Cell structure 6. Biology: cells

Any organism is an integral living system.

Despite the execution various functions And different sizes overall plan cell structure is similar.

It consists of three inextricably linked parts:

1. shells,

2. cytoplasm,

3. cores.

In a typical animal cell The following structures are distinguished:

1.membrane;

2.core;

3.cytoplasm;

4.endoplasmic reticulum (ER) ;

5.Golgi complex;

6.lysosomes;

7.mitochondria;

8.ribosomes;

9.cell center;

10. movement organoids .

7. What is osmotic pressure ?

Osmotic pressure, diffusion pressure, a thermodynamic parameter characterizing the tendency of a solution to decrease its concentration upon contact with a pure solvent due to counter diffusion of molecules of the solute and solvent.

The concentration of ions and sugars in the cell sap of the central vacuole is usually higher than in the cell wall; The tonoplast significantly slows down the diffusion of these substances from the vacuole and at the same time is easily permeable to water.

Therefore, water will flow into vacuole. This unidirectional process of water diffusion through a selectively permeable membrane is called osmosis A. Water entering the cell sap exerts pressure on the wall protoplast, and through it on the cell wall, causing its tense, elastic state, or cell turgor.

Turgor ensures that non-lignified plant organs retain their shape and position in space, as well as their resistance to mechanical factors.

If a cell is placed in hypertonic solution some non-toxic salt or sugar (i.e., into a solution of a higher concentration than the concentration of cell sap), then an osmotic release of water from the vacuole occurs. As a result of this, its volume is reduced, the elastic wall protoplast moves away from the cell wall, turgor disappears, and cell plasmolysis .

Plasmolysis is usually reversible. When a cell is placed in water or a hypotonic solution, water is again vigorously absorbed by the central vacuole, the protoplast is again pressed against the cell wall, and turgor is restored. Plasmolysis can serve as an indicator of the living state of a cell; a dead cell is not plasmolyzed, since it does not have selectively permeable membranes.

Loss of turgor causes the plant to wilt. When withering in air under conditions of insufficient water supply, the thin cell walls shrink simultaneously with the protoplast and become folded.

Turgor pressure not only maintains the shape of non-lignified plant parts, it is also one of the cell growth factors, ensuring cell growth by elongation, i.e., by absorbing water and increasing the size of the vacuole. In animal cells, there is no central vacuole; their growth occurs mainly due to an increase in the amount of cytoplasm, therefore the size of animal cells is usually smaller than that of plant cells.

Central vacuole occurs by the fusion of numerous small vacuoles that are present in meristematic (embryonic) cells. These cytoplasmic vacuoles are believed to be formed by the membranes of the endoplasmic reticulum or Golgi apparatus.

8. What is cytoplasm?

Cytoplasm - internal environment a living cell, other than the nucleus, bounded by a plasma membrane. It includes hyaloplasm - the main transparent substance of the cytoplasm, the essential cellular components found in it - organelles, as well as various non-permanent structures - inclusions.

The composition of the cytoplasm includes all types of organic and Not organic matter. It also contains insoluble waste metabolic processes and reserve nutrients. The main substance of the cytoplasm is water.

Cytoplasm is constantly moving, flowing inside a living cell, moving with it various substances, inclusions and organelles. This movement is called cyclosis. All metabolic processes take place in it.

The cytoplasm is capable of growth and reproduction and, if partially removed, can be restored. However, the cytoplasm functions normally only in the presence of the nucleus.

Without it, the cytoplasm cannot exist for a long time, just like the nucleus without the cytoplasm. The most important role of the cytoplasm is to unite all cellular structures(components) and ensuring their chemical interaction.

Following

Cell………………………………………………………1

Cell structure……………………………………………………2

Cytology………………………………………………………..3

Microscope and cell……………………………………………..4

Diagram of cell structure……………………………………………………….6

Cell division……………………………………………………10

Scheme of mitotic cell division…………………………...12

Cell

A cell is an elementary part of an organism, capable of independent existence, self-reproduction and development. The cell is the basis of the structure and life activity of all living organisms and plants. Cells can exist as independent organisms or as part of multicellular organisms(tissue cells). The term “Cell” was proposed by the English microscopist R. Hooke (1665). The cell is the subject of study of a special branch of biology - cytology. More systematic study of cells began in the nineteenth century. One of the largest scientific theories at that time there was Cell theory, which affirmed the unity of structure of all living nature. The study of all life at the cellular level is at the core of modern biological research.

In the structure and functions of each cell, signs are found that are common to all cells, which reflects the unity of their origin from primary organic substances. The particular characteristics of various cells are the result of their specialization in the process of evolution. Thus, all cells regulate metabolism in the same way, double and use their hereditary material, receive and utilize energy. At the same time, different single-celled organisms (amoebas, slippers, ciliates, etc.) differ quite greatly in size, shape, and behavior. The cells of multicellular organisms differ no less sharply. Thus, a person has lymphoid cells - small (about 10 microns in diameter) round cells involved in immunological reactions, and nerve cells, some of which have processes more than a meter long; These cells carry out the main regulatory functions in the body.

The first cytological research method was live cell microscopy. Modern options for intravital light microscopy - phase-contrast, luminescent, interference, etc. - allow you to study the shape of cells and general structure some of its structures, cell movement and division. Details of the cell structure are revealed only after special contrasting, which is achieved by staining the killed cell. New stage studying the structure of the cell - electron microscopy, which has a significantly higher resolution of the cell structure compared to light microscopy. The chemical composition of cells is studied by cyto- and histochemical methods, which make it possible to determine the localization and concentration of a substance in cellular structures, the intensity of the synthesis of substances and their movement in cells. Cytophysiological methods make it possible to study cell functions.

Cell structure

The cells of all organisms have a single structural plan, which clearly shows the commonality of all life processes. Each cell includes two inextricably linked parts: the cytoplasm and the nucleus. Both the cytoplasm and the nucleus are characterized by complexity and strict orderliness of structure and, in turn, they include many different structural units, performing very specific functions.

Shell. It directly interacts with the external environment and interacts with neighboring cells (in multicellular organisms).

The shell is the custom of the cell. She vigilantly ensures that unnecessary substances do not penetrate into the cage. this moment substances; on the contrary, the substances that the cell needs can count on its maximum assistance.

The core shell is double; consists of inner and outer nuclear membranes. Between these membranes is the perinuclear space. The outer nuclear membrane is usually associated with endoplasmic reticulum channels.

The core shell contains numerous pores. They are formed by the closure of the outer and inner membranes and have different diameters. Some nuclei, such as egg nuclei, have many pores and are located at regular intervals on the surface of the nucleus. The number of pores in the nuclear envelope varies various types cells. The pores are located at an equal distance from each other. Since the diameter of the pore can vary, and in some cases its walls have a rather complex structure, it seems that the pores are contracting, or closing, or, conversely, expanding. Thanks to the pores, the karyoplasm comes into direct contact with the cytoplasm. Quite large molecules of nucleosides, nucleotides, amino acids and proteins easily pass through the pores, and thus an active exchange takes place between the cytoplasm and the nucleus.

Cytology

The science that studies the structure and function of cells is called cytology.

Over the past decade, it has made great strides, largely due to the development of new methods for studying cells.

The main “tool” of cytology is a microscope, which allows one to study the structure of a cell at a magnification of 2400-2500 times. Cells are studied in living form, as well as after special treatment. The latter comes down to two main stages.

First, the cells are fixed, that is, they are killed with fast-acting substances that are toxic to the cells and do not destroy their structures. The second stage is coloring the preparation. It is based on the fact that different parts of the cell with to varying degrees intensity is perceived by some dyes. Thanks to this, it is possible to clearly identify different structural components cells that are not visible without staining due to their similar refractive index. The method of making sections is very often used. To do this, tissues or individual cells, after special treatment, are enclosed in a solid medium (paraffin, celloidin), after which, using a special device - a microtome equipped with a sharp razor, they are laid out into thin sections with a thickness of 3 microns (micron = 0.001 mm).

1. Not all organisms have a cellular structure.

Cellular organization was the result of a long evolution, which was preceded by non-cellular (precellular) forms of life. Before examination, fixed and colored preparations are placed in a medium with a high refractive index (glycerin, Canada balsam, etc.). Thanks to this, they become transparent, which facilitates the study of the drug.

In modern cytology, a number of new methods and techniques have been developed, the use of which has extremely deepened knowledge about the structure and physiology of the cell.

Very great importance To study cells, biochemical and cytochemical methods are used. Currently, we can not only study the structure of a cell, but also determine its chemical composition and its changes during the life of the cell. Many of these methods rely on the use of color reactions to distinguish between certain chemical substances or groups of substances. The study of the distribution of substances of different chemical compositions in a cell by color reactions is a cytochemical method. It is of great importance for the study of metabolism and other aspects of cell physiology.

Microscope and cell

Ultraviolet microscopy is widely used in modern cytology. Ultraviolet rays are invisible to the human eye, but are perceived by a photographic plate. Some playing especially important role In the life of a cell, organic substances (nucleic acids) selectively absorb ultraviolet rays. Therefore, from photographs taken in ultraviolet rays, one can judge the distribution of nucleic substances in the cell.

A number of sophisticated methods have been developed to study the penetration of various substances into the cell from the environment.

For this purpose, in particular, intravital (vital) dyes are used. These are dyes (for example, neutral red) that penetrate the cell without killing it. By observing a living, vitally stained cell, one can judge the routes of penetration and accumulation of substances in the cell.

A particularly important role in the development of cytology, as well as in the study thin structure electron microscopy played a role in the protozoa.

An electron microscope is based on a different principle than a light optical microscope. The object is studied in a beam of rapidly flying electrons. The wavelength of electron rays is many thousands of times less than the wavelength of light rays. This allows one to obtain significantly greater resolution, i.e., much greater magnification than in a light microscope. A beam of electrons passes through the object being studied and then falls on a fluorescent screen, on which an image of the object is projected. For an object to be transparent to the electron beam, it must be very thin. Conventional microtome sections with a thickness of 3-5 microns are completely unsuitable for this. They will completely absorb the electron beam. Special devices were created - ultramicrotomes, which make it possible to obtain sections of negligible thickness, on the order of 100-300 angstroms (an angstrom is a unit of length equal to one ten-thousandth of a micron). Differences in electron absorption in different parts cells are so small that without special processing on the screen electron microscope they cannot be detected. Therefore, the objects under study are pre-treated with substances that are impermeable or difficult to penetrate for electrons. Such a substance is osmium tetroxide (Os04). She in varying degrees is absorbed by different parts of the cell, which due to this retain electrons differently.

Using an electron microscope, magnifications of the order of 100,000 can be obtained.

Electron microscopy opens up new perspectives in the study of cell organization.

Cell structure diagram

In Fig. 15 and fig. 16 compares the diagram of the structure of the cell, as it was presented in the twenties of this century and as it appears at the present time.

Outside, the cell is delimited from the environment by a thin cell membrane, which plays an important role in regulating the entry of substances into the cytoplasm. The main substance of the cytoplasm has a complex chemical composition.

It is based on proteins that are in a state colloidal solution. Proteins are complex organic substances with large molecules (their molecular weight is very high, measured in tens of thousands relative to a hydrogen atom) and high chemical mobility. In addition to proteins, many other proteins are present in the cytoplasm organic compounds(carbohydrates, fats), among which complex organic substances - nucleic acids - play a particularly important role in the life of the cell. From inorganic components The cytoplasm should first of all be called water, which by weight makes up significantly more than half of all substances that make up the cell. Water is important as a solvent because metabolic reactions take place in a liquid medium. In addition, the cell contains salt ions (Ca2+, K+, Na+, Fe2+, Fe3+, etc.).

Organelles are located in the main substance of the cytoplasm - constantly present structures that perform certain functions in the life of the cell. Among them, mitochondria play an important role in metabolism. In a light microscope they are visible in the form of small rods, threads, and sometimes granules.

An electron microscope has shown that the structure of mitochondria is very complex. Each mitochondria has a shell consisting of three layers and an internal cavity.

From the shell into this cavity filled with liquid contents, numerous partitions protrude, not reaching the opposite wall, called cristae. Cytophysiological studies have shown that mitochondria are organelles with which the cell's respiratory processes (oxidative) are associated. In internal cavity, respiratory enzymes (organic catalysts) are localized on the shell and cristae, providing complex chemical transformations that make up the respiration process.

In the cytoplasm, in addition to mitochondria, there is a complex system membranes, which together form the endoplasmic reticulum (Fig. 16).

Electron microscopic studies have shown that the membranes of the endoplasmic reticulum are double. On the side facing the main substance of the cytoplasm, each membrane contains numerous granules (called “Pallas bodies” after the scientist who discovered them). These granules contain nucleic acids (namely ribonucleic acid), which is why they are also called ribosomes. On the endoplasmic reticulum, with the participation of ribosomes, one of the main processes of cell life is carried out - protein synthesis.

Some of the cytoplasmic membranes are devoid of ribosomes and form a special system called the Golgi apparatus.

This formation has been discovered in cells for quite some time, because it can be detected using special methods when examined under a light microscope. However fine structure The Golgi apparatus became known only as a result of electron microscopic studies. Functional meaning This organelle boils down to the fact that various substances synthesized in the cell are concentrated in the area of ​​the apparatus, for example, secretion grains in glandular cells, etc. The membranes of the Golgi apparatus are in connection with the endoplasmic reticulum. It is possible that a number of synthetic processes take place on the membranes of the Golgi apparatus.

The endoplasmic reticulum is connected to outer shell kernels. This connection apparently plays a significant role in the interaction between the nucleus and the cytoplasm. The endoplasmic reticulum also has a connection with the outer membrane of the cell and in some places directly passes into it.

Using an electron microscope, another type of organelle was discovered in cells - lysosomes (Fig. 16).

They resemble mitochondria in size and shape, but are easily distinguished from them by the absence of thin internal structure, so characteristic and typical of mitochondria. According to the views of most modern cytologists, lysosomes contain digestive enzymes associated with the breakdown of large molecules of organic substances entering the cell. These are like reservoirs of enzymes that are gradually used in the life of the cell.

In the cytoplasm of animal cells, a centrosome is usually located adjacent to the nucleus. This organelle has permanent structure. It is composed of nine ultramicroscopic rod-shaped formations, enclosed in a specially differentiated compacted cytoplasm. Centrosome is an organelle associated with cell division.

Rice. 16. Diagram of the cell structure, according to modern data, taking into account electron microscopic studies:

1 - cytoplasm; 2 - Golgi apparatus, 3 - centrosome; 4 - mitochondria; 5 - endoplasmic reticulum; 6 - core; 7 - nucleolus; 8 - lysosomes.

In addition to the listed cytoplasmic organelles of the cell, it may contain various special structures and inclusions associated with metabolism and the performance of various special functions characteristic of a given cell. Animal cells usually contain glycogen, or animal starch. This is a reserve substance consumed in the metabolic process as the main material for oxidative processes. There are often fatty inclusions in the form of small droplets.

In specialized cells such as muscle cells, there are special contractile fibers associated with the contractile function of these cells. A number of special organelles and inclusions are present in plant cells. In the green parts of plants, chloroplasts are always present - protein bodies containing the green pigment chlorophyll, with the participation of which photosynthesis is carried out - the process of aerial nutrition of the plant. Starch grains, which are absent in animals, are usually found here as a reserve substance. Unlike animals, plant cells have, except outer membrane, durable shells made of fiber and, which determines the special strength of plant tissues.

Cell division

The ability of cells to reproduce themselves is based on the unique property of DNA to self-copy and the strictly equivalent division of reproduced chromosomes during the process of Mitosis. As a result of division, two cells are formed, identical to the original one in genetic properties and with an updated composition of the nucleus and cytoplasm. The processes of self-reproduction of chromosomes, their division, the formation of two nuclei and division of the cytoplasm are separated in time, collectively constituting the mitotic cycle of the cell. If after division the cell begins to prepare for the next division, the mitotic cycle coincides with life cycle cells. However, in many cases, after division (and sometimes before it), cells leave the mitotic cycle, differentiate and perform one or another special function in the body. The composition of such cells can be updated due to divisions of poorly differentiated cells. In some tissues, differentiated cells are able to re-enter the mitotic cycle. In nervous tissue, differentiated cells do not divide; many of them live as long as the body as a whole, that is, in humans - several decades. At the same time, the kernels nerve cells do not lose the ability to divide: being transplanted into the cytoplasm cancer cells, the nuclei of neurons synthesize DNA and divide. Experiments with hybrid cells show the influence of the cytoplasm on the manifestation of nuclear functions. Inadequate preparation for division prevents mitosis or distorts its course. Thus, in some cases, cytoplasmic division does not occur and a binucleate cell is formed. Repeated division of nuclei in a non-dividing cell leads to the appearance of multinucleated cells or complex supracellular structures (symplasts), for example in striated muscles. Sometimes cell reproduction is limited to the reproduction of chromosomes, and polyploid cell, having a doubled (compared to the original cell) set of chromosomes. Polyploidization leads to increased synthetic activity and an increase in cell size and mass.

One of the main biological processes, ensuring the continuity of life forms and underlying all forms of reproduction, is the process of cell division. This process, known as karyokinesis, or mitosis, occurs with amazing consistency, with only some variations in detail, in the cells of all plants and animals, including protozoa. During mitosis occurs uniform distribution chromosomes undergoing duplication between daughter cells. From any part of each chromosome, daughter cells receive half. Without going into a detailed description of mitosis, we will only note its main points (Fig.).

In the first stage of mitosis, called prophase, chromosomes in the form of threads become clearly visible in the nucleus.

Rice. Scheme of mitotic cell division:

1 - non-fissile core;

2-6 - successive stages of nuclear change in prophase;

7-9 - metaphase;

10 - anaphase;

11-13 - telophase. different lengths.

In a non-dividing nucleus, as we have seen, chromosomes look like thin, irregularly located threads intertwined with each other. In prophase, they shorten and thicken. At the same time, each chromosome turns out to be double. A gap runs along its length, dividing the chromosome into two adjacent and completely similar halves.

At the next stage of mitosis - metaphase - the nuclear membrane is destroyed, the nucleoli are dissolved and the chromosomes find themselves lying in the cytoplasm. All chromosomes are arranged in one row, forming the so-called equatorial plate. The centrosome undergoes significant changes. It is divided into two parts, which diverge, and threads are formed between them, forming an achromatic spindle. The equatorial plate of chromosomes is located along the equator of this spindle.

At the anaphase stage, the process of divergence to opposite poles of the daughter chromosomes occurs, formed, as we have seen, as a result of the longitudinal splitting of the maternal chromosomes. Chromosomes diverging in anaphase slide along the threads of the achromatin spindle and eventually assemble in two groups in the centrosome region.

During last stage Mitosis - telophase - the structure of the non-dividing nucleus is restored. A nuclear envelope is formed around each group of chromosomes. Chromosomes stretch and thin, turning into long, randomly arranged thin threads. Nuclear sap is released, in which the nucleolus appears.

Simultaneously with the stages of anaphase and telophase, the cell cytoplasm is divided into two halves, which is usually carried out by simple constriction.

As can be seen from our brief description, the process of mitosis comes down primarily to the correct distribution of chromosomes between daughter nuclei. Chromosomes consist of bundles of thread-like DNA molecules arranged along longitudinal axis chromosomes. Visible beginning Mitosis is preceded, as has now been established by precise quantitative measurements, by DNA duplication, the molecular mechanism of which we have already discussed above.

Thus, mitosis and the splitting of chromosomes during it is only a visible expression of the processes of duplication (autoreproduction) of DNA molecules, carried out at the molecular level. DNA determines protein synthesis through RNA. The qualitative features of proteins are “encoded” in the structure of DNA. Therefore, it is obvious that the precise division of chromosomes in mitosis, based on the reduplication (autoreproduction) of DNA molecules, underlies the “hereditary information” in a number of successive generations of cells and organisms.

The number of chromosomes, as well as their shape, size, etc., is characteristic feature every type of organism. Humans, for example, have 46 chromosomes, perch - 28, common wheat - 42, etc.

1. Why is it necessary to use magnifying instruments to study cells?
2. Why is the microscope you are working with called a light microscope?

Each cell has three essential parts: the cell membrane, the cytoplasm and the genetic apparatus (Fig. 9).

Rice. 9. Animal and plant cells

Cell membrane not only limits the internal contents of the cell, but also protects it from adverse environmental influences and maintains a certain shape of the cells. Through the membrane, exchange of substances occurs between the contents of the cell and the external environment.

Cells of bacteria, fungi and plants, in addition to membranes, usually also have cell wall(shell). It is the external skeleton of the cell and determines its shape. The cell wall is permeable to water, salts and many organic substances.

Cytoplasm- semi-liquid contents of the cell. It contains various organelles (from the Greek organon - organ) and cellular inclusions. Cytoplasm unites all cellular structures and ensures their interaction.

Genetic apparatus- the most important part of the cell. It is he who controls all vital processes and determines the cell’s ability to reproduce itself. In the cells of plants, animals and fungi, the genetic apparatus is surrounded by a membrane and is called core. The nucleus contains carriers of hereditary information about the cell and the organism as a whole - chromosomes (from the Greek chromium - paint and soma - body). The similarity of parents and offspring depends on chromosomes. The nucleus may contain one or more nucleoli. Bacteria do not have a nucleus and the nuclear substance is located directly in the cytoplasm.

Features of cell structure. Cells of organisms belonging to different kingdoms of living nature have their own characteristics. Thus, only plant cells contain plastids in the cytoplasm. They are colorless or painted in various colors. Reserves accumulate in colorless plastids nutrients. Plastids, colored yellow and red, determine the color of flower petals, autumn leaves, and ripe fruits.

Most important have plastids that are colored green color, - chloroplasts (from the Greek chloros - green), containing chlorophyll. The process of photosynthesis occurs in chloroplasts.

Vacuoles(from Latin vacuum - empty) contain cell sap - water solution organic and inorganic compounds. The cell sap of plants may contain coloring substances (pigments) that give blue, purple, crimson color to petals and other parts of plants, as well as autumn leaves.

Bacterial cells have the simplest structure. Fungal cells, unlike plant and animal cells, usually contain many nuclei. But, despite the differences in structure, the cells of plants, animals and fungi have a similar set of organelles; there are no fundamental differences in the functioning of their genetic apparatus or in processes associated with metabolism.

Answer the questions

1. What is the function of the cell membrane?
2. Which cells have a cell wall (envelope)? What is her role?
3. What role does the genetic apparatus of the cell perform?
4. What is the fundamental difference in the structure of bacterial cells from the cells of plants, animals and fungi?

New concepts

Cell membrane. Cytoplasm. Genetic apparatus. Core. Chromosomes. Plastids. Vacuoles.

Think!

What does the similarity indicate? chemical composition and the structure of all cells?

My laboratory

Preparation and examination of a preparation of onion scale skin under a microscope

Fig. 10. Preparation of a microspecimen of onion scale skin

1. Consider the sequence of preparing the onion skin preparation shown in Figure 10.
2. Prepare the slide by wiping it thoroughly with gauze.
3. Use a pipette to place 1-2 drops of water onto the slide.
4. Using tweezers, carefully remove a small piece of clear skin from the inner surface of the onion scales. Place a piece of peel in a drop of water and straighten it with the tip of a dissecting needle.
5. Cover the peel with a cover slip as shown in the picture. Use filter paper to remove excess water.
6. Examine the prepared preparation at low magnification. Note which parts of the cell you see.
7. Stain the preparation with iodine solution. Use filter paper on the opposite side to pull off excess solution.
8. Examine the colored preparation. What changes have occurred?
9. Examine the specimen at high magnification. Find on it a dark stripe surrounding the cell - the membrane; underneath is a golden substance - the cytoplasm (it can occupy the entire cell or be located near the walls). The nucleus is clearly visible in the cytoplasm. Find the vacuole with cell sap (it differs from the cytoplasm in color).
10. Sketch 2-3 cells of onion skin. Label the membrane, cytoplasm, nucleus, vacuole with cell sap (Fig. 11).
11. Think about why the onion skin preparation was stained with iodine solution.

Figure 11. Cellular structure onion skins

Cell structure

Cell- an elementary unit of structure and vital activity of living organisms, which has its own metabolism and is capable of self-reproduction and development.

Eukaryotic cells contain a nucleus delimited from the cytoplasm by a membrane. They are characteristic of plants, fungi and animals.

During the development and differentiation of a eukaryotic cell, the nucleus can sometimes be destroyed, as occurs, for example, in mature mammalian erythrocytes.

Cytoplasm- the internal environment of the cell, ensuring the chemical interaction of all cellular structures.

It includes hyaloplasm(a transparent substance based on water) and the cellular components located in it ( organelles And inclusion). The cytoplasm of the cell continuously moves, and organelles and inclusions move along with it.

Cytoplasm capable of growth and reproduction; if partially removed, it can recover. However, the cytoplasm functions normally only in the presence of the nucleus. Without it, the cytoplasm cannot exist for a long time, just like the nucleus without the cytoplasm.

Features of the structure:

• Viscous colorless substance.
• Is in constant motion.
• Contains organelles - permanent structural components and cellular inclusions - non-permanent cell structures.
• Inclusions can be in the form of drops (fats) and grains (proteins, carbohydrates).

Functions performed:

• Connects all parts of the cell into a single whole.
• Transports substances.
• Chemical processes take place in it.
• Performs a supporting function.

The most important role of the cytoplasm is to unite all cellular structures (components) and ensure their chemical interaction.

Any cell has a very complex structure. The contents of the cell, as well as many intracellular structures, are limited biological membranes(lat. membrane- “skin”, “film”) - the thinnest films (3.5-10 nm thick), consisting mainly of proteins and lipids.

Cell membrane(or plasma membrane) separates the contents of any cell from external environment, ensuring its integrity.

The cell membrane is a double layer (bilayer) of molecules phospholipids. They have a hydrophilic (“head”) and a hydrophobic (“tail”) part. Hydrophobic areas face inward, and hydrophilic areas face outward.

The biological membrane contains proteins: integral(penetrating the membrane through), semi-integral(immersed at one end into the outer or inner lipid layer) and superficial(located on the outside or adjacent to inside membranes). Some of them contact the cell cytoskeleton and perform the function of channels and receptors.

Membranes may also contain carbohydrates associated with protein molecules ( glycoproteins) or lipids ( glycolipids). Carbohydrates are usually located on outer surface membranes and perform receptor functions.

Membrane functions

• barrier - ensures regulated, selective, passive and active metabolism with environment;
• transport - substances are transported into and out of the cell through the membrane (nutrients enter the cell, remove end products of metabolism, maintain a constant ion concentration);
• receptor (binding of hormones and other regulatory molecules);
• in multicellular organisms it ensures contacts between cells and the formation of tissues.

Cell membranes have semi-permeability, or selective permeability. They are designed in such a way that they regulate the process of transport of substances into the cell: some substances pass through, while others do not. Glucose, amino acids, and fatty acid and ions.

There are several mechanisms for the entry of substances into the cell or their removal out: diffusion, osmosis, active transport And exo- or endocytosis. Diffusion and osmosis are passive in nature - they do not require energy. The remaining mechanisms come with energy consumption.

Passive transport- the process of passing substances through a membrane without energy consumption. In this case, the substance moves from the area with its high concentration to the low side, i.e. along the concentration gradient.

The following types of passive transport are distinguished:

• simple diffusion(for small neutral molecules (H 2 O, CO 2, O 2), as well as hydrophobic low molecular weight organic substances that easily penetrate membrane phospholipids along a concentration gradient;
• facilitated diffusion(for hydrophilic molecules transported along a concentration gradient, but with the help of special integral proteins that form channels in the membrane that provide selective permeability. For elements such as K, Na and Cl, there are their own channels. Moreover, potassium channels are always open.

Active transport is the transfer of substances across a membrane against a concentration gradient. Such transfer requires energy expenditure by the cell. The energy source is usually ATP.