Who are eukaryotes and prokaryotes: comparative characteristics of cells of different kingdoms. The meaning of the word "eukaryotes What is eukaryote

Prokaryotic organisms include bacteria - primarily bacteria in the traditional sense of the term, then blue-green algae (cyanobacteria) and recently discovered green algal-like organisms (chloroxybacteria), as well as some multicellular organisms such as actinobacteria (actinomycetes) and myxobacteria that form fruit body.

These are all microbes. The name "prokaryotes" comes from the Greek words pro (before) and karyon (seed, nucleus). Most prokaryotic cells are smaller than eukaryotic cells. A prokaryotic structure carrying genes, sometimes misnamed bacterial chromosome should be called genophore... It is a circular strand of DNA that is not found in a membrane-surrounded nucleus; in an electron microscope, the genophore looks like a relatively transparent region, which is called nucleoid... In a eukaryotic cell, the carriers of genes are chromosomes located in the nucleus, bounded by a membrane. In exceptionally thin, transparent preparations, living chromosomes can be seen using a light microscope; more often they are studied in fixed and stained cells (in contrast to the prokaryotic geneophore, chromosomes are stained red with Völgen's reagent). Chromosomes are built from DNA, which is in a complex with five histone proteins, rich in arginine and lysine and making up a significant part of the mass of chromosomes (more than half) in most eukaryotes. Histones give chromosomes a number of characteristic properties - elasticity, the ability to compact packing and coloration. However, they are not involved in the ability of chromosomes to move, for which the mitotic spindle or similar microtubule systems are responsible.

All well-known organisms - algae, protozoa, molds, higher fungi, animals and plants - are composed of eukaryotic cells. The cells of these organisms (with the exception of some protoctists) divide by mitosis - the so-called indirect division, in which chromosomes are "split" longitudinally and diverge in two groups to opposite poles of the cell. The word mitosis in this book will be used in the classical sense - only when it comes about chromosomes and the mitotic apparatus; this concept does not include the exact direct distribution of genes that make up the linkage group (genophore) in bacteria. Prokaryotic cells can divide by constriction into equal parts or by budding into unequal parts, but they never divide by mitosis.

Prokaryotes usually reproduce asexually. For many of them, the sexual process is generally unknown and the offspring has only one parent (in this book, sexual reproduction refers to any process in which each offspring has more than one parent - usually two). In prokaryotes capable of sexual reproduction, the reproductive systems are unidirectional in the sense that donor cells (“male”) transfer their genes to recipient cells (“female”). The number of genes transferred varies from one conjugation to another: genes form a long DNA molecule, and usually only a small part of the genome (but sometimes almost the entire genome) is transferred. When bacteria are conjugated, the cytoplasm of cells does not merge, as is the case in all animals, in fungi (when hyphae are fused), and in many plants and protoctists. The new prokaryotic organism, called a recombinant, consists of the recipient cell itself, in which some genes have been replaced by those of the donor. Thus, in prokaryotes, parents almost never contribute equally. On the other hand, in a sexually arising eukaryotic cell (zygote), the contribution of the parents is the same or almost the same: a new eukaryotic individual usually receives half of the genes and some amount of nucleoplasm and cytoplasm from each parent.

Chromosomes are built from DNA and proteins, but isolated chromosome preparations often also contain significant admixtures of RNA from other regions of the nucleus. This RNA, probably both informational and ribosomal, easily adheres to isolated chromosomes. The eukaryotic nucleus also contains nucleoli, consisting of precursors of cytoplasmic ribosomes - from RNA chains of various lengths and a large number proteins. Other organelles that are unique to eukaryotic cells are mitochondria, plastids, centrioles, and kinetosomes with their undulipodia. With the exception of microtubules, which are found both inside and outside the nucleus, all of these organelles lie on the outside of the nuclear membrane.

All motor organelles of a eukaryotic cell are about 0.25 µm thick; of them, the longer ones (from 10 to 15 microns) and presented in a small number of each cell are traditionally called flagella, and the shorter and more numerous ones are called cilia. Electron microscopy revealed a striking structural similarity of all eukaryotic cilia and flagella: in all cases, the same arrangement of protein microtubules (9 + 2) can be seen in the cross section, each of which is about 0.024 μm in diameter. These organelles are much more complex than bacterial flagella and have a completely different structure and a different protein composition. It's time for their names to reflect new information; therefore, in our book for cilia, flagella and related organelles of eukaryotes (for example, for the axial filament in the tail of the sperm, for the structural units of cirrus in ciliates and other structures of the 9 + 2 type and their derivatives, developing from kinetosomes, which themselves have a cross section structure 9 + 0), the term undulipodium is used. The name flagellum is reserved for thin bacterial flagella and structures homologous to them, such as axial fibrils of spirochetes; usually flagella are too small to be seen with a conventional light microscope. This less ambiguous terminology is based on the considerations of T. Jan and his colleagues.

Commonly known prokaryotes and eukaryotes

	Prokaryotes	Eukaryotes
Single-celled heterotrophs	Real bacteria: hydrogen sulfide bacteria, E. coli, pseudomonads, some iron bacteria, bacilli, methane-forming bacteria, nitrogen-fixing bacteria, spirochetes, mycoplasmas, rickettsia, Chlamydia, Bedsonia	Protists: amoebae, radiolarians, foraminifera, ciliates, sporozoans, some dinoflagellates. Some yeast
Autotrophs	Blue-green and green prokaryotic algae (i.e. cyanobacteria and chloroxybacteria), other photosynthetic bacteria, chemoautotrophic bacteria	Algae: red, brown, charovy, diatoms; some dinoflagellates, chlorella, Cyanidium. Plants: mosses, liverworts, ferns, cicadas, conifers, flowering
Mycelial and multicellular organisms	Actinobacteria (actinomycetes), some sliding and budding bacteria	Water mold, chitrid, cap mushrooms, raincoats, ascomycetes, slime molds. Plants. Animals: sponges, ctenopods, coelenterates, brachiopods, bryozoans, annelids, gastropods, arthropods, echinoderms, tunicates, fish, mammals

Differences between prokaryotes and eukaryotes

Signs	Prokaryotes	Eukaryotes
Cell sizes	The cells are mostly small (1-10 microns); some are larger than 50 microns	The cells are mostly large (10-100 microns); some more than 1 mm
General features	Exclusively microorganisms. Unicellular or colonial. Filamentous or mycelial forms with “fruiting bodies” are the most complex morphologically. Nucleoid Without Boundary Membrane	Some are microorganisms; most are large organisms. Unicellular, colonial, mycelial, or multicellular. Vertebrates and angiosperms are the most complex morphologically. All have a core with a boundary membrane
Cell division	Non-mitotic, direct, most often by splitting in two or budding. Genofor contains DNA but no protein; does not give a Völgen reaction. No centrioles, mitotic spindle and microtubules	Various forms of mitosis. Usually there are many chromosomes containing DNA, RNA and proteins and giving a bright red Völgen color. Many forms have centrioles as well. mitotic spindle or ordered microtubules
Floor systems	Most forms are absent; if any, then carry out a unidirectional transfer of genetic material from donor to recipient	Most forms have; equal participation of both parents in fertilization
Development	There is no multicellular development starting from diploid zygotes; there is no pronounced differentiation of tissues. Solitary or colonial forms only. There are no complex intercellular connections. Metamorphosis is rare	Haploid forms are formed as a result of meiosis, diploid forms develop from zygotes; in multicellular organisms, a far-reaching differentiation of tissues. Plasmodesmata, desmosomes and other complex intercellular connections. Metamorphosis is common
Oxygen resistance	Strict or facultative anaerobes, microaerophiles, or aerobes	Mostly aerobes. Exceptions are clearly secondary modifications
Metabolism	Various metabolic patterns; no specialized membrane-limited organelles with enzymes designed to oxidize organic molecules (no mitochondria)	All kingdoms have the same scheme of oxidative metabolism: there are membrane organelles (mitochondria) with oxidation enzymes of tricarboxylic organic acids
Photosynthesis (if available); lipids, etc.	Photosynthetic enzymes are bound to cell membranes (chromatophores) rather than being packaged as separate organelles. There is anaerobic and aerobic photosynthesis with the release of sulfur, sulfate or oxygen. Hydrogen donors can be H 2, H 2 O, H 2 S, or (H 2 CO) n. Lipids: vaccine and oleic acids, hopans; steroids are extremely rare. Form aminoglycoside antibiotics	Photosynthetic enzymes are found in plastids bounded by membranes. Mostly photosynthesis with the release of oxygen; The hydrogen donor is always H 2 O. Lipids: linoleic and linolenic acids, steroids (ergosterol, cycloartenol, cholesterol) are common. Common (especially in plants) alkaloids, flavonoids, acetogenins and other secondary metabolites
Motor aids	Some have simple bacterial flagella composed of flagellin; others move by sliding. Intracellular movement is rare or absent; no phagocytosis, pinocytosis and cyclosis	Most have undulipodia: "flagella" or cilia of type 9 + 2. Structures 9 + 0 or 6 + 0 are evolutionary modifications of the 9 + 2 scheme. Pseudopodia containing an actin-like protein are common. Intracellular movement is characteristic (pinocytosis, phagocytosis, cyclosis), carried out with the help of specialized proteins - actin, myosin, tubulin
Cell wall	Glycopeptides are derivatives of diaminopimelic and muramic acids; glycoproteins are rare or absent; ascorbic acid is not required	Chitin or cellulose; glycoproteins with hydroxylated amino acids are common; ascorbic acid required
	Resistant to drying; heat-resistant endospores contain calcium dipicolinate; actinospores	Complex, vary depending on the type; no calcium dipicolinate; in disputes sporopollenin; no endospores

The emergence of eukaryotes is a major event. It changed the structure of the biosphere and opened up fundamentally new possibilities for progressive evolution. The eukaryotic cell is the result of a long evolution of the world of prokaryotes, a world in which various microbes have adapted to each other and looked for ways of effective cooperation.

chronology sketch (review)

Photosynthesizing prokaryotic complex Chlorochromatium aggregatum.

Eukaryotes are the result of the symbiosis of several types of prokaryotes. Prokaryotes are generally quite symbiotic (see Chapter 3 in The Birth of Complexity). Here is an interesting symbiotic system known as Chlorochromatium aggregatum. Lives in deep lakes, where there are anoxic conditions at depth. The central component is a mobile heterotrophic beta-proteobacteria. Around it are stacked from 10 to 60 photosynthetic green sulfur bacteria. All components are connected by outgrowths of the outer membrane of the central bacterium. The meaning of the commonwealth is that mobile beta-proteobacteria drags the whole company to places favorable for the life of fastidious sulfur bacteria, and sulfur bacteria are engaged in photosynthesis and provide food for themselves and beta-proteobacteria. Perhaps some ancient microbial associations of approximately this type were the ancestors of eukaryotes.

The theory of symbiogenesis. Merezhkovsky, Margulis. Mitochondria are descendants of alpha-proteobacteria, plastids are descendants of cyanobacteria. It is more difficult to understand who was the ancestor of everything else, that is, the cytoplasm and the nucleus. The nucleus and cytoplasm of eukaryotes combines the traits of archaea and bacteria, and also has many unique features.

About mitochondria. Perhaps it was the acquisition of mitochondria (and not the nucleus) that was the key moment in the formation of eukaryotes. Most of the genes of mitochondrial ancestors were transferred to the nucleus, where they came under the control of nuclear regulatory systems. These nuclear genes of mitochondrial origin encode not only mitochondrial proteins, but also many proteins that work in the cytoplasm. This suggests that the mitochondrial symbiont played a more important role in the formation of the eukaryotic cell than was assumed.

The coexistence of two different genomes in one cell required the development of an effective system of their regulation. And in order to effectively manage the work of a large genome, it is necessary to isolate the genome from the cytoplasm, in which the metabolism takes place and there are thousands chemical reactions... The nuclear membrane separates the genome from the violent chemical processes of the cytoplasm. The acquisition of symbionts (mitochondria) could become an important stimulus in the development of the nucleus and gene regulatory systems.

The same applies to sexual reproduction. Without sexual reproduction, you can live as long as your genome is small enough. Organisms with a large genome, but lacking sexual reproduction, are doomed to rapid extinction, with rare exceptions.

Alphaproteobacteria - this group included the ancestors of mitochondria.

Rhodospirillum is an amazing microorganism that can live both through photosynthesis, including under anaerobic conditions, and as an aerobic heterotroph, and even as an aerobic chemoautotroph. It can, for example, grow by oxidizing carbon monoxide CO, without using any other energy source. In addition to all this, he also knows how to fix atmospheric nitrogen. That is, it is a highly universal organism.

The immune system mistakes mitochondria for bacteria. When damaged mitochondria enter the bloodstream during trauma, characteristic molecules are released from them that are found only in bacteria and mitochondria (circular bacterial-type DNA and proteins that carry a special modified amino acid formylmethionine at one of their ends). This is due to the fact that the apparatus for protein synthesis in mitochondria remained the same as in bacteria. The cells of the immune system - neutrophils - react to these mitochondrial substances in the same way as to bacterial ones, and using the same receptors. This is the clearest confirmation of the bacterial nature of mitochondria.

The main function of mitochondria is oxygen respiration. Most likely, the need to protect against the toxic effects of oxygen was the stimulus for the unification of the anaerobic ancestor of the nucleus and cytoplasm with the "protomitochondria".

Where did bacteria, including alphaproteobacteria, have the molecular systems necessary for oxygen respiration? It seems that they were based on the molecular systems of photosynthesis. The electron transport chain formed in bacteria as part of the photosynthetic apparatus was adapted for oxygen respiration. In some bacteria, parts of the electron transport chains are still used simultaneously in photosynthesis and in respiration. Most likely, the ancestors of mitochondria were aerobic heterotrophic alpha-proteobacteria, which, in turn, evolved from photosynthetic alpha-proteobacteria such as rhodospirillum.

The number of common and unique protein domains in archaea, bacteria and eukaryotes. A protein domain is a part of a protein molecule that has a specific function and characteristic structure, that is, a sequence of amino acids. Each protein usually contains one or more of these structural and functional blocks, or domains.

4.5 thousand protein domains that eukaryotes have can be divided into 4 groups: 1) available only in eukaryotes, 2) common to all three super kingdoms, 3) common to eukaryotes and bacteria, but absent in archaea; 4) common for eukaryotes and archaea, but absent in bacteria. We will consider the last two groups (they are highlighted in the figure), since for these proteins one can speak with some certainty about their origin: bacterial or archaean, respectively.

The key point is that eukaryotic domains, supposedly inherited from bacteria and from archaea, have significantly different functions. Domains inherited from archaea (their functional spectrum is shown in the left graph) play a key role in the life of a eukaryotic cell. They are dominated by domains associated with storage, reproduction, organization and reading of genetic information. Most of the "archaeal" domains belong to those functional groups within which horizontal gene exchange in prokaryotes occurs least frequently. Apparently, eukaryotes received this complex by direct (vertical) inheritance from archaea.

Among the domains of bacterial origin, there are also proteins associated with information processes, but there are few of them. Most of them only work in mitochondria or plastids. Eukaryotic ribosomes of the cytoplasm are of archaeal origin, mitochondrial and plastid ribosomes are of bacterial origin.

Among the bacterial domains of eukaryotes, the proportion of signal-regulatory proteins is significantly higher. Eukaryotes have inherited many proteins from bacteria that are responsible for the mechanisms of cell response to environmental factors. And also - many proteins associated with metabolism (for more details see chapter 3 "The birth of complexity").

Eukaryotes have:

· Archaeal "core" (mechanisms of work with genetic information and protein synthesis)

· Bacterial "periphery" (metabolism and signal-regulatory systems)

· The simplest scenario: ARCHEY swallowed BACTERIA (the ancestors of mitochondria and plastids) and acquired all its bacterial traits from them.

· This scenario is too simple, because eukaryotes have many bacterial proteins that could not be borrowed from the ancestors of mitochondria or plastids.

Eukaryotes have many "bacterial" domains that are not characteristic of either cyanobacteria (ancestors of plastids) or alphaproteobacteria (ancestors of mitochondria). They were obtained from some other bacteria.

Birds and dinosaurs. It is difficult to reconstruct proto-eukaryotes. It is clear that the group of ancient prokaryotes that gave rise to the nucleus and cytoplasm had a number of unique features that are not present in prokaryotes that have survived to this day. And when we try to reconstruct the appearance of this ancestor, we are faced with the fact that the scope for hypotheses is too large.

Analogy. It is known that birds descended from dinosaurs, and not from some unknown dinosaurs, but from a very specific group - maniraptor dinosaurs, which belong to theropods, and theropods, in turn, are one of the groups of dinosaurs lizard-like. Many transitional forms have been found between flightless dinosaurs and birds.

But what could we say about the ancestors of birds if there were no fossil record? In the best case, we would find out that the closest relatives of birds are crocodiles. But could we recreate the appearance of the direct ancestors of birds, that is, dinosaurs? Unlikely. But it is in this position that we find ourselves when we are trying to restore the appearance of the ancestor of the nucleus and cytoplasm. It is clear that this was a group of some prokaryotic dinosaurs, a group that is extinct and did not leave, unlike real dinosaurs, distinct traces in the geological record. Modern archaea in relation to eukaryotes are like modern crocodiles in relation to birds. Try to restore the structure of dinosaurs, knowing only birds and crocodiles.

An argument in favor of the fact that in the Precambrian there lived a lot of all sorts of microbes, not similar to the current ones. Proterozoic stromatolites were much more complex and varied than modern ones. Stromatolites are a product of the vital activity of microbial communities. Does this mean that Proterozoic microbes were more diverse than modern ones, and that many groups of Proterozoic microbes simply did not survive to this day?

Ancestral community of eukaryotes and the origin of the eukaryotic cell (possible scenario)

The hypothetical “ancestral community” is a typical bacterial mat, only in its upper one lived the ancestors of cyanobacteria, which had not yet switched to oxygenic photosynthesis. They were engaged in anoxygenic photosynthesis. The electron donor was not water, but hydrogen sulfide. Sulfur and sulfates were isolated as a by-product.

The second layer was inhabited by purple photosynthetic bacteria, including alphaproteobacteria, the ancestors of mitochondria. Purple bacteria use long wavelength light (red and infrared). These waves have the best penetrating power. Purple bacteria still often live under a layer of cyanobacteria. Purple alpha-proteobacteria also use hydrogen sulfide as an electron donor.

In the third layer, there were fermenting bacteria that process organic matter; some of them emitted hydrogen as waste. This created the basis for sulfate-reducing bacteria. There could be methanogenic archaea as well. Among the archaea who lived here were the ancestors of the nucleus and cytoplasm.

The crisis was initiated by the transition of cyanobacteria to oxygenic photosynthesis. As an electron donor, cyanobacteria began to use ordinary water instead of hydrogen sulfide. This opened up great opportunities, but also had negative consequences. Instead of sulfur and sulfates, during photosynthesis, oxygen began to be released - a substance extremely toxic for all ancient inhabitants of the earth.

The first to encounter this poison were its producers - cyanobacteria. They were probably the first to develop means of protection against it. The electron transport chains that were used for photosynthesis were modified and began to serve for aerobic respiration. The original goal, apparently, was not to obtain energy, but only to neutralize oxygen.

Soon, the inhabitants of the second layer of the community - purple bacteria - had to develop similar defense systems. Just like cyanobacteria, they have formed aerobic respiration systems based on photosynthetic systems. It was in purple alphaproteobacteria that the most perfect respiratory chain developed, which now functions in the mitochondria of eukaryotes.

In the third layer of the community, the appearance of free oxygen should have caused a crisis. Methanogens and many sulfate reducers utilize molecular hydrogen using hydrogenase enzymes. Such microbes cannot live under aerobic conditions because oxygen inhibits hydrogenase. Many bacteria that produce hydrogen, in turn, do not grow in an environment where there are no microorganisms to utilize it. Of the fermenters, the community apparently retained forms that release low-organic compounds as end products (pyruvate, lactate, acetate, etc.). These fermenters have developed their own oxygen defenses, which are less efficacious. Archaea, the ancestors of the nucleus and cytoplasm, were also among the survivors.

Perhaps, at this critical moment, a key event took place - the weakening of genetic isolation in the ancestors of eukaryotes and the beginning of active borrowing of foreign genes. Proto-eukaryotes incorporated genes from various fermenters until they themselves became microaerophilic fermenters, fermenting carbohydrates to pyruvate and lactic acid.

The inhabitants of the third layer - the ancestors of eukaryotes - were now in direct contact with the new inhabitants of the second layer - aerobic alphaproteobacteria, which learned to use oxygen for energy. The metabolism of proto-eukaryotes and alpha-proteobacteria became complementary, which created the prerequisites for symbiosis. And the very location of alphaproteobacteria in the community (between the upper, oxygen-producing, and lower layers) predetermined their role as “protectors” of eukaryotic ancestors from excess oxygen.

Proto-eukaryotes probably ingested and acquired many different bacteria as endosymbionts. Experiments of this kind are still continuing in unicellular eukaryotes, which have a huge variety of intracellular symbionts. Of these experiments, the union with aerobic alphaproteobacteria was the most successful.

The most obvious the difference between prokaryotes and eukaryotes lies in the presence of nuclei in the latter, which is reflected in the name of these groups: "karyo" is translated from ancient Greek as the core, "pro" - before, "eu" - good. Hence, prokaryotes are pre-nuclear organisms, eukaryotes are nuclear.

However, this is far from the only and perhaps not the main difference between prokaryotic organisms and eukaryotes. There are no membrane organelles in prokaryotic cells at all(with rare exceptions) - mitochondria, chloroplasts, Golgi complex, endoplasmic reticulum, lysosomes. Their functions are performed by outgrowths (invaginations) of the cell membrane, on which various pigments and enzymes are located, which provide vital processes.

Prokaryotes do not have chromosomes characteristic of eukaryotes. Their main genetic material is nucleoid, usually in the form of a ring. In eukaryotic cells, chromosomes are complexes of DNA and histone proteins (play an important role in DNA packaging). These chemical complexes are called chromatin... The nucleoid of prokaryotes does not contain histones, and the RNA molecules associated with it give shape to it.

Eukaryotic chromosomes are found in the nucleus. In prokaryotes, the nucleoid is located in the cytoplasm and is usually attached in one place to the cell membrane.

In addition to the nucleoid, there are different amounts in prokaryotic cells. plasmids- nucleoids are significantly smaller than the main nucleoids.

The number of genes in the nucleoid of prokaryotes is an order of magnitude less than in chromosomes. Eukaryotes have many genes that play a regulatory role in relation to other genes. This enables eukaryotic cells of a multicellular organism containing the same genetic information to specialize; by changing your metabolism, you are more flexible in responding to changes in the external and internal environment. The structure of genes is also different. In prokaryotes, genes in DNA are located in groups - operons. Each operon is transcribed as a whole.

There are also differences between prokaryotes and eukaryotes in the processes of transcription and translation. The most important thing is that in prokaryotic cells these processes can occur simultaneously on one molecule of matrix (informational) RNA: while it is still being synthesized on DNA, ribosomes already “sit” at its finished end and synthesize protein. In eukaryotic cells, mRNA, after transcription, undergoes so-called maturation. And only after that protein can be synthesized on it.

Ribosomes in prokaryotes are smaller (sedimentation coefficient 70S) than in eukaryotes (80S). The number of proteins and RNA molecules in the ribosome subunits differs. It should be noted that the ribosomes (as well as genetic material) of mitochondria and chloroplasts are similar to prokaryotes, which may indicate their origin from ancient prokaryotic organisms trapped inside the host cell.

Prokaryotes are usually distinguished by a more complex structure of their shells. In addition to the cytoplasmic membrane and cell wall, they also have a capsule and other formations, depending on the type of prokaryotic organism. The cell wall performs a supporting function and prevents the penetration of harmful substances. The bacterial cell wall contains murein (glycopeptide). Among eukaryotes, plants have a cell wall (its main component is cellulose), and fungi have chitin.

Prokaryotic cells divide by binary division. They have No complex processes cell division (mitosis and meiosis) typical for eukaryotes. Although, before division, the nucleoid doubles, just like chromatin in chromosomes. In the life cycle of eukaryotes, an alternation of diploid and haploid phases is observed. In this case, the diploid phase usually predominates. Unlike them, prokaryotes do not have this.

Eukaryotic cells vary in size, but in any case, they are much larger than prokaryotic cells (dozens of times).

Nutrients enter the cells of prokaryotes only through osmosis. In addition to this, eukaryotic cells may also exhibit phago- and pinocytosis (“capture” of food and liquid by means of the cytoplasmic membrane).

In general, the difference between prokaryotes and eukaryotes lies in the unambiguously more complex structure of the latter. It is believed that cells of the prokaryotic type arose by abiogenesis (long-term chemical evolution under conditions early earth). Eukaryotes appeared later from prokaryotes, by combining them (symbiotic and also chimeric hypothesis) or the evolution of individual representatives (invagination hypothesis). The complexity of eukaryotic cells allowed them to organize a multicellular organism, in the process of evolution to provide all the basic diversity of life on Earth.

Table of differences between prokaryotes and eukaryotes

Sign	Prokaryotes	Eukaryotes
Cell nucleus	No	There is
Membrane organelles	No. Their functions are performed by invaginations of the cell membrane, on which pigments and enzymes are located.	Mitochondria, plastids, lysosomes, EPS, Golgi complex
Cell shell	More complex, there are different capsules. The cell wall is composed of murein.	The main component of the cell wall is cellulose (in plants) or chitin (in fungi). Animal cells do not have a cell wall.
Genetic material	Much less. It is represented by a nucleoid and plasmids, which have a circular shape and are located in the cytoplasm.	The amount of hereditary information is significant. Chromosomes (made up of DNA and proteins). Diploid is characteristic.
Division	Binary cell division.	There is mitosis and meiosis.
Multicellularity	Not typical for prokaryotes.	They are represented by both unicellular and multicellular forms.
Ribosomes	Smaller	Larger
Metabolism	More diverse (heterotrophs, photosynthetic and chemosynthetic autotrophs in various ways; anaerobic and aerobic respiration).	Autotrophy only in plants due to photosynthesis. Almost all eukaryotes are aerobes.
Origin	From inanimate nature in the process of chemical and prebiological evolution.	From prokaryotes in the process of their biological evolution.

All organisms on our planet are made up of cells. Cells are usually divided into eukaryotes and prokaryotes.

Eukaryotes

First, you need to define what eukaryotes are. If we translate this term from the Greek language, then it translates as owning the core. The nucleus of such organisms contains a genetic code. Such organisms include plants, fungi and animals.

The structure of the eukaryotic cell is different in different organisms. The eukaryotic cell has a rather complex structure. All eukaryotic cells are composed of a nucleus and cytoplasm.

A eukaryotic cell has an envelope called a plasmalemma. It protects the cell by selectively allowing certain substances to enter the cell. From the inside, the cytoplasm is adjacent to it. Various substances are stored in the cytoplasm. The cell has an endoplasmic reticulum, which promotes the circulation of substances through the cell, as well as their transfer from one cell to another. Ribosomes, which are also found in the cell, are responsible for protein synthesis. In addition, the cell may contain the Golgi complex, mitochondria, lysosomes, centrioles. The cell nucleus contains DNA and is responsible for metabolism. It is covered with a special membrane, with the help of which metabolism occurs between the nucleus and the cytoplasm.

Having considered the structure of eukaryotes, it becomes clear what eukaryotes are and that they cannot exist without a nucleus. Eukaryotic cells are mononuclear and multinucleated. The nucleus can have a variety of shapes, which depend on the shape of the cell itself.

What is the difference between eukaryotes and prokaryotes

Prokaryotes are organisms found in cells that lack a nucleus. The absence of a nucleus is the main difference between prokaryotes and eukaryotes. Prokaryotes include bacteria, for example.

Eukaryotes and prokaryotes also differ in size and volume. Eukaryotes are much larger than prokaryotes. Eukaryotes are usually multicellular organisms, while prokaryotes are unicellular. Prokaryotes reproduce by simply dividing the cell in half, while eukaryotes have a more complex reproduction mechanism. The DNA of eukaryotes is located in the nucleus, and prokaryotes in the cytoplasm.

In most cases, eukaryotic cells are part of multicellular organisms. However, in nature there are a considerable number of unicellular eukaryotes, which are structurally a cell, and physiologically - a whole organism. In turn, eukaryotic cells, which are part of a multicellular organism, are not capable of independent existence. They are usually divided into cells of plants, animals and fungi. Each of them have their own characteristics and have their own subtypes of cells that form different tissues.

Despite the diversity, all eukaryotes have a common ancestor, presumably emerging in the process.

In the cells of unicellular eukaryotes (protozoa) there are structural formations that perform the functions of organs at the cellular level. So ciliates have a cellular mouth and pharynx, powder, digestive and contractile vacuoles.

In all eukaryotic cells, it is isolated, separated from the external environment. In the cytoplasm there are cells separated from it by their membranes and various organelles. The nucleolus contains the nucleolus, chromatin, and nuclear juice. The cytoplasm contains numerous (larger than those of prokaryotes), various inclusions.

Eukaryotic cells are characterized by high ordering of their internal contents. Such compartmentation is achieved by dividing the cell into parts by membranes. Thus, the separation of biochemical processes is achieved in the cell. The molecular composition of membranes, the set of substances and ions on their surface are different, which determines their functional specialization.

The cytoplasm contains proteins-enzymes of glycolysis, the exchange of sugars, nitrogenous bases, amino acids and lipids. From certain proteins, microtubules are assembled. The cytoplasm performs a unifying and frame function.

Inclusions are relatively unstable components of the cytoplasm, which are reserves of nutrients, secretion granules (products for excretion from the cell), ballast (a number of pigments).

Organelles are permanent and perform vital functions. Among them there are organelles overall value(, ribosomes, polysomes, microfibrils and, centrioles, and others) and special in specialized cells (microvilli, cilia, synaptic vesicles, etc.).

The structure of an animal eukaryotic cell

Eukaryotic cells are capable of endocytosis (the capture of nutrients by the cytoplasmic membrane).

Eukaryotes (if any) are of a different chemical nature than prokaryotes. In the latter, it is based on murein. In plants, it is mainly cellulose, and in fungi, it is chitin.

The genetic material of eukaryotes is contained in the nucleus and is packed into chromosomes, which are a complex of DNA and proteins (mainly histones).