Mitochondria. Structural and functional organization of chloroplasts and mitochondria Processes occurring in chloroplasts and mitochondria of the cell
Mitochondria have the form of spherical bodies, rods, threads (length from 0.5 to 10 μm or more) (187). Sometimes mitochondria branch (for example, in protozoan cells, muscle fibers). The number of these organelles in a cell varies: from 1 to 100,000 or more. It depends on how actively metabolic processes occur in the cell. In the cells of green plants there are fewer mito-chondrules than in animal cells, since their functions (ATP synthesis) are also performed by chloroplasts.
Mitochondria in the cell are constantly renewed. For example, in liver cells, the lifespan of mitochondria is approximately 10 days.
The number of mitochondria depends on the activity of the cell and its energy expenditure. For the same reason, their number also changes during ontogenesis: in young embryonic cells they are more numerous than in relatively old postembryonic cells.
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What are plastids
Plastids (from the Greek plastos - fashioned) belong to a special class of organelles that are unique to plants and come in several types. Plant plastids have a single origin, a similar structure and can be mutually transformed. Their main feature is a double membrane and the presence of a DNA molecule, shaped like a ring. New plastids are formed by dividing old ones in half. As in mitochondria, the outer membrane performs protective and transport functions, and the inner one forms a complex system of membranes on which complex biochemical processes occur.
All photosynthetic reactions take place in special green plastids - chloroplasts (188) (from the Greek Chloros - green and plastos - fashioned), which is due to the presence in them of a special pigment called chlorophyll (from the Greek Chloros - green and Phyllon - leaf ). They are contained in the cytoplasm of cells of leaves, stems, fruits, perianth and other cells of green plant organs. are clearly visible in a light microscope (their sizes are 2-5 µm), most often they are oval in shape (189). Each cell has from 20 to 40 chloroplasts.
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Like all other plastids, the chloroplast is surrounded by a double membrane and has a complex internal membrane system. The main structure of chloroplasts is the thyloid, which consists of a single-layer membrane, is shaped like a flat sac and contains chlorophyll. Tila - Koids are stacked like coins. These structures are called grana. The entire space between them is filled with a liquid substance - matrix, which in chloroplasts is called stroma (from the Greek stroma - litter). Chloroplasts contain ribosomes, DNA, enzymes, they are capable of synthesizing proteins, lipids and starch, which makes them relatively independent from other cellular structures. Chloroplasts also contain starch grains and fatty inclusions, which represent the energy reserve of the cell. Under the influence of various factors, as well as with cell aging, the internal structure of chloroplasts becomes simpler and they turn into other types of plastids. Microbodies are also enzyme-bearing organelles that are EC derivatives and essential components.
17. Give a comparative description of the structure and functions of mitochondria and chloroplasts.
Rice. 6. Schemes of the structure of mitochondria ( A) and chloroplasts ( b)
Mitochondria (gr. mitos– thread and chondrion– granule) – intracellular organelles. Their shell consists of two membranes. The outer membrane is smooth, the inner one forms projections called cristae. Inside the mitochondria there is a semi-liquid matrix that contains RNA, DNA, proteins, lipids, carbohydrates, enzymes, ATP and other substances; the matrix also contains ribosomes. Mitochondria sizes range from 0.2–0.4 to 1–7 µm. The number depends on the type of cell (for example, a liver cell may have 1000–2500 mitochondria). Mitochondria can be spiral, round, elongated, cup-shaped, etc.; mitochondria can change shape (Fig. 6, A).
The inner membrane of mitochondria contains respiratory enzymes and ATP synthesis enzymes. Thanks to this, mitochondria provide cellular respiration and ATP synthesis.
Mitochondria can synthesize proteins themselves, because they have their own DNA, RNA and ribosomes. Mitochondria reproduce by fission in two.
In their structure, mitochondria resemble prokaryotic cells; in this regard, it is assumed that they originated from intracellular aerobic symbionts. Mitochondria are found in the cytoplasm of most plant and animal cells.
Chloroplasts belong to plastids - organelles found only in plant cells. These are green plates with a diameter of 3–4 microns, having an oval shape (Fig. 6, b). Chloroplasts, like mitochondria, have outer and inner membranes. The inner membrane forms outgrowths - thylakoids (cf. cristae in mitochondria). Thylakoids form stacks - grana, which are united by an internal membrane. One chloroplast can contain several tens of grana. The thylakoid membranes contain chlorophyll, and in the spaces between the grana in the matrix (stroma) of the chloroplast there are ribosomes, RNA and DNA (cf. the composition of the mitochondrial matrix). Chloroplast ribosomes, like mitochondrial ribosomes, synthesize proteins. The main function of chloroplasts is to ensure the process of photosynthesis: in the thylakoid membranes there is a light phase, and in the stroma of chloroplasts there is a dark phase of photosynthesis. The chloroplast matrix contains granules of primary starch, synthesized from glucose during photosynthesis. Chloroplasts, like mitochondria, reproduce by division. Thus, there are common features in the morphological and functional organization of mitochondria and chloroplasts. The main characteristic that unites these organelles is the presence of their own genetic information and the synthesis of their own proteins.
18. Reveal the features of the structure and functions of the endoplasmic reticulum of the cell.
Rice. 7. Schemes of the structure of rough ( A) and smooth ( b) endoplasmic reticulum
The endoplasmic reticulum (ER), or endoplasmic reticulum (ER), is a network of channels that penetrate the entire cytoplasm. The walls of these channels are formed by membranes that are in contact with all organelles of the cell. The EPS and organelles together form a single intracellular system, which carries out the metabolism of substances and energy in the cell and ensures the intracellular transport of substances. There are smooth and granular EPS. Granular or rough. The ER consists of membrane sacs (cisternae) covered with ribosomes, making it appear rough. Smooth ER may be devoid of ribosomes; its structure is closer to the tubular type. Proteins are synthesized on the ribosomes of the granular network, which then enter the ER channels, where they acquire a tertiary structure. On the membranes of the smooth ER, lipids and carbohydrates are synthesized, which also enter the ER channels (Fig. 7).
The ER performs the following functions: it participates in the synthesis of organic substances, transports synthesized substances to the Golgi apparatus, and divides the cell into compartments. In addition, in liver cells, EPS is involved in the neutralization of toxic substances, and in muscle cells it plays the role of a calcium depot, necessary for muscle contraction.
EPS is present in all cells, excluding bacterial cells and erythrocytes; it occupies from 30 to 50% of the cell volume.
19. Describe the structure of the ribosome. What is the role of ribosomes in metabolic processes?
Ribosomes are submicroscopic organelles with a diameter of 15–35 nm, visible under an electron microscope. Present in all cells. There can be several thousand ribosomes in one cell. Ribosomes are of nuclear, mitochondrial, and plastid origin (see answers to questions 11 and 17). The majority is formed in the nucleolus of the nucleus in the form of subunits (large and small) and then passes into the cytoplasm. There are no membranes. Ribosomes contain rRNA and proteins. Protein synthesis occurs on ribosomes. Most proteins are synthesized on rough ER (see answer to question 18); Protein synthesis partially occurs on ribosomes located in the cytoplasm in a free state. Groups of several dozen ribosomes form polysomes.
20. What is the biological role of the Golgi complex in the life of the cell?
The Golgi complex is a complex network of cavities, tubes and vesicles around the nucleus. It consists of three main components: a group of membrane cavities, a system of tubes extending from the cavities, and vesicles at the ends of the tubes. The Golgi complex performs the following functions: substances that are synthesized and transported along the ER accumulate in the cavities; here they undergo chemical changes. The modified substances are packaged into membrane vesicles, which are released by the cell in the form of secretions. In addition, the vesicles are used by the cell as lysosomes (Fig. 8).
The Golgi complex was discovered in 1898 in neurons.
21. What are cellular inclusions and what is their significance in the life processes of the cell? What is the biological role of lysosomes in cell life?
Cellular inclusions are unstable cell structures. These include drops and grains of proteins, carbohydrates, fats, as well as crystalline inclusions (organic crystals that can form proteins, viruses, oxalic acid salts, etc. in cells and inorganic crystals formed by calcium salts). Unlike organelles, these inclusions do not have membranes or cytoskeletal elements and are periodically synthesized and consumed.
Fat droplets are used as a reserve substance due to its high energy content. Carbohydrate grains (polysaccharides; in the form of starch in plants and in the form of glycogen in animals and fungi) - as a source of energy for the formation of ATP; protein grains - as a source of building material, calcium salts - to ensure the process of excitation, metabolism, etc.
Lysosomes (Greek) lyso– dissolve, soma- body) are small bubbles with a diameter of about 1 micron, bounded by a membrane and containing a complex of enzymes that ensures the breakdown of fats, carbohydrates and proteins. They are involved in the digestion of particles that enter the cell as a result of endocytosis (see answer to question 14) and in the removal of dying organs (for example, the tail of tadpoles), cells and organelles. During starvation, lysosomes dissolve some organelles without killing the cell. The formation of lysosomes occurs in the Golgi complex (see answer to question 20).
22. What inorganic compounds are part of the cell? What is the significance of the inorganic components of a cell in ensuring its vital processes? What is the biological role of water in a cell?
The inorganic compounds of the cell include water and various salts.
The role of salts in the body is to provide a transmembrane potential difference (due to the difference in the concentrations of potassium and sodium ions inside and outside the cell), buffer properties (due to the presence of phosphoric and carbonic acid anions in the cytoplasm), to create the osmotic pressure of the cell, etc. The composition of the inorganic substances of the cell includes microelements (their share is less than 0.1%). These include: zinc, manganese and cobalt, which are part of the active sites of enzymes; iron in hemoglobin; magnesium in chlorophyll; iodine in thyroid hormones, etc.
On average, a cell contains 80% water; in the cells of an embryo there is 95% water, in the cells of old organisms - 60%, i.e. the amount of water depends on the intensity of metabolism. The amount of water also depends on the type of tissue: in neurons it is 85%, in bones – 20%. When the body loses 20% of water, death occurs. Water determines the turgor (elasticity) of tissues, creates an environment for chemical reactions, participates in hydrolysis reactions, in the light phase of photosynthesis, in thermoregulation, and is a good solvent. Based on the type of interaction with water, substances are divided into hydrophilic, or polar, - highly soluble in water, and hydrophobic, or non-polar, - poorly soluble in water.
23. Describe the structure and functions of carbohydrates that make up the cell.
Carbohydrates are organic compounds containing hydrogen, carbon and oxygen. They are formed from water and carbon dioxide during photosynthesis in the chloroplasts of green plants (in bacteria - during bacterial photosynthesis or chemosynthesis).
There are monosaccharides (glucose, fructose, galactose, ribose, deoxyribose), disaccharides (sucrose, maltose), polysaccharides (starch, fiber, glycogen, chitin).
Carbohydrates perform the following functions: they are a source of energy (the breakdown of 1 g of glucose releases 17.6 kJ of energy), perform a construction function (cellulose membrane in plant cells, chitin in the skeleton of insects and in the cell wall of fungi), are part of DNA, RNA and ATP in the form of deoxyribose and ribose. Typically, animal cells contain about 1% carbohydrates (liver cells - up to 5%), and plant cells - up to 90%.
24. What are the structure and functions of fatty acids and lipoids that make up the cell.
Rice. 9. Spatial patterns of fatty acids
Rice. 10. Polycyclic structure of some steroids
Fats and lipoids belong to the group of non-polar organic compounds, i.e. are hydrophobic substances. Fats– these are triglycerides of higher fatty acids (Fig. 9), lipids- This is a large class of organic substances with hydrophobic properties (for example, cholesterol). Lipids include phospholipids (in their molecule one or two fatty acid residues are replaced by groups containing phosphorus and sometimes also nitrogen) and steroids (their structure is based on four carbon rings, Fig. 10).
These compounds perform an energetic function (the breakdown of 1 g of fat releases 38.9 kJ of energy), a structural function (phospholipids are the basis of biological membranes), and a protective function (shock protection, heat regulation, waterproofing).
25. What are the structural features and functions of the proteins that make up the cell?
Proteins are heteropolymers consisting of 20 different monomers - natural alpha amino acids. Proteins are irregular polymers.
The general structure of an amino acid can be represented as follows: R-(H)C(NH 2 )-COOH. Amino acids in protein are linked by a peptide bond -N(H)-C(=O). Amino acids are divided into replaceable (synthesized in the body itself) and essential, which the animal body receives from food. Among proteins, a distinction is made between proteins consisting only of amino acids and proteins that additionally contain a non-protein part (for example, hemoglobin, which consists of the globin protein and heme - porphyrin).
In the structure of a protein molecule, there are several levels of structural organization (Fig. 11). The primary structure is a sequence of amino acid residues connected by peptide bonds. Secondary structure is typically a helical structure (alpha helix) that is held together by multiple hydrogen bonds that occur between closely spaced –C=O and –NH groups. Another type of secondary structure is the beta layer, or folded layer; these are two parallel polypeptide chains connected by hydrogen bonds perpendicular to the chains. The tertiary structure of a protein molecule is a spatial configuration usually resembling a compact globule; it is supported by ionic, hydrogen and disulfide (S–S) bonds, as well as hydrophobic interactions. The quaternary structure is formed by the interaction of several subunits - globules (for example, the hemoglobin molecule consists of four such subunits). The loss of a protein molecule's structure is called denaturation; it can be caused by temperature, dehydration, radiation, etc. If the primary structure is not disturbed during denaturation, then when normal conditions are restored, the protein structure is completely recreated.
The functions of proteins in a cell are very diverse. They play the role of catalysts, i.e. accelerate chemical reactions in the body (enzymes speed up reactions tens and hundreds of thousands of times). Proteins also perform a construction function (they are part of cell membranes and organelles, as well as extracellular structures, for example, collagen in connective tissue). The movement of organisms is ensured by special proteins (actin and myosin). Proteins also perform a transport function (for example, hemoglobin transports O2). Proteins provide the functions of the body’s immune system (antibodies and antigens), blood clotting (blood plasma fibrinogen), i.e. perform protective functions. They also serve as one of the sources of energy (with the breakdown of 1 g of protein, 17.6 kJ of energy is released). The regulatory functions of proteins are also known, because many hormones are proteins (for example, hormones of the pituitary gland, pancreas, etc.). In addition, the body also has reserve proteins, for example, which are a source of nutrition for the development of the fetus.
26. Describe the structure and biological significance of ATP. Why is ATP called the main source of energy in the cell?
ATP is adenosine triphosphate, a nucleotide belonging to the group of nucleic acids. The concentration of ATP in the cell is low (0.04%; in skeletal muscles 0.5%). The ATP molecule consists of adenine, ribose and three phosphoric acid residues (Fig. 12). During the hydrolysis of a phosphoric acid residue, energy is released:
ATP + H 2 O = ADP + H 3 PO 4 + 40 kJ/mol.
The bond between phosphoric acid residues is high-energy; its cleavage releases approximately 4 times more energy than the cleavage of other bonds. The energy of ATP hydrolysis is used by the cell in the processes of biosynthesis and cell division, during movement, during the production of heat, during the conduction of nerve impulses, etc. After hydrolysis, the resulting ADP is usually quickly phosphorylated again with the help of cytochrome proteins to form ATP. ATP is formed in mitochondria during respiration, in chloroplasts during photosynthesis, as well as in some other intracellular processes. ATP is called a universal source of energy because the energy of the cell is based mainly on processes in which ATP is either synthesized or consumed.
27. Reveal the relationship between the structure and functions of DNA and RNA and indicate their similarities and differences.
DNA (deoxyribonucleic acid) is a molecule consisting of two helically twisted polynucleotide chains (Fig. 14). DNA forms a right-handed helix, with a diameter of approximately 2 nm, a length (in unfolded form) of up to 0.1 mm and a molecular weight of up to 6-10-12 kDa. The structure of DNA was first determined by D. Watson and F. Crick in 1953. The monomer of DNA is a deoxyribonucleotide, consisting of a nitrogenous base - adenine (A), cytosine (C), thymine (T) or guanine (G), - pentose (deoxyribose ) and phosphate. Nucleotides are connected into a chain due to phosphoric acid residues located between pentoses: a polynucleotide can contain up to 30,000 nucleotides. The nucleotide sequence of one chain is complementary (i.e. complementary) to the sequence in the other chain due to hydrogen bonds between complementary nitrogenous bases (two hydrogen bonds between A and T and three between G and C). In interphase, before cell division, DNA replication (reduplication) occurs: DNA unwinds from one end, and a new complementary strand is synthesized on each strand; This is an enzymatic process that uses ATP energy. DNA is found primarily in the nucleus (see answer to question #11); Extranuclear forms of DNA include mitochondrial and plastid DNA (see answer to question No. 17).
Rice. 13. Structural diagram of RNA: a – sugar-phosphate backbone; b – single chain
a – sugar phosphate backbone; b – complementary pairs of nitrogenous bases; c – double helix
RNA (ribonucleic acid) is a molecule consisting of a single chain of nucleotides (Fig. 13). A ribonucleotide consists of one of four nitrogenous bases, but RNA contains uracil (U) instead of thymine (T), and ribose instead of deoxyribose. There are different types of RNA in a cell: tRNA (transport - transports amino acids to ribosomes), messenger RNA (mRNA, transfers information about the sequence of amino acids from DNA to protein), ribosomal RNA (part of ribosomes; see answer to question N19), mitochondrial RNA, etc.
28. Features of the structure of nucleic acids.
DNA and RNA are polynucleotides consisting of deoxyribonucleotides and ribonucleotides, respectively (see answer to question 27). A nucleotide molecule consists of a pentose, a nitrogenous base and a phosphoric acid residue. DNA contains deoxyribose, RNA contains ribose; DNA contains nitrogenous bases A and G (belonging to the class of purines) and C and T (class of pyrimidines), and RNA contains U instead of T (see answer 27).
DNA and RNA are acids because they contain a phosphoric acid residue (–H 2 PO 4 ). A sugar, a nitrogenous base, and a phosphoric acid residue are combined to form a nucleotide molecule.
Two nucleotides form a dinucleotide by joining by condensation, which results in a phosphodiester bridge between the phosphate group of one nucleotide and the sugar of the other nucleotide. During the synthesis of a polynucleotide, this process is repeated many times. The unbranched sugar phosphate backbone is built by forming phosphodiester bridges between the 3rd and 5th carbon atoms of sugar residues. Phosphodiester bridges are formed by strong covalent bonds, which imparts strength and stability to the entire polynucleotide chain.
Nucleic acids have a primary structure (nucleotide sequence) and a three-dimensional structure. DNA consists of two helically twisted polynucleotide chains. The chains are directed in opposite directions: the 3-end of one chain is located opposite the 5-end of the other. The nitrogenous bases of the two chains located opposite each other are connected by hydrogen bonds (two bonds between A and T and three between G and C). Bases connected to each other by hydrogen bonds are called complementary (see also the answer to question 27).
29. Describe the process of protein biosynthesis. What is the biological significance of this process? What role does DNA play in the process of protein biosynthesis?
Proteins are synthesized by all cells except nuclear-free ones. The structure of a protein is determined by nuclear DNA. Information about the sequence of amino acids in one polypeptide chain is found in a section of DNA called a gene. DNA contains information about the primary structure of the protein. The DNA code is the same for all organisms. Each amino acid has three nucleotides that form a triplet, or codon. This coding is redundant: 64 combinations of triplets are possible, while there are only 20 amino acids. There are also control triplets, for example, indicating the beginning and end of a gene.
Protein synthesis begins with transcription, i.e. synthesis of mRNA from a DNA template. The process occurs with the help of the polymerase enzyme according to the principle of complementarity and starts from a specific section of DNA. The synthesized mRNA enters the cytoplasm onto ribosomes, where protein synthesis occurs.
tRNA has a structure similar to a clover leaf and carries amino acids to the ribosomes. Each amino acid is attached to the acceptor site of the corresponding tRNA, located on the “leaf petiole.” The opposite end of the tRNA is called an anticodon and carries information about the triplet corresponding to a given amino acid. There are more than 20 types of tRNA.
The transfer of information from mRNA to protein during its synthesis is called translation. Ribosomes assembled into polysomes move along mRNA; the movement occurs sequentially, in triplets. At the site of contact between the ribosome and mRNA, an enzyme works that assembles protein from amino acids delivered to the ribosomes by tRNA. In this case, the mRNA codon is compared with the tRNA anticodon; if they are complementary, the enzyme (synthetase) “crosslinks” the amino acids, and the ribosome moves forward one codon.
The synthesis of one protein molecule usually takes 1–2 minutes (one step takes 0.2 s).
Protein biosynthesis is a chain of reactions that uses the energy of ATP. Enzymes are involved in all protein synthesis reactions.
Protein biosynthesis is a matrix synthesis. The template is DNA in RNA synthesis and DNA or RNA in protein synthesis.
30. Reveal the role of enzymes in the regulation of vital processes and in protein biosynthesis.
A– simple enzyme; b – two-component enzyme; V– allosteric enzyme (A – active center, S-substrate, R – regulator, or allosteric center); 1 – catalytic section; 2 – contact areas; 3 – cofactor
Enzymes (Latin: leaven) are biological catalysts of a protein nature. They can consist only of protein or include a non-protein compound - vitamins or a metal ion. Enzymes are involved in both assimilation and dissimilation processes. They operate in a strictly defined sequence. Enzymes are specific to each substance and speed up only certain reactions. But there are enzymes that catalyze several reactions.
The active center of an enzyme is a small section of the enzyme where this reaction occurs (Fig. 15).
The physiological role of enzymes is that in their absence or insufficient activity, metabolic processes sharply slow down; in the presence of enzymes, reactions can accelerate 1011 times. The process of protein biosynthesis is also an enzymatic process (see answer to question 29).
31. Give a comparative description of autotrophic and heterotrophic organisms.
32. What is the importance of metabolic processes in the functioning of a cell, organism, biosphere?
Metabolism and energy are the most important functions of a living organism (see also the answer to question 7). During the metabolic process, the body receives the substances necessary to build and renew the structural elements of cells and tissues, and energy to support all life processes.
The set of all biosynthesis reactions, usually accompanied by the absorption of energy, is called assimilation (plastic exchange), and all decomposition reactions, usually accompanied by the release of energy, are called dissimilation (energy exchange). The totality of all reactions of assimilation and dissimilation is called metabolism.
Metabolism occurs at the cellular, tissue, organ and organism levels. Metabolic disorders affect all vital processes of the body and can lead to its death.
The biosphere is the geological shell of the Earth inhabited by living organisms. The biosphere is an open system; Like living organisms, the biosphere receives energy from the outside. Metabolism is constantly taking place in the biosphere. Biogeochemical processes take place in the biosphere, in which producer organisms and decomposer organisms participate. The non-stop process of natural cyclic redistribution of matter and energy in the biosphere is called a large circle of biotic exchange. Disturbances in this process lead to disruption of the homeostasis of the biosphere and can ultimately lead to its death.
33. In what structural units of the cell do oxygen oxidation processes occur? What is their chemistry and energetic effect?
The oxygen oxidation stage of energy metabolism occurs in mitochondria, on the inner membranes of which respiratory enzymes are located (see also the answer to question 17). At this stage, 18 ATP molecules are obtained from one lactic acid molecule, and in total, 38 ATP molecules are formed from one glucose molecule during glycolysis (an oxygen-free stage that occurs due to enzymes in the soluble part of the cell cytoplasm) and aerobic oxidation.
The efficiency of oxidative phosphorylation is 55%.
34. Reveal the essence and biological significance of the process of photosynthesis.
Photosynthesis is the process of synthesizing organic substances from inorganic substances using light energy.
Photosynthesis in plant cells occurs in chloroplasts (see also the answer to question 17).
Total formula:
6CO 2 + 6H 2 O = C 6 H 12 O 6 + 6O 2.
The light phase of photosynthesis occurs only in the light: a light quantum knocks out an electron from a chlorophyll molecule lying in the inner membrane of the thylakoid; the knocked out electron either returns back or ends up in a chain of enzymes that oxidize each other. A chain of enzymes transfers an electron to the outside of the thylakoid membrane to an electron transporter. The membrane is charged negatively from the outside.
The positively charged chlorophyll molecule lying in the center of the membrane oxidizes enzymes containing manganese ions lying on the inner side of the membrane. These enzymes participate in water photolysis reactions, which result in the formation of H+ ions; protons are released onto the inner surface of the thylakoid membrane and a positive charge appears on this surface. When the potential difference across the thylakoid membrane reaches 200 mV, protons begin to flow through the ATP synthetase channel, and ATP is synthesized.
During the dark phase of photosynthesis, glucose is synthesized from CO 2 and atomic hydrogen bound to carriers using the energy of ATP.
CO 2 binds with the help of the enzyme ribulose diphosphate carboxylase to ribulose-1,5-diphosphate, which is then converted into a three-carbon sugar.
Glucose synthesis occurs in the thylakoid matrix using enzyme systems. Total reaction of the dark stage:
6CO 2 + 24H = C 6 H 12 O 6 + 6H 2 O.
35. Give a comparative description of the processes of respiration and photosynthesis.
Respiration in plants is a process in which mainly the oxidation of carbohydrates occurs with the release of energy necessary for life. This process takes place in mitochondria (see answers to questions 17 and 33). When breathing in aerobic organisms, O 2 is absorbed and CO 2 is released. The total reaction of the aerobic respiration process:
C 6 H 12 O 6 + 6O 2 = 6CO 2 + 6H 2 0 + energy.
The energy released during the oxidation of a glucose molecule goes to the synthesis of ATP (see also the answer to question 33).
During photosynthesis, the process of formation of organic substances using light energy occurs (see the answer to question 34). In this case, O 2 is released into the atmosphere, and CO 2 is absorbed; energy is stored in the chemical bonds of organic substances, primarily carbohydrates.
Photosynthesis and respiration in plants are two sides of metabolism (assimilation and dissimilation).
36. What is the difference between photosynthesis and chemosynthesis and what is the significance of these processes for evolution?
The essence of the photosynthesis process is the synthesis of organic substances from CO 2 and H 2 O using light energy, and the essence of the chemosynthesis process is the synthesis of organic substances from inorganic substances using the energy of chemical reactions occurring during the oxidation of inorganic substances. During photosynthesis, O 2 is released into the atmosphere; The first photosynthetic organisms were cyanobacteria (blue-green algae), thanks to whose activity the Earth's atmosphere began to become saturated with O 2, which created conditions for the existence of all aerobic organisms. During chemosynthesis, O2 is not released into the atmosphere, because chemotrophs (nitrifying bacteria, sulfur bacteria, iron bacteria, etc.) do not use water, but H 2 S or molecular hydrogen as a source of hydrogen. If only chemotrophic bacteria existed on Earth, then aerobic organisms would not be able to live (see also answers to questions 31 and 34).
37. What is the essence of the process of mitosis and its biological significance? Give a brief description of the processes occurring in different phases of mitosis.
Mitosis (Greek) mios– thread) is the main method of cell division. In animal cells it lasts 30–60 minutes, in plant cells – 2–3 hours.
Mitosis consists of four phases: prophase, metaphase, anaphase and telophase (Fig. 16). Prophase is the 1st phase of division, during which the bichromatid chromosomes spiral and become visible. The nucleoli and nuclear membrane disintegrate and a spindle filament is formed. Metaphase is the phase of accumulation of chromosomes at the equator of the cell; The spindle filaments extend from the poles and attach to the centromeres of the chromosomes. Each chromosome has two strands coming from the two poles. Anaphase is the phase of chromosome segregation, during which centromeres divide and single-chromatid chromosomes are pulled apart by spindle threads to the poles of the cell. This is the shortest phase of mitosis. Telophase is the phase at the end of division, when despiralization of chromosomes occurs, a nucleolus is formed, the nuclear membrane is restored, a septum is formed at the equator (in plant cells) or a constriction occurs (in animal cells). The spindle threads disappear.
Before the start of mitosis, during interphase, the cell prepares for division (see answer to question 11).
As a result of mitosis, from one diploid cell with two-chromatid chromosomes and double the amount of DNA (2n4c; in this formula n is the number of chromosomes, c is the number of chromatids), two daughter cells with single-chromatid chromosomes and a single amount of DNA (2n2c) are formed. This is how somatic cells (body cells) divide.
The significance of mitosis is the accurate transfer of hereditary information to daughter cells, increasing the number of cells in the body, as well as ensuring the process of asexual reproduction of organisms and regeneration.
38. What are the functional and cytological differences between somatic and germ cells?
Somatic cells form the organs and tissues of the body of animals and plants; somatic cells themselves are formed as a result of mitosis and have a diploid set of chromosomes (2n); Each somatic cell contains two genes in a pair of homologous chromosomes that determine alternative characteristics (allelic genes).
Sex cells (gametes) are formed as a result of meiosis (reduction division; see also answers to questions 41 and 42) and have a haploid set of chromosomes (n). Each gamete contains one gene from each pair of homologous chromosomes. When gametes fuse, a zygote is formed.
39. Prove what is the evolutionary advantage of the separation of the sexes.
The separation of the sexes is the basis of sexual reproduction. In sexual reproduction, offspring are produced by the fusion of genetic material from haploid nuclei. These nuclei are contained in haploid gametes, the fusion of which forms a diploid zygote. From the zygote in the process of development a mature organism is obtained.
Sexual reproduction has a very large evolutionary advantage compared to asexual reproduction. This is due to the fact that the genotype of the offspring arises from a combination of genes belonging to both parents. As a result, the body's ability to adapt to environmental conditions increases.
40. What are the cytological basis for sex determination?
In the vast majority of dioecious animals, the sex of the individual developing from the egg is determined by genes. This is called genotypic sex determination. A diploid organism has two homologous sets of autosomes and, in most cases, one pair of sex chromosomes. In the autosomal set of both sexes, the paternal and maternal chromosomes are morphologically and functionally equivalent, while between the sex chromosomes, as a rule, there are morphological and in all cases functional differences. The chromosome that is represented in double numbers in one of the sexes is called the X chromosome. It is opposed to the Y chromosome, which is present in one copy.
A sex containing two X chromosomes in its cells is called homogametic, and a sex containing both X and Y chromosomes is called heterogametic.
In all mammals, many fish, some amphibians and insects, the homogametic sex is female, and the heterogametic sex is male. However, in birds, reptiles, tailed amphibians and some insects (butterflies), the female sex is heterogametic, and the male sex is homogametic. In some insects, the X0 genotype occurs due to the disappearance of the Y chromosome. In this case, the heterogametic sex produces two types of gametes: with and without the X chromosome.
41. Describe the main phases of meiotic division and reveal its biological significance.
Meiosis(Greek meiosis- reduction) is a method of dividing diploid cells with the formation of four daughter haploid cells from one mother diploid cell. Meiosis consists of two successive divisions of the nucleus and a short interphase between them (Fig. 17).
Fig. 18. Schematic representation of the successive stages of meiosis. A. Leptonema preceding chromosome conjugation. B. Beginning of conjugation at the zygonema stage. V. Pachinema. G. Diplonema. D. Metaphase I. E. Anaphase I. G. Telophase I. 3. Interphase between two divisions of meiosis. I. Prophase II. K. Metaphase II. L. Telophase II. For simplicity, only one pair of homologues is shown in the diagram.
The first division consists of prophase I, metaphase I, anaphase I and telophase I. In prophase I, paired chromosomes, each consisting of two chromatids, approach each other (this process is called conjugation of homologous chromosomes), cross over (crossing over), forming bridges (chiasmata), then exchange sections. Crossing over involves recombination of genes. After crossing over, the chromosomes are separated.
In metaphase I, paired chromosomes are located along the equator of the cell; spindle strands are attached to each chromosome. In anaphase I, bichromatid chromosomes diverge to the cell poles; in this case, the number of chromosomes at each pole becomes half that in the mother cell. Then comes telophase I - two cells with a haploid number of bichromatid chromosomes are formed; Therefore, the first division of meiosis is called reduction. Telophase I is followed by a short interphase (in some cases, telophase I and interphase are absent). In the interphase between two divisions of meiosis, chromosome duplication does not occur, because each chromosome already consists of two chromatids.
The second division of meiosis differs from mitosis only in that it is carried out by cells with a haploid set of chromosomes; in the second division, prophase II is sometimes absent. In metaphase II, bichromatid chromosomes are located along the equator; the process occurs in two daughter cells at once. In anaphase II, single-chromatid chromosomes move to the poles. In telophase II, nuclei and partitions (in plant cells) or constrictions (in animal cells) are formed in the four daughter cells. As a result of the second division of meiosis, four cells with a haploid set of chromosomes (1n1c) are formed; the second division is called equational (equalization) (Fig. 18). These are gametes in animals and humans or spores in plants.
The significance of meiosis is that it creates a haploid set of chromosomes and conditions for hereditary variability due to crossing over and probabilistic divergence of chromosomes
To be continued
Plant cell. Difference between a plant cell and an animal cell.
Briefly distinguish plant cells from animal cells.
A strong cell wall of considerable thickness;
Special organelles - plastids, in which the primary synthesis of organic substances from minerals occurs due to light energy;
A developed network of vacuoles, which largely determines the osmotic properties of cells.
A plant cell contains all the organelles characteristic of an animal cell: nucleus, endoplasmic reticulum, ribosomes, mitochondria, Golgi apparatus. However, the plant cell has significant differences.
A plant cell, like an animal cell, is surrounded by a cytoplasmic membrane, but besides it it is limited by a thick cell wall consisting of cellulose, which animal cells do not have. The cell wall has pores through which the endoplasmic reticulum channels of neighboring cells communicate with each other.
Plant cells have chloroplasts for photosynthesis, but animal cells do not have chloroplasts.
Another difference between plant and animal cells is that animal cells are round while plant cells are rectangular in shape.
In addition, all animal cells have centrioles, while only some lower forms of plants have centrioles (an intracellular organelle of a eukaryotic cell, representing bodies in the cell structure, the size of which is at the limit of the resolving power of a light microscope.
Animal cells have one or more small vacuoles, while plant cells have one large central vacuole, which can occupy up to 90% of the cell volume.
In plant cells, the vacuole performs the functions of storing water and maintaining cell elasticity. Functions of the vacuole in animal cells: storage of water, ions and waste.
Drawing of a plant cell with symbols.
Drawing of an animal cell with symbols.
Cell, the elementary unit of living things. The cell is delimited from other cells or from the external environment by a special membrane and has a nucleus or its equivalent, in which the bulk of the chemical information that controls heredity is concentrated. Cytology studies the structure of cells, and physiology deals with their functioning. The science that studies tissue made up of cells is called histology.
There are unicellular organisms whose entire body consists of one cell. This group includes bacteria and protists (protozoa and unicellular algae). Sometimes they are also called acellular, but the term unicellular is used more often. True multicellular animals (Metazoa) and plants (Metaphyta) contain many
Some structures of the body that do not participate in metabolism, in particular shells, pearls or the mineral basis of bones, are not formed by cells, but by the products of their secretion. Others, such as wood, bark, horns, hair and the outer layer of skin, are not of secretory origin, but are formed from dead cells.
Small organisms, such as rotifers, consist of only a few hundred cells. For comparison: in the human body there are about 1014 cells, every second 3 million red blood cells die and are replaced by new ones, and this is only one ten-millionth of the total number of body cells.
Cell structure.
At one time, the cell was considered as a more or less homogeneous drop of organic matter, which was called protoplasm or living substance. This term became obsolete after it was discovered that the cell consists of many clearly distinct structures called cellular organelles (“little organs”).
Chemical composition. Typically, 70–80% of the cell mass is water, in which various salts and low molecular weight organic compounds are dissolved. The most characteristic components of a cell are proteins and nucleic acids. Some proteins are structural components of the cell, others are enzymes, i.e. catalysts that determine the speed and direction of chemical reactions occurring in cells. Nucleic acids serve as carriers of hereditary information, which is realized in the process of intracellular protein synthesis.
Often cells contain a certain amount of storage substances that serve as a food reserve. Plant cells primarily store starch, a polymeric form of carbohydrates. Another carbohydrate polymer, glycogen, is stored in liver and muscle cells. Frequently stored foods also include fat, although some fats perform a different function, namely, they serve as essential structural components. Proteins in cells (with the exception of seed cells) are usually not stored.
Main parts of the cell. Some cells, mostly plant and bacterial, have an outer cell wall. In higher plants it consists of cellulose. The wall surrounds the cell itself, protecting it from mechanical stress. Cells, especially bacterial cells, can also secrete mucous substances, thereby forming a capsule around themselves, which, like the cell wall, has a protective function.
The cell itself consists of three main parts. Below the cell wall, if present, is the cell membrane. The membrane surrounds a heterogeneous material called cytoplasm. A round or oval nucleus is immersed in the cytoplasm.
Cell membrane.
The cell membrane is a very important part of the cell. It holds all cellular components together and delineates the internal and external environments. In addition, modified folds of the cell membrane form many of the cell's organelles.
The cell membrane is a double layer of molecules (bimolecular layer, or bilayer). These are mainly molecules of phospholipids and other substances related to them.
The main function of the cell membrane is to regulate the transport of substances into and out of the cell. Because the membrane is physically somewhat similar to oil, substances that are soluble in oil or organic solvents, such as ether, pass through it easily. The same applies to gases such as oxygen and carbon dioxide. At the same time, the membrane is practically impermeable to most water-soluble substances, in particular sugars and salts. Thanks to these properties, it is able to maintain a chemical environment inside the cell that differs from the outside.
Vacuole.
Plant cells often have one large central vacuole occupying almost the entire cell; the cytoplasm forms only a very thin layer between the cell wall and the vacuole. One of the functions of such a vacuole is the accumulation of water, allowing the cell to quickly increase in size. This ability is especially necessary during the period when plant tissues grow and form fibrous structures.
Cytoplasm.
The cytoplasm contains internal membranes that are similar to the outer membrane and form organelles of various types. These membranes can be thought of as folds of the outer membrane; sometimes the inner membranes are integral with the outer one, but often the inner fold is unlaced and contact with the outer membrane is interrupted. However, even if contact is maintained, the inner and outer membranes are not always chemically identical. In particular, the composition of membrane proteins differs in different cellular organelles.
Golgi apparatus.
The Golgi apparatus (Golgi complex) is a specialized part of the endoplasmic reticulum, consisting of stacked flat membrane sacs. It is involved in the secretion of proteins by the cell (packing of secreted proteins into granules occurs in it) and therefore is especially developed in cells that perform a secretory function. Important functions of the Golgi apparatus also include the attachment of carbohydrate groups to proteins and the use of these proteins to build the cell membrane and lysosome membrane. In some algae, cellulose fibers are synthesized in the Golgi apparatus.
Mitochondria and chloroplasts.
Mitochondria are relatively large sac-like structures with a rather complex structure. They consist of a matrix surrounded by an inner membrane, an intermembrane space and an outer membrane. The inner membrane is folded into folds called cristae. Clusters of proteins are located on the cristae. Many of them are enzymes that catalyze the oxidation of carbohydrate breakdown products; others catalyze reactions of fat synthesis and oxidation. Auxiliary enzymes involved in these processes are dissolved in the mitochondrial matrix.
Oxidation of organic substances occurs in mitochondria, coupled with the synthesis of adenosine triphosphate (ATP). The breakdown of ATP to form adenosine diphosphate (ADP) is accompanied by the release of energy, which is spent on various vital processes, for example, on the synthesis of proteins and nucleic acids, transport of substances into and out of the cell, transmission of nerve impulses or muscle contraction. Mitochondria are thus energy stations that process “fuel” – fats and carbohydrates – into a form of energy that can be used by the cell, and therefore the body as a whole.
Plant cells also contain mitochondria, but the main source of energy for their cells is light. Light energy is used by these cells to produce ATP and synthesize carbohydrates from carbon dioxide and water. Chlorophyll, a pigment that accumulates light energy, is found in chloroplasts. Chloroplasts, like mitochondria, have inner and outer membranes. From the outgrowths of the inner membrane during the development of chloroplasts, so-called chloroplasts arise. thylakoid membranes; the latter form flattened bags, collected in stacks like a column of coins; these stacks, called grana, contain chlorophyll. In addition to chlorophyll, chloroplasts contain all the other components necessary for photosynthesis.
Some specialized chloroplasts do not carry out photosynthesis, but have other functions, such as storing starch or pigments.
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 with the help of antibiotics, while they have no effect on the main system.
Organelle DNA is responsible for only a small part of organelle structure; most of their proteins are encoded in genes located on chromosomes.
Core.
The nucleus is surrounded by a double membrane. The very narrow (about 40 nm) space between two membranes is called perinuclear. The nuclear membranes pass into the membranes of the endoplasmic reticulum, and the perinuclear space opens into the reticular space. Typically the nuclear membrane has very narrow pores. Apparently, large molecules are transported through them, such as messenger RNA, which is synthesized on DNA and then enters the cytoplasm.
The bulk of the genetic material is located in the chromosomes of the cell nucleus. Chromosomes consist of long chains of double-stranded DNA, to which basic (i.e., alkaline) proteins are attached. Sometimes chromosomes have several identical DNA strands lying next to each other - such chromosomes are called polytene (multi-stranded). The number of chromosomes varies among species. Diploid cells of the human body contain 46 chromosomes, or 23 pairs.
Cell division.
Although all cells arise from the division of a previous cell, not all continue to divide. For example, nerve cells in the brain, once formed, do not divide. Their number is gradually decreasing; Damaged brain tissue is not able to recover through regeneration. If cells continue to divide, then they are characterized by a cell cycle consisting of two main stages: interphase and mitosis.
MITOSIS
After the chromosomes have been duplicated, each of the daughter cells should receive a full set of chromosomes. Simple cell division cannot achieve this - this result is achieved through a process called mitosis. The beginning of this process is the alignment of chromosomes in the equatorial plane of the cell. Then each chromosome splits longitudinally into two chromatids, which begin to diverge in opposite directions, becoming independent chromosomes. As a result, a complete set of chromosomes is located at both ends of the cell. The cell then divides into two, and each daughter cell receives a full set of chromosomes.
Mitosis in an animal cell is divided into four stages.
I. Prophase.
II. Metaphase.
III. Anaphase.
IV. Telophase.
The details of mitosis vary somewhat among different cell types. A typical plant cell forms a spindle but lacks centrioles. In fungi, mitosis occurs inside the nucleus, without previous disintegration of the nuclear membrane.
The division of the cell itself, called cytokinesis, does not have a strict connection with mitosis. Sometimes one or more mitoses occur without cell division; As a result, multinucleated cells are formed, often found in algae.
Reproduction by mitosis is called asexual reproduction, vegetative reproduction or cloning. Its most important aspect is genetic: with such reproduction, there is no divergence of hereditary factors in the offspring. The resulting daughter cells are genetically exactly the same as the mother cell. Mitosis is the only way of self-reproduction in species that do not have sexual reproduction, such as many single-celled organisms. However, even in species with sexual reproduction, body cells divide through mitosis and come from a single cell, the fertilized egg, and are therefore all genetically identical. Higher plants can reproduce asexually (using mitosis) by seedlings and tendrils (a well-known example is strawberries).
Meiosis.
Sexual reproduction of organisms is carried out with the help of specialized cells, the so-called. gametes - oocytes (eggs) and sperm (sperm). Gametes fuse to form one cell - a zygote. Each gamete is haploid, i.e. has one set of chromosomes. Within the set, all the chromosomes are different, but each chromosome of the egg corresponds to one of the chromosomes of the sperm. The zygote, therefore, already contains a pair of chromosomes corresponding to each other, which are called homologous. Homologous chromosomes are similar because they have the same genes or their variants (alleles) that determine specific characteristics. For example, one of the paired chromosomes may have a gene encoding blood type A, and the other may have a variant encoding blood type B. The zygote's chromosomes originating from the egg are maternal, and those originating from the sperm are paternal. .
Cytoplasmic division.
As a result of two meiotic divisions of a diploid cell, four cells are formed. When male reproductive cells are formed, four sperm of approximately the same size are obtained. When eggs are formed, the division of the cytoplasm occurs very unevenly: one cell remains large, while the other three are so small that they are almost entirely occupied by the nucleus. These small cells, the so-called. polar bodies serve only to accommodate excess chromosomes formed as a result of meiosis. The bulk of the cytoplasm necessary for the zygote remains in one cell - the egg.
Classification of fabrics.
Tissue is a phylogenetically formed system of cells and non-cellular structures, which has a common structure and is specialized to perform certain functions. Depending on this, epithelial, mesenchymal derivatives, muscle and nervous tissue are distinguished.
Epithelial tissue is morphologically characterized by a close union of cells into layers. Epithelium and mesothelium (a type of epithelium) line the surface of the body, serous membranes, the inner surface of hollow organs (alimentary canal, bladder, etc.) and form most of the glands.
Distinguish between integumentary and glandular epithelium
The integumentary epithelium belongs to the border epithelium, since it is located on the border of the internal and external environments and through it metabolism occurs (absorption and excretion). It also protects the underlying tissues from chemical, mechanical and other types of external influences.
The glandular epithelium has a secretory function, i.e. the ability to synthesize and secrete secret substances that have a specific effect on the processes occurring in the body.
The epithelium, all cells of which are connected to the basement membrane, is called single-layer.
In multilayered epithelium, only the lower layer of cells is connected to the basement membrane.
There are single- and multi-row single-layer epithelium. Single-row isomorphic epithelium is characterized by cells of the same shape with nuclei lying at the same level (in one row), and multirow, or anisomorphic, is characterized by cells of different shapes with nuclei lying at different levels and in several rows.
Multilayered epithelium, in which the cells of the upper layers turn into horny scales, is called multilayered keratinizing, and in the absence of keratinization - multilayered non-keratinizing.
A special form of multilayer epithelium is transitional, characterized by the fact that its appearance changes depending on the stretching of the underlying tissue (the walls of the renal pelvis, ureters, bladder, etc.).
All types of eukaryotic cells have mitochondria (Fig. 1). They look like either round bodies or rods, less often - threads. Their sizes range from 1 to 7 microns. The number of mitochondria in a cell ranges from several hundred to tens of thousands (in large protozoa).
Rice. 1. Mitochondria. At the top - mitochondria (?) in the urinary tract tubules visible under a light microscope. Below - three-dimensionalmodel of mitochondrial organization: 1 - cristae; 2 - externalmembrane; 3 - internal membrane; 4 - matrix
The mitochondrion is formed by two membranes -external And internal , between which is locatedintermembrane space . The inner membrane forms many invaginations - cristae, which are either plates or tubes. This organization provides a huge area of the internal membrane. It contains enzymes that ensure the conversion of energy contained in organic substances (carbohydrates, lipids) into ATP energy, necessary for the life of the cell. Therefore, the function of mitochondria is to participate inenergy cellular processes. That is why a large number of mitochondria are inherent, for example, in muscle cells that perform a lot of work.
Plastids. In plant cells, special organelles are found - plastids, which often have a spindle-shaped or rounded shape, sometimes more complex. There are three types of plastids - chloroplasts (Fig. 2), chromoplasts and leucoplasts.
Chloroplasts differ in green color, which is due to the pigment - chlorophyll, providing the process photosynthesis, i.e., the synthesis of organic substances from water (H 2 O) and carbon dioxide (CO 2) using the energy of sunlight. Chloroplasts are found mainly in leaf cells (in higher plants). They are formed by two membranes located parallel to each other, surrounding the contents of chloroplasts - stroma. The inner membrane forms numerous flattened sacs - thylakoids, which are stacked (like a stack of coins) - grains - and lie in the stroma. It is thylakoids that contain chlorophyll.
Chromoplasts determine the yellow, orange and red color of many flowers and fruits, in the cells of which they are present in large quantities. The main pigments in their composition are carotenes. The functional purpose of chromoplasts is to attract animals with color, ensuring pollination of flowers and dispersal of seeds.
Rice. 2. Plastids: A- chloroplasts in Elodea leaf cells, visible in a light microscope;b - diagram of the internal structure of a chloroplastwith grana, which are stacks of flat sacs,located perpendicular to the surface of the chloroplast;V - a more detailed diagram showing the anastomosingtubes connecting the individual gran chambers
Leukoplasts- these are colorless plastids contained in the cells of underground parts of plants (for example, in potato tubers), seeds and the core of stems. In leucoplasts, starch is mainly formed from glucose and accumulated in the storage organs of plants.
Plastids of one type can transform into another. For example, when leaves change color in autumn, chloroplasts transform into chromoplasts.
Lecture No. 6.
Number of hours: 2
MITOCHONDRIA AND PLASTIDES
1.
2. Plastids, structure, varieties, functions
3.
Mitochondria and plastids are double-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 structure and some common properties. However, in terms of basic metabolic processes they differ significantly from each other.
1. Mitochondria, structure, functional significance
General characteristics of mitochondria. Mitochondria (Greek “mitos” - thread, “chondrion” - grain, granule) are round, oval or rod-shaped double-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 because they are large and dense. The features of their internal structure can only be studied using an electron microscope.Mitochondria were discovered in 1894 by R. Altman, who gave them the name “bioblasts.”The term "mitochondrion" was introduced by K. Benda in 1897. Mitochondria are almost in all eukaryotic cells. Anaerobic organisms (intestinal amoebas, etc.) lack mitochondria. NumberThe number of mitochondria in a cell ranges from 1 to 100 thousand.and depends on the type, functional activity and age of the cell. Thus, in plant cells there are fewer mitochondria than in animal cells; and inmore in young cells than in old cells.The life cycle of mitochondria is several days. In a cell, mitochondria usually accumulate near areas of the cytoplasm where the need for ATP occurs. For example, in cardiac muscle, mitochondria are located near myofibrils, and in sperm they form a spiral sheath around the axis of the flagellum.
Ultramicroscopic structure of mitochondria. Mitochondria are bounded by two membranes, each of which is about 7 nm thick. The outer membrane is separated from the inner membrane by an intermembrane space about 10-20 nm wide. The outer membrane is smooth, and the inner one forms folds - cristae (Latin “crista” - ridge, outgrowth), increasing its surface. The number of cristae varies in the mitochondria of different cells. There can be from several dozen to several hundred. There are especially many cristae in the mitochondria of actively functioning cells, such as muscle cells. The cristae contain chains of electron transfer and associated phosphorylation of ADP (oxidative phosphorylation). The internal space of mitochondria is filled with a homogeneous substance called matrix. Mitochondrial cristae usually do not completely block the mitochondrial cavity. Therefore, the matrix is continuous throughout. The matrix contains circular DNA molecules, mitochondrial ribosomes, and deposits of calcium and magnesium salts. The synthesis of various types of RNA molecules occurs on mitochondrial DNA; 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 occurs 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 multiply by dividing by constriction or fragmentation of large mitochondria into smaller ones. Mitochondria formed in this way can grow and divide again.
Functions of mitochondria. The main function of mitochondria is to synthesize ATP. This process occurs as a result of the oxidation of organic substrates and the phosphorylation of ADP. The first stage of this process occurs in the cytoplasm under anaerobic conditions. Since the main substrate is glucose, the process is called glycolysis. At this stage, the substrate undergoes enzymatic breakdown to pyruvic acid with the simultaneous synthesis of a small amount of ATP. The second stage 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 using 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 glucose molecule with the formation of carbon dioxide and water provides energy for the recharge of 38 ATP molecules (2 molecules in the cytoplasm and 36 in mitochondria).
Analogues of mitochondria in bacteria. Bacteria do not have mitochondria. Instead, they have electron transport chains located in the cell membrane.
2. Plastids, structure, varieties, functions. The problem of the origin of plastids
Plastids (from Greek. plastides– creating, forming) - These are double-membrane organelles characteristic of photosynthetic eukaryotic organisms.There are three main types of plastids: chloroplasts, chromoplasts and leucoplasts. The collection of plastids in a cell is called plastidome. Plastids are related to each other by a single origin in ontogenesis from proplastids of meristematic cells.Each of these types, under certain conditions, can transform into one another. Like mitochondria, plastids contain their own DNA molecules. Therefore, they are also able to reproduce independently of cell division.
Chloroplasts(from Greek "chloros" - green, "plastos" - fashioned)- These are plastids in which photosynthesis occurs.
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. In higher plants, chloroplasts have biconvex or ellipsoidal shape. The number of chloroplasts in a cell can vary from one (some green algae) to a thousand (shag). INOn average, a cell of higher plants contains 15-50 chloroplasts.Usually chloroplasts are evenly distributed throughout the cytoplasm of the cell, but sometimes they are grouped near the nucleus or cell membrane. Apparently, this 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 has a folded structure. Between the folds are located thylakoids shaped like disks. Thylakoids form stacks like coins called grains. Mgrana are connected to each other by other thylakoids ( lamellas, frets). The number of thylakoids in one grana varies from a few to 50 or more. In turn, the chloroplast of higher plants contains about 50 grains (40-60), arranged in a checkerboard pattern. This arrangement ensures maximum illumination of each face. 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 pigmentlorophyll a with the formula C 55 H 72 O 5 N 4 M g . Contains chlorophyll as additional 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.The matrix containsenzymes of the “dark phase” of photosynthesis, DNA, RNA, ribosomes.In addition, primary deposition of starch in the form of starch grains occurs in the matrix.
Properties of chloroplasts:
· semi-autonomy (they have their own protein synthesizing apparatus, but most of the genetic information is located in the nucleus);
· ability to move independently (move away from direct sunlight);
· ability to reproduce independently.
Reproduction of chloroplasts. Chloroplasts develop from proplastids, which are capable of replicating by fission. In higher plants, division of mature chloroplasts also occurs, but extremely rarely. As leaves and stems age and fruits ripen, 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 the light phase of photosynthesis.
Chromoplasts(from Greek chromatos – color, paint and “ plastos " – fashioned)These 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 shells. 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: when filled with ripe juice, they turn yellow, pink or red.The main function of chromoplasts is to provide color to flowers, fruits, and seeds.
Unlike leucoplasts and especially chloroplasts, the inner membrane of chloroplasts does not form thylakoids (or forms single ones). Chromoplasts are the final result of plastid development (chloroplasts and plastids turn into chromoplasts).
Leukoplasts(from Greek leucos – white, plastos – fashioned, created). These are colorless plastidsround, ovoid, spindle-shaped. Found in the underground parts of plants, seeds, epidermis, and stem core. Especially rich leucoplasts of potato tubers.The inner shell forms a few thylakoids. In the light, chloroplasts are formed from chloroplasts.Leukoplasts in which secondary starch is synthesized and accumulated are called amyloplasts, oils – eylaloplasts, 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 about the origin of mitochondria and plastids. These are the theories of direct filiation and sequential endosymbioses. According to the theory of direct filiation, mitochondria and plastids were formed through compartmentalization of the cell itself. Photosynthetic eukaryotes evolved from photosynthetic prokaryotes. In the resulting autotrophic eukaryotic cells, mitochondria were formed through intracellular differentiation. As a result of the loss of plastids, animals and fungi evolved from autotrophs.
The most substantiated theory is the theory of sequential 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 such as anaerobic heterotrophic bacteria included free-living aerobic bacteria, which turned into mitochondria. In parallel with this, in the prokaryotic host cell the genophore is formed into a nucleus isolated from the cytoplasm. In this way, the first eukaryotic cell, which was heterotrophic, arose. The emerging eukaryotic cells, through repeated symbioses, included blue-green algae, which led to the appearance of chloroplast-type structures in them. Thus, heterotrophic eukaryotic cells already had mitochondria when the latter acquired plastids as a result of symbiosis. Subsequently, as a result of natural selection, mitochondria and chloroplasts lost part of their genetic material and turned into structures with limited autonomy.
Evidence for the endosymbiotic theory:
1. The 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 owna specific protein synthesis system (DNA, RNA, ribosomes). The specificity of this system lies in its autonomy and sharp difference from that in a cell.
3. The DNA of mitochondria and plastids issmall cyclic or linear molecule,which differs from the DNA of the nucleus and in its characteristics approaches the DNA of prokaryotic cells.DNA synthesis of mitochondria and plastids is notdepends on nuclear DNA synthesis.
4. Mitochondria and chloroplasts contain i-RNA, t-RNA, and r-RNA. The ribosomes and rRNA of these organelles differ sharply from those in the cytoplasm. In particular, the ribosomes of mitochondria and chloroplasts, 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. An increase in the number of chloroplasts occurs through changes in proplastids, which, in turn, multiply by division.
This theory well explains the preservation of remnants of replication systems in mitochondria and plastids and allows us 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 suppressed by the antibiotic streptomycin, and on yeast cells, where the formation of mitochondria was suppressed by other drugs. After such effects, 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 with the help of antibiotics, while they have no effect on the main system. Organelle DNA is responsible for the bulk of extrachromosomal, or cytoplasmic, inheritance. Extrachromosomal heredity does not obey Mendelian laws, since when a cell divides, the DNA of organelles is transmitted to daughter cells in a different way than chromosomes. The study of mutations that occur in organelle DNA and chromosomal DNA 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 evidence of their symbiotic origin.
