The structure of the human cell. The structure of the human cell, cell division and appearance, a description with pictures for children What is the main part of the cell

Cell- the elementary unit of a living system. Various structures of a living cell, which are responsible for the performance of a particular function, are called organelles, like the organs of the whole organism. Specific functions in the cell are distributed among organelles, intracellular structures that have a certain shape, such as the cell nucleus, mitochondria, etc.

Cell structures:

Cytoplasm. Mandatory part of the cell enclosed between plasma membrane and core. Cytosol is a viscous aqueous solution of various salts and organic matter, penetrated by a system of protein filaments - cytoskeletons. Most of the chemicals and physiological processes cells pass through the cytoplasm. Structure: Cytosol, cytoskeleton. Functions: includes various organelles, the internal environment of the cell
plasma membrane. Each cell of animals, plants, is limited from the environment or other cells by the plasma membrane. The thickness of this membrane is so small (about 10 nm) that it can only be seen with an electron microscope.

Lipids they form a double layer in the membrane, and proteins penetrate its entire thickness, are immersed to different depths in the lipid layer, or are located on the outer and inner surfaces of the membrane. The structure of the membranes of all other organelles is similar to the plasma membrane. Structure: a double layer of lipids, proteins, carbohydrates. Functions: restriction, preservation of the shape of the cell, protection against damage, regulator of the intake and removal of substances.

Lysosomes. Lysosomes are membranous organelles. They have an oval shape and a diameter of 0.5 microns. They contain a set of enzymes that break down organic matter. The membrane of lysosomes is very strong and prevents the penetration of its own enzymes into the cytoplasm of the cell, but if the lysosome is damaged by any external influences, then the entire cell or part of it is destroyed.
Lysosomes are found in all cells of plants, animals and fungi.

Carrying out the digestion of various organic particles, lysosomes provide additional "raw materials" for chemical and energy processes in the cell. During starvation, lysosome cells digest some organelles without killing the cell. Such partial digestion provides the cell with the necessary minimum of nutrients for a while. Sometimes lysosomes digest whole cells and groups of cells, which plays an essential role in the developmental processes in animals. An example is the loss of the tail during the transformation of a tadpole into a frog. Structure: oval-shaped vesicles, membrane outside, enzymes inside. Functions: breakdown of organic substances, destruction of dead organelles, destruction of spent cells.

Golgi complex. The products of biosynthesis entering the lumens of the cavities and tubules of the endoplasmic reticulum are concentrated and transported in the Golgi apparatus. This organelle is 5–10 µm in size.

Structure: cavities surrounded by membranes (vesicles). Functions: accumulation, packaging, excretion of organic substances, formation of lysosomes

Endoplasmic reticulum. The endoplasmic reticulum is a system for the synthesis and transport of organic substances in the cytoplasm of a cell, which is an openwork structure of connected cavities.
attached to the membranes of the endoplasmic reticulum big number ribosomes are the smallest organelles of the cell, having the form of a sphere with a diameter of 20 nm. and made up of RNA and protein. Ribosomes are where protein synthesis takes place. Then the newly synthesized proteins enter the system of cavities and tubules, through which they move inside the cell. Cavities, tubules, tubules from membranes, on the surface of ribosome membranes. Functions: synthesis of organic substances with the help of ribosomes, transport of substances.

Ribosomes. Ribosomes are attached to the membranes of the endoplasmic reticulum or are freely located in the cytoplasm, they are arranged in groups, and proteins are synthesized on them. Protein composition, ribosomal RNA Functions: provides protein biosynthesis (assembly of a protein molecule from).
Mitochondria. Mitochondria are energy organelles. The shape of mitochondria is different, they can be the rest, rod-shaped, filamentous with an average diameter of 1 micron. and 7 µm long. The number of mitochondria depends on the functional activity of the cell and can reach tens of thousands in the flying muscles of insects. Mitochondria are externally bounded by an outer membrane, under it is an inner membrane that forms numerous outgrowths - cristae.

Inside the mitochondria are RNA, DNA and ribosomes. Specific enzymes are built into its membranes, with the help of which the energy of food substances is converted into ATP energy in the mitochondria, which is necessary for the life of the cell and the organism as a whole.

Membrane, matrix, outgrowths - cristae. Functions: synthesis of an ATP molecule, synthesis of its own proteins, nucleic acids, carbohydrates, lipids, formation of its own ribosomes.

plastids. Only in the plant cell: leukoplasts, chloroplasts, chromoplasts. Functions: accumulation of reserve organic substances, attraction of pollinating insects, synthesis of ATP and carbohydrates. Chloroplasts are shaped like a disc or a ball with a diameter of 4-6 microns. With a double membrane - external and internal. Inside the chloroplast there are DNA ribosomes and special membrane structures - grana, connected to each other and to the inner membrane of the chloroplast. Each chloroplast contains about 50 grains, staggered for better light capture. Chlorophyll is found in the gran membranes, thanks to which the energy of sunlight is converted into the chemical energy of ATP. The energy of ATP is used in chloroplasts for the synthesis of organic compounds, primarily carbohydrates.
Chromoplasts. Pigments of red and yellow color, located in chromoplasts, give various parts plants red and yellow. carrots, tomato fruits.

Leukoplasts are the place of accumulation of a reserve nutrient - starch. There are especially many leukoplasts in the cells of potato tubers. In the light, leukoplasts can turn into chloroplasts (as a result of which potato cells turn green). In autumn, chloroplasts turn into chromoplasts and green leaves and fruits turn yellow and red.

Cell Center. It consists of two cylinders, centrioles, located perpendicular to each other. Functions: support for spindle threads

Cellular inclusions either appear in the cytoplasm or disappear during the life of the cell.

Dense inclusions in the form of granules contain reserve nutrients (starch, proteins, sugars, fats) or cell waste products that cannot yet be removed. All plastids of plant cells have the ability to synthesize and accumulate reserve nutrients. In plant cells, the accumulation of reserve nutrients occurs in vacuoles.

Grains, granules, drops Functions: non-permanent formations that store organic matter and energy

Nucleus. Nuclear envelope of two membranes, nuclear juice, nucleolus. Functions: storage of hereditary information in the cell and its reproduction, RNA synthesis - informational, transport, ribosomal. Spores are located in the nuclear membrane, through which an active exchange of substances between the nucleus and the cytoplasm is carried out. The nucleus stores hereditary information not only about all the features and properties of a given cell, about the processes that should proceed to it (for example, protein synthesis), but also about the characteristics of the organism as a whole. Information is recorded in DNA molecules, which are the main part of chromosomes. The nucleus contains a nucleolus. The nucleus, due to the presence in it of chromosomes containing hereditary information, performs the functions of a center that controls all vital activity and development of the cell.

The cells that form the tissues of plants and animals vary considerably in shape, size and internal structure. However, all of them show similarities in the main features of the processes of vital activity, metabolism, in irritability, growth, development, and the ability to change.

Biological transformations occurring in a cell are inextricably linked with those structures of a living cell that are responsible for the performance of a single or other function. Such structures are called organelles.

Cells of all types contain three main, inextricably linked components:

1. the structures that form its surface: the outer membrane of the cell, or the cell membrane, or the cytoplasmic membrane;
2. cytoplasm with a whole complex of specialized structures - organelles ( endoplasmic reticulum, ribosomes, mitochondria and plastids, the Golgi complex and lysosomes, the cell center), which are constantly present in the cell, and temporary formations called inclusions;
3. nucleus - separated from the cytoplasm by a porous membrane and contains nuclear juice, chromatin and nucleolus.

Cell structure

The surface apparatus of the cell (cytoplasmic membrane) of plants and animals has some features.

In unicellular organisms and leukocytes, the outer membrane ensures the penetration of ions, water, and small molecules of other substances into the cell. The process of entering the cell particulate matter is called phagocytosis, and the ingestion of droplets of liquid substances is called pinocytosis.

The outer plasma membrane regulates the exchange of substances between the cell and the external environment.

In eukaryotic cells there are organelles covered with a double membrane - mitochondria and plastids. They contain their own DNA and protein-synthesizing apparatus, multiply by division, that is, they have a certain autonomy in the cell. In addition to ATP, a small amount of protein is synthesized in mitochondria. Plastids are characteristic of plant cells and multiply by division.

Single-membrane organelles include the endoplasmic reticulum, the Golgi complex, lysosomes, different types vacuoles.

Modern means of research have allowed biologists to establish that, according to the structure of the cell, all living beings should be divided into organisms "non-nuclear" - prokaryotes and "nuclear" - eukaryotes.

Prokaryotic bacteria and blue-green algae, as well as viruses, have only one chromosome, represented by a DNA molecule (less often RNA), located directly in the cytoplasm of the cell.

Eukaryotes have a great wealth of organelles, have nuclei containing chromosomes in the form of nucleoproteins (a complex of DNA with a histone protein). Eukaryotes include most modern plants and animals, both unicellular and multicellular.

There are two levels of cellular organization:

• prokaryotic - their organisms are very simply arranged - they are unicellular or colonial forms that make up the kingdom of shotguns, blue-green algae and viruses
• eukaryotic - unicellular colonial and multicellular forms, from protozoa - rhizomes, flagellates, ciliates - to higher plants and animals that make up the kingdom of plants, the kingdom of fungi, the kingdom of animals

All organelles of the cell, despite the peculiarities of their structure and functions, are interconnected and "work" for the cell as a single system in which the cytoplasm is the link.

Special biological objects, occupying an intermediate position between animate and inanimate nature, are viruses discovered in 1892 by D.I. Ivanovsky, they currently constitute the object of a special science - virology.

Viruses reproduce only in plant, animal and human cells, causing various diseases. Viruses have a very simple structure and consist of a nucleic acid (DNA or RNA) and a protein shell. Outside the host cells, the viral particle does not show any vital functions: it does not feed, does not breathe, does not grow, does not multiply.

The chemical composition of living organisms

The chemical composition of living organisms can be expressed in two forms: atomic and molecular. The atomic (elemental) composition shows the ratio of the atoms of the elements that make up living organisms. Molecular (material) composition reflects the ratio of molecules of substances.

Chemical elements are part of cells in the form of ions and molecules of inorganic and organic substances. The most important inorganic substances in the cell - water and mineral salts, the most important organic substances - carbohydrates, lipids, proteins and nucleic acids.

Water is the predominant component of all living organisms. The average water content in the cells of most living organisms is about 70%.

Mineral salts in an aqueous solution of the cell dissociate into cations and anions. The most important cations are K+, Ca2+, Mg2+, Na+, NHJ, anions - Cl-, SO2-, HPO2-, H2PO-, HCO-, NO-.

Carbohydrates - organic compounds, consisting of one or more molecules of simple sugars. The content of carbohydrates in animal cells is 1-5%, and in some plant cells it reaches 70%.

Lipids - fats and fat-like organic compounds, practically insoluble in water. Their content in different cells varies greatly: from 2-3 to 50-90% in the cells of plant seeds and adipose tissue of animals.

Squirrels are biological heteropolymers whose monomers are amino acids. Only 20 amino acids are involved in the formation of proteins. They are called fundamental, or basic. Some of the amino acids are not synthesized in the organisms of animals and humans and must be supplied with plant foods (they are called essential).

Nucleic acids. There are two types of nucleic acids: DNA and RNA. Nucleic acids are polymers whose monomers are nucleotides.

Cell structure

The formation of cell theory

• Robert Hooke in 1665 discovered cells in a section of cork and was the first to use the term "cell".
• Anthony van Leeuwenhoek discovered unicellular organisms.
• Matthias Schleiden in 1838 and Thomas Schwann in 1839 formulated the basic principles cell theory. However, they erroneously believed that cells arise from the primary non-cellular substance.
• Rudolf Virchow in 1858 proved that all cells are formed from other cells by cell division.

Basic provisions of cell theory

1. The cell is the structural unit of all living things. All living organisms are made up of cells (viruses are an exception).
2. The cell is the functional unit of all living things. The cell shows the whole range of vital functions.
3. The cell is the unit of development of all living things. New cells are formed only as a result of the division of the original (mother) cell.
4. The cell is the genetic unit of all living things. The chromosomes of a cell contain information about the development of the whole organism.
5. The cells of all organisms are similar in chemical composition, structure and function.

Types of cell organization

Among living organisms, only viruses do not have cellular structure. All other organisms are represented by cellular life forms. There are two types of cellular organization: prokaryotic and eukaryotic. Bacteria are prokaryotes, and plants, fungi, and animals are eukaryotes.

Prokaryotic cells are relatively simple. They do not have a nucleus, the location of DNA in the cytoplasm is called a nucleoid, the only DNA molecule is circular and not associated with proteins, cells are smaller than eukaryotic cells, the cell wall contains a glycopeptide - murein, there are no membrane organelles, their functions are performed by invaginations of the plasma membrane, ribosomes are small, microtubules are absent, so the cytoplasm is immobile, and the cilia and flagella have a special structure.

Eukaryotic cells have a nucleus in which chromosomes are located - linear DNA molecules associated with proteins; various membrane organelles are located in the cytoplasm.

Plant cells are distinguished by the presence of a thick cellulose cell wall, plastids, and a large central vacuole that shifts the nucleus to the periphery. The cell center of higher plants does not contain centrioles. The storage carbohydrate is starch.

Fungal cells have a cell membrane containing chitin, there is a central vacuole in the cytoplasm, and there are no plastids. Only some fungi have a centriole in the cell center. The main reserve carbohydrate is glycogen.

Animal cells have, as a rule, a thin cell wall, do not contain plastids and a central vacuole; a centriole is characteristic of the cell center. The storage carbohydrate is glycogen.

The structure of a eukaryotic cell

A typical eukaryotic cell consists of three components: a membrane, a cytoplasm, and a nucleus.

Cell wall

Outside, the cell is surrounded by a shell, the basis of which is the plasma membrane, or plasmalemma, which has a typical structure and a thickness of 7.5 nm.

The cell membrane performs important and very diverse functions: it determines and maintains the shape of the cell; protects the cell from the mechanical effects of the penetration of damaging biological agents; carries out the reception of many molecular signals (for example, hormones); limits the internal contents of the cell; regulates the exchange of substances between the cell and environment, ensuring the constancy of the intracellular composition; participates in the formation of intercellular contacts and various kinds of specific protrusions of the cytoplasm (microvilli, cilia, flagella).

The carbon component in the membrane of animal cells is called the glycocalyx.

The exchange of substances between the cell and its environment occurs constantly. The mechanisms of transport of substances into and out of the cell depend on the size of the transported particles. Small molecules and ions are transported by the cell directly across the membrane in the form of active and passive transport.

Depending on the type and direction, endocytosis and exocytosis are distinguished.

The absorption and release of solid and large particles are called phagocytosis and reverse phagocytosis, respectively, liquid or dissolved particles - pinocytosis and reverse pinocytosis.

Cytoplasm

The cytoplasm is the internal contents of the cell and consists of hyaloplasm and various intracellular structures located in it.

Hyaloplasm (matrix) is an aqueous solution of inorganic and organic substances that can change its viscosity and are in constant motion. The ability to move or flow of the cytoplasm is called cyclosis.

The matrix is ​​an active medium in which many physical and chemical processes take place and which unites all elements of the cell into a single system.

The cytoplasmic structures of the cell are represented by inclusions and organelles. Inclusions are relatively non-permanent, occurring in certain types of cells at certain moments of life, for example, as a supply of nutrients (grains of starch, proteins, glycogen drops) or products to be excreted from the cell. Organelles are permanent and indispensable components of most cells that have a specific structure and perform a vital function.

The membrane organelles of a eukaryotic cell include the endoplasmic reticulum, the Golgi apparatus, mitochondria, lysosomes, and plastids.

Endoplasmic reticulum. The entire inner zone of the cytoplasm is filled with numerous small channels and cavities, the walls of which are membranes similar in structure to the plasma membrane. These channels branch, connect with each other and form a network called the endoplasmic reticulum.

The endoplasmic reticulum is heterogeneous in its structure. Two types of it are known - granular and smooth. On the membranes of the channels and cavities of the granular network there are many small round bodies - ribosomes, which give the membranes a rough appearance. The membranes of the smooth endoplasmic reticulum do not carry ribosomes on their surface.

The endoplasmic reticulum performs many different functions. The main function of the granular endoplasmic reticulum is participation in protein synthesis, which is carried out in ribosomes.

On the membranes of the smooth endoplasmic reticulum, lipids and carbohydrates are synthesized. All these synthesis products accumulate in channels and cavities, and then are transported to various cell organelles, where they are consumed or accumulated in the cytoplasm as cell inclusions. The endoplasmic reticulum connects the main organelles of the cell.

golgi apparatus

In many animal cells, such as nerve cells, it takes the form of a complex network located around the nucleus. In the cells of plants and protozoa, the Golgi apparatus is represented by individual sickle-shaped or rod-shaped bodies. The structure of this organoid is similar in the cells of plant and animal organisms, despite the variety of its shape.

The composition of the Golgi apparatus includes: cavities limited by membranes and located in groups (5-10 each); large and small bubbles located at the ends of the cavities. All these elements form a single complex.

The Golgi apparatus performs many important functions. Through the channels of the endoplasmic reticulum, the products of the synthetic activity of the cell - proteins, carbohydrates and fats - are transported to it. All these substances first accumulate, and then enter the cytoplasm in the form of large and small bubbles and are either used in the cell itself during its life activity, or removed from it and used in the body. For example, in the cells of the pancreas of mammals, digestive enzymes are synthesized, which accumulate in the cavities of the organoid. Then vesicles filled with enzymes form. They are excreted from the cells into the pancreatic duct, from where they flow into the intestinal cavity. Another important function of this organoid is that fats and carbohydrates (polysaccharides) are synthesized on its membranes, which are used in the cell and which are part of the membranes. Thanks to the activity of the Golgi apparatus, the renewal and growth of the plasma membrane occurs.

Mitochondria

The cytoplasm of most animal and plant cells contains small bodies (0.2-7 microns) - mitochondria (Greek "mitos" - thread, "chondrion" - grain, granule).

Mitochondria are clearly visible in a light microscope, with which you can see their shape, location, count the number. The internal structure of mitochondria was studied using an electron microscope. The shell of the mitochondrion consists of two membranes - outer and inner. The outer membrane is smooth, it does not form any folds and outgrowths. The inner membrane, on the contrary, forms numerous folds that are directed into the cavity of the mitochondria. The folds of the inner membrane are called cristae (lat. "crista" - comb, outgrowth). The number of cristae is not the same in the mitochondria of different cells. There can be from several tens to several hundreds, and there are especially many cristae in the mitochondria of actively functioning cells, for example, muscle cells.

Mitochondria are called the "power stations" of cells" since their main function is the synthesis of adenosine triphosphate (ATP). This acid is synthesized in the mitochondria of the cells of all organisms and is a universal source of energy necessary for the implementation of the vital processes of the cell and the whole organism.

New mitochondria are formed by the division of already existing mitochondria in the cell.

Lysosomes

They are small round bodies. Each lysosome is separated from the cytoplasm by a membrane. Inside the lysosome are enzymes that break down proteins, fats, carbohydrates, nucleic acids.

Lysosomes approach the food particle that has entered the cytoplasm, merge with it, and one digestive vacuole is formed, inside of which there is a food particle surrounded by lysosome enzymes. Substances formed as a result of the digestion of a food particle enter the cytoplasm and are used by the cell.

Possessing the ability to actively digest nutrients, lysosomes are involved in the removal of parts of cells, whole cells and organs that die in the process of vital activity. The formation of new lysosomes occurs in the cell constantly. Enzymes contained in lysosomes, like any other proteins, are synthesized on the ribosomes of the cytoplasm. Then these enzymes enter through the channels of the endoplasmic reticulum to the Golgi apparatus, in the cavities of which lysosomes are formed. In this form, lysosomes enter the cytoplasm.

plastids

Plastids are found in the cytoplasm of all plant cells. There are no plastids in animal cells. There are three main types of plastids: green - chloroplasts; red, orange and yellow - chromoplasts; colorless - leukoplasts.

Mandatory for most cells are also organelles that do not have a membrane structure. These include ribosomes, microfilaments, microtubules, and the cell center.

Ribosomes. Ribosomes are found in the cells of all organisms. These are microscopic bodies of rounded shape with a diameter of 15-20 nm. Each ribosome consists of two particles of different sizes, small and large.

One cell contains many thousands of ribosomes, they are located either on the membranes of the granular endoplasmic reticulum, or lie freely in the cytoplasm. Ribosomes are made up of proteins and RNA. The function of ribosomes is protein synthesis. Protein synthesis is a complex process that is carried out not by one ribosome, but by a whole group, including up to several dozen combined ribosomes. This group of ribosomes is called a polysome. The synthesized proteins are first accumulated in the channels and cavities of the endoplasmic reticulum and then transported to the organelles and cell sites where they are consumed. The endoplasmic reticulum and the ribosomes located on its membranes are a single apparatus for the biosynthesis and transport of proteins.

Microtubules and microfilaments

Filamentous structures, consisting of various contractile proteins and causing the motor functions of the cell. Microtubules have the form of hollow cylinders, the walls of which are composed of proteins - tubulins. Microfilaments are very thin, long, filamentous structures composed of actin and myosin.

Microtubules and microfilaments penetrate the entire cytoplasm of the cell, forming its cytoskeleton, causing cyclosis, intracellular movements of organelles, segregation of chromosomes during the division of nuclear material, etc.

Cell center (centrosome). In animal cells, an organoid is located near the nucleus, which is called the cell center. The main part of the cell center is made up of two small bodies - centrioles located in a small area of ​​​​densified cytoplasm. Each centriole has the shape of a cylinder up to 1 µm long. Centrioles play an important role in cell division; they are involved in the formation of the fission spindle.

In the process of evolution, different cells adapted to living in different conditions and performing specific functions. This required the presence in them of special organoids, which are called specialized, in contrast to the general-purpose organelles discussed above. These include contractile vacuoles of protozoa, myofibrils of muscle fibers, neurofibrils and synaptic vesicles of nerve cells, microvilli of epithelial cells, cilia and flagella of some protozoa.

Nucleus

The nucleus is the most important component of eukaryotic cells. Most cells have a single nucleus, but there are also multinucleated cells (in a number of protozoa, in the skeletal muscles of vertebrates). Some highly specialized cells lose nuclei (mammalian erythrocytes, for example).

The nucleus, as a rule, has a spherical or oval shape, less often it can be segmented or fusiform. The nucleus consists of the nuclear membrane and karyoplasm containing chromatin (chromosomes) and nucleoli.

The nuclear membrane is formed by two membranes (outer and inner) and contains numerous pores through which various substances are exchanged between the nucleus and the cytoplasm.

Karyoplasm (nucleoplasm) is a jelly-like solution that contains a variety of proteins, nucleotides, ions, as well as chromosomes and the nucleolus.

The nucleolus is a small rounded body, intensely stained and found in the nuclei of non-dividing cells. The function of the nucleolus is the synthesis of rRNA and their connection with proteins, i.e. assembly of ribosome subunits.

Chromatin - lumps, granules and filamentous structures that are specifically stained by some dyes, formed by DNA molecules in combination with proteins. Different parts of DNA molecules in the composition of chromatin have different degrees of helicity, and therefore differ in color intensity and the nature of genetic activity. Chromatin is a form of existence of genetic material in non-dividing cells and provides the possibility of doubling and realizing the information contained in it. In the process of cell division, DNA spiralization occurs and chromatin structures form chromosomes.

Chromosomes are dense, intensely staining structures that are units of the morphological organization of the genetic material and ensure its precise distribution during cell division.

The number of chromosomes in the cells of each biological species is constant. Usually in the nuclei of body cells (somatic) chromosomes are presented in pairs, in germ cells they are not paired. A single set of chromosomes in germ cells is called haploid (n), a set of chromosomes in somatic cells is called diploid (2n). Chromosomes different organisms differ in size and shape.

A diploid set of chromosomes in cells of a particular type of living organisms, characterized by the number, size and shape of chromosomes, is called a karyotype. In the chromosome set of somatic cells, paired chromosomes are called homologous, chromosomes from different pairs are called non-homologous. Homologous chromosomes are the same in size, shape, composition (one is inherited from the maternal, the other from the paternal organism). The chromosomes in the karyotype are also divided into autosomes, or non-sex chromosomes, which are the same in male and female individuals, and heterochromosomes, or sex chromosomes involved in sex determination and differing in males and females. The human karyotype is represented by 46 chromosomes (23 pairs): 44 autosomes and 2 sex chromosomes (the female has two identical X chromosomes, the male has X and Y chromosomes).

The nucleus stores and implements genetic information, controls the process of protein biosynthesis, and through proteins - all other life processes. The nucleus is involved in the replication and distribution of hereditary information between daughter cells, and, consequently, in the regulation of cell division and the development of the body.

Atlas: human anatomy and physiology. Complete practical guide Elena Yurievna Zigalova

The structure of the human cell

The structure of the human cell

All cells typically have a cytoplasm and a nucleus ( see fig. 1). The cytoplasm includes hyaloplasm, general-purpose organelles found in all cells, and special-purpose organelles that are found only in certain cells and perform special functions. In cells, there are also temporary cellular inclusion structures.

The size of human cells varies from a few micrometers (for example, a small lymphocyte) to 200 microns (an egg). In the human body, there are cells of various shapes: ovoid, spherical, spindle-shaped, flat, cubic, prismatic, polygonal, pyramidal, stellate, scaly, process, amoeboid.

Outside, each cell is covered plasma membrane (plasmolemma) 9–10 nm thick, which limits the cell from the extracellular environment. They perform the following functions: transport, protective, delimiting, receptor perception of signals from the external (for the cell) environment, participation in immune processes, providing surface properties of the cell.

Being very thin, the plasmalemma is not visible in a light microscope. AT electron microscope, if the cut is at right angles to the plane of the membrane, the latter is a three-layer structure, outside surface which is covered with fine fibrillar glycocalyx with a thickness of 75 to 2000 A° a set of molecules associated with plasma membrane proteins.

Rice. 3. The structure of the cell membrane, scheme (according to A. Ham and D. Cormack). 1 - carbohydrate chains; 2 - glycolipid; 3 - glycoprotein; 4 - hydrocarbon "tail"; 5 - polar "head"; 6 - protein; 7 - cholesterol; 8 - microtubules

The plasma membrane, like other membrane structures, consists of two layers of amphipathic lipid molecules (bilipid layer, or bilayer). Their hydrophilic "heads" are directed to the outer and inner sides membranes, and hydrophobic "tails" facing each other. Protein molecules are immersed in the bilipid layer. Some of them (integral, or internal transmembrane proteins) pass through the entire thickness of the membrane, others (peripheral, or external) lie in the inner or outer monolayer of the membrane. Some integral proteins are non-covalently bound to cytoplasmic proteins ( rice. 3). Like lipids, protein molecules are also amphipathic; their hydrophobic regions are surrounded by similar "tails" of lipids, and hydrophilic ones face outward or inside the cell or in one direction.

ATTENTION

Proteins carry out most of the membrane functions: many membrane proteins are receptors, others are enzymes, and others are carriers.

The plasma membrane forms a number of specific structures. These are intercellular connections, microvilli, cilia, cellular invaginations and processes.

microvilli- these are finger-like outgrowths of cells devoid of organelles, covered with a plasmalemma, 1–2 μm long and up to 0.1 μm in diameter. Some epithelial cells (for example, intestinal) have a very large number of microvilli, forming the so-called brush border. Along with the usual microvilli, on the surface of some cells there are large microvilli of the stereocilia (for example, hair sensory cells of the hearing and balance organs, epithelial cells of the epididymal duct, etc.).

Cilia and flagella perform the function of movement. Up to 250 cilia, 5–15 µm long, 0.15–0.25 µm in diameter, cover the apical surface of the upper epithelial cells. respiratory tract, fallopian tubes, seminiferous tubules. eyelash is an outgrowth of a cell surrounded by a plasmalemma. In the center of the cilium runs an axial filament, or axoneme, formed by 9 peripheral doublets of microtubules surrounding one central pair. Peripheral doublets, consisting of two microtubules, surround the central capsule. The peripheral doublets terminate in the basal body (kinetosome), which is formed from 9 triplets of microtubules. At the level of the plasmolemma of the apical part of the cell, triplets pass into doublets, and the central pair of microtubules also begins here. Flagella eukaryotic cells resemble cilia. Cilia perform coordinated oscillatory movements.

Cell Center formed by two centrioles(diplosome), located near the nucleus, located at an angle to each other ( rice. four). Each centriole is a cylinder, the wall of which consists of 9 triplets of microtubules about 0.5 µm long and about 0.25 µm in diameter. Triplets located at an angle of about 50° with respect to each other consist of three microtubules. Centrioles double in cell cycle. It is possible that, like mitochondria, centrioles contain their own DNA. Centrioles are involved in the formation of the basal bodies of cilia and flagella and in the formation of the mitotic spindle.

Rice. 4. The cell center and other structures of the cytoplasm (according to R. Krstic, as amended). 1 - centrosphere; 2 - centriole in a transverse section (triplets of microtubules, radial spokes, the central structure of the "cart wheel"); 3 - centriole (longitudinal section); 4 - satellites; 5 - bordered vesicles; 6 - granular endoplasmic reticulum; 7 - mitochondrion; 8 - internal reticular apparatus (Golgi complex); 9 - microtubules

microtubules, which are present in the cytoplasm of all eukaryotic cells, are formed by the protein tubulin. Microtubules form the cellular skeleton (cytoskeleton) and are involved in the transport of substances within the cell. cytoskeleton The cell is a three-dimensional network in which various organelles and soluble proteins are associated with microtubules. The main role in the formation of the cytoskeleton is played by microtubules, in addition to them, actin, myosin and intermediate filaments take part.