All are eukaryotic organisms. They can be unicellular or multicellular, but all have a common cell structure. All these very dissimilar organisms are believed to have a common origin, so the nuclear group is considered the highest-ranking monophyletic taxon. According to the most common hypotheses, eukaryotes appeared 1.5–2 billion years ago. An important role in the evolution of eukaryotes was played by symbiogenesis - a symbiosis between a eukaryotic cell, apparently already having a nucleus and capable of phagocytosis, and bacteria absorbed by this cell - the precursors of mitochondria and plastids.

Structure of a eukaryotic cell

See also category Structures of a eukaryotic cell

Eukaryotic cells are on average much larger than prokaryotic cells, the difference in volume reaches thousands of times. Eukaryotic cells include about a dozen types of different structures known as organelles (or organelles, which, however, somewhat distorts the original meaning of this term), of which many are separated from the cytoplasm by one or more membranes (in prokaryotic cells, internal organelles surrounded by a membrane are rare ). The nucleus is a part of the cell, surrounded in eukaryotes by a double membrane (two elementary membranes) and containing genetic material: DNA molecules, “packed” into chromosomes. There is usually one nucleus, but there are also multinucleated cells.

Division into kingdoms

There are several options for dividing the eukaryotic superkingdom into kingdoms. The plant and animal kingdoms were the first to be distinguished. Then the kingdom of fungi was identified, which, due to their biochemical characteristics, according to most biologists, cannot be classified as one of these kingdoms. Also, some authors distinguish the kingdoms of protozoa, myxomycetes, and chromists. Some systems have up to 20 kingdoms. According to the Thomas Cavalier-Smith system, all eukaryotes are divided into two monophyletic taxa - Unikonta And Bikonta. The position of eukaryotes such as Collodictyon ( Collodictyon) And Diphylleia, currently undefined.

Differences between eukaryotes and prokaryotes

The most important, fundamental feature of eukaryotic cells is associated with the location of the genetic apparatus in the cell. The genetic apparatus of all eukaryotes is located in the nucleus and is protected by the nuclear envelope (in Greek, “eukaryote” means having a nucleus). The DNA of eukaryotes is linear (in prokaryotes, the DNA is circular and is located in a special region of the cell - the nucleoid, which is not separated by a membrane from the rest of the cytoplasm). It is associated with histone proteins and other chromosomal proteins that bacteria do not have.

In the life cycle of eukaryotes, there are usually two nuclear phases (haplophase and diplophase). The first phase is characterized by a haploid (single) set of chromosomes, then, merging, two haploid cells (or two nuclei) form a diploid cell (nucleus) containing a double (diploid) set of chromosomes. Sometimes during the next division, and more often after several divisions, the cell again becomes haploid. Such a life cycle and, in general, diploidity are not typical for prokaryotes.

The third, perhaps the most interesting difference, is the presence in eukaryotic cells of special organelles that have their own genetic apparatus, reproduce by division and are surrounded by a membrane. These organelles are mitochondria and plastids. In their structure and life activity they are strikingly similar to bacteria. This circumstance has prompted modern scientists to believe that such organisms are descendants of bacteria that entered into a symbiotic relationship with eukaryotes. Prokaryotes are characterized by a small number of organelles, and none of them are surrounded by a double membrane. Prokaryotic cells do not have an endoplasmic reticulum, Golgi apparatus, or lysosomes.

Another important difference between prokaryotes and eukaryotes is the presence of endocytosis in eukaryotes, including phagocytosis in many groups. Phagocytosis (literally “eating by a cell”) is the ability of eukaryotic cells to capture, enclose in a membrane vesicle, and digest a wide variety of solid particles. This process provides an important protective function in the body. It was first discovered by I.I. Mechnikov in starfish. The appearance of phagocytosis in eukaryotes is most likely associated with average size (more about size differences is written below). The sizes of prokaryotic cells are disproportionately smaller, and therefore, in the process of evolutionary development of eukaryotes, they had the problem of supplying the body with a large amount of food. As a result, the first real, mobile predators appear among eukaryotes.

Most bacteria have a cell wall that is different from the eukaryotic one (not all eukaryotes have it). In prokaryotes, it is a durable structure consisting mainly of murein (in archaea, pseudomurein). The structure of murein is such that each cell is surrounded by a special mesh sac, which is one huge molecule. Among eukaryotes, many protists, fungi and plants have a cell wall. In fungi it consists of chitin and glucans, in lower plants it consists of cellulose and glycoproteins, diatoms synthesize a cell wall from silicic acids, in higher plants it consists of cellulose, hemicellulose and pectin. Apparently, for larger eukaryotic cells it has become impossible to create a cell wall of high strength from a single molecule. This circumstance could force eukaryotes to use different material for the cell wall. Another explanation is that the common ancestor of eukaryotes lost its cell wall due to the transition to predation, and then the genes responsible for the synthesis of murein were also lost. When some eukaryotes returned to osmotrophic nutrition, the cell wall appeared again, but on a different biochemical basis.

The metabolism of bacteria is also diverse. In general, there are four types of nutrition, and all are found among bacteria. These are photoautotrophic, photoheterotrophic, chemoautotrophic, chemoheterotrophic (phototrophic use the energy of sunlight, chemotrophic use chemical energy). Eukaryotes either synthesize energy from sunlight themselves or use ready-made energy of this origin. This may be due to the emergence of predators among eukaryotes, for which the need to synthesize energy has disappeared.

Another difference is the structure of the flagella. In bacteria they are thin - only 15–20 nm in diameter. These are hollow filaments made from the protein flagellin. The structure of eukaryotic flagella is much more complex. They are a cell outgrowth surrounded by a membrane and contain a cytoskeleton (axoneme) of nine pairs of peripheral microtubules and two microtubules in the center. Unlike rotating prokaryotic flagella, eukaryotic flagella bend or wriggle.

The two groups of organisms we are considering, as already mentioned, are very different in their average sizes. The diameter of a prokaryotic cell is usually 0.5–10 μm, while the same figure for eukaryotes is 10–100 μm. The volume of such a cell is 1000–10000 times greater than that of a prokaryotic cell.

Prokaryotic ribosomes are small (70S type). Eukaryotic cells contain both larger 80S-type ribosomes located in the cytoplasm and prokaryotic-type 70s ribosomes located in mitochondria and plastids.

Apparently, the time of emergence of these groups also differs. The first prokaryotes arose in the process of evolution about 3.5 billion years ago, from them about 1.2 billion years ago eukaryotic organisms evolved.

see also

	Eukaryotes in Wiktionary
	Eukaryotes on Wikimedia Commons

Foreign literature

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Which have a core. Almost all organisms are eukaryotes, except bacteria (viruses belong to a separate category, which not all biologists distinguish as a category of living beings). Eukaryotes include plants, animals, mushrooms and such living organisms as slime mold. Eukaryotes are divided into single-celled organisms And multicellular, but the principle of cell structure is the same for all of them.

It is believed that the first eukaryotes appeared about 2 billion years ago and evolved largely due to symbiogenesis- the interaction of eukaryotic cells and bacteria, which these cells absorbed, being capable of phagocytosis.

Eukaryotic cells They are very large in size, especially compared to prokaryotic ones. A eukaryotic cell has about ten organelles, most of which are separated by membranes from the cytoplasm, which is not the case in prokaryotes. Eukaryotes also have a nucleus, which we have already discussed. This is the part of the cell that is fenced off from the cytoplasm by a double membrane. It is in this part of the cell that the DNA contained in the chromosomes is located. The cells are usually mononucleated, but multinucleated cells are sometimes found.

Kingdoms of eukaryotes.

There are several options for dividing eukaryotes. Initially, all living organisms were divided only into plants and animals. Subsequently, the kingdom of mushrooms was identified, which differ significantly from both the first and the second. Even later, slime molds began to be isolated.

Slime mold is a polyphyletic group of organisms that some classify as the simplest, but the final classification of these organisms has not been fully classified. At one stage of development, these organisms have a plasmodic form - this is a slimy substance that does not have clear hard covers. In general, slime molds look like one multinucleate cell, which is visible to the naked eye.

Slime molds are related to fungi by sporulation, which germinate as zoospores, from which plasmodium subsequently develops.

Slime molds are heterotrophs capable of feeding inspectively, that is, absorb nutrients directly through the membrane, or endocytosis - take vesicles with nutrients inside. Slime molds include Acrasiaceae, Myxomycetes, Labyrinthulae and Plasmodiophorae.

Differences between prokaryotes and eukaryotes.

The main difference prokaryote and eukaryotes is that prokaryotes do not have a formed nucleus, separated by a membrane from the cytoplasm. In prokaryotes, circular DNA is found in the cytoplasm, and the place where the DNA is located is called the nucleoid.

Additional differences between eukaryotes.

1. Of the organelles, prokaryotes have only ribosomes 70S (small), and eukaryotes have not only large 80S ribosomes, but also many other organelles.
2. Since prokaryotes do not have a nucleus, they divide by fission in two - not with the help meiosis/mitosis.
3. Eukaryotes have histones that bacteria do not. Chromantin in eukaryotes contains 1/3 DNA and 2/3 protein; in prokaryotes the opposite is true.
4. A eukaryotic cell is 1000 times larger in volume and 10 times larger in diameter than a prokaryotic cell.

The structure of eukaryotic and prokaryotic cells. Eukaryotic cell. The structure of a prokaryotic cell. Comparison of prokaryotic and eukaryotic cells.

There are two types of cells known in modern and fossil organisms: prokaryotic and eukaryotic. They differ so sharply in structural features that this served to distinguish two superkingdoms of the living world - prokaryotes, i.e. prenuclear, and eukaryotes, i.e. real nuclear organisms. Intermediate forms between these largest living taxa are still unknown.

Main features and differences between prokaryotic and eukaryotic cells (table):

Signs	Prokaryotes	Eukaryotes
NUCLEAR MEMBRANE	Absent	Available
PLASMA MEMBRANE	Available	Available
MITOCHONDRIA	None	Available
EPS	Absent	Available
RIBOSOMES	Available	Available
VACUOLES	None	Available (especially typical for plants)
LYSOSOMES	None	Available
CELL WALL	Available, consists of a complex heteropolymer substance	Absent in animal cells, in plant cells it consists of cellulose
CAPSULE	If present, it consists of protein and sugar compounds	Absent
GOLGI COMPLEX	Absent	Available
DIVISION	Simple	Mitosis, amitosis, meiosis

The main difference between prokaryotic cells and eukaryotic cells is that their DNA is not organized into chromosomes and is not surrounded by a nuclear envelope. Eukaryotic cells are much more complex. Their DNA, associated with protein, is organized into chromosomes, which are located in a special formation, essentially the largest organelle of the cell - the nucleus. In addition, the extranuclear active content of such a cell is divided into separate compartments using the endoplasmic reticulum formed by the elementary membrane. Eukaryotic cells are usually larger than prokaryotic cells. Their sizes vary from 10 to 100 microns, while the sizes of prokaryotic cells (various bacteria, cyanobacteria - blue-green algae and some other organisms), as a rule, do not exceed 10 microns, often amounting to 2-3 microns. In a eukaryotic cell, gene carriers - chromosomes - are located in a morphologically formed nucleus, delimited from the rest of the cell by a membrane. In exceptionally thin, transparent preparations, living chromosomes can be seen using a light microscope. More often they are studied on fixed and colored preparations.

Chromosomes consist of DNA, which is complexed with histone proteins rich in the amino acids arginine and lysine. Histones make up a significant portion of the mass of chromosomes.

A eukaryotic cell has a variety of permanent intracellular structures - organelles (organelles) that are absent in a prokaryotic cell.

Prokaryotic cells can divide into equal parts by constriction or bud, i.e. produce daughter cells smaller than the mother cell, but never divide by mitosis. In contrast, cells of eukaryotic organisms divide by mitosis (except for some very archaic groups). In this case, the chromosomes “split” longitudinally (more precisely, each DNA strand reproduces its own likeness around itself), and their “halves” - chromatids (full copies of the DNA strand) disperse in groups to opposite poles of the cell. Each of the resulting cells receives the same set of chromosomes.

The ribosomes of a prokaryotic cell differ sharply from the ribosomes of eukaryotes in size. A number of processes characteristic of the cytoplasm of many eukaryotic cells - phagocytosis, pinocytosis and cyclosis (rotational movement of the cytoplasm) - have not been found in prokaryotes. A prokaryotic cell does not require ascorbic acid in the metabolic process, but eukaryotic cells cannot do without it.

The motile forms of prokaryotic and eukaryotic cells differ significantly. Prokaryotes have motor devices in the form of flagella or cilia, consisting of the protein flagellin. The motor devices of motile eukaryotic cells are called undulipodia, which are anchored in the cell with the help of special kinetosome bodies. Electron microscopy revealed the structural similarity of all undulipodia of eukaryotic organisms and their sharp differences from the flagella of prokaryotes

1. The structure of a eukaryotic cell.

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.
All types of cells contain two main components that are 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.

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% of proteins, the rest of the volume is made up of 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 grains called granules. 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 fuse to form stacks containing chlorophyll. 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 the uncolored parts of plants: in stems, roots, bulbs, etc. Starch grains accumulate in the leucoplasts of some cells, and 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, proteins, 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 - the 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.

2. Prokaryotic cell.

Prokaryotes include bacteria and blue-green algae (cyanea). The hereditary apparatus of prokaryotes is represented by one circular DNA molecule that does not form bonds with proteins and contains one copy of each gene - haploid organisms. The cytoplasm contains a large number of small ribosomes; internal membranes are absent or poorly expressed. Enzymes of plastic metabolism are located diffusely. The Golgi apparatus is represented by individual vesicles. Enzyme systems for energy metabolism are orderedly located on the inner surface of the outer cytoplasmic membrane. The outside of the cell is surrounded by a thick cell wall. Many prokaryotes are capable of sporulation under unfavorable living conditions; in this case, a small section of the cytoplasm containing DNA is isolated and surrounded by a thick multilayer capsule. Metabolic processes inside the spore practically stop. When exposed to favorable conditions, the spore transforms into an active cellular form. Prokaryotes reproduce by simple division in two.

The average size of prokaryotic cells is 5 microns. They do not have any internal membranes other than invaginations of the plasma membrane. There are no layers. Instead of a cell nucleus, there is its equivalent (nucleoid), devoid of a shell and consisting of a single DNA molecule. In addition, bacteria may contain DNA in the form of tiny plasmids, similar to the extranuclear DNA of eukaryotes.
Prokaryotic cells capable of photosynthesis (blue-green algae, green and purple bacteria) have differently structured large membrane invaginations - thylakoids, which in their function correspond to eukaryotic plastids. These same thylakoids or, in colorless cells, smaller membrane invaginations (and sometimes even the plasma membrane itself) functionally replace mitochondria. Other, complexly differentiated membrane invaginations are called mesasomes; their function is not clear.
Only some organelles of a prokaryotic cell are homologous to the corresponding organelles of eukaryotes. Prokaryotes are characterized by the presence of a murein sac - a mechanically strong element of the cell wall

Comparative characteristics of cells of plants, animals, bacteria, fungi

When comparing bacteria with eukaryotes, the only similarity that can be identified is the presence of a cell wall, but the similarities and differences of eukaryotic organisms deserve closer attention. The comparison should begin with components that are characteristic of plants, animals, and fungi. These are the nucleus, mitochondria, Golgi apparatus (complex), endoplasmic reticulum (or endoplasmic reticulum) and lysosomes. They are characteristic of all organisms, have a similar structure and perform the same functions. Now we need to focus on the differences. A plant cell, unlike an animal cell, has a cell wall consisting of cellulose. In addition, there are organelles characteristic of plant cells - plastids and vacuoles. The presence of these components is due to the need for plants to maintain their shape in the absence of a skeleton. There are differences in growth characteristics. In plants, it occurs mainly due to an increase in the size of vacuoles and cell elongation, while in animals there is an increase in the volume of the cytoplasm, and the vacuole is completely absent. Plastids (chloroplasts, leucoplasts, chromoplasts) are characteristic primarily of plants, since their main task is to provide an autotrophic method of nutrition. Animals, as opposed to plants, have digestive vacuoles that provide a heterotrophic method of nutrition. Fungi occupy a special position and their cells are characterized by characteristics characteristic of both plants and animals. Like animal fungi, they have a heterotrophic type of nutrition, a chitin-containing cell wall, and the main storage substance is glycogen. At the same time, they, like plants, are characterized by unlimited growth, inability to move, and nutrition by absorption.

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10. Vacuole 11. Hyaloplasm 12. Lysosome 13. Centrosome (Centriole)

Eukaryotes, or Nuclear(lat. Eucaryota from Greek εύ- - good and κάρυον - nucleus) - a superkingdom of living organisms whose cells contain nuclei. All organisms except bacteria and archaea are nuclear.

Structure of a eukaryotic cell Eukaryotic cells are on average much larger than prokaryotic cells, the difference in volume reaches thousands of times. Eukaryotic cells include about a dozen types of different structures known as organelles (or organelles, which, however, somewhat distorts the original meaning of this term), many of which are separated from the cytoplasm by one or more membranes. Prokaryotic cells always contain a cell membrane, ribosomes (significantly different from eukaryotic ribosomes) and genetic material - a bacterial chromosome, or genophore, but internal organelles surrounded by a membrane are rare. The nucleus is a part of the cell, surrounded in eukaryotes by a double membrane (two elementary membranes) and containing genetic material: DNA molecules, “packed” into chromosomes. There is usually one nucleus, but there are also multinucleated cells. Division into kingdoms There are several options for dividing the eukaryotic superkingdom into kingdoms. The plant and animal kingdoms were the first to be distinguished. Then the kingdom of fungi was identified, which, due to their biochemical characteristics, according to most biologists, cannot be classified as one of these kingdoms. Also, some authors distinguish the kingdoms of protozoa, myxomycetes, and chromists. Some systems have up to 20 kingdoms. Differences between eukaryotes and prokaryotes The most important, fundamental feature of eukaryotic cells is associated with the location of the genetic apparatus in the cell. The genetic apparatus of all eukaryotes is located in the nucleus and is protected by the nuclear envelope (in Greek, “eukaryote” means having a nucleus). Eukaryotic DNA is linear (in prokaryotes, DNA is circular and floats freely in the cytoplasm). It is associated with histone proteins and other chromosomal proteins that bacteria do not have. In the life cycle of eukaryotes, there are usually two nuclear phases (haplophase and diplophase). The first phase is characterized by a haploid (single) set of chromosomes, then, merging, two haploid cells (or two nuclei) form a diploid cell (nucleus) containing a double (diploid) set of chromosomes. After several divisions, the cell again becomes haploid. Such a life cycle and, in general, diploidity are not typical for prokaryotes. The third, perhaps the most interesting difference, is the presence in eukaryotic cells of special organelles that have their own genetic apparatus, reproduce by division and are surrounded by a membrane. These organelles are mitochondria and plastids. In their structure and life activity they are strikingly similar to bacteria. This circumstance has prompted modern scientists to believe that such organisms are descendants of bacteria that entered into a symbiotic relationship with eukaryotes. Prokaryotes are characterized by a small number of organelles, and none of them are surrounded by a double membrane. Prokaryotic cells do not have an endoplasmic reticulum, Golgi apparatus, or lysosomes. It is equally important, when describing the differences between prokaryotes and eukaryotes, to talk about such a phenomenon in eukaryotic cells as phagocytosis. Phagocytosis (literally “eating”) refers to the ability of eukaryotic cells to capture and digest a wide variety of solid particles. This process provides an important protective function in the body. It was first discovered by I.I. Mechnikov at the starfish. The appearance of phagocytosis in eukaryotes is most likely associated with average size (more about size differences is written below). The sizes of prokaryotic cells are disproportionately smaller and therefore, in the process of evolutionary development, eukaryotes faced the problem of supplying the body with large amounts of food, as a result, the first predators appeared in the group of eukaryotes. Most bacteria have a cell wall that is different from the eukaryotic one (not all eukaryotes have it). In prokaryotes, it is a durable structure consisting mainly of murein. The structure of murein is such that each cell is surrounded by a special mesh sac, which is one huge molecule. Among eukaryotes, fungi and plants have a cell wall. In fungi it consists of chitin and glucans, in lower plants from cellulose and glycoproteins, in diatoms they synthesize a cell wall from silicic acids, in higher plants from cellulose, hemicellulose and pectin. Apparently for larger eukaryotic cells it has become impossible to create a cell wall of high strength from a single molecule. This circumstance could force eukaryotes to use different material for the cell wall. The metabolism of bacteria is also diverse. In general, there are four types of nutrition, and all are found among bacteria. These are photoautotrophic, photoheterotrophic, chemoautotrophic, chemoheterotrophic (phototrophic use the energy of sunlight, chemotrophic use chemical energy). Eukaryotes either synthesize energy from sunlight themselves or use ready-made energy of this origin. This may be due to the emergence of predators among eukaryotes, for which the need to synthesize energy has disappeared. Another difference is the structure of the flagella. In bacteria they are thin - only 15-20 nm in diameter. These are hollow filaments made from the protein flagellin. The structure of eukaryotic flagella is much more complex. They are a cell outgrowth surrounded by a membrane and contain a cytoskeleton (axoneme) of nine pairs of peripheral microtubules and two microtubules in the center. Unlike rotating prokaryotic flagella, eukaryotic flagella bend or wriggle. The two groups of organisms we are considering, as already mentioned, are very different in their average sizes. The diameter of a prokaryotic cell is usually 0.5-10 microns, while the same figure for eukaryotes is 10-100 microns. The volume of such a cell is 1000-10000 times greater than that of a prokaryotic cell. Prokaryotes have small ribosomes (70S type). Eukaryotes have larger ribosomes (80S type). Apparently, the time of emergence of these groups also differs. The first prokaryotes arose in the process of evolution about 3.5 billion years ago, from them about 1.2 billion years ago eukaryotic organisms evolved. see also Sources, links Biological encyclopedic dictionary / edited by

The bulk of prokaryotes are represented by single-celled organisms that are very small in size. Units such as micrometer (µm) and nanometer (nm) are used to measure bacteria and their structures.

Bacteria are invisible to the naked human eye (the resolution limit of the human eye lies in the region of 70–80 microns). Light and electron microscopes are used to study the prokaryotic cell and its structures. The resolution limit of a light microscope is 0.2 microns, the resolution limit of the best electron microscopes is 0.15–0.3 nm. Thus, modern optical instruments increase the resolution of the human eye by 250–500 thousand times.

The average sizes of prokaryotes range from 0.5–3 µm. The size range of bacteria is very wide. Among them there are their giants and their pygmies. For example, the cells of Achromatium oxaliferum reach a length of 125 microns, the length of the filamentous sulfur bacterium Beggiatoa gigantea is several millimeters with a cell diameter of up to 55 microns. However, giant bacteria are very rare in nature. In most rod-shaped bacteria, the cell length usually does not exceed 5 µm with a diameter of 1 µm. Spherical bacteria have a cell diameter from 0.5 to 1–2 μm. The smallest prokaryotic organisms are represented by the group of mycoplasmas; their cell size is 0.12–0.15 microns. It is estimated that a mycoplasma cell contains 1200 protein molecules and about 100 enzymatic reactions occur. Obviously, this is the minimum that can provide the most primitive cellular level of life.

The sizes of microorganism cells are interconnected with their structural organization and can vary depending on the age of the culture, the composition of the nutrient medium and the action of other factors. The small size of prokaryotes has ecological significance. Due to their minimal mass and size, microbes are spread by air currents over vast distances and become ubiquitous.

Based on the shape of the cells, the bulk of prokaryotes can be divided into three groups: spherical, or cocci, rod-shaped and convoluted. Depending on the plane of division and the nature of the relative arrangement of cells relative to each other, cocci are distinguished: micrococci and staphylococci - cells divide in different planes (in micrococci, after division, the cells diverge and lie alone, in staphylococci, cells form irregular clusters shaped like a bunch of grapes); diplococci and streptococci - cells divide in the same plane (the former form pairs, the latter - chains of different lengths); tetracocci and sarcina - cells divide in two and three mutually perpendicular planes, respectively (in the former, the cells are arranged in the form of a tetrad, and in the latter they form packets of 8 individuals).

Rod-shaped bacteria represent the largest group of prokaryotes. They have a cylindrical body shape with rounded or pointed ends and vary greatly in the ratio of length to width. In the environment, rod-shaped bacteria are located singly or form short or long chains. Rod-shaped bacteria with a gram-positive membrane that can form endospores are called bacilli.

Convoluted microorganisms differ in the degree of cell curvature and the number of turns. Thus, spirochete cells are highly curved and have from 6 to 15 turns; spirilla cells have from 4 to 6 turns; Vibrio cells are short, comma- or crescent-shaped rods.

In addition to the main forms of bacteria described above, other very diverse forms of microorganisms are found in nature. For example, mycobacteria have rod-shaped cells with varying degrees of branching. The body of the actinomycete is represented by one highly branching cell, similar to the mycelium of a fungus. There are star shapes. Some bacteria are shaped like rings or half rings (toroids).

Stembed and budding bacteria have a unique cell shape, bearing outgrowths called prostek. Prostecs are cell outgrowths containing cytoplasm and surrounded by elements of the cell membrane.

One cell can have from 1 to 8 layers of different lengths. The functions of prostek are different. For example, in bacteria of the genus Hyphomicrobium, prostecae, at the ends of which buds are formed, perform a reproductive function. In stalked bacteria of the genus Caulobacter, the stalks serve to attach the cell to the substrate. Obviously, prosteks, by increasing the surface of contact between the cell and the substrate, promote the penetration of nutrients. Small bacteria - mycoplasmas, lacking a cell wall, are characterized by pronounced pleomorphism and have coccoid, rod-shaped or filamentous cells.

Differences between prokaryotes and eukaryotes

The presence of a single cytoplasmic membrane that limits the contents of the cell.

Absence of membrane-bound organelles.

Absence of a nuclear membrane. Nuclear material in the form of a ring-shaped chromosome or nucleoid.

Chromosome DNA is stabilized not by histones, but by polyamines.

Characterized by the presence of autonomous extrachromosomal units of heredity - plasmids.

The cell wall contains peptidoglycan - murein and other specific substances.

Type 70-S ribosomes are present in the cytoplasm.

Reproduction is based on simple division; mitosis is not typical.

There is no phagocytosis or pinocytosis.

There is no movement of cytoplasm.

It is possible for endospores to form as a type of resting cell to withstand unfavorable conditions.

The presence of special reserve substances: volutin, granulosa, other organic and inorganic substances (sulfur, calcium carbonate).

The presence of suprathecal cell structures: capsules, fimbriae, flagella.

Possibility of forming prostek.