Unity of cell structure.
The contents of any cell are separated from the external environment by a special structure - plasma membrane (plasmalemma). This isolation allows you to create a very special environment inside the cell, unlike what surrounds it. Therefore, processes that do not occur anywhere else can occur in the cell; they are called life processes.
The internal environment of a living cell, bounded by the plasma membrane, is called cytoplasm. It includes hyaloplasm(basic transparent substance) and cell organelles, as well as various non-permanent structures - inclusions. Organelles that are present in any cell also include ribosomes, where it happens protein synthesis.
The structure of eukaryotic cells.
Eukaryotes- These are organisms whose cells have a nucleus. Core- this is the very organelle of the eukaryotic cell in which the hereditary information recorded in the chromosomes is stored and from which the hereditary information is transcribed. Chromosome is a DNA molecule integrated with proteins. The core contains nucleolus- the place where other important organelles involved in protein synthesis are formed - ribosomes. But ribosomes are only formed in the nucleus, and they work (i.e. synthesize protein) in the cytoplasm. Some of them are free in the cytoplasm, and some are attached to membranes, forming a network, which is called endoplasmic.
Ribosomes- non-membrane organelles.
Endoplasmic reticulum is a network of membrane-bounded tubules. There are two types: smooth and granular. Ribosomes are located on the membranes of the granular endoplasmic reticulum, so proteins are synthesized and transported there. And the smooth endoplasmic reticulum is the site of synthesis and transport of carbohydrates and lipids. There are no ribosomes on it.
The synthesis of proteins, carbohydrates and fats requires energy, which is produced in the eukaryotic cell by the “energy stations” of the cell - mitochondria.
Mitochondria- double-membrane organelles in which the process of cellular respiration occurs. Organic compounds are oxidized on mitochondrial membranes and chemical energy is accumulated in the form of special energy molecules (ATP).
There is also a place in the cell where organic compounds can accumulate and from where they can be transported - this is Golgi apparatus, system of flat membrane bags. It is involved in the transport of proteins, lipids, and carbohydrates. The Golgi apparatus also produces organelles for intracellular digestion - lysosomes.
Lysosomes- single-membrane organelles, characteristic of animal cells, contain enzymes that can break down proteins, carbohydrates, nucleic acids, and lipids.
A cell may contain organelles that do not have a membrane structure, such as ribosomes and a cytoskeleton.
Cytoskeleton- this is the musculoskeletal system of the cell, includes microfilaments, cilia, flagella, the cell center, which produces microtubules and centrioles.
There are organelles characteristic only of plant cells - plastids. There are: chloroplasts, chromoplasts and leucoplasts. The process of photosynthesis occurs in chloroplasts.
In plant cells also vacuoles- waste products of the cell, which are reservoirs of water and compounds dissolved in it. Eukaryotic organisms include plants, animals and fungi.
The structure of prokaryotic cells.
Prokaryotes- single-celled organisms whose cells do not have a nucleus.
Prokaryotic cells are small in size and store genetic material in the form of a circular DNA molecule (nucleoid). Prokaryotic organisms include bacteria and cyanobacteria, which were formerly called blue-green algae.
If the process of aerobic respiration occurs in prokaryotes, then special protrusions of the plasma membrane are used for this - mesosomes. If bacteria are photosynthetic, then the process of photosynthesis occurs on photosynthetic membranes - thylakoids.
Protein synthesis in prokaryotes occurs at ribosomes. Prokaryotic cells have few organelles.
Hypotheses of the origin of organelles of eukaryotic cells.
Prokaryotic cells appeared on Earth earlier than eukaryotic cells.
1) symbiotic hypothesis explains the mechanism of emergence of some organelles of the eukaryotic cell - mitochondria and photosynthetic plastids.
2) Intussusception hypothesis- states that the origin of the eukaryotic cell comes from the fact that the ancestral form was an aerobic prokaryote. The organelles in it arose as a result of invagination and detachment of parts of the shell, followed by functional specialization into the nucleus, mitochondria, chloroplasts of other organelles.
Eukaryotic cells from the simplest organisms to the cells of higher plants and mammals, they are distinguished by the complexity and diversity of structure. Typical eukaryotic cell does not exist, but common features can be identified from thousands of cell types. Each eukaryotic cell consists of cytoplasm and nucleus.
Structure eukaryotic cell.
Plasmalemma(cell membrane) of animal cells is formed by a membrane covered on the outside with a layer of glycocalyx 10-20 nm thick. Plasmalemma performs delimiting, barrier, transport and receptor functions. Due to the property of selective permeability, the plasmalemma regulates the chemical composition of the internal environment of the cell. The plasmalemma contains receptor molecules that selectively recognize certain biologically active substances (hormones). In layers and layers, neighboring cells are held together due to the presence of different types of contacts, which are represented by sections of the plasmalemma that have a special structure. The cortical layer is adjacent to the membrane from the inside cytoplasm thickness 0.1-0.5 microns.
Cytoplasm. The cytoplasm contains a number of formed structures that have regular features of structure and behavior at different periods of the life of the cell. Each of these structures has a specific function. Hence their comparison with the organs of the whole organism arose, and therefore they received the name organelles, or organoids. Various substances are deposited in the cytoplasm - inclusions (glycogen, fat droplets, pigments). The cytoplasm is permeated with membranes endoplasmic reticulum.
Endoplasmic reticulum (EDR). The endoplasmic reticulum is a branched network of channels and cavities in the cytoplasm of a cell, formed by membranes. On the membranes of the channels there are numerous enzymes that ensure the vital activity of the cell. There are 2 types of EMF membranes - smooth and rough. On membranes smooth endoplasmic reticulum There are enzyme systems involved in fat and carbohydrate metabolism. Main function rough endoplasmic reticulum- protein synthesis, which occurs in ribosomes attached to membranes. Endoplasmic reticulum- this is the general intracellular circulatory system, through the channels of which substances are transported inside the cell and from cell to cell.
Ribosomes carry out the function of protein synthesis. Ribosomes are spherical particles with a diameter of 15-35 nm, consisting of 2 subunits of unequal sizes and containing approximately equal amounts of proteins and RNA. Ribosomes in the cytoplasm are located or attached to the outer surface of the membranes of the endoplasmic reticulum. Depending on the type of protein synthesized, ribosomes can be combined into complexes - polyribosomes. Ribosomes are present in all types of cells.
Golgi complex. Main structural element Golgi complex is a smooth membrane that forms packets of flattened cisterns, or large vacuoles, or small vesicles. The cisternae of the Golgi complex are connected to the channels of the endoplasmic reticulum. Proteins, polysaccharides, and fats synthesized on the membranes of the endoplasmic reticulum are transported to the complex, condensed inside its structures and “packaged” in the form of a secretion, ready for release, or used in the cell itself during its life.
Mitochondria. The universal distribution of mitochondria in the animal and plant world indicates the important role that mitochondria playing in a cage. Mitochondria have the shape of spherical, oval and cylindrical bodies, and can be filamentous. The size of mitochondria is 0.2-1 microns in diameter, up to 5-7 microns in length. The length of the filamentous forms reaches 15-20 microns. The number of mitochondria in the cells of different tissues is not the same; there are more of them where synthetic processes are intense (liver) or energy costs are high. The mitochondrial wall consists of 2 membranes - outer and inner. The outer membrane is smooth, and septa - ridges, or cristae - extend from the inner membrane into the organoid. The membranes of the cristae contain numerous enzymes involved in energy metabolism. Main function of mitochondria - ATP synthesis.
Lysosomes- small oval bodies with a diameter of about 0.4 µm, surrounded by one three-layer membrane. Lysosomes contain about 30 enzymes that can break down proteins, nucleic acids, polysaccharides, lipids and other substances. The breakdown of substances using enzymes is called lysis, which is why the organoid is named lysosome. It is believed that lysosomes are formed from the structures of the Golgi complex or directly from the endoplasmic reticulum. Functions of lysosomes : intracellular digestion of nutrients, destruction of the structure of the cell itself when it dies during embryonic development, when embryonic tissues are replaced with permanent ones, and in a number of other cases.
Centrioles. The cell center consists of 2 very small cylindrical bodies located at right angles to each other. These bodies are called centrioles. The centriole wall consists of 9 pairs of microtubules. Centrioles are capable of self-assembly and belong to the self-replicating organelles of the cytoplasm. Centrioles play an important role in cell division: they begin the growth of microtubules that form the division spindle.
Core. The nucleus is the most important component of the cell. It contains DNA molecules and therefore performs two main functions: 1) storage and reproduction of genetic information, 2) regulation of metabolic processes occurring in the cell. Lost cell core, cannot exist. The core is also incapable of independent existence. Most cells have one nucleus, but 2-3 nuclei can be observed in one cell, for example in liver cells. Multinucleated cells with the number of nuclei of several dozen are known. The shapes of the nuclei depend on the shape of the cell. The kernels are spherical and multi-lobed. The core is surrounded by a shell consisting of two membranes having the usual three-layer structure. The outer nuclear membrane is covered with ribosomes, the inner membrane is smooth. The main role in the life of the nucleus is played by the exchange of substances between the nucleus and the cytoplasm. The contents of the nucleus include nuclear sap, or karyoplasm, chromatin and the nucleolus. The composition of nuclear sap includes various proteins, including most nuclear enzymes, free nucleotides, amino acids, products of the activity of the nucleolus and chromatin, moving from the nucleus to the cytoplasm. Chromatin contains DNA, proteins and represents spiralized and compacted sections of chromosomes. Nucleolus It is a dense round body located in the nuclear juice. The number of nucleoli ranges from 1 to 5-7 or more. Nucleoli are present only in non-dividing nuclei; during mitosis they disappear, and after completion of division they are formed again. The nucleolus is not an independent cell organelle; it lacks a membrane and is formed around the region of the chromosome in which the rRNA structure is encoded. Ribosomes are formed in the nucleolus, which then move into the cytoplasm. Chromatin are called lumps, granules and network-like structures of the nucleus, which are intensely stained with some dyes and differ in shape from the nucleolus.
A cell is an elementary unit of structure and vital activity of all alive organisms(except viruses, which are often referred to as non-cellular forms of life), possessing their own metabolism, capable of independent existence, self-reproduction and development. All living organisms are either multicellular animals, plants And mushrooms, consist of many cells, or, like many protozoa And bacteria, are single-celled organisms. The branch of biology that studies the structure and functioning of cells is called cytology. Recently, it has also become common to talk about cell biology, or cell biology.
Distinctive characteristics of plant and animal cells
|
Signs |
plant cell |
animal cell |
|
Plastids |
Chloroplasts, chromoplasts, leucoplasts |
None |
|
Nutrition method |
Autotrophic (phototrophic, chemotrophic) | |
|
ATP synthesis |
In chloroplasts, mitochondria |
In mitochondria |
|
ATP breakdown |
In chloroplasts and all parts of the cell where energy is required |
In all parts of the cell where energy is required |
|
Cell center |
In lower plants |
In all cells |
|
Cellulose cell wall |
Located outside the cell membrane |
Absent |
|
Inclusions |
Spare nutrients in the form of grains of starch, protein, drops of oil; vacuoles with cell sap; salt crystals |
Spare nutrients in the form of grains and drops (proteins, fats, carbohydrates, glycogen); end products of metabolism, salt crystals, pigments |
|
Large cavities filled with cell sap - an aqueous solution of various substances (reserve or final products). Osmotic reservoirs of the cell. |
Contractile, digestive, excretory vacuoles. Usually small. |
General features 1. Unity of structural systems - cytoplasm and nucleus. 2. The similarity of metabolic and energy processes. 3. Unity of the principle of hereditary code. 4. Universal membrane structure. 5. Unity of chemical composition. 6. Similarities in the process of cell division.
Cell structure
All cellular life forms on Earth can be divided into two superkingdoms based on the structure of their constituent cells:
prokaryotes (prenuclear) - simpler in structure and arose earlier in the process of evolution;
eukaryotes (nuclear) - more complex, arose later. The cells that make up the human body are eukaryotic.
Despite the variety of forms, the organization of cells of all living organisms is subject to common structural principles.
The contents of the cell are separated from the environment by the plasma membrane, or plasmalemma. Inside the cell is filled with cytoplasm, in which various organelles and cellular inclusions are located, as well as genetic material in the form of a DNA molecule. Each organelle of the cell performs its own special function, and together they all determine the vital activity of the cell as a whole.
Prokaryotic cell
Structure of a typical prokaryotic cell: capsule, cell wall, plasmalemma, cytoplasm,ribosomes, plasmid, drank, flagellum,nucleoid.
Prokaryotes (from lat. pro- before, before and Greek κάρῠον - core, nut) - organisms that, unlike eukaryotes, do not have a formed cell nucleus and other internal membrane organelles (with the exception of flat tanks in photosynthetic species, for example, cyanobacteria). A single large circular (in some species linear) double-stranded molecule DNA, which contains the bulk of the cell’s genetic material (the so-called nucleoid) does not form a complex with proteins - histones(so-called chromatin). Prokaryotes include bacteria, including cyanobacteria(blue-green algae), and archaea. The descendants of prokaryotic cells are organelles eukaryotic cells - mitochondria And plastids. The main content of the cell, filling its entire volume, is viscous granular cytoplasm.
Eukaryotic cell
Eukaryotes are organisms that, unlike prokaryotes, have a cellular structure core, delimited from the cytoplasm by the nuclear envelope. The genetic material is contained in several linear double-stranded DNA molecules (depending on the type of organism, their number per nucleus can range from two to several hundred), attached from the inside to the membrane of the cell nucleus and forming in the vast majority (except dinoflagellates) complex with proteins - histones, called chromatin. Eukaryotic cells have a system of internal membranes that, in addition to the nucleus, form a number of other organoids (endoplasmic reticulum, Golgi apparatus and etc.). In addition, the vast majority have permanent intracellular symbionts-prokaryotes - mitochondria, and in algae and plants - also plastids.
Structure of a eukaryotic cell
Schematic representation of an animal cell. (By clicking on any of the names of the component parts of the cell, you will be taken to the corresponding article.)
Surface complex of an animal cell
Consists of the glycocalyx, the plasmalemma and the underlying cortical layer. cytoplasm. The plasma membrane is also called plasmalemma, the outer membrane of the cell. This is a biological membrane, about 10 nanometers thick. Provides primarily a delimiting function in relation to the environment external to the cell. In addition, she performs transport function. The cell does not waste energy to maintain the integrity of its membrane: the molecules are held together according to the same principle by which fat molecules are held together - hydrophobic It is thermodynamically more advantageous for parts of molecules to be located in close proximity to each other. The glycocalyx is molecules of oligosaccharides, polysaccharides, glycoproteins and glycolipids “anchored” in the plasmalemma. The glycocalyx performs receptor and marker functions. Plasma membrane animals cells mainly consists of phospholipids and lipoproteins interspersed with protein molecules, in particular, surface antigens and receptors. In the cortical (adjacent to the plasma membrane) layer of the cytoplasm there are specific cytoskeletal elements - actin microfilaments ordered in a certain way. The main and most important function of the cortical layer (cortex) is pseudopodial reactions: ejection, attachment and contraction of pseudopodia. In this case, the microfilaments are rearranged, lengthened or shortened. The shape of the cell (for example, the presence of microvilli) also depends on the structure of the cytoskeleton of the cortical layer.
The cells that form the tissues of animals and plants vary significantly in shape, size and internal structure. However, they all show similarities in the main features of life processes, metabolism, irritability, growth, development, and the ability to change.
Cells of all types contain two main components, closely related to each other - the cytoplasm and the nucleus. The nucleus is separated from the cytoplasm by a porous membrane and contains nuclear sap, chromatin and the nucleolus. Semi-liquid cytoplasm fills the entire cell and is penetrated by numerous tubules. On the outside it is covered with a cytoplasmic membrane. It has specialized organelle structures, permanently present in the cell, and temporary formations - inclusions.Membrane organelles : outer cytoplasmic membrane (OCM), endoplasmic reticulum (ER), Golgi apparatus, lysosomes, mitochondria and plastids. The structure of all membrane organelles is based on a biological membrane. All membranes have a fundamentally uniform structural plan and consist of a double layer of phospholipids, into which protein molecules are immersed at different depths on different sides. The membranes of organelles differ from each other only in the sets of proteins they contain.
Scheme of the structure of a eukaryotic cell. A - cell of animal origin; B - plant cell: 1 - nucleus with chromatin and nucleolus, 2 - cytoplasmic membrane, 3 - cell wall, 4 - pores in the cell wall through which the cytoplasm of neighboring cells communicates, 5 - rough endoplasmic reticulum, b - smooth endoplasmic reticulum, 7 - pinocytotic vacuole, 8 - Golgi apparatus (complex), 9 - lysosome, 10 - fatty inclusions in the channels of the smooth endoplasmic reticulum, 11 - cell center, 12 - mitochondria, 13 - free ribosomes and polyribosomes, 14 - vacuole, 15 - chloroplast
Cytoplasmic membrane. All plant cells, multicellular animals, protozoa and bacteria have a three-layer cell membrane: the outer and inner layers consist of protein molecules, the middle layer consists of lipid molecules. It limits the cytoplasm from the external environment, surrounds all cell organelles and is a universal biological structure. In some cells, the outer membrane is formed by several membranes tightly adjacent to each other. In such cases, the cell membrane becomes dense and elastic and allows the cell to maintain its shape, as, for example, in euglena and slipper ciliates. Most plant cells, in addition to the membrane, also have a thick cellulose shell on the outside - cell wall. It is clearly visible in a conventional light microscope and performs a supporting function due to the rigid outer layer, which gives the cells a clear shape.
On the surface of cells, the membrane forms elongated outgrowths - microvilli, folds, invaginations and protrusions, which greatly increases the absorption or excretory surface. With the help of membrane outgrowths, cells connect with each other in the tissues and organs of multicellular organisms; various enzymes involved in metabolism are located on the folds of the membranes. By delimiting the cell from the environment, the membrane regulates the direction of diffusion of substances and at the same time actively transports them into the cell (accumulation) or out (excretion). Due to these properties of the membrane, the concentration of potassium, calcium, magnesium, and phosphorus ions in the cytoplasm is higher, and the concentration of sodium and chlorine is lower than in the environment. Through the pores of the outer membrane, ions, water and small molecules of other substances penetrate into the cell from the external environment. Penetration of relatively large solid particles into the cell is carried out by phagocytosis(from the Greek “phago” - devour, “drink” - cell). In this case, the outer membrane at the point of contact with the particle bends into the cell, drawing the particle deep into the cytoplasm, where it undergoes enzymatic cleavage. Drops of liquid substances enter the cell in a similar way; their absorption is called pinocytosis(from the Greek “pino” - drink, “cytos” - cell). The outer cell membrane also performs other important biological functions.
Cytoplasm 85% consists of water, 10% - proteins, the rest of the volume accounts for lipids, carbohydrates, nucleic acids and mineral compounds; all these substances form a colloidal solution similar in consistency to glycerin. The colloidal substance of a cell, depending on its physiological state and the nature of the influence of the external environment, has the properties of both a liquid and an elastic, denser body. The cytoplasm is penetrated by channels of various shapes and sizes, which are called endoplasmic reticulum. Their walls are membranes that are in close contact with all organelles of the cell and together with them constitute a single functional and structural system for the metabolism and energy and movement of substances within the cell.
The walls of the tubules contain tiny granules called ribosomes. This network of tubules is called granular. Ribosomes can be located scattered on the surface of the tubules or form complexes of five to seven or more ribosomes, called polysomes. Other tubules do not contain granules; they form a smooth endoplasmic reticulum. Enzymes involved in the synthesis of fats and carbohydrates are located on the walls.
The internal cavity of the tubules is filled with waste products of the cell. Intracellular tubules, forming a complex branching system, regulate the movement and concentration of substances, separate various molecules of organic substances and the stages of their synthesis. On the inner and outer surfaces of membranes rich in enzymes, proteins, fats and carbohydrates are synthesized, which are either used in metabolism, or accumulate in the cytoplasm as inclusions, or are excreted.
Ribosomes found in all types of cells - from bacteria to cells of multicellular organisms. These are round bodies consisting of ribonucleic acid (RNA) and proteins in almost equal proportions. They certainly contain magnesium, the presence of which maintains the structure of ribosomes. Ribosomes can be associated with the membranes of the endoplasmic reticulum, with the outer cell membrane, or lie free in the cytoplasm. They carry out protein synthesis. In addition to the cytoplasm, ribosomes are found in the cell nucleus. They are formed in the nucleolus and then enter the cytoplasm.
Golgi complex in plant cells it looks like individual bodies surrounded by membranes. In animal cells, this organelle is represented by cisterns, tubules and vesicles. Cell secretion products enter the membrane tubes of the Golgi complex from the tubules of the endoplasmic reticulum, where they are chemically rearranged, compacted, and then pass into the cytoplasm and are either used by the cell itself or removed from it. In the tanks of the Golgi complex, polysaccharides are synthesized and combined with proteins, resulting in the formation of glycoproteins.
Mitochondria- small rod-shaped bodies bounded by two membranes. Numerous folds - cristae - extend from the inner membrane of the mitochondrion; on their walls there are various enzymes, with the help of which the synthesis of a high-energy substance - adenosine triphosphoric acid (ATP) is carried out. Depending on the activity of the cell and external influences, mitochondria can move, change their size and shape. Ribosomes, phospholipids, RNA and DNA are found in mitochondria. The presence of DNA in mitochondria is associated with the ability of these organelles to reproduce by forming a constriction or budding during cell division, as well as the synthesis of some mitochondrial proteins.
Lysosomes- small oval formations, bounded by a membrane and scattered throughout the cytoplasm. Found in all cells of animals and plants. They arise in extensions of the endoplasmic reticulum and in the Golgi complex, here they are filled with hydrolytic enzymes, and then separate and enter the cytoplasm. Under normal conditions, lysosomes digest particles that enter the cell by phagocytosis and organelles of dying cells. Lysosome products are excreted through the lysosome membrane into the cytoplasm, where they are included in new molecules. When the lysosome membrane ruptures, enzymes enter the cytoplasm and digest its contents, causing cell death.
Plastids found only in plant cells and found in most green plants. Organic substances are synthesized and accumulated in plastids. There are three types of plastids: chloroplasts, chromoplasts and leucoplasts.
Chloroplasts - green plastids containing the green pigment chlorophyll. They are found in leaves, young stems, and unripe fruits. Chloroplasts are surrounded by a double membrane. In higher plants, the internal part of the chloroplasts is filled with a semi-liquid substance, in which the plates are laid parallel to each other. Paired membranes of the plates, merging, form stacks containing chlorophyll (Fig. 6). In each stack of chloroplasts of higher plants, layers of protein molecules and lipid molecules alternate, and chlorophyll molecules are located between them. This layered structure provides maximum free surfaces and facilitates the capture and transfer of energy during photosynthesis.
Chromoplasts - plastids containing plant pigments (red or brown, yellow, orange). They are concentrated in the cytoplasm of cells of flowers, stems, fruits, and leaves of plants and give them the appropriate color. Chromoplasts are formed from leucoplasts or chloroplasts as a result of the accumulation of pigments carotenoids.
Leukoplasts - colorless plastids located in uncolored parts of plants: in stems, roots, bulbs, etc. Starch grains accumulate in the leucoplasts of some cells, oils and proteins accumulate in the leucoplasts of other cells.
All plastids arise from their predecessors - proplastids. They revealed DNA that controls the reproduction of these organelles.
Cell center, or centrosome, plays an important role in cell division and consists of two centrioles . It is found in all animal and plant cells, except for flowering fungi, lower fungi and some protozoa. Centrioles in dividing cells take part in the formation of the division spindle and are located at its poles. In a dividing cell, the cell center is the first to divide, and at the same time an achromatin spindle is formed, which orients the chromosomes as they diverge to the poles. One centriole leaves each of the daughter cells.
Many plant and animal cells have special purpose organoids: cilia, performing the function of movement (ciliates, respiratory tract cells), flagella(protozoa unicellular, male reproductive cells in animals and plants, etc.). Inclusions - temporary elements that arise in a cell at a certain stage of its life as a result of a synthetic function. They are either used or removed from the cell. Inclusions are also reserve nutrients: in plant cells - starch, droplets of fat, blocks, essential oils, many organic acids, salts of organic and inorganic acids; in animal cells - glycogen (in liver cells and muscles), drops of fat (in subcutaneous tissue); Some inclusions accumulate in cells as waste - in the form of crystals, pigments, etc.
Vacuoles - these are cavities bounded by a membrane; well expressed in plant cells and present in protozoa. They arise in different areas of the endoplasmic reticulum. And they gradually separate from it. Vacuoles maintain turgor pressure; cellular or vacuolar sap is concentrated in them, the molecules of which determine its osmotic concentration. It is believed that the initial products of synthesis - soluble carbohydrates, proteins, pectins, etc. - accumulate in the cisterns of the endoplasmic reticulum. These clusters represent the rudiments of future vacuoles.
Cytoskeleton . One of the distinctive features of a eukaryotic cell is the development in its cytoplasm of skeletal formations in the form of microtubules and bundles of protein fibers. The elements of the cytoskeleton are closely associated with the outer cytoplasmic membrane and the nuclear envelope and form complex weaves in the cytoplasm. The supporting elements of the cytoplasm determine the shape of the cell, ensure the movement of intracellular structures and the movement of the entire cell.
Core The cell plays a major role in its life; with its removal, the cell ceases its functions and dies. Most animal cells have one nucleus, but there are also multinucleated cells (human liver and muscles, fungi, ciliates, green algae). Mammalian red blood cells develop from precursor cells containing a nucleus, but mature red blood cells lose it and do not live long.
The nucleus is surrounded by a double membrane, permeated with pores, through which it is closely connected with the channels of the endoplasmic reticulum and the cytoplasm. Inside the core is chromatin- spiralized sections of chromosomes. During cell division, they turn into rod-shaped structures that are clearly visible under a light microscope. Chromosomes are complex complexes of proteins and DNA called nucleoprotein.
The functions of the nucleus are to regulate all the vital functions of the cell, which it carries out with the help of DNA and RNA material carriers of hereditary information. In preparation for cell division, DNA doubles; during mitosis, chromosomes separate and are passed on to daughter cells, ensuring the continuity of hereditary information in each type of organism.
Karyoplasm - liquid phase of the nucleus, in which the waste products of nuclear structures are found in dissolved form
Nucleolus- isolated, densest part of the core. The nucleolus contains complex proteins and RNA, free or bound phosphates of potassium, magnesium, calcium, iron, zinc, as well as ribosomes. The nucleolus disappears before the start of cell division and is re-formed in the last phase of division.
Thus, the cell has a fine and very complex organization. The extensive network of cytoplasmic membranes and the membrane principle of the structure of organelles make it possible to distinguish between the many chemical reactions occurring simultaneously in the cell. Each of the intracellular formations has its own structure and specific function, but only through their interaction is the harmonious functioning of the cell possible. Based on this interaction, substances from the environment enter the cell, and waste products are removed from it into the external environment - this is how metabolism occurs. The perfection of the structural organization of a cell could only arise as a result of long-term biological evolution, during which the functions it performed gradually became more complex.
The simplest unicellular forms represent both a cell and an organism with all its life manifestations. In multicellular organisms, cells form homogeneous groups - tissues. In turn, tissues form organs, systems, and their functions are determined by the general vital activity of the whole organism.
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