II. cytoplasm

The cytoplasm is the internal contents of the cell and consists of the main substance, or 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 is in constant motion. The ability to move, or flow of the cytoplasm, is called cyclosis. The matrix is an active medium in which many chemical and physiological processes take place and which unites all the components of the cell into a single system.

The cytoplasmic structures of the cell are represented by inclusions and organelles.

Organelles are permanent and indispensable components of most cells, having a specific structure and performing vital functions. Organoids are of general purpose and special purpose.

Organelles of general importance are present in all cells and, depending on the structural features, are divided into non-membrane, single-membrane and two-membrane.

Organelles of special significance are present only in the cells of certain tissues; for example, myofibrils in muscle tissues, neurofibrils in nervous tissue.

non-membrane organelles.

This group includes ribosomes, microtubules and microfilaments, as well as the cell center.

RIBOSOMES.

Ribosomes - very small organelles, present in all cell types. They have a rounded shape, consist of approximately equal mass amounts of rRNA and protein and are represented by two subunits: large and small. Between the subunits is a space where the mRNA attaches.

In cells, ribosomes are freely localized in the cytoplasm, on EPS membranes, in the mitochondrial matrix, on the outer membrane of the nucleus, and in plants in plastids.

The function of ribosomes is the assembly of protein molecules.

During active protein synthesis, polyribosomes are formed. Polyribosomes- a complex of ribosomes (from 5 to 70 ribosomes). There is a connection between individual ribosomes, which is carried out with the help of mRNA molecules.

Rice. 5. The structure of the ribosome (scheme)

1- small subunit; 2 - i-RNA; 3 - large subunit of 4-rRNA

MICROTUBES AND MICROFILAMENTS

Microtubules and microfilaments filamentous structures made up of various contractile proteins. Microtubules look like long 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, chromosome segregation during the division of nuclear material. In addition to free microtubules penetrating the cytoplasm, cells have microtubules organized in a certain way that form the centrioles of the cell center, basal bodies, cilia and flagella.

CELL CENTER

Cell center or centrosome- usually located near the nucleus, consists of two centrioles located perpendicular to each other. Each centriole has the form of a hollow cylinder, the wall of which is formed by 9 triplets of microtubules. There are no microtubules in the center. Therefore, the centriole microtubule system can be described by the formula (9×3)+0.

During the preparation of the cell for division, doubling occurs - duplication centrioles: maternal and daughter diverge to the poles of the cell, outlining the direction of future division, near each, a new centriole is formed from microtubules of the cytoplasm. The main functions of the cell center are:

1) participation in the processes of cell division, the divergence of centrioles determines the orientation of the division spindle and the movement of chromosomes;

2) the structure and function of cilia and flagella (basal bodies) is associated with this organoid; thus, centrioles are associated with the processes of movement in the cell.

Single membrane organelles

These include the endoplasmic reticulum, the Golgi apparatus, lysosomes, and peroxisomes.

5.2.1 Endoplasmic reticulum (ER).

It is a network in the inner layers of the cytoplasm (endoplasm) - an endoplasmic reticulum, which is a complex system tubules, tubules and cisterns bounded by membranes.

There are EPS (EPR):

Smooth (agranular) (does not contain ribosomes on membranes)	Rough (granular) (on membranes - ribosomes)
1. Synthesis of glycogen and lipids (sebaceous glands, liver). 2. Accumulation of synthesis products. 3. Secret transport.	1. Protein synthesis (protein gland cells). 2. Participation in secretory processes, secretion transport. 3. Accumulation of synthesis products.
	4. Provides communication with cell organelles. 5. Provides transport of secrets to cell organelles. 6. Provides communication between the nucleus and cellular organelles and the cytoplasmic membrane. 7. Provides circulation of various substances through the cytoplasm. 8. Participation in pinocytosis (transport of various substances that enter the cell from the outside).

The greatest development of EPS is characteristic of secretory cells. EPS is weakly developed in spermatozoa.

The formation of EPS occurs during cell division from the growths of the outer cytoplasmic membrane and nuclear membrane, is transmitted from cell to cell during cell division.

GOLGI COMPLEX

Golgi complex opened in 1898 by Golgi.

The form of the complex can be in the form of a network around the nucleus, in the form of a cap or belt around the nucleus, in the form of individual elements - rounded, sickle-shaped bodies called dictyosomes.

The Golgi complex consists of three elements that can pass one into another and are interconnected with each other:

1) a system of flat tanks, arranged in packs of five to eight, in the form of a stack of coins and tightly adjacent to each other;

2) a system of tubules extending from the tanks, anastomosing with each other and forming a network;

3) large and small vesicles that close the end sections of the tubules.

This organoid is most well developed in glandular cells, for example, in leukocytes and oocytes, as well as in other cells that produce protein products, polysaccharides and lipids.

Weak development of the Golgi complex is observed in undifferentiated and tumor cells.

Composition: phospholipids, proteins, enzymes for the synthesis of polysaccharides and lipids.

1) participation in the secretory activity of the cell;

2) the accumulation of finished or almost finished products;

3) transportation of secretion products throughout the cell through a system of tubules and vesicles;

4) condensation of secretory granules (osmotic removal of water);

5) isolation and accumulation of substances that are toxic to cells from outside (toxins, anesthetic substances), which are then removed from the cell;

6) formation of yolk grains in oocytes;

7) formation of cell partitions (in plant cells).

The Golgi complex during cell division is transferred from the mother to the daughter.

LYSOSOME

They perform the function of intracellular digestion of food macromolecules and foreign components entering the cell during phago- and pinocytosis, providing the cell with additional raw materials for chemical and energy processes. To carry out these functions, lysosomes contain about 40 hydrolytic enzymes - hydrolases that destroy proteins, nucleic acids, lipids, carbohydrates at acidic pH (proteinases, nucleases, phosphatases, lipases). There are primary lysosomes, secondary lysosomes (phagolysosomes and autophagosomes) and residual bodies. Primary lysosomes are microvesicles detached from the cavities of the Golgi apparatus, surrounded by a single membrane and containing a set of enzymes. After the fusion of primary lysosomes with some substrate to be cleaved, various secondary lysosomes are formed. An example of secondary lysosomes are the digestive vacuoles of protozoa. Such lysosomes are called phagolysosomes, or heterophagosomes. If the fusion occurs with the altered organelles of the cell itself, then autophagosomes are formed. Lysosomes, in the cavities of which undigested products accumulate, are called telolisosomes or residual bodies.

EPS, the Golgi apparatus, and lysosomes are functionally related intracellular structures separated from the cytoplasm by a single membrane. They constitute a single tubular-vacuolar system of the cell.

Peroxisomes

They have an oval shape. Crystal-like structures are located in the central part of the matrix. The matrix contains enzymes for the oxidation of amino acids, during which hydrogen peroxide is formed. The enzyme catalase is also present, which destroys peroxide. (Characteristic of liver and kidney cells)

Double membrane organelles

Mitochondria

The shape of the mitochondria can be oval, rod-shaped, filamentous, highly branched. The forms of mitochondria can change from one to another with changes in pH, osmotic pressure, and temperature. The shape can be different in different cells, and in different parts of the same cell.

Outside, mitochondria are bounded by a smooth outer membrane. The inner membrane forms numerous outgrowths - cristae. The internal content of mitochondria is called the matrix. Mitochondria are semi-autonomous organelles, since they contain their own apparatus for protein biosynthesis (circular DNA, RNA, ribosomes, amino acids, enzymes).

Matrix- the substance is denser than the cytoplasm, homogeneous.

krist a lot in the liver cells, they are located tightly relative to each other; less in muscles.

Fig.7. The structure of the mitochondria (scheme)

1- smooth outer membrane; 2 - inner membrane; 3 - cristae; 4 - matrix (and it contains a circular DNA molecule, many ribosomes, enzymes).

The size of mitochondria varies from 0.2 to 20 microns.

The number of mitochondria is different in different cell types: from 5-7 to 2500, depending on the functional activity of the cells. A large number of mitochondria in liver cells, working muscles (more in young than in old ones).

The location of mitochondria can be uniform throughout the cytoplasm, such as in epithelial cells, nerve cells, protozoan cells, or uneven, for example, in the area of the most active cellular activity. In secretory cells, these are the areas where the secret is produced, in the cells of the heart muscle and gametes (surround the nucleus). A structural connection between mitochondria and the cell nucleus was found in the periods preceding cell division. It is believed that during this period, the processes of metabolism and energy are actively taking place and it is carried out according to structures resembling tubes.

Chemical composition: proteins - 70%, lipids - 25%, nucleic acids (DNA, RNA - slightly), vitamins A, B 12, B 6, K, E, enzymes.

Mitochondria are the most sensitive organelles to the effects of various factors: drugs, fever, poisons lead to swelling, an increase in the volume of mitochondria, their matrix liquefies, the number of cristae decreases and folds appear on the outer membrane. These processes lead to disruption of cellular respiration and can become irreversible with frequent and extreme exposures.

In mitochondria, ATP is synthesized as a result of the processes of oxidation of organic substrates and ADP phosphorylation and the synthesis of steroid hormones.

Such organelles are present only in the cells of certain tissues, for example, myofibrils - in muscle, neurofibrils - in nerve, tono-fibrils, cilia and flagella - in epithelial.

INCLUSIONS

Unlike organelles, inclusion are temporary structures that appear in the cell at certain periods of the cell's life. The main place of localization of inclusions is the cytoplasm, but sometimes the nucleus.

Inclusions are products of cellular metabolism, they can take the form of granules, grains, drops, vacuoles and crystals; are used either by the cell itself as needed, or serve for the entire macroorganism.

Inclusions classified according to their chemical composition:

fatty:	carbohydrate:	protein:	pigmented:
1) in any cell in the form of droplets of fat; 2) white fat - specialized adipose tissue of adults; 3) brown fat - specialized adipose tissue of embryos; 4) as a result of pathological processes - fatty degeneration of cells (liver, heart); 5) in plants - seeds contain up to 70% inclusions;	1) glycogen - in skeletal muscle cells, liver, neurons; 2) in cells of endoparasites (anaerobic type of respiration); 3) starch - in plant cells;	1) in eggs, liver cells, protozoa;	1) lipofuscin - aging pigment; 2) lipochromes - in the cortical substance-venapdrenal and corpus luteum of the ovary; 3) retinin - visual purple of the eye; 4) melanin - in pigment cells; 5) hemoglobin - respiratory - in erythrocytes;
	secretory: can be proteins, fats, carbohydrates, or mixed and are located in the cells of the corresponding glands: 1) sebaceous gland; 2) endocrine glands; 3) glands of the digestive system; 4) mammary glands; 5) mucus in goblet cells; 6) essential oils of plants.

CELL NUCLEUS

The cell nucleus is involved in the differentiation of cells in shape, number, location, and size. The shape of the nucleus is often related to the shape of the cell, but it can also be completely wrong. In spherical, cubic, and polyhedral cells, the nucleus is usually spherical; in cylindrical, prismatic and spindle-shaped - the shape of an ellipse (smooth myocyte).

Fig 8. Smooth myocyte

An example of an irregularly shaped nucleus is the nuclei of leukocytes (segmented - segmented neutrophilic leukocyte). Blood monocytes have a bean-shaped nucleus.

Rice. 9. blood monocyte Rice. ten Segmented

neutrophilic leukocyte

Most cells have one nucleus. But there are binuclear cells: liver cells, hepatocytes and cartilage chondrocytes, and multinuclear cells: osteoclasts of bone tissue and megakaryocytes of the red bone marrow - up to 100 nuclei. Nuclei are especially numerous in symplasts and syncytia (striated muscle fibers and reticular tissue), but these formations are not actually cells.

Fig.11. Hepatocyte Rice. 12.Megakariacite

The arrangement of nuclei is individual for each cell type. Usually in undifferentiated cells, the nucleus is located in the geometric center of the cell. With maturation, accumulation of reserve nutrients and organelles, the nucleus shifts to the periphery. There are cells in which the nucleus occupies a sharply eccentric position. The most striking example of this is white fat cells, adipocytes, in which almost the entire volume of the cytoplasm is occupied by a drop of fat. In any case, no matter how the nucleus is located in the cell, it is almost always surrounded by a zone of undifferentiated cytoplasm.

Rice. 13Adipocytes

The size of the nucleus depends on the type of cell and is usually directly proportional to the volume of the cytoplasm. The ratio between the volume of the nucleus and the cytoplasm is usually expressed by the so-called nuclear-plasmic (N-C) Hertwig ratio: with an increase in the volume of the cytoplasm, the volume of the nucleus also increases. The moment of onset of cell division, apparently, is determined by a change in the R-C ratio and is due to the fact that only a certain volume of the nucleus is able to control a certain volume of the cytoplasm. Usually larger nuclei are found in young, tumor cells, cells preparing for division. At the same time, the volume of the nucleus is a feature characteristic of each tissue. There are tissues whose cells have a small nucleus relative to the volume of the cytoplasm, these are the so-called cells cytoplasmic type. These include most cells of the body, for example, all types of epithelia.

Others - have a large nucleus, which occupies almost the entire cell and a thin rim of the cytoplasm - cells nuclear such as blood lymphocytes.

Fig.16 The structure of the nucleus (diagram)

1- ribosomes on the outer membrane; 2 - nuclear pores; 3 - outer membrane; 4 - inner membrane; 5 - nuclear envelope; (karyolemma, nucleolemma); 6 - slit-like perinuclear space; 7 - nucleolus;

8 - nuclear juice (karyoplasm, nucleoplasm); 9 - heterochromatin;

10 - euchromatin.

nuclear envelope formed by two elementary biological membranes, between which there is a slit-like perinuclear space. The nuclear envelope serves to delimit the intranuclear space from the cytoplasm of the cell. It is not continuous and has the smallest holes - pores. The nuclear pore is formed by the fusion of nuclear membranes and is a complexly organized globular-fibrillar structure that fills the perforation in the nuclear envelope. This so-called nuclear pore complex. Along the border of the hole there are three rows of granules (eight in each). The first row is adjacent to the intranuclear space, the second to the cytoplasm, and the third is located between them. Fibrillar processes depart from the granules, which are connected in the center with the help of the granule and create a septum, diaphragm across the pore. The number of pores is not constant and depends on the metabolic activity of the cell.

nuclear juice- an uncolored mass that fills the entire internal space of the nucleus between its components and is a colloidal system and has turgor.

Nucleoli- one or more steroid bodies, often quite large (in neurocytes and oocytes). Nucleoli - nucleoli- the most dense structure of the nucleus, they stain well with basic dyes, as they are rich in RNA. It is heterogeneous in its structure, has a fine-grained or fine-fibered structure. Serves as a place of education ribosome.

Chromatin- zones of dense substance, which are well perceived by dyes, are characteristic of a non-dividing cell. Chromatin has a different state of aggregation - during cell division, it turns by condensation and spiralization into chromosomes. Each chromosome has centromere- the place of attachment to the threads of the spindle of division during mitosis of the centromere divides the chromosome into two arms.

In addition to the centromere (primary constriction), the chromosome may have secondary constriction and separated by her satellite. On the outside, each chromosome is covered pellicle, under which there is a protein matrix. In the matrix are chromatids. Chromatids are made up of lameness, and those from filaments. The set of chromosomes in each organism is chromosome set.

Fig17. Chromosome structure (diagram)

1 - centromere (primary constriction); 2-shoulders; 3 - secondary constriction; 4-satellite; 5 - pellicle; 6 - protein matrix; 7 - chromatin

REPRODUCTION OF CELLS.

All living organisms are made up of cells. In the process of vital activity, part of the cells of the body wears out, ages and dies. The only way to form cells is the division of the previous ones. Cell division is a vital process for all organisms.

Life (cell) cycle.

The life of a cell from the moment of its origin as a result of the division of the maternal cell to its own division or death is called life (cell) cycle. An essential component of the cell cycle is mitotic cycle, including the period of cell preparation for division and division itself. Cell preparation for division, or interphase, is a significant part of the time of the mitotic cycle and consists of periods:

1. Presynthetic (postmitotic) G1 - occurs immediately after cell division. Biosynthesis processes take place in the cells, new organelles are formed. The young cell is growing. This period is the most variable in duration.

2. Synthetic S is the main one in the mitotic cycle. DNA replication takes place. Each chromosome becomes double-stranded, that is, it consists of two chromatids - identical DNA molecules. In addition, the cell continues to synthesize RNA and proteins. In dividing mammalian cells, it lasts about 6-10 hours.

3. Postsynthetic (premitotic) G2 is relatively short, in mammalian cells it is about 2-5 hours. At this time, the number of centrioles and mitochondria doubles, active metabolic processes take place, proteins and energy are accumulated for the upcoming division. The cell starts dividing.

7.2 CELL DIVISION.

Three methods of eukaryotic cell division have been described:

1) amitosis (direct division),

2) mitosis (indirect division).

3) meiosis (reduction division).

7.2.1 Amitosis- cell division without spiralization of chromosomes, arose earlier than mitosis. In this way they reproduce prokaryotes, highly specialized and degrading cells. At the same time, the nuclear membrane and nucleoli do not disappear, the chromosomes remain spiralized.

Types of amitosis:

1) lacing(characteristic of bacteria)

2) fragmentation(megakaryoblast, megakaryocyte)

3)budding(platelet buds from megakaryocytes)

By distribution genetic material

Irradiation, tissue degeneration, and the action of various agents that disrupt the entry of cells into mitosis lead to division without a mitotic apparatus.

Mitosis

It is characterized by the destruction of the nuclear membrane and nucleoli, the spiralization of chromosomes. In mitosis, there are prophase, metaphase, anaphase and telophase.

Fig.18. Diagram of mitosis

I. Prophase:

1) The shape of the cell becomes round, its contents become more viscous, the chromosomes take the form of long thin threads twisted inside the nucleus. Each chromosome is made up of two chromatids.

2) Chromatids gradually shorten and approach the nuclear membrane, which is a sign of the beginning of the destruction of the karyolemma.

3) The spindle develops: the centrioles diverge towards the poles and double, between them the fission spindle threads are formed.

4) The destruction of the nuclear membrane occurs, in the center of the cell a zone of liquid cytoplasm is formed, where the chromosomes rush.

late metaphase

Chromosomes line up in the equatorial plane, forming metaphyseal plate. Spindle threads are attached to the centromeres of chromosomes.

There are two types of spindle fibers: some of them are associated with chromosomes and are called chromosomal, while others stretch from pole to pole and are called continuous.

maternal

IV. Telophase.

The migration of two daughter groups of chromosomes to opposite poles of the cell is completed. reconstruction nuclei and decondensation chromosomes, they despiralize, the karyolemma is restored, nucleoli appear. Nuclear fission is completed.

Begins cytokinesis (cytotomy)- the process of ligation and division of the cytoplasm with the formation of constriction. There is a "boiling" of the cell surface due to its intensive growth. The cytoplasm loses its viscosity, the centrioles lose their activity, the organelles are divided approximately in half between the daughter cells.

Fig.24 Cytokinesis

Mitosis types:

1) Any tissue is a self-regulating system, in connection with this, the number of cells that died in the tissue is balanced by the number of their formed.

2) Exist daily allowance rhythms of mitotic activity. The greatest mitotic activity coincides with periods of tissue rest, and an increase in tissue function leads to inhibition of mitoses (in nocturnal animals - in the early morning, and in animals leading a daytime lifestyle - at night).

3) The inhibitory effect on mitotic activity is exerted by stress hormones: adrenaline and norepinephrine, and the stimulating effect is exerted by growth hormone. Changes in mitotic activity occur due to changes in the duration of interphase. Each cell initially has the ability to divide, but under certain conditions this ability inhibited. Inhibition can be of varying degrees, up to irreversible.

cell lifespan can be thought of as a period from one division to another. In stable cell populations, in which there is practically no cell reproduction, their life span is maximum (liver, nervous system).

Endoreproduction- all cases when chromosome reduplication or DNA replication occurs, cell division does not occur. This leads to polyplodia, an increase in the volume of the nucleus and cells. It can occur with violations of the mitotic apparatus, it is observed both in normal and pathological conditions. It is characteristic of cells of the liver, urinary tract.

Endomitosis proceeds with an indestructible nuclear envelope. Chromosome reduplication occurs as in normal division, resulting in the formation of giant chromosomes. All the figures characteristic of mitosis are observed, but they occur inside the nucleus. Distinguish endoprophase,endometaphase,endoanaphase,endothelophase. Since the kernel shell is preserved, the result is polyploid cell. The significance of endomitosis lies in the fact that during it the main activity of the cell does not stop.

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- relatively unstable, 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 released from the cell. Organelles - permanent and obligatory components of most cells, having a specific structure and performing a vital function.

To membrane organelles eukaryotic cells include the endoplasmic reticulum, the Golgi apparatus, mitochondria, lysosomes, 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 ( see fig.4). 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 organelle is similar in the cells of plant and animal organisms, despite the diversity 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 mitochondria 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" - crest, 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 "power stations" of cells "since their main function is the synthesis of adenosine triphosphoric acid (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 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) (see Fig. 3). 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 \u200b\u200bdensified 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 organelles, which are called specialized in contrast to the general-purpose organelles discussed above. Among them are contractile vacuoles protozoa, myofibrils muscle fiber, neurofibrils and synaptic vesicles nerve cells, microvilli epithelial cells, cilia and flagella some of the simplest.

With the help of cilia and flagella, cells can move in a liquid medium, since these organelles are able to make rhythmic movements. If there are a large number of hair-like outgrowths of small length on the surface of the cell, then they are called cilia; if there are few such outgrowths and their length is significant, then they are called flagella. Cells of higher plants and higher fungi, as well as sporozoans, do not have cilia and flagella, even in male germ cells. Myofibrils Myofibrils are special differentiated contractile elements...

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Lecture #4

SPECIAL ORGANOSES AND INCLUSIONS

Special purpose organelles

Special purpose organelles are found in many animal and plant cells. They differ from common organelles in that they are characteristic only of certain highly differentiated cells and perform a strictly defined function characteristic of these cells.

Classification of special purpose organelles:

1. Organelles of movement: cilia, flagella, myofibrils.

2. Support structures: tonofibrils.

3. Organelles involved in the transmission of excitation: neurofibrils.

4. Organelles that perceive external stimuli: photoreceptors, statoreceptors, phonoreceptors.

5. Cell surface organelles: microvilli, cuticle.

6. Organelles of defense and attack in unicellular organisms: trichocysts - in ciliates; conoid, roptria - in representatives of the Sporozoa class.

Let us consider in more detail the main of these organelles.

Cilia and flagella

Cilia and flagella are filamentous or hairlike outgrowths of the free surface of cells. With the help of cilia and flagella, cells can move in a liquid medium, since these organelles are able to make rhythmic movements. If cilia and flagella are present in cells attached to some substrate, then they cause the movement of the surrounding fluid.

There are no differences in the fine organization of these structures. If there are a large number of hair-like outgrowths of small length on the surface of the cell, then they are called cilia , but if there are few such outgrowths and their length is significant, then they are called flagella.

In animals, cilia and flagella are found: a) in the cells of the ciliary epithelium (the epithelium of the trachea, some parts of the genital tract); b) in spermatozoa (in nematodes and decapods, sperm cells do not have a tourniquet); c) in protozoa (flagellates, ciliates, rhizopods). In the plant world, they are found in mobile zoospores of algae, mosses, ferns, lower fungi, and myxomycetes. Cells of higher plants and higher fungi, as well as sporozoans, do not have cilia and flagella, even in male germ cells.

The thickness of cilia and flagella is about 200 nm (0.2 µm). Since there are no fundamental differences in the structure of cilia and flagella, let us consider the ultrastructure of these formations using the example of a cilium. Outside, the cilium is covered with a cytoplasmic membrane. Inside it is located axoneme (or axial cylinder), consisting of microtubules. The lower proximal part of the ciliumbasal body, embedded in the cytoplasm. The diameters of the axoneme and the basal body are the same.

The basal body is similar in structure to the centriole and consists of 9 triplets of microtubules. The axoneme in its composition, in contrast to the basal body, has 9 pairs (doublets) of microtubules that form the outer wall of the axoneme cylinder. The microtubule doublets are slightly rotated (about 10 0 ) with respect to the radius of the axoneme. In addition to peripheral doublets of microtubules, a pair of central microtubules is located in the center of the axoneme. These two central microtubules, unlike the peripheral ones, do not reach the basal bodies. Since the basal bodies contain a contractile protein like actomyosin, the peripheral microtubules perform a motor function, while the central ones only support.

Roots are often found at the base of cilia and flagella. kinetodesmata , which are bundles of thin (6 nm) fibrils with transverse striation. Often such striated kinetodesmata extend from the basal bodies deep into the cytoplasm towards the nucleus. The role of these structures is still not well understood.

Deviations from the above structural plan are rare, but in some cells, for example, in the flagella of spermatozoa and some flagellates, 9 additional fibrils were found located between the central and peripheral microtubules. These additional fibrils are connected to the tubules of the axoneme by very thin fibers.

myofibrils

Myofibrils are special differentiated contractile elements of the cell, due to which complex and perfect muscle movements occur. There are two types of myofibrils: smooth and striated. Both types of myofibrils are widespread in multicellular animals and in protozoa.

Striated myofibrils are widely known in the somatic and cardiac muscles of arthropods and chordates. Smooth myofibrils are typical of the musculature of the internal organs of vertebrates and of the somatic muscles of many lower invertebrates.

The structure of myofibrils has been most thoroughly studied in striated muscle fibers. Myofibril has a thickness of 0.5 microns and a length that is from 10-20 microns to several millimeters and even centimeters. In a light microscope, it can be seen that the bundles of myofibrils are colored unevenly: at equal intervals of length, alternation of dark and light areas is visible in them. The dark areas are birefringent and are calledanisotropic disks(A-disks) . Light areas of birefringence are not detected and are calledisotropic disks(I-discs) .

Each A-disk is divided into two halves by a less dense than the rest of its sections, a strip called H-zone (Hansen strip). in the middle of each I -disc has a dark line called Z -line (telophragm). The section of myofibril between two Z -lines are called sarcomere. It is a unit of structure and functioning of the myofibril.

Details of the structure of the sarcomere were obtained only by studying myofibrils in an electron microscope. Each myofibril consists of a bundle of very thin filaments - myofilaments. There are two types of myofilaments: thick and thin. Thin myofilaments are about 7 nm in diameter and about 1 µm long; they are composed primarily of the actin protein. They are located within I -disk and enter the A-disk to the H-zone. Thick myofilaments up to 1.5 μm long and about 15 nm thick are composed of the protein myosin; they are located only within the A-disk. In thin myofilaments, in addition to actin, there are also proteins tropomyosin and troponin. Z -lines contain protein α-actinin and desmin.

Neither actin nor myosin individually has contractile ability. Actin, a protein with a molecular weight of 43.5 thousand, is a globular protein about 3 nm in size. In the presence of ATP and some protein factors, it is capable of aggregation in the form of filamentous structures up to 7 nm thick. Such actin fibrils consist of two spirals wrapped around each other. Myosin, which is part of thick filaments, is a very large protein (molecular weight 470 thousand), consisting of six chains: two long, spirally wrapped around one another, and four short ones, which bind to the ends of long chains and form globular “heads” . The latter have ATPase activity, can react with fibrillar actin, formingactomyosin complex,contractive.

Actin myofilaments are connected at one end with Z -line, which consists of branching molecules of the α-actinin protein, forming a fibrillar network running across the myofibril. On both sides to Z -lines are attached to the ends of the actin filaments of neighboring sarcomeres. Function Z -lines is, as it were, in the binding of neighboring sarcomeres to each other; Z -lines are not reducible structures.

The mechanism of muscle contraction is the simultaneous shortening of all sarcomeres along the entire length of the myofibril. G. Huxley showed that contraction is based on the movement of thick and thin threads relative to each other. At the same time, thick myosin filaments seem to enter the space between the actin filaments, bringing them closer to each other. Z -lines. This model of sliding filaments can explain not only the contraction of striated muscles, but also any contractile structures.

There are also actin and myosin filaments in smooth muscle cells, but they are not as regularly arranged as in striated muscles. There are no sarcomeres here, but simply among the bundles of actin myofilaments, myosin molecules are arranged in no particular order.

Tonofibrils

Tonofibrils are characteristic of cells of unicellular organisms and epithelial cells of multicellular animals. An electron microscopic study showed that they consist of a beam tonofilaments - the thinnest threads with a diameter of 6-15 nm. One bundle may contain from 3 to several hundred tonofilaments.

Tonofibrils are arranged in bundles in the cell in different directions, attached either to desmosomes or to any part of the cytoplasmic membrane and never pass from one cell to another.

Tonofibrils perform a supporting function in the cell.

neurofibrils

Neurofibrils were discovered in 1855 by F.V. Ovsyannikov. They are characteristic of nerve cells (neurons). Made up of finer threadsneurofilaments.

In the body of a neuron, neurofibrils are arranged randomly, and in the processes they form a bundle parallel to the length of the process. There are only two exceptions to this rule: the parallel, ordered arrangement of neurofibrils in the body of a neuron was first discovered in rabid animals, and then in animals that hibernate.

The discovery of neurofibrils led to the emergenceneurofibrillary theoryconduction of nervous excitement. Proponents of this theory believed that neurofibrils are a continuous conductive element of the nervous system. However, later it was found that neurofibrils do not pass from one neuron to another. We are currently followingneural theory, according to which the main role in the conduction of a nerve impulse belongs to the plasmalemma of a neuron, and substances involved in the formation of nerve impulses are transmitted through neurofibrils from the body of a neuron to its end. And excitation is transmitted from one cell to another with the help of a synapse (the structure of the synapse was described earlier when considering communication intercellular contacts). In the synapse, excitation is transmitted chemically with the help of a mediator.

Non-permanent inclusions in the cell

Unlike organoids, both general and special purpose, inclusions are non-permanent formations, either appearing or disappearing during the life of the cell. The main location of inclusions is the cytoplasm, but they are sometimes found in the nucleus.

By their nature, all inclusions are products of cellular metabolism. According to their chemical composition and functions, they are classified as follows:

1. trophic (protein, carbohydrate, fat);

2. secretory;

3. excretory;

4. pigmented.

Trophic inclusions

Protein inclusions. Have the form of grains, granules, disks. They can be present in all cells, but are less common than fats and carbohydrates. An example of protein inclusions is the yolk in the eggs, aleurone grains in the endosperm of seeds. In this case, protein granules serve as a reserve nutrient material for the embryo; in other cells, it is a trophic (building) material for the further construction of cell elements. Protein inclusions can serve as an energy reserve in the most extreme case, when carbohydrate and fat reserves are completely used up.

In plant cells, starch is most often deposited in the form of grains of various shapes and sizes, and the shape of starch grains is specific for each plant species and for certain tissues. The cytoplasm of potato tubers, grains of cereals, legumes, etc. is rich in starch deposits. Other polysaccharides are found in lower plants: paramyloid, starch of red algae.

Carbohydrate inclusions are the main energy reserve of the cell. With the breakdown of 1 g of carbohydrate, 17.6 kJ of energy is released, which accumulates in the form of ATP.

Fat inclusions. Fats in the cytoplasm are deposited in the form of small droplets. They are found in both animals and plants. In some cells, there are very few fatty inclusions and they are constantly used by the cell itself in the process of metabolism, in other cells they accumulate in large quantities, for example, connective tissue fat cells, fish and amphibian liver epithelial cells. A large number of fat droplets are also found in the cytoplasm of many types of protozoa, for example, ciliates. A lot of fat is contained in the seeds of plants, and its amount can reach up to 70% of the dry weight of the seeds (oilseeds).

The process of fat deposition is not associated with any cell organelles; they are deposited in the main substance of the cytoplasm. Under certain conditions, fat drops can merge with each other, increasing in size, eventually a giant fat drop fills the entire cell, the cytoplasm with the nucleus dies off and the cell turns into a bag of fat. This phenomenon is calledfatty degeneration of the cell. This process can be pathological (for example, with fatty degeneration of the liver, heart muscle, etc.) or be a natural process in the life of the body (for example, sebaceous gland cells, subcutaneous fat cells of whales, seals).

Fat inclusions can perform the following functions:

1) are a long-term energy reserve of the cell (38.9 kJ of energy is released during the breakdown of 1 g of fat);

2) thermoregulation (for example, in animals living in cold climates, the layer of fat in the subcutaneous tissue reaches 1 m);

3) shock absorption during movement (for example, layers of fat on the soles of the feet, on the paws of terrestrial animals, the palms of the hands, around the internal organs);

4) supply of nutrients in hibernating animals (for example, bear, badger, hedgehog);

5) the source of metabolic water in the body of animals living in arid conditions (the breakdown of 1 kg of fat produces 1.1 kg of water).

Secretory inclusions

Secrets are products of anabolic cell reactions that perform various vital functions in the body.

Secretory inclusions accumulate in secretory cells in the form of grains, granules, drops. Their chemical nature is very diverse. It can be proteins, lipids, ketones, alcohols, hydrochloric acid and others. Crystalline inclusions are also found in the cells of many plants, and most often these are calcium oxalates.

Functions of secretory inclusions:

1) humoral regulation of the vital activity of the body (hormones in the cells of the endocrine glands);

2) catalyzing the processes of digestion of food (enzymes in the cells of the glands of the digestive tract);

3) transmission of excitation in synapses (mediators in the presynaptic endings of neurons);

4) nutrients for young (milk in the mammary glands of mammals);

5) protective function (mucus in amphibians protects the skin from drying out; poisons, toxins in animals protect against enemies and help kill prey).

Secretions are removed from cells in various ways. According to the method of removing the secret from the cell, 3 types of secretion are distinguished:

1) merocrine - the secret is removed through the pores without damaging the cell; such a cell functions continuously (for example, the glands of the fundus of the stomach);

2) apocrine - drops of secret are laced with a part of the cytoplasm; such a cell functions with interruptions necessary for its restoration (for example, salivary glands, part of the sweat glands)

3) holocrine - the secret fills the whole cell, the cytoplasm dies, the cell dies and turns into a bag with a secret; such a cell functions only once (for example, the sebaceous glands).

excretory inclusions

Excretory inclusions are products of catabolic reactions that are not used by the cell and the body, are often poisonous and must be removed. Excretions can accumulate in liquid (drops) and solid (grains, granules) state.

Examples of excretory inclusions are drops of sweat in the cells of the sweat glands, urine in the cells of the renal tubules. Many invertebrates have special cells - nephrocytes that function as storage kidneys. They accumulate excretions, and then either carry them into the intestine or onto the surface of the body, or leave them as part of their cytoplasm. An important role in the isolation of toxic excretions is played by the Golgi complex. Examples of nephrocytes are chloragogenic cells in annelids, pericardial cells in mollusks and insects, excretory cells in ciliary worms and ascidians.

pigment inclusions

Pigment inclusions can exist in the form of granules, grains, occasionally in the form of drops. Their main function is to give color to plant and animal cells and the body as a whole. But in some cases, pigment inclusions perform more complex functions. Consider, as an example, some pigments of the animal and plant world.

Pigments of the animal world:

one). Melanin - a brown pigment, located in the cells of the basal layer of the skin, gives color to the epithelium of the skin and all its derivatives (human hair, animal hair, nails, claws, feathers in birds, scales in reptiles), as well as the iris of the eye. In animals, melanin creates various types of protective coloration, and in humans it performs the function of protecting against ultraviolet radiation.

2). Lipofuscin - a yellow pigment, the granules of which accumulate during the life of cells and, especially, as they age, as well as during various dystrophic processes (“ageing pigment”).

3). Lutein - a yellow pigment contained in the corpus luteum of pregnancy.

four). Retinin - a characteristic pigment that is part of the visual purple of the retina.

5). Respiratory pigments of animals:

– hemocyanin - a pigment containing copper in its composition; it can change its color from blue (in the oxidized state) to colorless (in the reduced state); found in crustaceans, some snails, cephalopods (dissolved in blood plasma or hemolymph);

– hemoerythrin - a pigment containing iron in its composition; it can change its color from red (in the oxidized state) to colorless (in the reduced state); found in some annelids (found in blood cells);

– chlorocruorin - a pigment that also contains iron in its composition; it can change its color from red (in the oxidized state) to green (in the reduced state); found in some polychaete worms (dissolved in blood plasma);

– hemoglobin - iron-containing pigment, changes its color from orange-red (in the oxidized state) to purple-red (in the reduced state). This is the most widely distributed respiratory pigment in nature, found in some molluscs (dissolved in blood plasma), in some annelids (in plasma or in cells), in all vertebrates (in red blood cells).

Plant pigments:

one). Chlorophyll - a green pigment located in the grains of chloroplasts and is involved in the process of photosynthesis.

2). group of carotenoids carotene (orange), xanthophyll (red), lycopene (yellow); these pigments are contained in chromoplasts and provide color to fruits, seeds, and other plant organs.

5). Phycobilins are the pigments of lower plants; blue-green algae are phycocyanin (blue pigment), and in the composition of red algae - phycoerythrin (red pigment).

The change in cell color is due to the redistribution of pigments.

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In a school biology course, students often, like a palisade in front of an apple orchard, face specific terminology. The terms "organelles" and "inclusions" arise in the field of cytology or cell theory, but what do they mean and what is the difference between them? These questions remain unanswered for most students.

Definition

Inclusions- These are formations that can appear in a living cell in the course of its life.

Organelles- These are the essential structures of the cell that ensure its functioning.

Comparison

Inclusions in a living cell may or may not appear. Traditional inclusions include:

trophic inclusions, or the result of the accumulation of nutrients - proteins, lipids and carbohydrates. For example, starch polysaccharide is stored in plant cells as a storage form of carbohydrates. In the endosperm of some cultures, it forms especially large granules called aleurone grains. Animal cells can accumulate "animal starch" - glycogen. The largest amount of this inclusion is observed in liver cells, in muscles. When the body needs to work urgently, it is glycogen that is consumed in the first place. Inclusions of the protein vitellin in the cytoplasm of the egg have the form of granules;
excretory inclusions. These are accumulations of metabolic products that, for some reason, were not removed from the cell. Foreign agents also belong to this group. With these inclusions, the lysosomes are “dealt with”, and the residues are excreted - removed from the cell;
secretory inclusions. They are synthesized in specialized cells and are secreted outward by special ducts or with the help of blood, lymph. Hormones are a classic example of secretory inclusions;
pigment inclusions. These are highly specialized pigmentocytes that are present in the cells of the dermis and in the structures of the eye and protect the “inside” of the organs from intense sunlight. This group also includes hemoglobin, which provides oxygen transport, and the pigment lipofuscin, which accumulates in aging somatic cells.

Cell organelles can be compared to human organs. Almost every cell, except for highly specialized ones, has a standard set of organelles. These include:

the cell membrane, which limits the internal contents of the cell, performs a protective and permeable function;
endoplasmic reticulum, which transports nutrients and is involved in protein synthesis;
ribosomes that synthesize proteins;
mitochondria, in which the breakdown of organic substances and the release of energy;
leukoplasts, chromoplasts, chloroplasts are present only in plant cells. Participate in the process of photosynthesis, accumulate inclusions. Plastids can move from one stage to another, changing color and function;
the Golgi apparatus, which is involved in the metabolic process and manages the structure of the cell membrane;
the cellular center that organizes the process of reproduction in lower plants and primitive animals;
organelles of movement;
nucleus and its structures - nuclear membrane, nucleolus, chromosomes and nuclear juice. They are responsible for the reproduction of a cell or the whole organism and transmit genetic information to offspring.

Cell organelles are structures without which the existence and reproduction of a cell or a separate organism is impossible.

Organelles

Findings site

The main difference between inclusions and organelles is their functionality. Without organelles, the cell will be incapacitated. The absence or presence of inclusions for most cells is not a life-affirming factor.
Organelles are constantly present in the cell, inclusions disappear and appear in the process of metabolism.
The narrow specialization of some cells is associated with inclusions. At the same time, some of their organelles may atrophy.

BASICS OF CYTOLOGY

I. General principles of the structural and functional organization of the cell and its components. Plasmolemma, its structure and functions.

A cell is an elementary structural, functional and genetic unit in all living organisms.

The morphological characteristics of a cell vary depending on its function. The process during which cells acquire their structural and functional properties and characteristics (specialization) - cell differentiation. Molecular genetic basis differentiation - the synthesis of specific i-RNA and on them - specific proteins.

Cells of all types are characterized by the similarity of the general organization and structure of the most important components .

Each eukaryotic cell consists of two main components: nuclei and cytoplasm, limited cell membrane (plasmolemma).

Cytoplasm separated from the external environment plasma membrane and contains:

organelles

inclusion immersed in

cell matrix (cytosol, hyaloplasm).

Organelles – permanent components of the cytoplasm that have a characteristic structure and are specialized in performing certain functions in a cage.

Inclusions – fickle components of the cytoplasm formed as a result of the accumulation of metabolic products of cells.

PLASMATIC MEMBRANE (plasmolemma, cytolemma, outer cell membrane )

All eukaryotic cells have a boundary membrane - plasmalemma. The plasma membrane plays a role semi-permeable selective barrier, and on the one hand, separates the cytoplasm from the environment surrounding the cell, and on the other hand, provides its connection with this environment.

Plasma membrane functions:

Maintaining the shape of the cell;

Regulation of the transfer of substances and particles into and out of the cytoplasm;

Recognition by this cell of other cells and intercellular substance, attachment to them;

Establishment of intercellular contacts and transmission of information from one cell to another;

Interaction with signal molecules (hormones, mediators, cytokines) due to the presence of specific receptors for them on the surface of the plasmalemma;

The implementation of cell movement due to the connection of the plasmalemma with the contractile elements of the cytoskeleton.

The structure of the plasmalemma:

Molecular structure plasmalemma is described as fluid mosaic model: lipid bilayer, in which protein molecules are immersed (Fig. 1.).

Fig.1.

Thickness p lazmolema varies from 7,5 before 10 nm;

lipid bilayer It is represented mainly by phospholipid molecules consisting of two long non-polar (hydrophobic) fatty acid chains and a polar (hydrophilic) head. In the membrane, the hydrophobic chains face the inside of the bilayer, while the hydrophilic heads face the outside.

The chemical composition of the plasmalemma:

· lipids(phospholipids, sphingolipids, cholesterol);

· proteins;

· oligosaccharides, covalently associated with some of these lipids and proteins (glycoproteins and glycolipids).

Plasma membrane proteins . Membrane proteins make up more than 50% of the mass of membranes. They are retained in the lipid bilayer due to hydrophobic interactions with lipid molecules. Proteins provide specific properties membranes and play various biological roles:

structural molecules;

enzymes;

carriers;

receptors.

Membrane proteins are divided into 2 groups: integral and peripheral:

peripheral proteins usually located outside the lipid bilayer and loosely associated with the membrane surface;

integral proteins are proteins, either completely (proper integral proteins) or partially (semi-integral proteins) immersed in the lipid bilayer. Part of the proteins completely penetrates the entire membrane ( transmembrane proteins); they provide channels through which small water-soluble molecules and ions are transported on both sides of the membrane.

Proteins are distributed within the cell membrane mosaic. Lipids and membrane proteins are not fixed within the membrane, but have mobility: proteins can move in the plane of the membranes, as if "floating" in the thickness of the lipid bilayer (like "icebergs in the lipid" ocean ").

Oligosaccharides. Chains of oligosaccharides associated with protein particles (glycoproteins) or with lipids (glycolipids) can protrude beyond the outer surface of the plasmalemma and form the basis glycocalyx, the supra-membrane layer, which is revealed under an electron microscope in the form of a loose layer of moderate electron density.

Carbohydrate sites give the cell a negative charge and are an important component of specific molecules - receptors. Receptors provide such important processes in the life of cells as recognition of other cells and intercellular substance, adhesive interactions, response to the action of protein hormones, immune response, etc. The glycocalyx is also the site of concentration of many enzymes, some of which may not be formed by the cell itself, but only adsorbed in the glycocalyx layer.

Membrane transport. The plasmalemma is the place where material is exchanged between the cell and the environment surrounding the cell:

Membrane transport mechanisms (Fig. 2):

passive diffusion;

Facilitated diffusion;

active transport;

Endocytosis.

Fig.2.

Passive transport is a process that does not require energy, since the transfer of small water-soluble molecules (oxygen, carbon dioxide, water) and some of the ions is carried out by diffusion. Such a process is not specific and depends on the concentration gradient of the transported molecule.

Lightweight transport also depends on the concentration gradient and allows the transport of larger hydrophilic molecules such as glucose and amino acids. This process is passive, but requires the presence of carrier proteins, which are specific for transported molecules.

active transport- a process in which the transport of molecules is carried out using carrier proteins against electrochemical gradient. To carry out this process, energy is required, which is released due to ATP splitting. An example of active transport is the sodium-potassium pump: by means of the Na + -K + -ATP-ase carrier protein, Na + ions are removed from the cytoplasm, and K + ions are simultaneously transferred into it.

Endocytosis- the process of transport of macromolecules from the extracellular space into the cell. In this case, the extracellular material is captured in the area of invagination (invagination) of the plasmalemma, the edges of the invagination then close, and thus a endocytic vesicle (endosome), surrounded by a membrane.

Varieties of endocytosis are (Fig. 3):

pinocytosis,

phagocytosis,

receptor-mediated endocytosis.

Fig.3.

pinocytosis liquids together with the substances soluble in it ("the cell drinks"). In the cytoplasm of the cell pinocytic vesicles usually merge with primary lysosomes, and their contents are subjected to intracellular processing.

Phagocytosis- capture and absorption by the cell dense particles(bacteria, protozoa, fungi, damaged cells, some extracellular components).

Phagocytosis is usually accompanied by the formation of protrusions of the cytoplasm ( pseudopodia, filopodia) that cover dense material. The edges of the cytoplasmic processes close, and are formed phagosomes. Phagosomes fuse with lysosomes to form phagolysosomes, where lysosome enzymes digest biopolymers into monomers.

Receptor-mediated endocytosis. Receptors for many substances are located on the cell surface. These receptors bind to ligands(molecules of absorbed substance with high affinity for the receptor).

Receptors, as they move, can accumulate in special areas called fringed fossae. Around such pits and formed from them bordered bubbles a reticular sheath is formed, consisting of several polypeptides, the main of which is a protein clathrin. Fringed endocytic vesicles carry the receptor-ligand complex into the cell. Later, after absorption of substances, the receptor-ligand complex is cleaved, and the receptors return to the plasmalemma. With the help of bordered vesicles, immunoglobulins, growth factors, low density lipoproteins (LDL) are transported.

Exocytosis is the reverse process of endocytosis. At the same time, membrane exocytic vesicles containing products of their own synthesis or undigested, harmful substances approach the plasmalemma and merge with it with their membrane, which is embedded in the plasmalemma - the contents of the exocytic vesicle are released into the extracellular space.

Transcytosis- a process that combines endocytosis and exocytosis. An endocytic vesicle is formed on one cell surface, which is transferred to the opposite cell surface and, becoming an exocytic vesicle, releases its contents into the extracellular space. This process is characteristic of the cells lining the blood vessels - endotheliocytes, especially in the capillaries.

During endocytosis, a portion of the plasmalemma becomes an endocytic vesicle; during exocytosis, on the contrary, the membrane is embedded in the plasmalemma. This phenomenon is called membrane conveyor.

II. CYTOPLASM. Organelles. Inclusions.

Organelles- structures constantly present in the cytoplasm, having a certain structure and specialized in performance of certain (specific) functions in a cage.

Organelles are divided into:

organelles of general importance

special organelles.

Organelles of general importance are present in all cells and are necessary for their vital activity. These include:

mitochondria,

ribosomes

endoplasmic reticulum (ER),

golgi complex