Eukaryotic cell, main structural components, their structure and functions: organelles, cytoplasm, inclusions. Cellular center: functions and structure, distribution of genetic information

Cytoplasm is the internal contents of the cell, enclosed between the plasma membrane and the nucleus. It consists of a basic substance, or hyaloplasma, and those in it organoids And inclusions.

Hyaloplasm (cytosol) is an active metabolic environment; many chemical and physiological processes take place in it; it unites all components of the cell into a single system. It is an aqueous solution of inorganic and organic substances, capable of changing its viscosity and being in constant motion. The ability to move, or flow, the cytoplasm is called cyclosis. IN During the process of cyclosis, the movement of substances and structures located in the cytoplasm occurs.

Organoids - permanent cytoplasmic structures of cells that have a specific structure and perform vital functions. To membrane organelles include the endoplasmic reticulum, lamellar Golgi complex, isosomes, peroxisomes, mitochondria and plastids. Mandatory for most cells are also organelles that do not have a membrane structure. TO These include ribosomes, microfilaments, microtubules, cell center, centrioles, basal bodies, flagella, cilia.

The genetic control of eukaryotic cells includes: core, which contains the majority of the DNA molecules of eukaryotic cells (a small part of the DNA is contained in mitochondria and plastids); ribosomes, which use nucleic acid information to synthesize proteins. Proteins control metabolism and determine the specialization of cells in a multicellular organism.

Most cells have one nucleus, but multinucleated cells are also found (in a number of protozoa, in the skeletal muscles of vertebrates). Some highly specialized cells lose their nuclei (erythrocytes in mammals and sieve tube cells in angiosperms). The nucleus, as a rule, has a spherical or oval shape, less often it can be segmented or fusiform. IN the composition of the core includes nuclear envelope And nucleoplasm(karyoplasm) containing chromatin(chromosomes).

The nuclear envelope is formed by outer and inner membranes and contains numerous pores through which various substances are exchanged between the nucleus and the cytoplasm.

Nucleoplasm is a jelly-like solution containing various proteins, nucleotides, ions, as well as chromatin and the nucleolus.

The nucleolus is a small round body, intensely stained and found in the nuclei of nondividing cells. The function of the nucleolus is the synthesis of rRNA and its connection with proteins, i.e. assembly of ribosomal subunits.

Chromatin is clumps, granules and filamentous structures formed by DNA molecules in complex with proteins that are specifically stained with certain dyes. Different sections of DNA molecules within chromatin have different degrees of helicity, and therefore differ in color intensity and the nature of genetic activity. Fragments designated as euchromatic, characterized by lower packing density. They contain genetic information and can be transcribed (encode RNA synthesis). Heterochromatic chromosome fragments are characterized by denser packing. Genetically, they are inert and are not transcribed. Chromatin is a form of existence of genetic material in non-dividing cells and provides the possibility of doubling and implementing the information contained in it.

During cell division, DNA spiralization occurs and chromatin structures form chromosomes. Chromosomes- dense, intensely stained structures that are units of structural organization of genetic material and ensure its precise distribution during cell division. Chromosomes are best seen (and studied) during the metaphase stage of mitosis. Each metaphase chromosome consists of two chromatid(strongly spiraled identical DNA molecules formed as a result of replication). The chromatids are connected to each other in the region of the primary constriction, or centromeres. The centromere divides the chromosome into two arms. Depending on the location of the centromere, there are equal-armed (metacentric), unequal-armed (submetacentric) and rod-shaped (telocentric) chromosomes (see Fig. 2.4). Some chromosomes have secondary constrictions that separate satellites (acrocentric with satellite). Secondary constrictions of a number of chromosomes participate in the formation of the nucleolus and contain ribosomal genes.

Rice. 2.4.

A- metacyclic (equal-armed); b- submetacentric (unequal-armed); V- acrocentric (rod-shaped); g - chromosome with satellite

A set of chromosomes of cells of a particular type of organism, characterized by the number, size and shape of chromosomes, is called a karyotype(Fig. 2.5). In the karyotype of somatic cells, paired chromosomes are called homologous, chromosomes from different pairs - non-homologous. Homologous chromosomes are identical in size, shape, composition and order of genes (one is inherited from the paternal organism, the other from the maternal organism). Chromosomes within a karyotype are also divided into autosomes, are the same in males and females, and sexual chromosomes involved in sex determination and differing between males and females. In humans, the karyotype of somatic cells consists of 46 chromosomes (23 pairs): 44 autosomes and 2 sex chromosomes (a woman has 2 homologous X chromosomes, a man has X and Y chromosomes, which have non-homologous and homologous regions). Chromosomes of karyotypes of organisms of different species differ in number, size and shape. In germ cells, the chromosomes are unpaired (due to meiosis, the gamete contains one chromosome from each pair). A single set of chromosomes in germ cells is called haploid (n), set of chromosomes in somatic cells - diploid (2p).

Rice. 2.5.A- stash; 6 - mosquito; V- chicken; G- green algae; d- salmon; e- locusts; and- Drosophila

Ribosomes are found in pro- and eukaryotic cells. Ribosomes are spherical bodies that consist of big And small subunits. They contain approximately equal amounts of rRNA and protein by mass. Ribosomes are located either freely in the cytoplasm or on the surface of the membranes of the endoplasmic reticulum. Mitochondria and cell plastids also contain ribosomes. The function of ribosomes is to assemble protein molecules based on information from mRNA (see Chapter 3).

The intracellular membrane system performs a variety of functions in eukaryotic cells. Membranes of different organelles can have direct transitions (endoplasmic reticulum, Golgi complex, nuclear membrane) or communicate through membrane sacs (vesicles). The intracellular membrane system includes the nuclear envelope, endoplasmic reticulum, Golgi complex, lysosomes, vacuoles and plasma membrane. The latter cannot be attributed to intracellular membranes in terms of localization, but is nevertheless associated with the endoplasmic reticulum and other internal membranes.

The endoplasmic reticulum (ER) is a branched network of membranes that permeates the entire cytoplasm of the cell, connecting to the perinuclear space and the cavities of the Golgi complex. The endoplasmic reticulum forms a system of interconnected channels, cisterns, tubes and vesicles, the cavities of which are delimited from the hyaloplasm by membranes. There are two types of endoplasmic reticulum: rough And smooth. Ribosomes are located on the membranes of the rough (granular) endoplasmic reticulum. Some of the proteins they synthesize are included in the membrane of the endoplasmic reticulum, others enter the lumen of its channels, where they are converted and transported to the Golgi apparatus.

The membranes of the smooth (agranular) endoplasmic reticulum are involved in cell metabolism, lipid synthesis, carbohydrate metabolism, neutralization of toxic products, and also carry out transport within the cell.

The Golgi complex consists of a stack of flattened disc-shaped membrane cavities and the vesicles formed from them (lysosomes and vacuoles). Proteins and lipids entering the cavity of the Golgi complex undergo various transformations, accumulate, sort, package into secretory vesicles and are transported to various intracellular structures or outside the cell. The membranes of the Golgi complex are also capable of synthesizing polysaccharides and forming lysosomes.

Lysosomes are formed in the Golgi complex and perform the function of intracellular digestion of macromolecules and foreign components entering the cell during phago- and pinocytosis and provide the cell with additional raw materials for chemical and energy processes. During starvation, lysosome cells digest some organelles and replenish their supply of nutrients for a while. During the development process in animals, the death of individual cells and even organs often occurs, which occurs with the indispensable participation of lysosomes. To carry out these functions, lysosomes contain hydrolytic enzymes that destroy proteins, nucleic acids, lipids, carbohydrates, etc. There are primary and secondary lysosomes. Primary lysosomes separated from the cavities of the Golgi complex in the form of microbubbles, surrounded by a single membrane and containing a set of enzymes. After the fusion of primary lysosomes with some substrate that is subject to cleavage, various secondary lysosomes. An example of secondary lysosomes are the digestive vacuoles of protozoa.

Peroxisomes are formed in the smooth ER and are spherical structures covered with a membrane. They contain enzymes that neutralize toxic products of lipid peroxidation and some toxic substances.

Eukaryotic cells also have organelles isolated from the hyaloplasm by two membranes. Mitochondria and plastids transform energy in cells from one type to another. According to the symbiotic hypothesis about the origin of eukaryotic cells, they are descendants of ancient prokaryotic symbiont cells - bacteria and blue-green algae. These organelles are called semi-autonomous because they have their own protein biosynthesis apparatus (DNA, ribosomes, RNA, enzymes) and synthesize part of the proteins that function in them.

Mitochondria have very variable sizes and shapes (rod-shaped, oval, round). Externally, mitochondria are bounded by an outer membrane. The inner membrane of mitochondria forms numerous cristae (outgrowths) and contains numerous enzymes involved in the conversion of food energy into adenosine triphosphate (ATP) energy. Some special biosyntheses also occur in mitochondria (steroid hormones in the cells of the adrenal cortex, bile acids in liver cells). Between the cristae of mitochondria there is a matrix containing circular DNA, different types of RNA, and ribosomes. Mitochondria are capable of synthesizing a small number of proteins involved in the processes of ATP synthesis. The main part of the necessary proteins is encoded by nuclear DNA and, after assembly on ribosomes, is transported to mitochondria.

Plastids are organelles found in the cells of photosynthetic eukaryotic organisms. Depending on the color, there are three main types: chloroplasts, chromoplasts And leukoplasts. Chloroplasts are characterized by an oval or disc-shaped shape and are covered with an outer membrane. The inner membrane of chloroplasts forms flattened membrane sacs - thylakoids, stacked - gran. IN The thylakoid membranes contain chlorophyll, which gives the chloroplast a green color and ensures the light phase of photosynthesis. The liquid content of the chloroplast, which is not part of the thylakoids, is called stroma. It contains DNA, ribosomes and various enzymes involved in the dark phase of photosynthesis. Chromoplasts are simpler in structure, have no granules, are not capable of photosynthesis, and contain a variety of pigments: yellow, orange and red. They give bright colors to flowers and fruits, attracting animals and thus facilitating plant pollination and seed dispersal. Leukoplasts are almost devoid of thylakoids; the pigments in them are in an inactive form (protochlorophylls). Leukoplasts are colorless, contained in the cells of underground or uncolored parts of plants (roots, rhizomes, tubers). Capable of accumulating reserve nutrients, primarily starch, sometimes proteins, less often fats. In the light they can turn into chloroplasts (for example, during the germination of potato tubers).

The cytoplasm of eukaryotic cells is penetrated by a network of fibrillar (filamentous) formations that form the cytoskeleton of cells, which plays an important role in organizing the structure of cells, as well as in ensuring their activity.

Microtubules And microfilaments- thread-like structures consisting of various contractile proteins and determining the motor functions of the cell. Microtubules look like long hollow cylinders, the walls of which consist of proteins - tubulins. Microfilaments are very thin, long, thread-like structures composed of actin and myosin. Microtubules and microfilaments penetrate the entire cytoplasm of the cell, forming its cytoskeleton, causing the flow of cytoplasm (cyclosis), intracellular movements of organelles, divergence of chromosomes during the division of nuclear material, etc. In addition to free microtubules that penetrate the cytoplasm, cells have microtubules organized in a certain way that form centrioles cell center, basal bodies, cilia And flagella.

Cell center usually located near the nucleus, consists of two centrioles located perpendicular to each other. The centriole has the shape of a flat cylinder, the wall of which is formed by nine triplets of microtubules (9x3). Centrioles of the cell center are involved in the formation of the mitotic spindle of the cell.

Flagella and cilia- these are organelles of movement, which are peculiar outgrowths of the cytoplasm of some cells. The skeleton of a flagellum or cilium has the form of a cylinder, along the perimeter of which there are nine paired microtubules, and in the center two - single 9 (9 x 2 + 2).

During the process of evolution, different cells adapted to live in different conditions and perform specific functions. This required the presence of special organelles in them, which are called specialized Unlike organelles of general importance. Specialized organelles include contractile vacuoles of protozoa, muscle fiber myofibrils, neurofibrils and synaptic vesicles of nerve cells, microvilli of intestinal epithelial cells, cilia and flagella of some protozoa, etc.

Inclusions -relatively unstable cytoplasmic structures of cells found in certain types of cells at certain moments of life, for example, as a reserve of nutrients (starch grains, proteins, drops of glycogen) or products to be removed from the cell (secretion granules), etc.

The cytoplasm is represented by the main substance (matrix, hyaloplasm), in which inclusions and organelles are distributed. The main substance of the cytoplasm fills the space between the plasmalemma, nuclear membrane, organelles and other structures. Even an electron microscope does not reveal any internal organization in it. It is represented by a variety of organic and inorganic substances dissolved in water, including enzymes and other proteins. Precursors and intermediate products of many biochemical cycles are concentrated in the main substance of the cytoplasm. Glycolysis occurs in it, which plays an important role in the formation of energy flow.

Inclusions are relatively unstable components of the cytoplasm that serve as reserve nutrients (starch, glycogen), products to be released from the cell (secretion granules), and ballast substances (some pigments). Organelles are permanent structures of the cytoplasm that perform certain functions in the cell.

Organelles of general importance and special ones are distinguished. The latter are found in most cells, but are present in significant quantities only in cells specialized to perform a specific function. These include microvilli of the absorptive surface of intestinal epithelial cells, cilia of the epithelium of the trachea and bronchi. Such special organelles as synaptic vesicles, transporting mediators-carriers of nervous excitation from one neurocyte to another or a working organ cell, as well as myofibrils, ensuring the act of muscle contraction, are present only in cells of a certain functional specialization. A detailed examination of special organelles is part of the histology course.

Organelles of general importance include elements of the tubular and vacuolar system of the cytoplasm in the form of a rough and smooth cytoplasmic reticulum, a lamellar complex, mitochondria, ribosomes and polysomes, lysosomes, peroxisomes, microfibrils and microtubules, centrioles of the cell center. Plant cells also contain chloroplasts, which carry out photosynthesis.

The tubular and vacuolar system is formed by communicating or isolated tubular and flattened cisterns, bounded by a membrane and spreading throughout the cytoplasm of the cell. Often tanks have bubble-like expansions. In this system, rough and smooth cytoplasmic reticulum are distinguished. A peculiarity of the structure of the rough network is the attachment of polysomes to the membranes. Because of this, its function is the synthesis of certain proteins, for example, secreted by gland cells. The densely packed layers of the rough reticulum cisterna represent areas of the most active protein synthesis and are called ergastoplasm. The membranes of the smooth cytoplasmic reticulum are devoid of polysomes. Some stages of the metabolism of carbohydrates, fats and other non-protein substances occur in it. It is assumed that in areas of the smooth network the process of formation of all intracellular membranes begins. Transport of substances occurs through the tubules.

A ribosome is a round ribonucleoprotein particle with a diameter of 20-30 nm. It consists of small and large subunits, the combination of which occurs in the presence of messenger RNA (mRNA). One RNA molecule often combines several ribosomes like a string of beads. This structure is called a polysome. Polysomes are freely located in the main substance of the cytoplasm or attached to the membranes of the rough cytoplasmic reticulum. In both cases, they serve as the site of protein synthesis. In this case, proteins that are used in the life of the cell itself are formed on free polysomes, and proteins that function outside the cell body are formed on attached polysomes.

The lamellar complex is formed by a collection of dictyosomes numbering from several hundred to several thousand per cell. The dictyosome is represented by a stack of 3-12 flattened disc-shaped cisterns, from the edges of which vesicles are attached. Local expansion of the cisterns leads to the formation of vacuoles. In differentiated cells of vertebrates, dictyosomes are usually collected in the perinuclear zone of the cytoplasm. In the lamellar complex, secretory vesicles or vacuoles are formed, the contents of which are represented by the so-called exported proteins and other compounds that are to be removed from the cell. In this case, the secretion entering the dictyosome from the sites of synthesis undergoes some chemical transformations in it. It is also isolated (segregated) in the form of “portions”, which here acquire a membrane shell. Primary lysosomes are formed in the lamellar complex. In the tanks of dictyosomes, polysaccharides are synthesized, complexes of these compounds with proteins (glycoproteins) and fats (glycolipids) are formed, which can then be found in the glycocalyx of the plasmalemma.

Mitochondria are round or rod-shaped structures, usually from 1.0 to 5.0 µm in length, present in most cells in the amount of 150-1500 copies. The mitochondrial shell consists of two membranes that differ in chemical composition, set of enzymes and functions. The inner membrane forms leaf-shaped (cristae) or tubular (tubules) invaginations. The space delimited by the inner membrane is filled with an organelle matrix, in which, using an electron microscope, granules with a diameter of 20-50 nm are detected, accumulating calcium and magnesium ions, and particles of carbohydrates, such as glycogen. The matrix contains its own protein biosynthesis apparatus. It is represented by 2-6 copies of a circular DNA molecule devoid of histones, ribosomes, transfer RNA (tRNA), enzymes for transcription and translation of hereditary information. In terms of basic indicators, such as the size and internal structure of ribosomes, the organization of its own hereditary material (DNA), this apparatus is similar to that of prokaryotes and differs from the apparatus of protein biosynthesis in the cytoplasm of a eukaryotic cell. This speaks in favor of the symbiotic hypothesis of the origin of the latter. The genes of the mitochondria's own DNA encode the nucleotide sequences of mitochondrial ribosomal and transport RNAs, as well as the primary structure of some proteins, mainly the inner membrane of the organelle. The amino acid sequence of most mitochondrial proteins is encoded in the DNA of the chromosomes of the cell nucleus and is formed outside the organelle in the cytoplasm. The main function of mitochondria is to extract energy from organic substances by oxidizing them and storing energy in a biologically usable form in adenosine triphosphoric acid (ATP) molecules. All structural components of the mitochondrion participate in the energy function, but the leading role belongs to the inner membrane. It contains complexes of electron transport enzymes (respiratory chain), dehydrogenases that catalyze the oxidation of respiration substrates, enzymes that couple the process of electron transport, accompanied by the release of energy, with the process of ATP synthesis. Side functions of mitochondria are the synthesis of steroid hormones and some amino acids (glutamic).

Lysosomes are vesicles with a diameter of up to 2 microns, which contain a set of acid hydrolase enzymes that catalyze the hydrolytic (in an aqueous medium) breakdown of nucleic acids, proteins, fats, and carbohydrates. They have a shell of one membrane, sometimes covered on the outside with a fibrous layer of protein. The function of lysosomes is the intracellular digestion of various chemical compounds and structures. Primary lysosomes are inactive organelles, secondary are organelles in which the digestion process occurs. Secondary lysosomes are formed from primary ones. They are divided into heterolysosomes (phagolysosomes) and autolysosomes (cytolysosomes). In the first, material entering the cell from the outside is digested by pinocytosis or phagocytosis, in the second, the cell’s own structures are destroyed. Secondary lysosomes, in which the digestion process is completed, are called residual bodies. They lack hydrolases and contain undigested material.

A collective group of organelles consists of microbodies. These are vesicles with a diameter of 0.1-1.5 µm, limited by one membrane, with a fine-grained matrix and often crystalloid or amorphous protein inclusions. Peroxisomes belong to this group. They contain oxidase enzymes that catalyze the formation of hydrogen peroxide, which is then destroyed by the enzyme peroxidase. These reactions are used in various metabolic cycles, for example in the metabolism of uric acid in liver and kidney cells.

Organelles of general importance include some permanent structures of the cytoplasm that lack membranes. Microtubules are tubular structures of various lengths with a diameter of 24 nm, which are found in a free state in the cytoplasm or as structural elements of centrioles, mitotic spindles, flagella and cilia. Free microtubules and microtubules of flagella, cilia and centrioles have different resistance to destructive influences. In a free state, microtubules perform a supporting function and determine the directions of movement of vesicles and other structures within the cell. Microfilaments are long, thin formations found throughout the cytoplasm, but often concentrated under the plasmalemma and near the nuclear membrane. There appear to be several different classes of microfilaments. Microfilaments from the contractile protein actin determine the flow of the cytoplasm, for example, in plant cells around the central vacuole, intracellular movements of vesicles, chloroplasts, nuclei, amoeboid movement, division of cell bodies by constriction.

Animal cells dividing by mitosis, some plant cells, fungi and algae are characterized by a cell center, an important element of which is centrioles. The centriole has the appearance of a hollow cylinder with a diameter of about 150 nm and a length of 300-500 nm. Its wall is formed by 27 microtubules, grouped into 9 triplets. The function of centrioles includes the formation of mitotic spindle threads. They polarize the process of cell division, ensuring the natural separation of chromatids (daughter chromosomes) in anaphase of mitosis.

The vital activity of a cell as a unit of biological activity is ensured by a set of interconnected metabolic processes confined to certain intracellular structures, ordered in time and space. These processes form three flows - information, energy and matter.

Information flow

Thanks to the presence of a flow of information, the cell, using the centuries-old evolutionary experience of its ancestors, creates an organization that meets the criteria of a living thing, preserves and maintains this organization over time, despite changing environmental conditions, and passes it on over a series of generations. The flow of information involves the nucleus (DNA of chromosomes), macromolecules that carry information into the cytoplasm (mRNA), and the cytoplasmic transcription apparatus (ribosomes and polysomes, tRNA, amino acid activation enzymes). At the final stage of this flow, polypeptides synthesized on polysomes acquire tertiary and quaternary structures and are used as catalysts or building blocks (Fig. 7). In addition to the nuclear genome, which is the main one in terms of the volume of information contained, the genomes of mitochondria also function in eukaryotic cells, and in green plants, chloroplasts.

From the above diagram it can be seen that in the flow under consideration, information is transferred from DNA to protein. What are the codes that record information in DNA and protein? What is the recoding mechanism?

Coding consists of recording certain information using special characters in order to make the information compact, ensure its use repeatedly and in parts, and create convenience during transportation. A typical example of coding is the recording of human thought in the form of written text. In the encoding process, code groups are formed by combinations of symbols that serve to designate an essential element of information. The entire volume of the message is represented by a certain sequence of code groups. The set of symbols makes up the alphabet, and the set of code groups makes up the code dictionary.

The symbols of the DNA code are deoxyribonucleotides, which differ in their nitrogenous base (adenyl, guanyl, thymidyl, cytidyl), so the alphabet is four-letter. The code group is a codon - a section of a DNA molecule consisting of three nucleotides. This makes the code triplet. Information is recorded in linear order along the length of the DNA molecule as a sequence of codons. The DNA code is non-overlapping because each nucleotide is included in one codon. It does not have commas and within a block of information corresponding, for example, to one polypeptide, codons follow each other without interruptions.

The protein code symbol is amino acids. They also correspond to code groups. Information is also recorded in linear order along the length of the polypeptide molecule as a sequence of amino acids.

A comparison of a section of a DNA molecule as the starting point and a polypeptide corresponding to it in content as the final point of the information flow indicates collinearity of the DNA and protein codes: codons follow in the same order as the amino acid residues they encode.

The position of a specific amino acid residue in a polypeptide molecule can be designated in DNA using one of several synonymous codons, which indicates the degeneracy of the DNA code. This property follows from the ratio of the volumes of the DNA and protein code dictionaries. By combining three out of four possible deoxyribonucleotides, 64 different codons are formed, while the protein contains 20 amino acids. The degeneracy of the DNA code is regular: most of the information is contained in the first two nucleotides of the column. Each amino acid corresponds to no more than two such initial doublets, while the number of synonymous codons can reach up to six (for example, for arginine). The degeneracy of the code and the informational disparity of nucleotides in a codon affect the phenotypic expression of point mutations. Indeed, along with changes leading to the replacement of one amino acid residue with another, “silent” mutations are possible if the change converts a codon into a synonym. Although replacing a codon with a synonym does not change the amino acid sequence of a polypeptide, it can affect the rate of its synthesis. Three of the 64 codons, called nonsense, do not code for amino acids. They serve as terminators and indicate the point where information reading stops. The DNA code is universal in the sense that it is identical in all organisms. The only facts that do not agree with this conclusion concern details of punctuation (for example, the designation of the start of reading in E. coli and in a mammalian cell) and the reading of nonsense codons.

Recoding of information occurs during the process of protein biosynthesis. In the first step, referred to as transcription, the original DNA information is read through the synthesis of ribonucleic acids. The latter are complementary to only one of the polynucleotide chains of DNA; the place of thymine in them is occupied by a nitrogenous base close to it - uracil. In a eukaryotic cell, this stage occurs in the nucleus and also independently in mitochondria and chloroplasts. As a result of transcription, several types of RNA are formed, while mRNA acquires information about the sequence of amino acids in polypeptides, and rRNA and tRNA ensure the transfer of information from mRNA to polypeptides.

A feature of transcription from the nuclear DNA of a eukaryotic cell is the formation of an initially larger amount of RNA than that which will then directly participate in the synthesis of polypeptides. Excess RNA, the nature and function of which is not clear, is destroyed during the transformation (processing) of RNA before its transport from the nucleus to the cytoplasm.

Reading of mRNA information and its transfer to protein (translation stage) occurs in the cytoplasm. The central role here belongs to various tRNAs, of which there are several dozen in the cell. Each tRNA sample is capable of attaching a specific amino acid in an activated state (rich in energy). As a result of activation of an amino acid and its addition to tRNA, an “aminoacyl-tRNA” complex is formed. Due to the presence of an anticodon - a sequence of three nucleotides complementary to the nucleotides of the codon of a given amino acid - tRNA recognizes the location of this amino acid in the polypeptide in accordance with the sequence of mRNA codons. Since the transfer of information to protein is carried out not from DNA, but from mRNA, the codons of certain amino acids are designated in accordance with the nucleotide composition of RNA. Thus, it is tRNA that reads information from mRNA.

The assembly of polypeptide molecules occurs on the ribosome, which ensures the required arrangement of the participants in the translation process: mRNA, aminoacyl-tRNA complexes and tRNA-under-construction polypeptide. An idea of ​​the function of ribosomes is given by the ribosomal cycle of protein synthesis.

A functioning ribosome consists of large and small subunits and an mRNA molecule. In one of its two active sites - the peptide (I) - the polypeptide is built up, and to the other - the aminoacyl (II) tRNA with activated amino acids is attached. The “aminoacyl-tRNA” complex, which arrived first, initiates reading and occupies site I. In site II, a second similar complex is fixed, corresponding to the first sense code of the mRNA. After the formation of a peptide bond between the amino acids, the tRNA of site I is released. In its place, tRNA moves to its place in the form of a complex with two amino acid residues, occupying site II. The next aminoacyl-tRNA complex, corresponding to the next sense codon of the mRNA, is connected to site II at-1. The described cycle is repeated until the termination codon of the mRNA (UAA, UAG or UGA) is reached, in relation to which the tRNA does not exist. At this stage, the ribosome breaks down into subunits, releasing mRNA and polypeptide.

Energy flow

The flow of energy in representatives of different groups of organisms is represented by intracellular energy supply mechanisms - fermentation, photo- or chemosynthesis, respiration.

The central role in the bioenergetics of animal cells belongs to respiratory metabolism. It involves the breakdown of low-calorie organic “fuel” in the form of glucose, fatty acids, amino acids and the use of the released energy for the synthesis of high-calorie cellular “fuel” in the form of ATP. ATP and other compounds rich in energy in a biologically utilized form are called high-energy. ATP energy, directly or being transferred to other high-energy compounds, for example creatine phosphate, used in muscles, is converted in various processes into one or another type of work - chemical (synthesis), osmotic (maintaining gradients of substances), electrical, mechanical, regulatory. Among the organelles of such a cell, a special place in the respiratory metabolism belongs to mitochondria, with the inner membrane of which enzymes of the respiratory chain are associated, as well as to the cytoplasmic matrix, in which the process of oxygen-free breakdown of glucose occurs - anaerobic glycolysis. Of the energy converters of ATP chemical bonds into work, the mechanochemical system of striated muscle is the most studied. It consists of contractile proteins and an enzyme that breaks down high-energy compounds to release energy.

A feature of the energy flow of a plant cell is photosynthesis - a mechanism for converting the energy of sunlight into the energy of chemical bonds of organic substances.

The energy supply mechanisms of the cell are highly efficient. The efficiency of chloroplast reaches 25%, and mitochondria - 45-60%, significantly exceeding that of a steam engine (8%) or an internal combustion engine (17%).

Respiratory metabolic reactions not only supply energy, but also provide the cell with the building blocks for the synthesis of a variety of molecules. They serve as many products of the breakdown of nutrients. A special role in this belongs to the central link of respiratory metabolism - the Krebs cycle, carried out in mitochondria. Through this cycle passes the path of carbon atoms (carbon skeletons) of most compounds that serve as intermediate products in the synthesis of chemical components of the cell, as well as the switching of cell metabolism from one predominant path to another, for example, from carbohydrate to fat. Thus, respiratory metabolism simultaneously constitutes the leading link in the flow of substances, combining metabolic pathways for the breakdown and synthesis of carbohydrates, proteins, fats, and nucleic acids.

Eukaryotes include the kingdoms of plants, animals, and fungi.

Basic characteristics of eukaryotes.

1. The cell is divided into cytoplasm and nucleus.
2. Most of the DNA is concentrated in the nucleus. It is nuclear DNA that is responsible for most of the life processes of the cell and for the transmission of heredity to daughter cells.
3. Nuclear DNA is divided into strands that are not closed in rings.
4. DNA strands are linearly elongated within chromosomes and are clearly visible during mitosis. The set of chromosomes in the nuclei of somatic cells is diploid.
5. A system of external and internal membranes has been developed. The internal ones divide the cell into separate compartments - compartments. Take part in the formation of cell organelles.
6. There are many organelles. Some organelles are surrounded by a double membrane: nucleus, mitochondria, chloroplasts. In the nucleus, along with the membrane and nuclear juice, the nucleolus and chromosomes are found. The cytoplasm is represented by the main substance (matrix, hyaloplasm) in which inclusions and organelles are distributed.
7. A large number of organelles are limited by a single membrane (lysosomes, vacuoles, etc.)
8. In a eukaryotic cell, organelles of general and special importance are distinguished. For example: general meaning – nucleus, mitochondria, EPS, etc.; of special significance are microvilli of the absorptive surface of the intestinal epithelial cell, cilia of the epithelium of the trachea and bronchi.
9. Mitosis is a characteristic mechanism of reproduction in generations of genetically similar cells.
10. Characteristic of the sexual process. True sex cells - gametes - are formed.
11. Not capable of fixing free nitrogen.
12. Aerobic respiration occurs in mitochondria.
13. Photosynthesis takes place in chloroplasts containing membranes, which are usually arranged in grana.
14. Eukaryotes are represented by unicellular, filamentous and truly multicellular forms.

Main structural components of a eukaryotic cell

organoids

Core. Structure and functions.

A cell has a nucleus and cytoplasm. Cell nucleus consists of a membrane, nuclear juice, nucleolus and chromatin. Functional role nuclear envelope consists in the isolation of the genetic material (chromosomes) of a eukaryotic cell from the cytoplasm with its numerous metabolic reactions, as well as the regulation of bilateral interactions between the nucleus and the cytoplasm. The nuclear envelope consists of two membranes separated by a perinuclear space. The latter can communicate with the tubules of the cytoplasmic reticulum.

The nuclear envelope is penetrated by a pore with a diameter of 80-90 nm. The pore region or pore complex with a diameter of about 120 nm has a certain structure, which indicates a complex mechanism for regulating the nuclear-cytoplasmic movements of substances and structures. The number of pores depends on the functional state of the cell. The higher the synthetic activity in the cell, the greater their number. It is estimated that in lower vertebrates, in erythroblasts, where hemoglobin is intensively formed and accumulated, there are about 30 pores per 1 μm 2 of the nuclear membrane. In mature erythrocytes of these animals, which retain their nuclei, up to five pores remain per 1 μg of membrane, i.e. 6 times less.

In the area of ​​the feather complex the so-called dense plate - the protein layer underlying the entire inner membrane of the nuclear envelope. This structure primarily performs a supporting function, since in its presence the shape of the nucleus is preserved even if both membranes of the nuclear envelope are destroyed. It is also assumed that the regular connection with the substance of the dense lamina promotes the ordered arrangement of chromosomes in the interphase nucleus.

The basis nuclear juice, or matrix, make up proteins. Nuclear sap forms the internal environment of the nucleus, and therefore plays an important role in ensuring the normal functioning of the genetic material. Nuclear juice contains filamentous, or fibrillar, proteins, with which the performance of the support function is associated: the matrix also contains the primary transcription products of genetic information - heteronuclear RNAs (hn-RNAs), which are also processed here, turning into m-RNA (see 3.4.3.2).

Nucleolus represents the structure in which formation and maturation occurs ribosomal RNA (rRNA). rRNA genes occupy certain sections (depending on the type of animal) of one or several chromosomes (in humans there are 13-15 and 21-22 pairs) - nucleolar organizers, in the area of ​​which nucleoli are formed. Such areas in metaphase chromosomes look like narrowings and are called secondary constrictions. WITH Using an electron microscope, filamentous and granular components are identified in the nucleolus. The filamentous (fibrillar) component is represented by complexes of protein and giant RNA precursor molecules, from which smaller molecules of mature rRNA are then formed. During the process of maturation, fibrils are transformed into ribonucleoprotein grains (granules), which represent the granular component.

Chromatin structures in the form of clumps, scattered in the nucleoplasm, are an interphase form of existence of cell chromosomes

cytoplasm

IN cytoplasm distinguish between the main substance (matrix, hyaloplasm), inclusions and organelles. Basic substance of the cytoplasm fills the space between the plasmalemma, nuclear envelope and other intracellular structures. An ordinary electron microscope does not reveal any internal organization in it. The protein composition of hyaloplasm is diverse. The most important proteins are represented by enzymes of glycolysis, metabolism of sugars, nitrogenous bases, amino acids and lipids. A number of hyaloplasmic proteins serve as subunits from which structures such as microtubules are assembled.

The main substance of the cytoplasm forms the true internal environment of the cell, which unites all intracellular structures and ensures their interaction with each other. The performance of a unifying and scaffolding function by the matrix may be associated with a microtrabecular network, detected using a high-power electron microscope, formed by thin fibrils 2-3 nm thick and penetrating the entire cytoplasm. A significant amount of intracellular movement of substances and structures occurs through the hyaloplasm. The main substance of the cytoplasm should be considered in the same way as a complex colloidal system capable of transitioning from a sol-like (liquid) state to a gel-like state. In the process of such transitions, work is done. For the functional significance of such transitions, see Section. 2.3.8.

Inclusions(Fig. 2.5) are called relatively unstable components of the cytoplasm, which serve as reserve nutrients (fat, glycogen), products to be removed from the cell (secretion granules), and ballast substances (some pigments).

Organelles - These are permanent structures of the cytoplasm that perform vital functions in the cell.

Organelles are isolated general meaning And special. The latter are present in significant quantities in cells specialized to perform a specific function, but in small quantities they can also be found in other types of cells. These include, for example, microvilli of the absorptive surface of the intestinal epithelial cell, cilia of the epithelium of the trachea and bronchi, synaptic vesicles, transporting substances that carry nervous excitation from one nerve cell to another or a cell of the working organ, myofibrils on which muscle contraction depends. A detailed examination of special organelles is part of the histology course.

Organelles of general importance include elements of the tubular and vacuolar system in the form of a rough and smooth cytoplasmic reticulum, a lamellar complex, mitochondria, ribosomes and polysomes, lysosomes, peroxisomes, microfibrils and microtubules, centrioles of the cell center. Plant cells also contain chloroplasts, in which photosynthesis occurs.

Kanaltsevaya And vacuolar system formed by communicating or separate tubular or flattened (cistern) cavities, bounded by membranes and spreading throughout the cytoplasm of the cell. Often tanks have bubble-like expansions. In the named system there are rough And smooth cytoplasmic reticulum(see Fig. 2.3). A structural feature of the rough network is the attachment of polysomes to its membranes. Because of this, it performs the function of synthesizing a certain category of proteins that are predominantly removed from the cell, for example, secreted by gland cells. In the area of ​​the rough network, the formation of proteins and lipids of cytoplasmic membranes occurs, as well as their assembly. The cisterns of the rough network, densely packed in a layered structure, are the sites of the most active protein synthesis and are called ergastoplasma.

The membranes of the smooth cytoplasmic reticulum are devoid of polysomes. Functionally, this network is associated with the metabolism of carbohydrates, fats and other non-protein substances, such as steroid hormones (in the gonads, adrenal cortex). Through the tubules and cisterns, substances, in particular material secreted by the glandular cell, move from the site of synthesis to the zone of packaging into granules. In areas of liver cells rich in smooth network structures, harmful toxic substances and some drugs (barbiturates) are destroyed and neutralized. In the vesicles and tubules of the smooth network of striated muscles, calcium ions are stored (deposited), which play an important role in the contraction process.

Ribosome - it is a round ribonucleoprotein particle with a diameter of 20-30 nm. It consists of small and large subunits, the combination of which occurs in the presence of messenger RNA (mRNA). One molecule of mRNA usually links several ribosomes together like a string of beads. This structure is called polysome. Polysomes are freely located in the main substance of the cytoplasm or attached to the membranes of the rough cytoplasmic reticulum. In both cases, they serve as a site of active protein synthesis. Comparison of the ratio of the number of free and membrane-attached polysomes in embryonic undifferentiated and tumor cells, on the one hand, and in specialized cells of an adult organism, on the other, led to the conclusion that proteins are formed on hyaloplasma polysomes for their own needs (for “home” use) of a given cell, while on the polysomes of the granular network proteins are synthesized that are removed from the cell and used for the needs of the body (for example, digestive enzymes, breast milk proteins).

Golgi lamellar complex formed by a collection of dictyosomes ranging in number from several tens (usually about 20) to several hundreds and even thousands per cell.

Dictyosome(Fig. 2.6, A) is represented by a stack of 3-12 flattened disc-shaped cisterns, from the edges of which vesicles (vesicles) are laced. Limited to a certain area (local) expansion of the cisterns gives rise to larger vesicles (vacuoles). In differentiated cells of vertebrates and humans, dictyosomes are usually collected in the perinuclear zone of the cytoplasm. In the lamellar complex, secretory vesicles or vacuoles are formed, the contents of which are proteins and other compounds that must be removed from the cell. In this case, the precursor of the secretion (prosecret), entering the dictyosome from the synthesis zone, undergoes some chemical transformations in it. It is also isolated (segregated) in the form of “portions”, which are also covered with a membrane shell. Lysosomes are formed in the lamellar complex. Dictyosomes synthesize polysaccharides, as well as their complexes with proteins (glycoproteins) and fats (glycolipids), which can then be found in the glycocalyx of the cell membrane.

The mitochondrial shell consists of two membranes that differ in chemical composition, set of enzymes and functions. The inner membrane forms leaf-shaped (cristae) or tubular (tubules) invaginations. The space bounded by the inner membrane is matrix organelles. Using an electron microscope, grains with a diameter of 20-40 nm are detected in it. They accumulate calcium and magnesium ions, as well as polysaccharides such as glycogen.

The matrix contains the organelle's own protein biosynthesis apparatus. It is represented by 2 copies of a circular DNA molecule devoid of histones (as in prokaryotes), ribosomes, a set of transfer RNAs (tRNAs), enzymes for DNA replication, transcription and translation of hereditary information. In terms of its basic properties: the size and structure of ribosomes, the organization of its own hereditary material, this apparatus is similar to that of prokaryotes and differs from the apparatus of protein biosynthesis in the cytoplasm of a eukaryotic cell (which confirms the symbiotic hypothesis of the origin of mitochondria; see § 1.5). Genes of their own DNA encode nucleotide sequences mitochondrial rRNA and tRNA, as well as amino acid sequences of some proteins of the organelle, mainly its inner membrane. The amino acid sequences (primary structure) of most mitochondrial proteins are encoded in the DNA of the cell nucleus and are formed outside the organelle in the cytoplasm.

The main function of mitochondria is to enzymatically extract energy from certain chemicals (by oxidizing them) and storing energy in a biologically usable form (by synthesizing adenosine triphosphate -ATP molecules). In general this process is called oxidative(disbandment. Matrix components and the inner membrane actively participate in the energy function of mitochondria. It is with this membrane that the electron transport chain (oxidation) and ATP synthetase, which catalyzes the oxidation-associated phosphorylation of ADP into ATP, are associated. Among the side functions of mitochondria is participation in the synthesis of steroid hormones and some amino acids (glutamic).

Lysosomes(Fig. 2.6, IN) are bubbles with a diameter of usually 0.2-0.4 μm, which contain a set of acid hydrolase enzymes that catalyze the hydrolytic (in an aqueous environment) breakdown of nucleic acids, proteins, fats, and polysaccharides at low pH values. Their shell is formed by a single membrane, sometimes covered on the outside with a fibrous protein layer (in electron diffraction patterns there are “bordered” bubbles). The function of lysosomes is the intracellular digestion of various chemical compounds and structures.

Primary lysosomes(diameter 100 nm) are called inactive organelles, secondary - organelles in which the digestion process occurs. Secondary lysosomes are formed from primary ones. They are divided into heterolysosomes(phagolysosomes) and autolysosomes(cytolysosomes). Firstly (Fig. 2.6, G) material entering the cell from the outside is digested through pinocytosis and phagocytosis, and secondly, the cell’s own structures, which have completed their function, are destroyed. Secondary lysosomes, in which the digestion process is completed, are called residual bodies(telolysosomes). They lack hydrolases and contain undigested material.

Microbodies form a collective group of organelles. These are vesicles with a diameter of 0.1-1.5 μm limited by one membrane with a fine-grained matrix and often crystalloid or amorphous protein inclusions. This group includes, in particular, peroxisomes. They contain oxidase enzymes that catalyze the formation of hydrogen peroxide, which, being toxic, is then destroyed by the action of the peroxidase enzyme. These reactions are involved in various metabolic cycles, for example in the exchange of uric acid in liver and kidney cells. In a liver cell, the number of peroxisomes reaches 70-100.

Organelles of general importance also include some permanent structures of the cytoplasm that lack membranes. Microtubules(Fig. 2.6, D) - tubular formations of various lengths with an outer diameter of 24 nm, a lumen width of 15 nm and a wall thickness of about 5 nm. They are found in a free state in the cytoplasm of cells or as structural elements of flagella, cilia, mitotic spindles, and centrioles. Free microtubules and microtubules of cilia, flagella and centrioles have different resistance to destructive influences, for example chemical (colchicine). Microtubules are built from stereotypical protein subunits through their polymerization. In a living cell, polymerization processes occur simultaneously with depolymerization processes. The ratio of these processes determines the number of microtubules. In a free state, microtubules perform a supporting function, determining the shape of cells, and are also factors in the directional movement of intracellular components.

Microfilaments(Fig. 2.6, E) are called long, thin structures, sometimes forming bundles and found throughout the cytoplasm. There are several different types of microfilaments. Actin microfilaments due to the presence of contractile proteins (actin) in them, they are considered as structures that provide cellular forms of movement, for example, amoeboid. They are also credited with a scaffolding role and participation in the organization of intracellular movements of organelles and areas of hyaloplasm.

Along the periphery of cells under the plasmalemma, as well as in the perinuclear zone, bundles of microfilaments 10 nm thick are found - intermediate filstents. In epithelial, nerve, glial, muscle cells, fibroblasts, they are built from different proteins. Intermediate filaments apparently perform a mechanical, scaffolding function.

Actin microfibrils and intermediate filaments, like microtubules, are built from subunits. Because of this, their quantity depends on the ratio of the polymerization and depolymerization processes.

Characteristic for animal cells, parts of plant cells, fungi and algae cell center, which contains centrioles. Centriole(under an electron microscope) has the appearance of a “hollow” cylinder with a diameter of about 150 nm and a length of 300-500 nm. Its wall is formed by 27 microtubules, grouped into 9 triplets. The function of centrioles includes the formation of mitotic spindle threads, which are also formed by microtubules. Centrioles polarize the process of cell division, ensuring the separation of sister chromatids (chromosomes) in anaphase of mitosis.

A eukaryotic cell has a cellular skeleton (cytoskeleton) of intracellular fibers (Rings) - early 20th century, rediscovered at the end of 1970. This structure allows the cell to have its own shape, sometimes changing it. Cytoplasm is in motion. The cytoskeleton is involved in the process of organelle transfer and participates in cell regeneration.

Mitochondria are complex formations with a double membrane (0.2-0.7 µm) and different shapes. The inner membrane has cristae. The outer membrane is permeable to almost all chemicals, the inner membrane is permeable only to active transport. Between the membranes is the matrix. Mitochondria are located where energy is needed. Mitochondria have a ribosome system, a DNA molecule. Mutations may occur (more than 66 diseases). As a rule, they are associated with insufficient ATP energy and are often associated with cardiovascular failure and pathologies. The number of mitochondria is different (there is 1 mitochondria in a trypanosome cell). The amount depends on age, function, tissue activity (liver - more than 1000).

Lysosomes are bodies surrounded by an elementary membrane. Contains 60 enzymes (40 lysosomal, hydrolytic). Inside the lysosome there is a neutral environment. They are activated by low pH values, entering the cytoplasm (self-digestion). Lysosome membranes protect the cytoplasm and cell from destruction. They are formed in the Golgi complex (intracellular stomach; they can recycle spent cell structures). There are 4 types. 1-primary, 2-4 – secondary. Through endocytosis, a substance enters the cell. The primary lysosome (storage granule) with a set of enzymes absorbs the substance and a digestive vacuole is formed (with complete digestion, breakdown occurs to low molecular weight compounds). Undigested residues remain in residual bodies, which can accumulate (lysosomal storage diseases). Residual bodies that accumulate in the embryonic period lead to gargaleism, deformities, and mucopolysaccharidoses. Autophagy lysosomes destroy the cell's own structures (unnecessary structures). May contain mitochondria, parts of the Golgi complex. Often formed during fasting. May occur when exposed to other cells (red blood cells).

Cytoplasm is the internal contents of the cell and consists of the main substance, or hyaloplasm, and the various intracellular structures contained in it.

Hyaloplasm (matrix) is an aqueous solution of inorganic and organic substances, capable of changing its viscosity and in constant motion. The ability to move, or flow, the cytoplasm is called cyclosis. The matrix is ​​an active medium in which many chemical and physiological processes occur and which combines all the components of the cell into a single system.

The cytoplasmic structures of the cell are represented by inclusions and organelles.

Organelles are permanent and essential components of most cells, having a specific structure and performing vital functions. Organoids are of general purpose and special purpose types.

Organelles of general importance are present in all cells and, depending on their structural features, are divided into non-membrane, single-membrane and double-membrane.

Organelles of special importance are present only in the cells of certain tissues; for example, myofibrils in muscle tissue, neurofibrils in nervous tissue.

Non-membrane organelles.

This group includes ribosomes, microtubules and microfilaments, as well as the cell center.

RIBOSOMES.

Ribosomes - very small organelles present in all types of cells. They have a round shape, consist of approximately equal amounts of rRNA and protein by mass, and are represented by two subunits: large and small. Between the subunits there is a space where the mRNA attaches.

In cells, ribosomes are localized freely in the cytoplasm, on the membranes of the ER, in the mitochondrial matrix, on the outer membrane of the nucleus, and in plastids in plants.

The function of ribosomes is to assemble protein molecules.

During active protein synthesis, polyribosomes are formed. Polyribosomes- ribosome complex (from 5 to 70 ribosomes). There is a connection between individual ribosomes, which is carried out using mRNA molecules.

Rice. 5. Structure of the ribosome (diagram)

1- small subunit; 2 – mRNA; 3 – large subunit of 4-rRNA

MICROTUBLES AND MICROFILAMENTS

Microtubules and microfilaments – thread-like structures consisting of various contractile proteins. Microtubules look like long hollow cylinders, the walls of which consist of proteins - tubulins. Microfilaments are very thin, long, thread-like structures consisting of actin and myosin. Microtubules and microfilaments permeate the entire cytoplasm of the cell, forming its cytoskeleton, causing cyclosis, intracellular movements of organelles, and divergence of chromosomes during the division of nuclear material. In addition to free microtubules that penetrate the cytoplasm, cells have microtubules organized in a certain way, forming centrioles of the cell center, basal bodies, cilia and flagella.

CELL CENTER

Cell center or centrosome– usually located near the nucleus, consists of two centrioles located perpendicular to each other. Each centriole looks like a hollow cylinder, the wall of which is formed by 9 triplets of microtubules. There are no microtubules in the center. Therefore, the microtubule system of the centriole can be described by the formula (9 × 3) + 0.

During the preparation of the cell for division, doubling occurs - duplication centrioles: mother and daughter diverge to the poles of the cell, outlining the direction of future division; near each, a new centriole is formed from microtubules of the cytoplasm. The main functions of the cell center are:

1) participation in the processes of cell division, the divergence of centrioles determines the orientation of the division spindle and the movement of chromosomes;

2) the structure and function of cilia and flagella (basal bodies) are associated with this organelle; Thus, centrioles are associated with movement processes in the cell.

Single-membrane organelles

These include the endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes.

5.2.1 Endoplasmic reticulum (ER).

It is a network in the inner layers of the cytoplasm (endoplasm) - the endoplasmic reticulum, which is a complex system tubules, straws And tanks, bounded by membranes.

There are EPS (EPR):

Smooth (agranular) (does not contain ribosomes on the membranes)	Rough (granular) (on membranes - ribosomes)
1. Synthesis of glycogen and lipids (sebaceous glands, liver). 2. Accumulation of synthesis products. 3. Secret transport.	1. Protein synthesis (protein gland cells). 2. Participation in secretory processes, secretion transport. 3. Accumulation of synthesis products.
	4. Provides communication with cell organelles. 5. Provides transport of secretions to cell organoids. 6. Provides connection between the nucleus and cellular organelles and the cytoplasmic membrane. 7. Provides circulation of various substances throughout the cytoplasm. 8. Participation in pinocytosis (transport of various substances entering the cell from the outside).

The greatest development of the EPS is characteristic of secretory cells. The ER is poorly developed in sperm.

The formation of EPS occurs during cell division from proliferations of the outer cytoplasmic membrane and nuclear membrane, and is transmitted from cell to cell during cell division.

GOLGI COMPLEX

Golgi complex discovered in 1898 by Golgi.

The shape of the complex can be in the form of a network around the nucleus, in the form of a cap or belt around the nucleus, or in the form of individual elements - round, crescent-shaped bodies called dictyosomes.

The Golgi complex consists of three elements that can transform into one another and are interconnected:

1) a system of flat tanks, arranged in packs of five to eight, in the form of a stack of coins and tightly adjacent to each other;

2) a system of tubes extending from the cisterns, anastomosing with each other and forming a network;

3) large and small bubbles closing the end sections of the tubes.

This organelle is most well developed in glandular cells, for example, in leukocytes and oocytes, as well as in other cells that produce protein products, polysaccharides and lipids.

Weak development of the Golgi complex is observed in undifferentiated and tumor cells.

Composition: phospholipids, proteins, enzymes for the synthesis of polysaccharides and lipids.

1) participation in the secretory activity of the cell;

2) accumulation of finished or almost finished products;

3) transportation of secretion products throughout the cell through a system of tubes and vesicles;

4) condensation of secretory granules (osmotic removal of water);

5) isolation and accumulation of substances that are toxic to cells from outside (toxins, anesthetic substances), which are then removed from the cell;

6) formation of yolk grains in oocytes;

7) formation of cell partitions (in plant cells).

During cell division, the Golgi complex is transferred from mother to daughter cells.

LYSOSOMES

They perform the function of intracellular digestion of food macromolecules and foreign components entering the cell during phago- and pinocytosis, providing the cell with additional raw materials for chemical and energy processes. To carry out these functions, lysosomes contain about 40 hydrolytic enzymes - hydrolases that destroy proteins, nucleic acids, lipids, carbohydrates at acidic pH (proteinases, nucleases, phosphatases, lipases). There are primary lysosomes, secondary lysosomes (phagolysosomes and autophagosomes) and residual bodies. Primary lysosomes are microbubbles detached from the cavities of the Golgi apparatus, surrounded by a single membrane and containing a set of enzymes. After the fusion of primary lysosomes with some substrate that is subject to cleavage, various secondary lysosomes are formed. An example of secondary lysosomes are the digestive vacuoles of protozoa. Such lysosomes are called phagolysosomes, or heterophagosomes. If fusion occurs with altered organelles of the cell itself, then autophagosomes are formed. Lysosomes, in the cavities of which undigested products accumulate, are called telolysosomes or residual bodies.

The ER, Golgi apparatus and lysosomes are functionally connected intracellular structures, delimited from the cytoplasm by a single membrane. They form a single tubular-vacuolar system of the cell.

Peroxisomes

They have an oval shape. In the central part of the matrix there are crystal-like structures. The matrix contains amino acid oxidation enzymes, which produce hydrogen peroxide. The enzyme catalase is also present, which destroys peroxide. (Characteristic of liver and kidney cells)

Double membrane organelles

Mitochondria

Mitochondria can be oval, rod-shaped, filamentous, or highly branched in shape. The forms of mitochondria can change from one to another with changes in pH, osmotic pressure, and temperature. The shape can be different in different cells, and in different parts of the same cell.

Externally, mitochondria are bounded by a smooth outer membrane. The inner membrane forms numerous outgrowths - cristae. The internal contents of mitochondria are called the matrix. Mitochondria are semi-autonomous organelles, since they contain their own protein biosynthesis apparatus (circular DNA, RNA, ribosomes, amino acids, enzymes).

Matrix- a substance denser than the cytoplasm, homogeneous.

Krist a lot in liver cells, they are located tightly relative to each other; in muscles - less.

Fig.7. Mitochondria structure (diagram)

1- smooth outer membrane; 2 - internal membrane; 3 – cristae; 4 – matrix (and it contains a circular DNA molecule, many ribosomes, enzymes).

The size of mitochondria varies from 0.2 to 20 microns.

The number of mitochondria varies in different types of cells: from 5-7 to 2500, depending on the functional activity of the cells. A large number of mitochondria in liver cells and working muscles (more in young than in old).

The arrangement of mitochondria can be uniform throughout the cytoplasm, such as in epithelial cells, nerve cells, protozoan cells, or uneven, for example, in the area of ​​the most active cellular activity. In secretory cells, these are the areas where secretions are produced, in cardiac muscle cells and gametes (surrounding the nucleus). A structural connection between mitochondria and the cell nucleus has been discovered in the periods preceding cell division. It is believed that during this period, metabolic and energy processes actively occur and are carried out through structures resembling tubes.

Chemical composition: proteins - 70%, lipids - 25%, nucleic acids (DNA, RNA - insignificant), vitamins A, B12, B6, K, E, enzymes.

Mitochondria are the most sensitive organelles to the effects of various factors: drugs, increased temperature, poisons lead to swelling, an increase in the volume of mitochondria, their matrix liquefies, the number of cristae decreases and folds appear on the outer membrane. These processes impair cellular respiration and can become irreversible with frequent and extreme exposure.

In mitochondria, ATP is synthesized as a result of the processes of oxidation of organic substrates and phosphorylation of ADP and the synthesis of steroid hormones

During the process of evolution, different cells adapted to live in different conditions and perform specific functions. This required the presence of special organelles in them, which are called specialized.

Such organelles are present only in the cells of certain tissues, for example, myofibrils in muscle cells, neurofibrils in nerve cells, and tonofibrils, cilia and flagella in epithelial tissues.

INCLUSIONS

Unlike organelles, inclusion are temporary structures that appear in the cell during certain periods of the cell’s life. The main location of inclusions is the cytoplasm, but sometimes the nucleus.

Inclusions are products of cellular metabolism and can take the form of granules, grains, droplets, vacuoles and crystals; are used either by the cell itself as needed, or serve for the entire macroorganism.

Inclusions classified by chemical composition:

CELL NUCLEUS

The cell nucleus is involved in the differentiation of cells by shape, number, location and size. The shape of the nucleus is often related to the shape of the cell, but it can also be completely irregular. In spherical, cubic and polyhedral cells, the nucleus is usually spherical; in cylindrical, prismatic and fusiform - the shape of an ellipse (smooth myocyte).

Figure 8. Smooth myocyte

An example of an irregularly shaped nucleus is the nuclei of leukocytes (segmented - segmented neutrophil leukocyte). Blood monocytes have a bean-shaped nucleus.

Rice. 9. Blood monocyte Rice. 10 Segmented

neutrophil leukocyte

Most cells have one nucleus. But there are binucleate cells: liver cells, hepatocytes and cartilage chondrocytes, and multinucleated cells: osteoclasts of bone tissue and megakaryocytes of red bone marrow - up to 100 nuclei. Nuclei are especially numerous in symplasts and syncytia (striated muscle fibers and reticular tissue), but these formations are not cells themselves.

Fig.11. Hepatocyte Rice. 12.Megakaryacite

The location of the nuclei is individual for each cell type. Typically, in undifferentiated cells, the nucleus is located at the geometric center of the cell. As it matures and accumulates reserve nutrients and organelles, the nucleus shifts to the periphery. There are cells in which the nucleus occupies a sharply eccentric position. The most striking example of this is white fat adipocytes, in which almost the entire volume of the cytoplasm is occupied by a drop of fat. In any case, no matter how the nucleus is located in the cell, it is almost always surrounded by a zone of undifferentiated cytoplasm.

The size of the nucleus depends on the cell type and is usually directly proportional to the volume of the cytoplasm. The relationship between the volume of the nucleus and the cytoplasm is usually expressed by the so-called nuclear-plasmic (N-P) Hertwig relation: as the volume of the cytoplasm increases, the volume of the nucleus also increases. The moment of onset of cell division is apparently determined by a change in the R-C ratio and is due to the fact that only a certain volume of the nucleus is able to control a certain volume of the cytoplasm. Typically, larger nuclei are found in young, tumor cells, and cells preparing to divide. At the same time, the volume of the nucleus is a feature characteristic of each tissue. There are tissues whose cells have a small nucleus relative to the volume of the cytoplasm; these are the so-called cells cytoplasmic type. These include most cells of the body, for example, all types of epithelia.

Others have a large nucleus that occupies almost the entire cell and a thin rim of cytoplasm - the cell nuclear type, these are blood lymphocytes.

Fig.16 Structure of the nucleus (diagram)

1- ribosomes on the outer membrane; 2 - nuclear pores; 3 - outer membrane; 4 - internal membrane; 5 - nuclear membrane; (karyolemma, nucleolemma); 6 - slit-like perinuclear space; 7 - nucleolus;

8 - nuclear juice (karyoplasm, nucleoplasm); 9 - heterochromatin;

10 – euchromatin.

Nuclear envelope formed by two elementary biological membranes, between which there is a slit-like perinuclear space. The nuclear envelope serves to delimit the intranuclear space from the cell cytoplasm. It is not continuous and has tiny holes - pores. The nuclear pore is formed due to the fusion of nuclear membranes and is a complex globular-fibrillar structure that fills the perforation in the nuclear envelope. This is the so-called nuclear pore complex. Along the border of the hole there are three rows of granules (eight in each). The first row is adjacent to the intranuclear space, the second to the cytoplasm, and the third is located between them. Fibrillar processes extend from the granules, which connect at the center of the granule and create a septum, aperture across the pore. The number of pores is not constant and depends on the metabolic activity of the cell.

Nuclear juice- an uncolored mass that fills the entire internal space of the nucleus between its components and is a colloidal system and has turgor.

Nucleoli- one or more steroid bodies, often quite large (in neurocytes and oocytes). Nucleoli - nucleoli- the densest structure of the nucleus, they are well stained with basic dyes, as they are rich in RNA. It is heterogeneous in structure and has a fine-grained or fine-fiber structure. Serve as a place of education ribosomes.

Chromatin- zones of dense matter that readily perceive dyes are characteristic of a non-dividing cell. Chromatin has a different state of aggregation - during cell division it is transformed by condensation and spiralization into chromosomes. Each chromosome has centromere- the place of attachment to the spindle threads during mitosis, the centromere divides the chromosome into two arms.

In addition to the centromere (primary constriction), a chromosome may have secondary constriction and separated by her satellite. The outside of each chromosome is covered pellicle, under which there is a protein matrix. The matrix contains chromatids. Chromatids are made up of chromonema, and those from filaments. The totality of the chromosomes of each organism is chromosome set.

Fig17. Chromosome structure (diagram)

1 - centromere (primary constriction); 2- shoulders; 3 – secondary constriction; 4-satellite; 5 – pellicle; 6 – protein matrix; 7 - chromatin

CELL REPRODUCTION.

All living organisms are made up of cells. In the process of life, some of the body's cells wear out, age and die. The only way to form cells is by dividing the previous ones. Cell division is a vital process for all organisms.

Life (cellular) cycle.

The life of a cell from the moment of its origin as a result of the division of the mother cell until its own division or death is called life (cellular) cycle. An essential component of the cell cycle is mitotic cycle, including the period of cell preparation for division and division itself. Preparing a cell for division, or interphase, makes up a significant part of the time of the mitotic cycle and consists of periods:

1. Presynthetic (postmitotic) G1 - occurs immediately after cell division. Biosynthesis processes take place in cells and new organelles are formed. The young cell is growing. This period is the most variable in duration.

2. Synthetic S is the main one in the mitotic cycle. DNA replication occurs. Each chromosome becomes double-stranded, that is, it consists of two chromatids - identical DNA molecules. In addition, the cell continues to synthesize RNA and proteins. In dividing mammalian cells it lasts about 6–10 hours.

3. Postsynthetic (premitotic) G2 is relatively short, in mammalian cells it is about 2–5 hours. At this time, the number of centrioles and mitochondria doubles, active metabolic processes take place, proteins and energy are accumulated for the upcoming division. The cell begins to divide.

7.2 CELL DIVISION.

Three methods of division of eukaryotic cells have been described:

1) amitosis (direct division),

2) mitosis (indirect division).

3) meiosis (reduction division).

7.2.1 Amitosis- cell division without chromosome spiralization, which occurred before mitosis. This is how they reproduce prokaryotes, highly specialized And degrading cells. In this case, the nuclear membrane and nucleoli do not disappear, the chromosomes remain spiralized.

Types of amitosis:

1) ligation(typical of bacteria)

2) fragmentation(megakaryoblast, megakaryocyte)

3)budding(platelet buds from megakaryocyte)

By distribution genetic material

Division without a mitotic apparatus is caused by irradiation, tissue degeneration, and the action of various agents that disrupt the entry of cells into mitosis.

Mitosis

Characterized by destruction of the nuclear membrane and nucleoli, spiralization of chromosomes. In mitosis there are prophase, metaphase, anaphase And telophase.

Fig.18. Mitosis diagram

I. Prophase:

1) The shape of the cell becomes rounded, its contents become more viscous, chromosomes take on the appearance of long thin threads twisted inside the nucleus. Each chromosome consists of two chromatids.

2) Chromatids gradually shorten and approach the nuclear envelope, which is a sign of the beginning of the destruction of the karyolemma.

3) The spindle develops: the centrioles diverge to the poles and double, and spindle filaments form between them.

4) The nuclear membrane is destroyed, and a zone of liquid cytoplasm is formed in the center of the cell, where chromosomes rush.

Late metaphase

Chromosomes line up in the equatorial plane, forming metaphyseal plate. The spindle strands are attached to the centromeres of the chromosomes.

There are two types of spindle filaments: some of them are associated with chromosomes and are called chromosomal, and others stretch from pole to pole and are called continuous.

Maternal

IV. Telophase.

The migration of two daughter groups of chromosomes to opposite poles of the cell is completed. reconstruction cores and decondensation chromosomes, they despiral, the karyolemma is restored, and nucleoli appear. Nuclear fission is completed.

Begins cytokinesis (cytotomy)- the process of ligation and separation of the cytoplasm with the formation of a constriction. A “boiling” of the cell surface is observed due to its intensive growth. The cytoplasm loses its viscosity, the centrioles lose activity, and the organelles are divided approximately in half between the daughter cells.

Types of mitosis:

1) Any tissue is a self-regulating system; therefore, the number of cells that die in the tissue is balanced by the number of cells formed.

2) There are daily allowance rhythms of mitotic activity. The greatest mitotic activity coincides with periods of tissue rest, and increased tissue function leads to inhibition of mitoses (in nocturnal animals - in the early morning, and in diurnal animals - at night).

3) Stress hormones have an inhibitory effect on mitotic activity: adrenaline and norepinephrine, and growth hormone has a stimulating effect. Changes in mitotic activity occur due to changes in the duration of interphase. Each cell initially has the ability to divide, but under certain conditions this ability inhibited. Inhibition can be of varying degrees, even irreversible.

Cell lifespan can be considered as the period from one division to another. In stable cell populations, in which practically no cell reproduction occurs, their life expectancy is maximum (liver, nervous system).

Endoreproduction- all cases where chromosome reduplication or DNA replication occurs, cell division does not occur. This leads to polyploidy, an increase in the volume of the nucleus and cell. It can occur due to disorders of the mitotic apparatus and is observed both in normal and pathological conditions. Characteristic of liver cells and urinary tract.

Endomitosis occurs with a non-destructive nuclear shell. Chromosome reduplication occurs as during normal division, resulting in the formation of giant chromosomes. All the figures characteristic of mitosis are observed, but they occur inside the nucleus. Distinguish endoprophase,endometaphase,endoanaphase,endotelophase. Since the core shell is preserved, the result is polyploid cell. The significance of endomitosis is that during it the main activity of the cell does not stop.

Cell organelles, also known as organelles, are specialized structures of the cell itself, responsible for various important and vital functions. Why “organelles” after all? It’s just that here these cell components are compared with the organs of a multicellular organism.

What organelles make up the cell?

Also, sometimes organelles mean only the permanent structures of the cell that are located in it. For the same reason, the cell nucleus and its nucleolus are not called organelles, just as cilia and flagella are not organelles. But the organelles that make up the cell include: complex, endoplasmic reticulum, ribosomes, microtubules, microfilaments, lysosomes. In fact, these are the main organelles of the cell.

If we are talking about animal cells, then their organelles also include centrioles and microfibrils. But the number of organelles of a plant cell still includes only plastids characteristic of plants. In general, the composition of organelles in cells can differ significantly depending on the type of cell itself.

Drawing of the structure of a cell, including its organelles.

Double membrane cell organelles

Also in biology, there is such a phenomenon as double-membrane cell organelles, these include mitochondria and plastids. Below we will describe their inherent functions, as well as all other main organelles.

Functions of cell organelles

Now let us briefly describe the main functions of animal cell organelles. So:

• The plasma membrane is a thin film around the cell consisting of lipids and proteins. A very important organelle that transports water, minerals and organic substances into the cell, removes harmful waste products and protects the cell.
• Cytoplasm is the internal semi-liquid environment of the cell. Provides communication between the nucleus and organelles.
• The endoplasmic reticulum is also a network of channels in the cytoplasm. Takes an active part in the synthesis of proteins, carbohydrates and lipids, and is involved in the transport of nutrients.
• Mitochondria are organelles in which organic substances are oxidized and ATP molecules are synthesized with the participation of enzymes. Essentially, mitochondria are a cell organelle that synthesizes energy.
• Plastids (chloroplasts, leucoplasts, chromoplasts) - as we mentioned above, are found exclusively in plant cells; in general, their presence is the main feature of the plant organism. They play a very important function, for example, chloroplasts, containing the green pigment chlorophyll, are responsible for the phenomenon in plants.
• The Golgi complex is a system of cavities delimited from the cytoplasm by a membrane. Carry out the synthesis of fats and carbohydrates on the membrane.
• Lysosomes are bodies separated from the cytoplasm by a membrane. The special enzymes they contain accelerate the breakdown of complex molecules. The lysosome is also an organelle that ensures protein assembly in cells.
• - cavities in the cytoplasm filled with cell sap, a place of accumulation of reserve nutrients; they regulate the water content in the cell.

In general, all organelles are important, because they regulate the life of the cell.

Basic cell organelles, video

And finally, a thematic video about cell organelles.