Yeast mother cells sacrifice everything for their offspring. Yeast cell morphology

Shape and dimensions. Yeast cells have a variety of shapes: round, oval or elliptical, lemon-shaped, cylindrical, sometimes very elongated in the form of hyphae.

Compared to other microorganisms, yeasts are quite large. The diameter of yeast cells reaches 1-8 microns, length - 1-10 microns. With such cell sizes, their surface in 1 liter of fermented grape juice can reach 10 m2. It is this large surface of yeast cells that determines the intensity of their metabolism and the process of exchange of substances with the environment. A mass of yeast cells equal in mass to an animal (500 kg) synthesizes more than 50 tons of protein per day, while an animal synthesizes only about 0.5 kg.

The density (specific gravity) of yeast cells differs little from the density of bacterial cells. The relative density of bacterial cells ranges from 1.050 to 1.112; yeast - from 1.055 to 1.060.

Morphologically unchanged forms of yeast cells are observed only in young cultures on a standard nutrient medium. The same yeast culture may consist of glue

current, varying in shape and size, especially depending on developmental stages and environmental conditions. Thus, during the fermentation period, sherry yeast is, as a rule, large and has an elliptical or round cell shape, whereas during the film formation stage it becomes smaller and acquires a more elongated configuration. Yeasts of the genus Hansenula, which usually form a film on the surface of wine from clusters of highly elongated, small cells, when developing in an environment with limited air access, for example at the bottom of a bottle, become larger and round in shape. The shape and size of yeast cells changes significantly when CO2 is introduced into the medium. The size of the cells of brewer's yeast (Sacch. carlsbergensis) increases and, on the contrary, the cells of weakly fermenting yeast (Torula latvica) become much smaller.

Structure. The yeast cell has a complex structure and composition. The dimensions of its individual elements are beyond the resolution of a light microscope. Significant advances currently achieved in deciphering the structures of yeast cells and their functions are based on new research methods - electron microscopy and complex cytology.

A yeast cell is a unicellular microscopic organism and has basically the same structure as the cells of animals and plants (Fig. 16, 17). Inside the cell there are components that are usually divided into organelles and inclusions. Organelles include the cell membrane, cytoplasmic membrane, cytoplasm, mitochondria, vacuoles, Golgi apparatus, and nucleus. Inclusions are temporary cell formations (glycogen, trehalose, fat, metachromatin, etc.) that appear and disappear during the metabolic process.

The cell membrane is dense, thin and elastic, surrounds the cytoplasm and gives the yeast cell its characteristic shape, protects it from harmful environmental factors, and carries an electrical charge. The membrane maintains intracellular osmotic pressure by regulating the entry of salts and other low-molecular compounds into the cell through the pores.

The chemical composition of the cell membrane includes protein-nolysaccharide complexes, phosphates and lipids. At Sacch. cerevisiae, the polysaccharide part of the complex consists of approximately equal amounts of glucan and mannan, the sum of which amounts to about 90% of the dry mass of the shell; the rest of it comes from protein, lipids, and glucosamine. Protein complexes are saturated with disulfide and sulfhydryl groups. Lipids mainly (about 2/3) consist of free fatty acids (oleic, palmitoleic, palmitic and stearic); the rest of them are triglycerides, sterols and phospholipids.

The cell walls of other yeast species contain glucan, which is not always associated with mainan. Mannan may be contained in the cell walls of yeast in small quantities (Sacch. guttulata and Endomycopsis capsularis), or completely absent (Nadsonia fulvescens, Schizosaccharomyces). Yeast cell walls lacking mannan, with the exception of species of the genus Schizosaccharomyces, contain increased amounts of chitin.

In the periplasm region, which is located between the inner surface of the cell membrane and the outer surface of the cytoplasmic membrane, a number of enzymes are found. These are mainly hydrolytic enzymes, including 3-fructofuranosidase (invertase) and acid phosphatase.

The phenomenon of flocculation in yeast is usually associated with the composition of the cell wall. For yeast, as well as for bacteria, it has been established that the transition of S-forms (smooth) to R-forms (rough) and the formation of flakes in a liquid medium are due to the transformation of hydrophilic groups of the cell wall into hydrophobic ones. However, analysis of yeast cell walls isolated from floc-forming and non- floc-forming strains of Sacch. cerevisiae and Sacch. carls-bergensis grown on a synthetic medium did not reveal significant differences in the content of the main components, although as both of them grew, the amount of mannan gradually decreased and the amount of glucan increased. And only the active connection of phosphomannan and calcium ions in two neighboring flocculent yeast cells can explain the phenomenon of flocculation.

Electron microscopy also did not reveal any features of cell walls in flocculating yeast. Apparently, the yeast flocculation effect is due to the heredity of the strain.

The cytoplasmic membrane forms the outer layer of the cytoplasm. It surrounds the protoplast - a separate structural fraction of the cell, which can be obtained by removing the cell membrane using enzymes. The cytoplasmic membrane consists of electronic layers of different densities with a total thickness of no more than 10 nm, closely associated with the cytoplasm. When microscopying in a dark field, the membrane can be observed in the form of a thin luminous rim.

In the cell of a microorganism, the cytoplasmic membrane performs 4 main functions: acts as an osmotic barrier, regulates the penetration of nutrients from solution into the cell and the removal of metabolic products, performs the biosynthesis of some constituent parts of the cell (components of the cell membrane), and is the site of localization of some enzymes and organelles ( ribosomes).

The entry (transport) of substances into the cell is associated with a change in membrane permeability, which in turn depends on the activity of phospholipase and lipase.

The cytoplasm of a yeast cell contains all the basic structures inherent in highly differentiated cells, namely: mitochondria, ribosomes, a more or less developed endoplasmic reticulum (endoplasmic reticulum), storage substances and other intracellular inclusions of lipid and carbohydrate nature. They take an active part in the implementation of important enzymatic processes in the cytoplasm.

As a result of electron microscopic and biochemical studies, it was established that the cytoplasm is a colloidal system consisting of proteins, carbohydrates, lipids, minerals, water and other types of compounds. In terms of consistency, it is characterized by a high viscosity, 800-8000 times higher than the viscosity of water, which corresponds to the viscosity of glycerin or thick syrup. The viscosity of the cytoplasm increases with cell aging.

The structure of the cytoplasm changes depending on the culture conditions and the age of the cell. Changes in the cytoplasm can be reversible (paranecrosis) and irreversible (necrotic), which is determined by the degree of exposure to external factors. In young yeast cells, the cytoplasm is homogeneous; with aging, vacuoles, granularity, fat droplets, granules of polyphosphates and lipoids appear, i.e. the cytoplasm becomes heterogeneous.

During prolonged differential centrifugation, the cytoplasm is divided into a soluble fraction (cell sap) and a particle fraction consisting of membranes and ribosomes. The soluble fraction, which fills the space between particles in the contact cell, is the main component that ensures the interaction of metabolites with the environment where the structural elements are located and function. The soluble fraction contains substances with high molecular weight, mainly enzymes and transfer RNA (tRNA), as well as low molecular weight compounds, which include storage carbohydrates, for example, trehalose and a pool of amino acids and nucleotides. As a result of the presence of low molecular weight compounds in the cytoplasm, a noticeable difference arises in the osmotic pressure of the cellular contents and the external environment. The osmotic pressure in yeast is usually about 1.2 MPa.

Electron microscopic studies of ultrathin sections of cells showed that the cytoplasm is penetrated by an endoplasmic reticulum (reticulum), which has a granular or smooth structure. The close contact of the reticulum with other components of the cell allows it to play an important role in the metabolic processes of the cell.

One of the components of the endoplasmic reticulum are ribosomes. These are ultramicroscopic dense spherical granules. They contain almost equal amounts of RNA. and protein, minor - lipids. The synthesis of cellular proteins is associated with the functional feature of ribosomes. On them, condensation of activated amino acids occurs and they are laid into a polypeptide chain in accordance with the genetic information transmitted from the nucleus through messenger RNA.

Mitochondria are highly specialized, obligate, tiny organelles of the yeast cell. They are usually distributed evenly between the cell membrane and the vacuole. They can be found not only in resting yeast cells, but also in budding cells at various stages of growth and reproduction, in buds and spores. The monograph by A.V. Kotelnikova and R.A. Zvyagilskaya “Biochemistry of Yeast Mitochondria” examines in detail the structural and functional organization of yeast mitochondria.

The shape and size of yeast mitochondria are inherently variable. Their length in various yeast organisms ranges from 0.2 to 7.5 microns. In shape they can be in the form of small granules, sticks, threads, chains.

Typically, the number of mitochondria in a cell ranges from one to 50. But even if there is only one mitochondria in a cell, it occupies at least 20% of the cell volume. In yeast Sacch. cerevisiae, the volume of these organelles can reach 80% of the volume of the entire cell. At a low concentration of glucose in the medium, the yeast cell contains 100-200 mitochondria, at a high concentration - 30-40.

Mitochondria perform their functions during continuous movement with changes in size and shape, and are characterized by a rapid and specific response to changes in culture conditions and the physiological state of the cell.

Mitochondria are separated from the cytoplasm by an outer two-layer membrane. The inner membrane forms protrusions-cristae, most often of a vesicular-tubular structure. The space between the cristae is filled with a homogeneous substance - the matrix. The mitochondrial outer membrane layer adjacent to the cytoplasm has a rough surface and contains irregular perforations.

The fact of the obligatory presence of mitochondria in the cell structure indicates the localization in them of special processes important in metabolism. Yeast mitochondria consist mainly of lipids - about 30% and proteins - 65-70% of the dry mass of mitochondria, of which about 25-35% are in the form of structural protein.

Yeast mitochondria contain enzymes that provide the main energy function (oxidation of substrates in the Krebs cycle, electron transfer through the respiratory chain and oxidative phosphorylation) - they participate in the complex mechanism of reproduction of mitochondrial DNA, transcription and translation of genetic information, in the biosynthesis of phospholipid sterols, in the activation of fatty acids. acids, etc. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) constitute a quantitatively small structural component of mitochondria.

One of the most significant achievements of molecular biology in recent years is the discovery of specific DNA in mitochondria. Mitochondrial DNA (mDNA) is significantly different from the nuclear component of a cell's DNA.

Vacuoles in yeast cells are an obligatory organelle. In addition to electrolytes dissolved in water, vacuoles contain colloidal proteins, fats, carbohydrates and enzymes. The following accumulate in vacuoles: Na, K, Ca, Mg, Cl, SO4 and PO4 in the form of individual elements and salts, and their concentration is many times higher than the salt content in the environment.

The size and shape of vacuoles in yeast cells are subject to significant changes. In one yeast cell there can be either several of them, or one - the central vacuole.

Vacuoles contain various inclusions both in solution and in the form of crystalline or amorphous granules and drops; pH of vacuole juice 5.9-6.0; the osmotic pressure is a value close to the osmotic pressure of 2.5-3.5% solutions of sodium chloride. The contents of the vacuole are usually optically empty. Only crystalline and lipoid inclusions that are in energetic Brownian motion glow. Active redox processes take place in vacuoles.

The Golgi apparatus (dictyosome) consists of a series of double membranes, concentrically curved. Sometimes round vesicles emerge from the ends of the membranes, which can then turn into large vacuoles. It is believed that the function of the Golgi apparatus is to control the general course of physiological processes.

The nucleus is a permanent structural component of the yeast cell. In living yeast, nuclei can be clearly observed in cells that have spent several days in sterile water, the cytoplasm of which, due to starvation, becomes more homogeneous and transparent, as well as with fluorescence microscopy after treating yeast cells with solutions of fluorochromes and with phase-contrast microscopy.

The yeast nucleus has a shell, a nucleolus (karyosome) and the main contents - karyoplasm. The shell takes part in the regulation of intranuclear processes by changing the permeability and direct communications between the nucleus and the extracellular environment and the nucleus and cytoplasm.

The diameter of the yeast kernel is about 2 microns; most often the kernels have a spherical or elliptical shape. Their location can change during the life of the cell.

The size of the yeast nucleus varies not only in cells of different genera, but also in cells of the same culture under different physiological conditions. Large nuclei with a distinct karyosome are observed in yeast representatives of the genera Saccharomyces, Schizosaccharomyces, Saccharomycodes. In contrast, cells belonging to weakly fermenting and non-fermenting yeasts (Torulopsis pulcherrima, T. utilis) usually have relatively small nuclei. In growing yeast cells, the nuclei become enriched in nucleic acids and often move to active growth sites closer to areas where budding or cell division occurs.

The main chemical ingredient of the nucleus is DNA. The transfer of genetic information from one generation of cells to another is associated with it and only with it. The nucleus also necessarily contains RNA, but usually in smaller quantities than DNA. The nucleus also contains proteins that are not associated with nucleic acids. DNA and RNA molecules determine the synthesis and characteristics of cell proteins. The flow of genetic information is directed from DNA through RNA to protein (DNA->RNA-protein).

0.18-0.17% based on dry biomass. The DNA content in wine yeast cells remains almost unchanged after heat treatment.

The nucleotide composition of DNA depends in a very specific way on the type of organism. However, the number of adenine residues is equal to the number of thymine residues (i.e. A = T), and the number of guanine residues is equal to the number of cytosine residues (G = C), i.e., the number of pyrimidine residues is equal to the number of purine residues (A + G = T + C). Closely related species of yeast have a similar nucleotide composition, but evolutionarily distant organisms differ quite noticeably in this indicator. Therefore, the nucleotide composition of DNA is often used as a taxonomic character.

When cells multiply, structures containing DNA are found in the nuclei - chromosomes, which in yeast can sometimes be seen on fixed preparations when stained. In chromosomes, DNA bound to histone proteins forms nucleoprotein threads about 10 nm thick. For a particular yeast species, the number of chromosomes, their length and shape are constant.

However, data on the number of chromosomes in yeast are very contradictory and have not yet been supported by either the results of electron microscopic studies or genetic data.

Under certain conditions in nature and in experiment, when exposed to various chemical and physical agents, the number of chromosomes in a cell can change. Haploid cells have a single set of chromosomes (p), while diploid cells have a double set of chromosomes (2 p). With an increase in the number of chromosome sets, the DNA content increases and the size of the cell, its nucleus and nucleolus increases. Haploid cells contain exactly half the amount of DNA found in diploid cells.

Intracellular inclusions and reserve substances located in the cytoplasm and vacuoles of cells during their life are morphologically differentiated formations of glycogen granules, metachromatin (volutin), fat-like substances and fat droplets, accumulations of sulfur, crystals of acids and sugars. They are present in the cell in an osmotically inert form and are usually insoluble in water.

Trehalose (a non-reducing disaccharide) and glycogen (a polysaccharide) are contained in the cell within significant limits: trehalose - from 20 to 150 mg, glycogen - from 30 to 100 mg per 1 g of cell dry matter. In yeast grown under aerobic conditions, trehalose predominates, while in yeast grown anaerobically, glycogen predominates. With increasing environmental temperature, the amount of trehalose in cells increases. Glycogen granules appear at the beginning of fermentation and gradually disappear towards the end. According to modern information, glycogen in the yeast Sacch. cerevisiae is localized outside the cytoplasmic membrane in the periplasmic space and is associated with insoluble components of cell walls.

Lipids (fats and fat-like substances), being important components of the cytoplasmic membranes of cell organelles, also serve as reserve nutritional material. Yeast fats are a mixture of true fats (fatty acid glycerides) with phospholipids (lecithin, cephalin) and sterols (ergosterol). Fat droplets in yeast cells are clearly visible in a light microscope due to the fact that they strongly refract light. They are especially visible in filmy yeasts.

Phospholipids, along with sterols, carotenoids and squalenes, promote the growth of yeast and the performance of biosynthetic reactions in anaerobiosis.

Depending on the type and cultivation conditions, yeast is capable of synthesizing from 1 to 74% of lipids based on dry weight. The yeast Rhodotorula gracile has the greatest ability to accumulate lipids in cells. In yeast cells of this genus, the amount of lipids can reach 74% of the dry mass of the cells.

Metachromatin (volutin), which consists mainly of polyphosphates, is in the state of a colloidal solution in vacuoles and is usually indistinguishable without staining or precipitation with fixatives. However, sometimes, as a result of some deviations in cellular metabolism, metachromatin can be observed in the vacuoles, partially precipitated from the solution in the form of spherical granules that refract light well and vigorously brown. Metachromatin is used in the construction of cellular protoplast and as an energy source. Therefore, its amount decreases during active synthetic processes and increases during delayed reproduction.

Sulfur, sometimes contained in the protoplasm of yeast, is detected by the strong refraction of light. The deposition of sulfur in yeast cells leads to the appearance of involutional forms.

The cells of some yeast genera (Rhodotorula, Sporobolo-myces salmonicolor) are colored orange and red, due to the content of pigments belonging to the group of carotenoids. Yeasts of the genus Candida (C. pulcherrima, C. reu-kaufii) on media containing iron form dark red colonies due to the red pyrazine pigment, which includes complex-bound iron.

So, the structure of a yeast cell contains various substances, the changes in which as a result of biochemical processes must be perfectly coordinated with each other.

The transition of yeast cells from aerobic to anaerobic existence (fermentation) is accompanied by enlargement of the nucleus and caryosomes, and a decrease in the content of deoxyribonucleic acid; the surface of mitochondria in relation to the biomass of their substance decreases; Metachromatin and glycogen usually accumulate in fermentative yeast cells.

The structure of yeast cells of different genera is not the same. It has been established that mitochondria, endoplasmic reticulum and cell walls of yeast Sacch. vini and Rhodotorula glutinis differ significantly. Ultrastructural organization of yeast cells Rh. glutinis is more developed and specialized than Sacch yeast. vini. In Rh. glutinis mitochondria are more numerous, with a more ordered internal structure, with a significantly developed and peculiarly constructed reticulum.

Scientists have found that when yeast cells budding, they pass on more mitochondria to their offspring than they keep for themselves.

The impulse to sacrifice everything, even one’s own health, for the benefit of one’s offspring is inherent in various types of living beings, not just humans. Female polar bears starve to death, mother dolphins stop sleeping, and some species of spiders sacrifice themselves to provide food for their offspring.

Science and life // Illustrations

An extraordinary discovery was made by scientists from the University of California, San Francisco (UCSF). It turned out that even yeast has a “parental instinct”, and they can sacrifice themselves so that their offspring can survive. Researchers at UCSF have discovered that baker's yeast (Saccharomyces cerevisiae) passes on most of its mitochondria to its offspring, according to data published in the journal Science. Mitochondria are miniature “power plants” of plant, animal and fungal cells (which include yeast), generating energy for basic biochemical processes.

For a long time it was believed that during mitosis, the process of cell division, all cellular organelles are divided equally. But this does not happen with all cells. Human stem cells, for example, often divide in such a way that the resulting cells both “look” and “behave” differently. The same thing happens with some cancer cells. The process of mitosis in yeast is called budding. Its peculiarity is that during the budding (division) of the mother cell, the offspring receives more mitochondria than remains in the parent cell. “Pumping” of additional mitochondria occurs with the participation of cytoskeletal proteins. Usually, “maternal capital” is enough for 10 divisions; by the 20th, almost all maternal cells die. It is curious that the “daughter” cell itself is smaller in size than the “mother” cell.

Most of all, scientists were surprised by the fact that “mother” yeast passes on mitochondria to its offspring, thereby hastening its own death.

Study leader Wallace Marshall, a doctor of biochemical and biophysical sciences, said that the mother cell will transfer as many mitochondria as the new cells need. “The mother gives everything, receiving nothing as the offspring grow,” he emphasized.

If scientists can control the process of mitochondrial transmission in yeast, it may be another step towards understanding how cancer cells grow.

Illustrations: 1. The budding process of Saccharomyces cerevisiae. 2. Computer image of the yeast mitochondrial network.

Yeast cell wall- a strong, rigid and at the same time elastic structure that performs the same functions as bacteria. Its thickness varies in different types of yeast from 0.15 to 0.28 microns, its mass is 15-30% of the dry weight (DM) of the cell. The chemical composition of the wall includes 60-90% polysaccharides (hemicelluloses, consisting of equal amounts of glucan and manan), 3-10% lipids, 10-25% proteins, 7-9% minerals, 0.5-3% chitin. The yeast wall, as a rule, has a three-layer structure: the first, outer, layer is lipoprotein, the middle is the mannan-protein complex, the inner, adjacent to the cytoplasmic membrane, is glucan, which plays a major role in maintaining the shape of the cell. Enzymes were found in the cell wall - invertase, alkaline and acid phosphatases, lipase, which contribute to the breakdown and oxidation of nutrients entering the cell, as well as glucanase and manase, which are involved in softening the cell wall before the formation of a bud - a daughter cell.

Cytoplasmic membrane of a yeast cell- three-layer structure with a thickness of 0.008-0.010 microns. It fits tightly to the inner cell wall and at the same time is closely connected with the cytoplasm. Its surface is usually smooth, but sometimes there are small protrusions. The cytoplasmic membrane consists of approximately equal amounts of phospholipids, proteins and a small amount of carbohydrates. Performs the following main functions: acts as an osmotic barrier, regulates the entry of nutrients into the cell and the removal of final metabolic products, participates in the regulation of the synthesis of cell wall components, and is the site of localization of many enzymes (redox enzymes, permeases, cofactors) and organelles (ribosomes).

Yeast cell cytoplasm- a thick liquid whose viscosity is 800-8000 times higher than the viscosity of water. Its structure depends on the age of the yeast. In young cells, the cytoplasm is homogeneous; in mature cells, it becomes heterogeneous as a result of the appearance of polysaccharide and polyphosphate granules and small vacuoles; in old cells, heterogeneity increases due to fat droplets and an increase in vacuoles.

The cytoplasm consists of a liquid part (cell sap), membranes and ribosomes. Dissolved in the liquid part are enzymes, transfer RNA (ribonucleic acid), low molecular weight compounds - storage carbohydrates, amino acids, nucleotides, which create an osmotic pressure in the cell of 1.2 MPa. The cytoplasm is penetrated by the endoplasmic reticulum (reticulum). This is a system of double membranes (tubules, vesicles) with a smooth or rough surface, which are in close contact with the CPM, nucleus, outer membrane of mitochondria and other cell structures. Ribosomes are attached to the rough areas of the network. The smooth parts contain enzymes of carbohydrate and lipid metabolism. It is believed that the endoplasmic reticulum is involved in many cellular processes and ensures continuous and orderly delivery of low molecular weight and building substances to the places of their consumption.

Core- this is the most important and permanent structure of the yeast cell and performs genetic, informational and metabolic functions. The yeast nucleus controls the synthesis of proteins, enzymes, lipids, cellular structures and other metabolic processes, and plays a major role in cell reproduction. The nucleus in yeast has a round shape with dimensions of 1-2 microns, surrounded by two membranes in which there are many pores. Through the pores, the necessary nutrients enter the nucleus, and from the nucleus into the cytoplasm - information and transport RNA, ribosome precursors, etc.

In the nucleoplasm of the nucleus (inner part) there is a nucleolus, which plays a certain role in the synthesis of ribosomal substances. A large number of ribosomes are attached to the outer surface of the nuclear shell, indicating a high level of protein synthesis in the immediate vicinity of the nucleus. The main chemical component of the nucleus is deoxyribonucleic acid (DNA). The DNA composition is constant (0.17-0.18% SM). When the cell is at rest, DNA is found in the nucleus in the form of thin chromatin strands. When a cell multiplies, DNA is concentrated in the chromosomes, where it is bound to proteins - histones. Chromosomes are the basic material structures that determine heredity and variability. The number of chromosomes, their length and shape are assumed to be constant for certain yeast species.

Mitochondria- obligatory structures of a eukaryotic cell. Depending on the living conditions of yeast, the shape, size, number and internal structure of mitochondria change. They can be spherical, rod-shaped or filamentous in shape. Dimensions range from 0.2 to 3.0 µm in length and from 0.05 to 1.5 µm in diameter. Mitochondria are surrounded by a double membrane - internal and external. In cells grown under aerobic conditions, the inner membrane forms protrusions - cristae, which penetrate the internal homogeneous content - the matrix. Mitochondria of cells growing under anaerobic conditions are simpler and do not contain cristae.

Mitochondria contain a large number of enzymes: redox enzymes, the tricarboxylic acid cycle, and electron transport. Mitochondria play a major role in the production, accumulation and distribution of energy in the cell and the synthesis of ATP (adenosine triphosphate).

Golgi apparatus consists of membrane disks with a diameter of 0.02 to 0.25 microns. Vesicles emerge from the ends of the membranes, which then turn into lysosomes and vacuoles. It is believed that the Golgi apparatus controls many physiological processes in the cell: it transports various substances to the place of their use or release from the cell, participates in the formation of cell walls, spores, buds, and removes toxic substances from the cell.

Lysosomes- dense granules, bounded by a lipoprotein membrane. They contain active hydrolytic and lysing enzyme systems capable of breaking down large molecules of polysaccharides, proteins, nucleic acids, and fats. Star-shaped lysosomes cover damaged cytoplasmic structures and “digest” them. If the membrane surrounding the lysosomes ruptures, cell autolysis occurs.

Vacuole- an organelle that is located in the central part of the cell and is separated from the cytoplasm by a vacuolar membrane, plays a significant role in the metabolism of the cell. In the vacuole, sodium, potassium, calcium, magnesium, chlorine are present in a dissolved state in the form of ions and salts; hydrolytic enzymes, metachromatin, fats, carbohydrates and proteins are present in the form of colloids. These substances are used by the cell as needed. The size of the vacuole depends on the age and physiological state of the yeast cell. In mature cells, the vacuole increases, and in old cells it reaches its maximum size. It is noted that during the budding process, the vacuole breaks up into small parts, which are distributed between the mother cell and the daughter cell. After completion of the reproduction cycle, small vacuoles merge into one. It is possible that the vacuole is involved in the removal of toxic substances from the cell.

Yeast cell storage substances contained in the cytoplasm and vacuoles are glycogen granules, trehalose sugar, metachromatin and fat droplets.

Glycogen (polysaccharide) is contained in the cell in large quantities: from 30 to 100 mg per 1 g of cell mass. Glycogen granules appear in yeast at the beginning of fermentation when there is an excess of carbohydrate nutrition in the medium; Until the end of fermentation, the amount of glycogen decreases. Entrained glycogen in the cytoplasm, however, there is an assumption that it accumulates in the space between the CPM and the cell wall.

Under aerobic conditions, trehalose predominates in yeast cells, the amount of which can reach 20-150 mg per 1 g of cell mass. The vacuole contains a reserve substance, metachromatin, consisting of lipoproteins, polyphosphates, magnesium and RNA. The content of metachromatin decreases during active cell growth and increases when reproduction is delayed.

Fat in the cytoplasm and vacuole, which acts as a reserve nutrient material, is a mixture of triglycerides with phospholipids and sterols (ergosterol). Fat droplets in old cells are visible in a light microscope due to the fact that they strongly refract light.

Also, in addition to melanins, many typically terrestrial fungi are characterized by the presence of carotenoid pigments, which perform the same function of protection from solar insolation. Typical representatives of intensely pigmented fungi with a high content of carotenoids are many widespread basidiomycete yeasts from the genera Rhodotorula, Sporobolomyces, Lipomyces and etc.

Ascomycete yeast Metschnikowia pulcherrima form the red-cherry pigment pulherrimin, which diffuses into the medium. The so-called “black yeast” forms dark brown or black colonies due to the accumulation of melanoid pigments.

Drawing - Lipomyces starkey– soil yeast. Cells in a 7-day culture on wort agar.

Soil yeast genus Lipomyces– lipomycetes are distributed exclusively in soils and are not found in other habitats. Their ability to survive well in mineral soil horizons poor in organic matter has long been known. However, lipomycetes are not oligotrophs; in laboratory cultures they are not capable of growth on diluted nutrient media. A distinctive feature of these yeasts is their very large cells; in rich media they quickly accumulate a large amount of reserve lipids - up to 80% of the total cell mass (Fig.). Apparently, the survival strategy of lipomycetes in the soil is the rapid accumulation of reserve substances when accessible substrates, such as fresh litter, enter, and then their long-term consumption in “waiting” for new arrivals.

Literature

Babeva, I.P., Gorin, S.E. Soil yeast. M.: Moscow State University Publishing House, 1987. – P. 50–63.
Kudryavtsev, A.A., Gurevich, G.A., Fichte, B.A. Mechanical properties of microbial membranes. – Pushchino, 1988.
Kates M. Lipidology techniques. M.: Mir, 1975. – 322 p.
Revis, D.D., Botstein, J. Roth “Genetics of bacteria” M., Mir 1984. – P. 101.
Rudic, V., Popova, N., Crivova, A. et al. “Biosynthesis of Lipoxygenase, Lipids and its Fatty Acid Composition of Actinomycetes and Yeast”, Bucureşti, Roumanian Biotechnological Letters. – 2002. – V. 7. – N 3. – R. 711-716.
Rudic, V., Topală, L. Teza de doctorat şi autoreferatul pot fi consultate la biblioteca Academiei de Ştiinţe a Moldovei – bd. Ştefancel Mare, 1, MD – 2001, Chişinău.

Oxidative phosphorylation in bacteria

In prokaryotic cells capable of oxidative phosphorylation, the elements of the tricarboxylic acid cycle are localized directly in the cytoplasm, and the respiratory chain and phosphorylation enzymes are associated with the cell plasma membrane. This was first shown by cytochemical methods. Thus, the enzyme succinate dehydrogenase is associated with the plasma membrane and with its protrusions protruding into the cytoplasm, with the so-called mesosomes (Fig. 212). It should be noted that such bacterial mesosomes can be associated not only with the processes of aerobic respiration, but also in some species participate in cell division, in the process of distributing DNA among new cells, in the formation of a cell wall, etc. Coupling factors for oxidation and ATP synthesis are also localized on the plasma membrane in the mesosomes of some bacteria. In an electron microscope, spherical particles similar to those found in the mitochondria of eukaryotic cells were found in fractions of bacterial plasma membranes. Thus, in bacterial cells capable of oxidative phosphorylation, the plasma membrane plays a role similar to the inner membrane of the mitochondria of eukaryotic cells.

Just like other cytoplasmic organelles, mitochondria can increase in number, which is especially noticeable during cell division or when the functional load of the cell increases; moreover, mitochondria are constantly renewed. Thus, in the liver, the average lifespan of mitochondria is about 10 days. Therefore, the question naturally arises of how this increase in the number of mitochondria occurs, due to what processes and what structures new mitochondria are formed.

The bulk of experimental data suggests that the increase in the number of mitochondria occurs through the growth and division of preceding mitochondria. This assumption was first made by Altman (1893), who described mitochondria under the term “bioblasts.” Later, with the help of time-lapse filming, it was possible to observe intravital division and fragmentation of long mitochondria into shorter ones. This process is especially clearly visible during cell division of some unicellular algae and lower fungi, in which mitochondrial division is coordinated with cell division. With an electron microscope, one can often see the division of mitochondria through the formation of a constriction in many cells (Fig. 213), for example, in liver cells (although without evidence of the dynamics of this process, such observations are not very convincing). Outwardly, all these pictures are very reminiscent of the binary method of bacterial division.

The reality of increasing the number of mitochondria by fission was proven by studying the behavior of mitochondria in living tissue culture cells. It was found that during the cell cycle, mitochondria can grow to several microns and then fragment and divide into smaller bodies.

In addition, mitochondria can fuse with each other. Thus, in a culture of endothelial cells of the heart of a xenopus tadpole, up to 40 cases of mitochondrial fusion and fission were observed in 1 hour. In embryonic kidney culture cells, growth and branching of mitochondria were observed in the S-period of the cell cycle. However, already in the G 2 period, small mitochondria formed due to fission during the fragmentation of long mitochondria predominated.

Thus, the reproduction of mitochondria proceeds according to the principle: omnis mitochondrion e mitochondrion.

It is interesting to observe the fate of mitochondria in yeast cells. Under aerobic conditions, yeast cells have typical mitochondria with clearly defined cristae. When cells are transferred to anaerobic conditions (for example, when they are subcultured or when transferred to a nitrogen atmosphere), typical mitochondria are not detected in their cytoplasm, and small membrane vesicles are visible instead. It turned out that under anaerobic conditions, yeast cells do not contain a complete respiratory chain (they lack cytochrome b and a). When the culture is aerated, there is a rapid induction of the biosynthesis of respiratory enzymes, a sharp increase in oxygen consumption, and normal mitochondria appear in the cytoplasm. These observations led to the idea that in yeast, under anaerobic conditions, promitochondrial structures with a reduced oxidation system exist in the cytoplasm. Such promitochondria, when cells are transferred to an aerobic environment, begin to rearrange themselves; elements of the complete chain of oxidation and phosphorylation are included in their membranes, which is accompanied by a change in their morphology. Thus, from primitive, inactive promitochondria, ordinary functioning mitochondria are formed through their completion and growth.

Probably, similar processes occur during the division of mitochondria: an increase in the mass of mitochondrial membranes with all specific components occurs due to the synthesis and inclusion of individual proteins - enzymes and lipids, an increase in the mass of matrix proteins, and then the division of the structure, as if doubled or multiplied, occurs.

These ideas are supported by facts concerning the organization and composition of the mitochondrial matrix or mitoplasm, in which DNA, various types of RNA and ribosomes are found.

Research in recent years has led to surprising discoveries: double-membrane organelles have a complete autoreproduction system. This system is complete in the sense that DNA is open in mitochondria and plastids, on which informational, transfer and ribosomal RNAs and ribosomes that carry out the synthesis of mitochondrial and plastid proteins are synthesized. However, as it turned out, these systems, although autonomous, are very limited in their capabilities.

DNA in mitochondria is represented by cyclic molecules that do not form bonds with histones; in this respect, they resemble bacterial chromosomes. Their size is small, about 7 microns; one cyclic molecule of animal mitochondria contains 16-19 thousand. DNA nucleotide pairs. In humans, mitochondrial DNA contains 16.5 thousand bp, it is completely deciphered. It was found that the mitochondrial DNA of various objects is very homogeneous; their difference lies only in the size of introns and non-transcribed regions. All mitochondrial DNA is represented by multiple copies, collected in groups or clusters. Thus, one rat liver mitochondria can contain from 1 to 50 cyclic DNA molecules. The total amount of mitochondrial DNA per cell is about one percent. Mitochondrial DNA synthesis is not associated with DNA synthesis in the nucleus.

Just like in bacteria, mitochondrial DNA is collected in a separate zone - the nucleoid, its size is about 0.4 microns in diameter. Long mitochondria can have from 1 to 10 nucleoids. When a long mitochondrion divides, a section containing a nucleoid is separated from it (similar to the binary fission of bacteria). The amount of DNA in individual mitochondrial nucleoids can fluctuate up to 10-fold depending on the cell type.

In vivo, mitochondrial nucleoids can be stained with special fluorochromes. It turned out that in some cell cultures, from 6 to 60% of mitochondria do not have a nucleoid, which may be explained by the fact that the division of these organelles is more likely associated with fragmentation rather than with the distribution of nucleoids.

As already mentioned, mitochondria can both divide and merge with each other. In normal human Hela cell culture, all mitochondria contain nucleoids. However, one of the mutant lines of this culture contained mitochondria in which nucleoids were not detected using fluorochromes. But if these mutant cells are fused with the cytoplasts of cells of the original type, then nucleoids were found in all mitochondria. This suggests that when mitochondria fuse with each other, an exchange of their internal components can occur.

It is important to emphasize that the rRNA and ribosomes of mitochondria are sharply different from those in the cytoplasm. If 80s ribosomes are found in the cytoplasm, then the ribosomes of plant cell mitochondria belong to 70s ribosomes (consist of 30s and 50s subunits, contain 16s and 23s RNA, characteristic of prokaryotic cells), and smaller ribosomes (about 50s) are found in the mitochondria of animal cells.

Mitochondrial ribosomal RNA is synthesized on mitochondrial DNA. In mitoplasm, protein synthesis occurs on ribosomes. It stops, in contrast to synthesis on cytoplasmic ribosomes, under the action of the antibiotic chloramphenicol, which suppresses protein synthesis in bacteria.

Transfer RNAs are also synthesized on the mitochondrial genome; a total of 22 tRNAs are synthesized. The triplet code of the mitochondrial synthetic system is different from that used in the hyaloplasm. Despite the presence of seemingly all the components necessary for protein synthesis, small mitochondrial DNA molecules cannot encode all mitochondrial proteins, only a small part of them. So DNA is 15 thousand bp in size. can encode proteins with a total molecular weight of about 6x10 5. At the same time, the total molecular weight of the proteins of the particle of the complete respiratory ensemble of the mitochondria reaches a value of about 2x10 6. If we consider that in addition to proteins of oxidative phosphorylation, mitochondria include enzymes of the tricarboxylic acid cycle, enzymes of DNA and RNA synthesis, amino acid activation enzymes and other proteins, it is clear that in order to encode these numerous proteins and rRNA and tRNA, the amount of genetic information in the short molecule of mitochondrial DNA is clearly lacking. Deciphering the nucleotide sequence of human mitochondrial DNA showed that it encodes only 2 ribosomal RNAs, 22 transfer RNAs and a total of 13 different polypeptide chains.

There is now convincing evidence that most mitochondrial proteins are under genetic control from the cell nucleus and are synthesized outside the mitochondria. Thus, in particular, cytochrome c is formed in the hyaloplasm, and of the nine polypeptide chains in the ATP synthetase, only one is synthesized in the matrix of animal mitochondria. Mitochondrial DNA encodes only a few mitochondrial proteins, which are localized in membranes and are structural proteins responsible for the correct integration of individual functional components in mitochondrial membranes.

Most mitochondrial proteins are synthesized on ribosomes in the cytosol. These proteins have special signal sequences that are recognized by receptors on the outer membrane of mitochondria. These proteins can be incorporated into them (see the analogy with the peroxisome membrane) and then move to the inner membrane. This transfer occurs at the points of contact between the outer and inner membranes, where such transport is noted (Fig. 214). Most mitochondrial lipids are also synthesized in the cytoplasm.

All these discoveries, showing the relatively independent structure and functioning of the mitochondrial protein synthesis system, revived the hypothesis about the endosymbiotic origin of mitochondria, that mitochondria are organisms such as bacteria that are in symbiosis with a eukaryotic cell.

In yeast mitochondrial DNA, only 2 ribosomal RNA genes and only 1 ribosomal protein gene were found. This protein is located in the small subunit of the ribosome. The ribosomal protein gene is quite variable in size even among different strains, which is why it received the name variable ( Var l). The remaining proteins and RNA of mitochondrial ribosomes are encoded by nuclear genes. 24 transfer RNA genes ensure the transport of all amino acids to the site of protein synthesis, and only one transfer RNA, transporting lysine, is imported from the cytoplasm and encoded by the nucleus. All transfer RNAs of yeast mitochondria are encoded by the same DNA strand, and only one of them is encoded by the opposite strand. None of the transport DNA genes have introns. Cytochrome b protein genes and cytochrome C protein genes can have many introns - from 5 to 9.

From the data presented it follows that the structural proteins encoded by the mitochondrial genome of yeast are clearly insufficient for the functioning of these organelles and most of them are encoded by the nuclear genome.

Characteristic features of the organization and expression of the mitochondrial genome of fungi:

1. significant diversity in the sets and arrangement of mitochondrial genes in different species;

a wide variety of ways to organize genetic material - from the compact organization of the genome to the free distribution of genes along mtDNA with extended non-coding sequences between genes;

3. mosaic structure of a number of genes;
4. significant intraspecific variations in mtDNA size associated with the presence of “optional” introns;
5. the ability of individual mtDNA segments to be excised and amplified with the formation of a defective mitochondrial genome;
6. the presence of one or more ORIs, in each of which replication is initiated bidirectionally;
7. location of all mitochondrial genes on one strand of mtDNA and asymmetric transcription of mtDNA;
8. multiplicity of mtDNA transcription units;
9. a variety of signals for processing primary transcripts, which can be either tRNA or oligonucleotide blocks of another type - depending on the species;
10. In most cases, mRNAs contain extended terminal non-coding sequences.

The most complex organization of the mitochondrial genome is in higher plants. Their mitochondrial genome is a set of supercoiled double-stranded circular and/or linear molecules. All mitochondrial genome sequences can be organized into one large circular "chromosome", and the observed different size classes of mitochondrial DNA are most likely the result of recombination processes. At least on spinach, species of two genera Brassica And Raphanus, sugar beets and wheat, it was shown that the reason for such dispersion of the mitochondrial genome is the recombination of homologous regions of mitochondrial DNA. Due to the presence of directly oriented two or three families of repeats ranging in size from 1 to 14 kb, mitochondrial DNA molecules are capable of active inter- and intragenomic rearrangements. As a result of such rearrangements, mitochondrial DNA can be present in the form of molecules of various size classes.

So, for example, in cruciferous Brassica campestris Mitochondrial DNA is present in the form of three types of circular molecules. The first type contains the complete genome - 218 kb, the second - 135 and the third - 83 kb. Subgenomic rings are formed as a result of recombination of genomic rings having a pair of direct repeats 2 kb in length.

In wheat, the size of the mitochondrial genome is much larger - 430 kb, and there are more than 10 direct recombination repeats, as a result, during electron microscopic observation, many rings of various sizes can be seen, but no one has observed one large circular molecule, perhaps in this state, the wheat mitochondrial genome is never present. In Marchantia moss and other cruciferous Brassica hirta There are no direct recombination repeats and, perhaps, this is why mitochondrial DNA is in the form of circular molecules of the same size class. However, for mitochondrial DNA of higher plants this is the exception rather than the rule. In most higher plants, the mitochondrial genome contains both recombination repeats and mitochondrial DNA molecules of various size classes.

The number of molecules of the same size class can vary greatly in different plant tissues, depending on the state of the plant and environmental conditions. A change in the numerical ratios of mitochondrial DNA molecules of different size classes was noted during plant cultivation in vivo And in vitro. Perhaps changes in the numerical relationships between molecules of different size classes reflect the adaptability of plants through increased amplification of the desired genes.

In addition, the mitochondrial genome may contain plasmids, both linear and circular, with both DNA and RNA sequences, ranging in size from 1 to 30 kb. Mitochondrial plasmids likely originated from other cellular genomes or even other organisms. Sometimes their presence or absence can be associated with cytoplasmic male sterility of plants, but, however, not always. Plasmids are present in some species, but sterility is not observed. In at least one case, it was clearly demonstrated that in the mitochondria of lines with the so-called S-type of maize sterility, a correlation was found between the presence of plasmid-like mitochondrial DNA and the manifestation of the phenomenon of cytoplasmic male sterility. The ability of mitochondrial plasmids to integrate into both the mitochondrial genome and nuclear chromosomes was noted. However, in other cases, the presence of plasmid DNA does not always cause pollen sterility.

The size of the mitochondrial genome of plants is most variable - from 200 to 2500 kb. The size of the mitochondrial genome of higher plants is larger than the size of their chloroplast genome.

Significant variation in the size of the mitochondrial genome is the second feature of the plant mitochondrial genome. The genome is not only very large, but can also be different, even among closely related species, and in some cases low variability can be observed - species of the genus Brassica, in others it is very large. The highest size variability is observed in pumpkin plants. Within this family, the size of the mitochondrial genome is most variable - from 330 kb. in watermelon up to 2500 kb. at the melon. Therefore, the share of mitochondrial DNA in the total volume of the plant genome can also vary significantly - about 1% in most plants, up to 15% in melon hypocotyl cells.

Various reasons have been attempted to explain the presence of large mitochondrial genomes.

The presence of additional genes or special sequences necessary for the functioning of mitochondria.

The presence of DNA that is used by the plant, but not as a coding one, but for some other function.

DNA that is not used for mitochondrial functioning is called “selfish” DNA.

Apparently, there is another possibility for increasing the size of the mitochondrial genome - these are sequences homologous to nuclear and chloroplast DNA. Sequences homologous to nuclear DNA, for example, in Arabidopsis account for up to 5% of the mitochondrial genome. Initially, the chloroplast genome sequence incorporated into the mitochondrial genome was discovered in maize. It included a region of about 14 kb containing altered chloroplast 16S-ribosomal RNA genes and a region of the large subunit RDPK/O. Subsequently, chloroplast insertions were discovered in the mitochondrial genome of many higher plant species. Typically, they account for 1-2% of mitochondrial sequences and include three major sequences.

The sequence is 12 kb long. from a reverse repeat of chloroplast DNA. It contains sequences for the 3" exon of four transfer RNAs and sequence 16 S ribosomal RNA.

A 1.9 to 2.7 kb sequence that completely encodes the large subunit of Rubisco.

Sequence no longer than 2 kb. In the chloroplast genome, this region encodes the 3" end of the 23S ribosomal RNA, 4.5S and 5S rRNA, as well as three transfer RNAs. Of all the chloroplast genome sequences that are present in the plant mitochondrial genome, only the transfer RNA sequences are actually transcribed .

Since the same chloroplast sequences are present in the mitochondrial genome of many plant species, it can be assumed that they have some functional significance. At the same time, their role, the mechanism of transfer and the timing of this transfer remain unknown. Did this transfer occur at a distant time in the evolution of the formation of a eukaryotic cell, or did the presence of chloroplast insertions in the mitochondrial genome indicate that this is a normal process of information exchange between organelles, which occurs now, or does it occur periodically in the relatively recent evolutionary time of the formation of specific species and plant genera?

In addition, some of the mitochondrial genome sequences are sequences homologous to viral ones.

To establish the number of genes in the genome of plant mitochondria that actually function, a number of researchers determined the number of translation products. It was shown that the number of detectable protein bands was the same even for plants with 10-fold differences in genome size. Although the methods used do not directly answer the question about the total number of genes in the mitochondrial genome, it is nevertheless interesting that the same number of translation products was identified in the analyzed angiosperm species and was close to the number of genes encoding proteins in animal and mitochondrial mitochondria. yeast.

For the first time, the complete nucleotide sequence of mitochondrial DNA in plants was determined in 1986 in one species - Marchantia ( Marchantia polymorpha), and later in Arabidopsis and several species of algae.

The mitochondrial DNA molecule in Marchantia has a size of 186,608 bp. It encodes genes for 3 rRNAs, 29 genes for 27 tRNAs and 30 genes for known functional proteins (16 ribosomal proteins, 3 subunits of cytochrome C oxidase, cytochrome b, 4 subunits of ATP synthetase and 9 subunits of NADH dehydrogenase). The genome also contains 32 unidentified open reading frames. In addition, 32 introns were found located in 16 genes. The number of genes for a particular complex may vary in different plants, since one or more genes of this complex may be transferred to the nucleus. Among the unidentified genes, at least 10 are consistently found in almost all plant species, indicating the importance of their functions.

The number of mitochondrial genes encoding transfer RNAs of plant mitochondria is highly variable. In many species, their own mitochondrial transfer RNAs are clearly insufficient and are therefore exported from the cytoplasm (encoded by the nucleus or plastid genome). For example, in Arabidopsis, 12 transfer RNAs are mitochondrial encoded, 6 are chloroplast and 13 are nuclear; in Marchantia, 29 are mitochondrial and 2 are nuclear, and none of the transport RNAs have chloroplast coding; in potatoes, 25 are mitochondrial, 5 are chloroplast and 11 are nuclear; in wheat, 9 are mitochondrial, 6 are chloroplast and 3 are nuclear (Table 3).

Unlike animal mitochondrial DNA and chloroplast genes, plant mitochondrial DNA genes are dispersed throughout the genome. This applies to both genes encoding transfer RNAs and genes encoding proteins.

Table 3

The nature of mitochondrial transfer RNAs in plants

Like the genome of fungal mitochondria, the genome of plant mitochondria has introns that the genomes of animal mitochondria do not have.

In some species, a number of genes in the genome are duplicated. Thus, in corn and broad beans, rRNA genes are not repeated, but in wheat they are repeated several times. Genes encoding mitochondrial proteins may also be repeated in their genome.

Naturally, mitochondria, like chloroplasts, contain much more enzyme proteins than their genome of genes. And, therefore, most proteins are controlled by the nuclear genome, assembled in the cytoplasm on cytoplasmic rather than mitochondrial ribosomes, and transported into mitochondrial membranes.

Thus, the mitochondrial genome of plants is an extremely variable system in structure, but quite stable in the number of genes. In contrast to the compact genome of chloroplasts, in the mitochondrial genome of plants, genes make up less than 20% of the genome. The enlargement of the mitochondrial genome compared to fungi or animals is caused by the presence of introns, various repeating sequences, insertions from the genome of chloroplasts, the nucleus and viruses. The functions of approximately 50% of the plant mitochondrial genome have not yet been elucidated. In addition to the fact that many structural genes that control the function of mitochondria are located in the nucleus, many genes that control the processes of transcription, processing, and translation of mitochondrial genes are also located there. Consequently, mitochondria are even less autonomous organelles than plastids.