Features of cell structure. Comparative characteristics of the structure of prokaryotic and eukaryotic cells Structure of eukaryotic and prokaryotic cells conclusion

1. A prokaryotic cell is characterized by the presence
A) ribosomes
B) mitochondria
B) formed core
D) plasma membrane
D) endoplasmic reticulum
E) one circular DNA

Answer

2. Prokaryotic cells are different from eukaryotic cells
A) the presence of ribosomes
B) absence of mitochondria
B) lack of a formed core
D) the presence of a plasma membrane
D) absence of organelles of movement
E) the presence of one ring chromosome

Answer

3. Establish a correspondence between the structure of cells and their type: 1-prokaryotic, 2-eukaryotic
A) do not have a formed core
B) have a nuclear membrane
B) diploid or haploid
D) always haploid
D) do not have mitochondria or Golgi complex
E) contain mitochondria, Golgi complex

Answer

A1 B2 C2 D1 D1 E2

4. Why are bacteria classified as prokaryotes?
A) contain a nucleus in the cell, separated from the cytoplasm
B) consist of many differentiated cells
B) have one ring chromosome
D) do not have a cell center, Golgi complex and mitochondria
D) do not have a nucleus isolated from the cytoplasm
E) have cytoplasm and plasma membrane

Answer

5. A bacterial cell belongs to the group of prokaryotic cells, since it
A) does not have a shell-covered core
B) has cytoplasm
B) has one DNA molecule immersed in the cytoplasm
D) has an outer plasma membrane
D) does not have mitochondria
E) has ribosomes where protein biosynthesis occurs

Answer

6. Cells of eukaryotic organisms, unlike prokaryotic ones, have
A) cytoplasm
B) core covered with a shell
B) DNA molecules
D) mitochondria
D) dense shell
E) endoplasmic reticulum

Answer

7. Establish a correspondence between the characteristics of the cell and its type: 1-prokaryotic, 2-eukaryotic
A) There are no membrane organelles
B) There is a cell wall made of murein
B) Hereditary material is represented by a nucleoid
D) Contains only small ribosomes
D) Hereditary material is represented by linear DNA
E) Cellular respiration occurs in mitochondria

Answer

A1 B1 C1 D1 D2 E2

8. Prokaryotic cells are different from eukaryotic cells
A) the presence of a nucleoid in the cytoplasm
B) the presence of ribosomes in the cytoplasm
B) ATP synthesis in mitochondria
D) the presence of the endoplasmic reticulum
D) absence of a morphologically distinct nucleus
E) the presence of invaginations of the plasma membrane that perform the function of membrane organelles

Prokaryotic cells are smaller and simpler in structure than eukaryotic cells. Among them there are no multicellular organisms; only sometimes they form something like colonies. Prokaryotes do not only have a cell nucleus, but also all membrane organelles (mitochondria, chloroplasts, EPS, Golgi complex, centrioles, etc.).

Prokaryotes include bacteria, blue-green algae (cyanobacteria), archaea, etc. Prokaryotes were the first living organisms on Earth.

The functions of membrane structures are performed by outgrowths (invaginations) cell membrane into the cytoplasm. They come in tubular, lamellar, and other shapes. A number of them are called mesosomes. Photosynthetic pigments, respiratory and other enzymes are located on such various formations and thus perform their functions.

In prokaryotes, in the central part of the cell there is only one large chromosome ( nucleoid), which has a ring structure. It contains DNA. Instead of proteins that give the chromosome its shape like in eukaryotes, there is RNA. The chromosome is not separated from the cytoplasm by a membrane membrane, therefore they say that prokaryotes are nuclear-free organisms. However, in one place the chromosome is attached to the cell membrane.

In addition to the nucleoid, the structure of prokaryotic cells contains the presence of plasmids (small chromosomes also with a ring structure).

Unlike eukaryotes, the cytoplasm of prokaryotes is immobile.

Prokaryotes have ribosomes, but they are smaller than the ribosomes of eukaryotes.

Prokaryotic cells are distinguished by the complex structure of their membranes. In addition to the cytoplasmic membrane (plasmalemma), they have a cell wall, as well as a capsule and other formations, depending on the type of prokaryotic organism. The cell wall performs a supporting function and prevents penetration harmful substances. The bacterial cell wall contains murein (a glycopeptide).

On the surface of prokaryotes there are often flagella (one or many) and various villi. Cells move with the help of flagella liquid medium. The villi perform different functions (provide non-wetting, attachment, transport substances, participate in the sexual process, forming a conjugation bridge).

Prokaryotic cells divide by binary fission. They do not have mitosis or meiosis. Before dividing, the nucleoid doubles.

Prokaryotes often form spores, which are a way of experiencing unfavorable conditions. Spores of a number of bacteria remain viable at high and extremely low temperatures. When a spore is formed, the prokaryotic cell is covered with a thick, dense membrane. Her internal structure changes somewhat.

The first prokaryotic cells in evolution appeared about 3-3.5 billion years ago. Their name comes from the Greek. pro-do, karion-kernel, since they do not have a formalized kernel. Their genetic material in the form of a single ring-shaped DNA molecule is not surrounded by a membrane shell, lies directly in the cytoplasm and is called genophore(or nucleoid).

In the cytoplasm of the organelles there are only small ribosomes (70 S instead of 80 S- in eukaryotes).

In addition, bacteria may contain DNA in the form of tiny plasmids, similar to the extranuclear DNA of eukaryotes. Plasmids are carriers of cytoplasmic inheritance and determine some specific properties of bacteria.

Fig. 10. The structure of a prokaryotic cell.

Located on top of the cytoplasm in prokaryotes cell membrane, consisting of a plasma membrane and a cell wall. The plasma membrane in prokaryotes has folded invaginations into the cytoplasm of the mesosome, on the surface of which there are respiratory enzymes, and ATP synthesis occurs. Similar membrane formations are also involved in nitrogen fixation.

In prokaryotic cells capable of photosynthesis (blue-green algae, green and purple bacteria), there are structured large membrane invaginations - thylakoids containing pigments (including bacteriochlorophyll). All enzymes that provide vital processes are diffusely scattered in the cytoplasm or fixed on the membrane. In many prokaryotes, storage substances are deposited inside the cytoplasm: fats, polysaccharides, etc.

Outside the plasma membrane in prokaryotes there is a mechanically strong formation - the cell wall, built in most cases from murein.

The cell wall maintains the shape of cells, provides their rigidity and antigenic properties. It serves as additional protection for cells and, in some cases, for the formation of cell colonies. The cell wall of some bacteria is surrounded by a thick layer of mucus made of polysaccharides and polypeptides.

Bacteria reproduce asexually - by dividing in two. After reduplication of the ring chromosome and cell elongation, a transverse septum is formed. Then the daughter cells disperse.

Reproduction is sometimes preceded by the sexual process in the form of the emergence of new combinations of genes on the chromosome. There are three known methods for the formation of recombinants: transformation, conjugation, transduction.

At transformation A small fragment of DNA emerges from the donor cell, which is actively absorbed by the recipient cell and incorporated into its DNA, replacing a similar, although not necessarily identical, fragment in it.

Conjugation is the transfer of DNA between cells in contact with each other. In this case, plasmids with a sex factor, or F+ factor, are involved in gene transfer.

Transduction is the transfer of a DNA fragment from one cell to another by a bacteriophage.

Many bacteria tend to form spores when there is a lack of nutrients in the environment or metabolic products accumulate in excess. Sporulation begins with the detachment of part of the cytoplasm from the mother cell. The detached part contains a chromosome and is surrounded by a membrane and then a cell wall, often multilayered. In this case, vital processes practically cease. The resulting spores are very stable in a dry state and can remain viable for hundreds and thousands of years, withstanding sharp temperature fluctuations. When exposed to favorable conditions, the spores transform into an active bacterial cell.

Prokaryotic cells- these are the most primitive, very simply structured organisms that retain the features of deep antiquity. TO prokaryotic(or prenuclear) organisms include bacteria and blue-green algae (cyanobacteria). Based on the similarity of structure and sharp differences from other cells, prokaryotes are classified into the independent kingdom of crushed cells.

Let's look at the structure prokaryotic cell using bacteria as an example. The genetic apparatus of a prokaryotic cell is represented by the DNA of a single circular chromosome, is located in the cytoplasm and is not delimited from it by a membrane. This analogue of the nucleus is called a nucleoid. DNA does not form complexes with proteins and therefore all genes that are part of the chromosome “work”, i.e. information is continuously read from them.

Prokaryotic cell surrounded by a membrane separating the cytoplasm from the cell wall, formed from a complex, highly polymeric substance. There are few organelles in the cytoplasm, but numerous small ribosomes are present (bacterial cells contain from 5,000 to 50,000 ribosomes).

Structure of a prokaryotic cell

The cytoplasm of a prokaryotic cell is penetrated by membranes that form the endoplasmic reticulum; it contains ribosomes that carry out protein synthesis.

The inner part of the cell wall of a prokaryotic cell is represented by a plasma membrane, the protrusions of which into the cytoplasm form mesosomes, which are involved in the construction of cell walls, reproduction, and are the site of DNA attachment. Respiration in bacteria occurs in mesosomes, and in blue-green algae in cytoplasmic membranes.

Many bacteria deposit reserve substances inside the cell: polysaccharides, fats, polyphosphates. Reserve substances, when included in metabolism, can prolong the life of a cell in the absence of external energy sources.

(1-cell wall, 2-outer cytoplasmic membrane, 3-chromosome (circular DNA molecule), 4-ribosome, 5-mesosome, 6-invagination of the outer cytoplasmic membrane, 7-vacuoles, 8-flagella, 9-stacks of membranes, in which photosynthesis occurs)

As a rule, bacteria reproduce by dividing in two. After cell elongation, a transverse partition is gradually formed, which is laid in the direction from the outside to the inside, then the daughter cells disperse or remain connected in characteristic groups - chains, packets, etc. Bacteria - coli Every 20 minutes it doubles its numbers.

Bacteria are characterized by spore formation. It begins with the detachment of part of the cytoplasm from the mother cell. The detached part contains one genome and is surrounded by a cytoplasmic membrane. Then a cell wall, often multilayered, grows around the spore. In bacteria, the sexual process occurs in the form of an exchange of genetic information between two cells. The sexual process increases the hereditary variability of microorganisms.

Most living organisms are united in the superkingdom of eukaryotes, which includes the kingdom of plants, fungi and animals. Eukaryotic cells are larger prokaryotic cells, consist of a surface apparatus, a nucleus and a cytoplasm.

Eukaryotic cell

Eukaryotic(eukaryotic)cells contain a nucleus coordinating the vital activity of the cell, in which the hereditary apparatus of the body is located, and numerous organoids, performing various functions. Most eukaryotes are aerobes, that is, they use energy metabolism air oxygen.

The most obvious The difference between prokaryotes and eukaryotes is that they have latest kernels , which is reflected in the names of these groups: “karyo” is translated from ancient Greek as core, “pro” - before, “eu” - good. Hence, prokaryotes are prenuclear organisms, eukaryotes are nuclear.

However, this is far from the only and perhaps not the main difference between prokaryotic organisms and eukaryotes. Prokaryotic cells do not have membrane organelles 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 that ensure vital processes.

Prokaryotes do not have the chromosomes characteristic of eukaryotes. Their main genetic material is nucleoid, usually shaped like a ring. In eukaryotic cells, chromosomes are complexes of DNA and histone proteins (play 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 it its shape.

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, prokaryotic cells have different amounts plasmids- nucleoids significantly smaller in size than the main one.

The number of genes in the nucleoid of prokaryotes is an order of magnitude less than in chromosomes. Eukaryotes have many genes that perform regulatory function in relation to other genes. This allows eukaryotic cells of a multicellular organism that contain the same genetic information to specialize; by changing your metabolism, respond more flexibly to changes in external and internal environment. The structure of the genes is also different. In prokaryotes, genes in DNA are arranged in groups called operons. Each operon is transcribed as a single unit.

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 messenger (messenger) RNA: while it is still being synthesized on DNA, ribosomes are already “sitting” at its finished end and synthesizing protein. In eukaryotic cells, mRNA undergoes so-called maturation after transcription. And only after that can protein be synthesized on it.

The ribosomes of prokaryotes are smaller (sedimentation coefficient 70S) than those of eukaryotes (80S). The number of proteins and RNA molecules in the ribosomal subunits differs. It should be noted that the ribosomes (as well as the genetic material) of mitochondria and chloroplasts are similar to prokaryotes, which may indicate their origin from ancient prokaryotic organisms that ended up 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 structures, 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 (a glycopeptide). Among eukaryotes, plants have a cell wall (its main component is cellulose), and fungi have chitin.

Prokaryotic cells divide by binary fission. They have No complex processes cell division(mitosis and meiosis), characteristic of eukaryotes. Although before division the nucleoid doubles, just like chromatin in chromosomes. IN life cycle In eukaryotes, there is an alternation of diploid and haploid phases. 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 significantly larger than prokaryotic cells (tens of times).

Nutrients enter prokaryotic cells only through osmosis. In eukaryotic cells, in addition, phago- and pinocytosis (“capture” of food and liquid using the cytoplasmic membrane) can also be observed.

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

Table of differences between prokaryotes and eukaryotes