the form of plates, rods, grains and scales.

Depending on the pigment pigment present in the plastid, these organelles are divided into groups:

• chloroplasts(gr. сchloros– green) – green in color,
• chromoplasts– yellow, orange and reddish color,
• leucoplasts- colorless plastids.

Note 2

As the plant develops, plastids of one type are able to transform into plastids of another type. This phenomenon is widespread in nature: changes in the color of leaves, the color of fruits changes during the ripening process.

Most algae have plastids instead chromatophores(usually there is only one in the cell, it is of significant size, and has the shape of a spiral ribbon, bowl, mesh or stellate plate).

Plastids have a rather complex internal structure.

Chloroplasts have their own DNA, RNA, ribosomes, inclusions: starch grains, fat droplets. Externally, chloroplasts are bounded by a double membrane, the internal space is filled stroma– semi-liquid substance) which contains grains- special structures characteristic only of chloroplasts.

Granas are represented by packets of flat round sacs ( thylakoids), which are stacked like a column of coins perpendicular to the wide surface of the chloroplast. The thylakoids of neighboring grana are connected to each other into a single interconnected system by membrane channels (intermembrane lamellae).

In the thickness and on the surface, the grains are located in a certain order chlorophyll.

Chloroplasts have different numbers of grains.

Example 1

The chloroplasts of spinach cells contain 40-60 grains.

Chloroplasts are not attached to certain places in the cytoplasm, but can change their position either passively or actively move oriented towards the light ( phototaxis).

The active movement of chloroplasts is especially clearly observed with a significant increase in one-sided illumination. In this case, chloroplasts accumulate at the side walls of the cell, and are oriented edgewise. In low light, chloroplasts are oriented towards the light with their wider side and are located along the cell wall facing the light. At average light intensity, chloroplasts occupy a middle position. In this way, the most favorable conditions for the process of photosynthesis are achieved.

Thanks to the complex internal spatial organization of structural elements, chloroplasts are able to effectively absorb and use radiant energy, and there is also a differentiation in time and space of numerous and diverse reactions that make up the process of photosynthesis. The light-dependent reactions of this process occur only in the thylakoids, and the biochemical (dark) reactions occur in the stroma of the chloroplast.

Note 3

The chlorophyll molecule is very similar to the hemoglobin molecule and differs mainly in that at the center of the hemoglobin molecule there is an iron atom, and not a magnesium atom, like chlorophyll.

There are four types of chlorophyll in nature: a, b, c, d.

Chlorophylls a and b found in the chloroplasts of higher plants and green algae; diatoms contain chlorophylls a and c, red – a and d. Chlorophylls a and b studied better than others (they were first identified at the beginning of the twentieth century by the Russian scientist M.S. Tsvet).

Besides these, there are four types bacteriochlorophylls– green pigments of green and purple bacteria: a, b, c, d.

Most bacteria capable of photosynthesis contain bacteriochlorophyll A, some are bacteriochlorophyll b, green bacteria - c and d.

Chlorophyll absorbs radiant energy quite efficiently and transfers it to other molecules. Thanks to this, chlorophyll is the only substance on Earth that can support the process of photosynthesis.

Plastids, like mitochondria, are characterized to a certain extent by autonomy within the cell. They are able to reproduce mainly by division.

Along with photosynthesis, the synthesis of other substances, such as proteins, lipids, and some vitamins, occurs in chloroplasts.

Due to the presence of DNA in plastids, they play a certain role in the transmission of traits by inheritance. (cytoplasmic inheritance).

Mitochondria are the energy centers of the cell

The cytoplasm of most animal and plant cells contains fairly large oval organelles (0.2–7 μm), covered with two membranes.

Mitochondria They are called the power stations of cells because their main function is the synthesis of ATP. Mitochondria convert the energy of chemical bonds of organic substances into the energy of phosphate bonds of the ATP molecule, which is a universal source of energy for all life processes of the cell and the whole organism. ATP synthesized in mitochondria freely enters the cytoplasm and then goes to the nucleus and organelles of the cell, where its chemical energy is used.

Mitochondria are found in almost all eukaryotic cells, with the exception of anaerobic protozoa and erythrocytes. They are located chaotically in the cytoplasm, but more often they can be identified near the nucleus or in places with high energy demand.

Example 2

In muscle fibers, mitochondria are located between the myofibrils.

These organelles can change their structure and shape, and also move within the cell.

The number of organelles can vary from tens to several thousand depending on the activity of the cell.

Example 3

One mammalian liver cell contains more than 1000 mitochondria.

The structure of mitochondria differs to some extent in different types of cells and tissues, but all mitochondria have a fundamentally the same structure.

Mitochondria are formed by fission. During cell division, they are more or less evenly distributed between daughter cells.

Outer membrane smooth, does not form any folds or outgrowths, and is easily permeable to many organic molecules. Contains enzymes that convert substances into reactive substrates. Participates in the formation of the intermembrane space.

Inner membrane poorly permeable to most substances. Forms many protrusions inside the matrix - Krist. The number of cristae in the mitochondria of different cells is not the same. There can be from several tens to several hundreds, and there are especially many of them in the mitochondria of actively functioning cells (muscle cells). Contains proteins that are involved in three important processes:

• enzymes that catalyze redox reactions of the respiratory chain and electron transport;
• specific transport proteins involved in the formation of hydrogen cations in the intermembrane space;
• ATP synthetase enzymatic complex that synthesizes ATP.

Matrix- the internal space of the mitochondrion, limited by the inner membrane. It contains hundreds of different enzymes that are involved in the destruction of organic substances down to carbon dioxide and water. In this case, the energy of chemical bonds between the atoms of molecules is released, which is subsequently converted into the energy of high-energy bonds in the ATP molecule. The matrix also contains ribosomes and mitochondrial DNA molecules.

Note 4

Thanks to the DNA and ribosomes of the mitochondria themselves, the synthesis of proteins necessary for the organelle itself is ensured, and which are not formed in the cytoplasm.

Mitochondria are organelles the size of bacteria (about 1 x 2 microns). Mitochondria are an integral part of all living eukaryotic cells; usually a cell contains about 2000 mitochondria, the total volume of which is up to 25% of the total cell volume. The shape, size and number are constantly changing. The number of mitochondria varies from several tens to hundreds. There are especially many of them in the secretory tissues of plants.

Mitochondria, regardless of their size or shape, have a universal structure, their ultrastructure is uniform. Mitochondria are bounded by two membranes. The outer mitochondrial membrane separates it from the hyaloplasm. Usually it has smooth contours and does not form indentations or folds. It accounts for about 7% of the area of ​​all cell membranes. The thickness of this membrane is about 7 nm, it is not connected to any other membranes of the cytoplasm and is closed on itself, so that it is a membrane sac. The outer membrane is separated from the inner membrane by an intermembrane space about 10-20 nm wide. The inner membrane (about 7 nm thick) limits the actual internal contents of the mitochondrion, its matrix, or mitoplasm. A characteristic feature of the inner membranes of mitochondria is their ability to form numerous invaginations into the mitochondria. Such invaginations most often take the form of flat ridges, or cristae.

Rice. Diagram of the general organization of mitochondria

1 -- outer membrane; 2 -- inner membrane; 3 -- invaginations of the inner membrane - cristae; 4 -- places of invagination, view from the surface of the inner membrane

Mitochondria are capable of nuclear-independent synthesis of their proteins on their own ribosomes under the control of mitochondrial DNA. Mitochondria are formed only by fission.

The main function of mitochondria is to provide the energy needs of the cell through respiration. Energy-rich ATP molecules are synthesized during the oxidative phosphorylation reaction. The energy stored by ATP is obtained as a result of the oxidation of various energy-rich substances, mainly sugars, in mitochondria. The mechanism of oxidative phosphorylation by chemiosmotic coupling was discovered in 1960 by the English biochemist P. Mitchell

The main function of ribosomes is translation, that is, protein synthesis. In photographs taken using an electron microscope, they look like round bodies with a diameter of 20 - 30 nm.

Each ribosome consists of 2 subunits of unequal size, shape and structure. Ribosomal subunits are designated by their sedimentation coefficients (that is, sedimentation during centrifugation).

Apparently, the small subunit is located on top of the large one so that space (“tunnel”) is maintained between the particles. The tunnel is used to accommodate mRNA during protein synthesis.

Ribosomes are a large ribonucleoprotein complex with a molecular weight of about 2.5 mDa, consisting of ribosomal proteins, rRNA molecules and associated translation factors. Ribosomes are non-membrane organelles on which protein synthesis occurs in the cell. They are spherical structures with a diameter of about 20 nm. These smallest cellular organelles are extremely complex. Not a single molecule that makes up ribosomes is repeated twice. The ribosomes of the bacterium E. coli have been studied better than others.

Margoulitz, Cayer and Clares were the first to propose the Endosymbiotic Theory, and Liin continued it.

The most widespread hypothesis is the endosymbiotic origin of mitochondria, according to which modern animal mitochondria originate from alpha-proteobacteria (to which modern Rickettsia prowazekii belongs), which penetrated into the cytosol of precursor cells. It is believed that during endosymbiosis, bacteria transferred most of their vital genes to the chromosomes of the host cell, retaining in their genome (in the case of human cells) information about only 13 polypeptides, 22 tRNAs and two rRNAs. All polypeptides are part of the enzymatic complexes of the mitochondrial oxidative phosphorylation system.

Mitochondria are formed by endocytosis of an ancient large anaerobic prokaryote that has absorbed a smaller aerobic prokaryote. The relationship of such cells was at first symbiotic, and then the large cell began to control the processes occurring in the mitochondria.

Proof:

The difference in the structure of the inner and outer membranes of mitochondria

The presence in mitochondria of their own circular DNA (like bacteria), which contains genes for certain mitochondrial proteins

The presence of its own protein-synthesizing apparatus in the membrane, and the ribosomes in it are of the prokaryotic type

Mitochondrial division occurs in a simple binary way, or by budding, and does not depend on cell division.

Despite a certain independence, mitochondria are under the control of the eukaryotic cell. For example, in the hyaloplasm some proteins necessary for the normal functioning of mitochondria and some protein factors that regulate mitochondrial division are synthesized.

The DNA of mitochondria and plastids, unlike the DNA of most prokaryotes, contains introns.

Only part of their proteins are encoded in the own DNA of mitochondria and chloroplasts, while the rest are encoded in the DNA of the cell nucleus. During evolution, part of the genetic material “flowed” from the genome of mitochondria and chloroplasts into the nuclear genome. This explains the fact that neither chloroplasts nor mitochondria can no longer exist (reproduce) independently.

The question of the origin of the nuclear-cytoplasmic component (NCC), which captured proto-mitochondria, has not been resolved. Neither bacteria nor archaea are capable of phagocytosis, feeding exclusively osmotrophically. Molecular biological and biochemical studies indicate the chimeric archaeal-bacterial nature of JCC. How the fusion of organisms from two domains occurred is also not clear.

Theory The endosymbiotic origin of chloroplasts was first proposed in 1883 by Andreas Schimper, who showed their self-replication inside the cell. Famintzin in 1907, based on the work of Schimper, also came to the conclusion that chloroplasts are symbionts, like algae in lichens.

In the 1920s, the theory was developed by B. M. Kozo-Polyansky, it was suggested that mitochondria are also symbionts

Cell nucleus, nucleocytoplasm

The mixing in eukaryotes of many properties characteristic of archaea and bacteria allowed us to assume the symbiotic origin of the nucleus from a methanogenic archaebacterium that invaded the myxobacterium cell. Histones, for example, are found in eukaryotes and some archaea, and the genes encoding them are very similar. Another hypothesis explaining the combination of molecular characteristics of archaea and eubacteria in eukaryotes is that at some stage of evolution, the archaeal-like ancestors of the nucleocytoplasmic component of eukaryotes acquired the ability to enhance the exchange of genes with eubacteria through horizontal gene transfer

In the last decade, the hypothesis of viral eukaryogenesis has also been formed. It is based on a number of similarities in the structure of the genetic apparatus of eukaryotes and viruses: the linear structure of DNA, its close interaction with proteins, etc. The similarity of the DNA polymerase of eukaryotes and poxyviruses was shown, which made their ancestors the main candidates for the role of the nucleus.

Flagella and cilia

Lynn Margulis also suggested the origin of flagella and cilia from symbiotic spirochetes. Despite the similarity in size and structure of these organelles and bacteria and the existence of Mixotricha paradoxa, which uses spirochetes for movement, no specifically spirochete proteins were found in flagella. However, the FtsZ protein, common to all bacteria and archaea, is known to be homologous to tubulin and possibly its precursor. Flagella and cilia do not possess such characteristics of bacterial cells as a closed outer membrane, their own protein-synthesizing apparatus, and the ability to divide. Data about the presence of DNA in basal bodies, which appeared in the 1990s, were subsequently refuted. An increase in the number of basal bodies and centrioles homologous to them occurs not by division, but by completing the construction of a new organelle next to the old one.

Peroxisomes

Christian de Duve discovered peroxisomes in 1965. He also suggested that peroxisomes were the first endosymbionts of a eukaryotic cell, allowing it to survive with an increasing amount of free molecular oxygen in the earth’s atmosphere. Peroxisomes, however, unlike mitochondria and plastids, have neither genetic material nor an apparatus for protein synthesis. It has been shown that these organelles are formed de novo in the cell in the ER and there is no reason to consider them endosymbionts