Topic 4. MORPHOLOGY AND FUNCTIONS OF THE CYTOPLASMA AND CELL ORGANELLES

Cytology is the science of the structure, development and vital activity of cells. Consequently, cytology studies the regularities of the structural and functional organization of the first (cellular) level of organization of living matter. A cell is the smallest unit of living matter that has independent vital activity and the ability to reproduce itself. Subcellular formations (nucleus, mitochondria and other organelles), although they are living structures, do not have independent vital activity.

A cell is an ordered, structured system of biopolymers limited by an active membrane, forming a nucleus and cytoplasm, participating in a single set of metabolic and energy processes that maintain and reproduce the entire system as a whole.

A cell is a living system consisting of a cytoplasm and a nucleus and is the basis of the structure, development and life of all animal organisms.

The main components of the cell:

1) core;

2) cytoplasm.

According to the ratio of the nucleus and cytoplasm (nuclear-cytoplasmic ratio), cells are divided into:

1) cells of the nuclear type (the volume of the nucleus prevails over the volume of the cytoplasm);

2) cells of the cytoplasmic type (the cytoplasm prevails over the nucleus).

In shape, the cells are round (blood cells), flat, cubic or prismatic (cells of different epithelium), spindle-shaped (smooth muscle cells), process (nerve cells), etc. Most cells contain one nucleus, but one cell can have 2, 3 and more nuclei (multinuclear cells). In the body there are structures (symplasts, syncytium) containing several tens or even hundreds of nuclei. However, these structures are formed either as a result of the fusion of individual cells (symplasts) or as a result of incomplete cell division (syncytium). The morphology of these structures will be considered in the study of tissues.

Structural components of the cytoplasm of an animal cell:

1) plasmolemma (cytolemma);

2) hyaloplasm;

3) organelles;

4) inclusions.

The plasma membrane surrounding the cytoplasm is often considered as one of the organelles of the cytoplasm.

Plasmolemma (cytolemma)

The plasmalemma is the shell of an animal cell that delimits its internal environment and ensures the interaction of the cell with the extracellular environment.

Plasma membrane functions:

1) delimiting (barrier);

2) receptor;

3) antigenic;

4) transport;

5) formation of intercellular contacts.

The chemical composition of plasma membrane substances: proteins, lipids, carbohydrates.

The structure of the plasmalemma:

1) a double layer of lipid molecules, which forms the basis of the plasmolemma, in which protein molecules are sometimes included;

2) supramembrane layer;

3) submembrane layer found in some cells.

Each lipid molecule has two parts:

1) hydrophilic head;

2) hydrophobic tails.

The hydrophobic tails of lipid molecules bind to each other and form a lipid layer. Hydrophilic heads are in contact with the external and internal environment.

Protein molecules are built into the bilipid layer of the membrane locally and do not form a continuous layer. According to the function performed, plasma membrane proteins are divided into:

1) structural;

2) transport;

3) receptor proteins;

4) enzyme proteins;

5) antigenic determinants.

Proteins and hydrophilic lipid heads located on the outer surface of the plasmalemma are usually associated with chains of carbohydrates and form complex polymeric molecules. It is these macromolecules that make up the epimembrane layer - the glycocalyx. A significant part of the surface glycoproteins and glycolipids normally performs receptor functions: it perceives hormones and other biologically active substances. Such cellular receptors transmit perceived signals to intracellular enzyme systems, enhancing or inhibiting metabolism, and thereby affect cell function.

There are the following methods of transport of substances:

1) a method of diffusion of substances (ions, some low molecular weight substances) through the plasmalemma without energy consumption;

2) active transport of substances (amino acids, nucleotides, etc.) with the help of carrier proteins with energy consumption;

3) vesicular transport (produced by means of vesicles (vesicles)). It is divided into endocytosis - the transport of substances into the cell, exocytosis - the transport of substances from the cell.

In turn, endocytosis is divided into:

1) phagocytosis - capture and movement into the cell;

2) pinocytosis - the transfer of water and small molecules.

The process of phagocytosis is divided into several phases:

1) adhesion (sticking) of the object to the cytolemma of the phagocytic cell;

2) the absorption of the object by first forming a deepening of the invagination, and then moving it into the hyaloplasm.

In those tissues in which cells or their processes are tightly adjacent to each other (epithelial, smooth muscle, etc.), connections are formed between the plasma membranes of contacting cells - intercellular contacts.

Types of intercellular contacts:

1) simple contact - 15 - 20 nm (communication is carried out due to the contact of glycocalyx macromolecules). Simple contacts occupy the most extensive areas of adjoining cells. With the help of simple contacts, a weak bond is carried out - adhesion, which does not prevent the transport of substances into the intercellular spaces. A variation of a simple contact is a lock-type contact, when the plasmolemms of neighboring cells, together with sections of the cytoplasm, seem to bulge into each other, which results in an increase in the area of ​​\uXNUMXb\uXNUMXbcontacting surfaces and a stronger mechanical bond;

2) desmosomal contact - 0,5 µm. Desmosomal junctions (or adhesion patches) are small areas of interaction between cells. Each such site has a three-layer structure and consists of two semi-desmosomes - electron-dense sections located in the cytoplasm at the points of cell contact, and an accumulation of electron-dense material in the intermembrane space - 15 - 20 nm. The number of desmosomal contacts in one cell can reach 2000. The functional role of desmosomes is to provide mechanical contact between cells;

3) tight contact. This contact is also called end plates. They are localized in organs (stomach, intestines), in which the epithelium delimits the aggressive contents of these organs, for example, gastric juice containing hydrochloric acid. Tight junctions are located only between the apical parts of the cells, covering each cell along the entire perimeter. There are no intermembrane spaces in these areas, and bilipid membranes of neighboring cells merge into a single bilipid membrane. In adjacent areas of the cytoplasm of adjoining cells, an accumulation of electron-dense material is noted. The functional role of tight junctions is a strong mechanical connection of cells, an obstacle to the transport of substances through intercellular spaces;

4) gap-like contact (or nexuses) - 0,5 - 3 microns (both membranes are pierced in the transverse direction by protein molecules (or connexons) containing hydrophilic channels through which the exchange of ions and micromolecules of neighboring cells is carried out, which ensures their functional connection) . These contacts are limited areas of contacts of neighboring cells. An example of gap-like junctions (nexuses) are the contacts of cardiomyocytes, while through them there is a distribution of biopotentials and a friendly contraction of the cardiac muscles;

5) synaptic contact (or synapse) - specific contacts between nerve cells (interneuronal synapses) or between nerve and muscle cells (myoneural synapses). The functional role of synapses is the transmission of a nerve impulse or a wave of excitation (inhibition) from one cell to another or from a nerve cell to a muscle cell.

Hyaloplasm

Hyaloplasm (or cytoplasm matrix) makes up the internal environment of the cell. It consists of water and various biopolymers (proteins, nucleic acids, polysaccharides, lipids), of which the main part is proteins of various chemical and functional specificities. The hyaloplasm also contains amino acids, monosugars, nucleotides and other low molecular weight substances.

Biopolymers form a colloidal medium with water, which, depending on the conditions, can be dense (in the form of a gel) or more liquid (in the form of a sol), both in the entire cytoplasm and in its individual sections. In the hyaloplasm, various organelles and inclusions are localized and interact with each other and with the environment of the hyaloplasm. Moreover, their location is most often specific to certain cell types. Through the bilipid membrane, the hyaloplasm interacts with the extracellular environment. Consequently, hyaloplasm is a dynamic environment and plays an important role in the functioning of individual organelles and the vital activity of cells as a whole.

Organelles

Organelles are permanent structural elements of the cytoplasm of a cell that have a specific structure and perform certain functions.

Organelle classification:

1) common organelles inherent in all cells and providing various aspects of the cell's vital activity;

2) special organelles that are present in the cytoplasm of only certain cells and perform specific functions of these cells.

In turn, common organelles are divided into membranous and non-membrane.

Special organelles are divided into:

1) cytoplasmic (myofibrils, neurofibrils, tonofibrils);

2) cell surface organelles (cilia, flagella).

Membrane organelles include:

1) mitochondria;

2) endoplasmic reticulum;

3) lamellar complex;

4) lysosomes;

5) peroxisomes.

Non-membrane organelles include:

1) ribosomes;

2) cell center;

3) microtubules;

4) microfibrils;

5) microfilaments.

The principle of the structure of membrane organelles

Membrane organelles are closed and isolated areas (compartments) in the hyaloplasm, having their own internal structure. Their wall consists of a bilipid membrane and proteins like a plasmalemma. However, bilipid membranes of organelles have specific features: the thickness of bilipid membranes of organelles is less than that of plasmolemms (7 nm versus 10 nm), membrane membranes differ in the number and content of proteins built into them.

However, despite the differences, the membranes of organelles have the same structural principle, therefore they have the ability to interact with each other, integrate, merge, disconnect, lace up.

The general principle of the structure of organelle membranes can be explained by the fact that they are all formed in the endoplasmic reticulum, and then their functional rearrangement occurs in the Golgi complex.

Mitochondria

Mitochondria are the most isolated structural elements of the cytoplasm of the cell, which have a largely independent vital activity.

There is an opinion that in the past mitochondria were independent living organisms, after which they penetrated into the cytoplasm of cells, where they lead a saprophytic existence. Proof of this may be the presence of a genetic apparatus (mitochondrial DNA) and a synthetic apparatus (mitochondrial ribosomes) in mitochondria.

The shape of mitochondria can be oval, round, elongated, and even branched, but oval-elongated prevails. The mitochondrial wall is formed by two bilipid membranes separated by a space of 10–20 nm. At the same time, the outer membrane covers the entire mitochondrion in the form of a bag along the periphery and delimits it from the hyaloplasm. The inner membrane delimits the internal environment of the mitochondria, while it forms folds inside the mitochondria - cristae. The internal environment of the mitochondria (mitochondrial matrix) has a fine-grained structure and contains granules (mitochondrial DNA and ribosomes).

The function of mitochondria is the production of energy in the form of ATP.

The source of energy in mitochondria is pyruvic acid (pyruvate), which is formed from proteins, fats and carbohydrates in the hyaloplasm. Pyruvate oxidation occurs in the mitochondrial matrix, and on the mitochondrial cristae, electron transfer, ADP phosphorylation, and ATP formation take place. The ATP produced in the mitochondria is the only form of energy that is used by the cell to carry out various processes.

Endoplasmic reticulum

The endoplasmic reticulum (ER) in different cells can be presented in the form of flattened cisterns, tubules, or individual vesicles. The wall consists of a bilipid membrane.

There are two types of EPS:

1) granular (granular, or rough);

2) non-granular (or smooth). On the outer surface of the membranes of the granular ER contains attached ribosomes.

In the cytoplasm during electron microscopic examination, two types of EPS can be detected, however, one of them predominates, which determines the functional specificity of the cell. These two varieties of EPS are not independent and isolated forms, since a more detailed study can reveal the transition of one variety to another.

Functions of granular EPS:

1) synthesis of proteins intended for removal from the cell (for export);

2) separation (segregation) of the synthesized product from the hyaloplasm;

3) condensation and modification of the synthesized protein;

4) transport of the synthesized products to the tanks of the lamellar complex;

5) synthesis of lipid membrane components.

Functions of smooth EPS:

1) participation in the synthesis of glycogen;

2) lipid synthesis;

3) detoxification function (neutralization of toxic substances by combining them with other substances).

Golgi lamellar complex

The lamellar complex is called the transport apparatus of the cell.

The lamellar Golgi complex (mesh apparatus) is represented by an accumulation of flattened cisterns and small vesicles bounded by a bilipid membrane. The lamellar complex is subdivided into subunits - dictyosomes. Each dictyosome is a stack of flattened cisterns, along the periphery of which small vesicles are localized. At the same time, in each flattened tank, the peripheral part is somewhat expanded, and the central part is narrowed. There are two poles in the dictyosome: the cispole (directed by the base towards the nucleus) and the transpole (directed towards the cytolemma). It has been established that transport vacuoles approaching the cispole carry products synthesized in EPS to the Golgi complex. Vesicles are laced from the transpole, carrying the secret to the plasmalemma for its release from the cell. Some of the small vesicles filled with enzyme proteins remain in the cytoplasm and are called lysosomes.

Function of the lamellar complex:

1) transport (removes the products synthesized in it from the cell);

2) condensation and modification of substances synthesized in granular EPS;

3) formation of lysosomes (together with granular ER);

4) participation in carbohydrate metabolism;

5) synthesis of molecules that form the glycocalyx of the cytolemma;

6) synthesis, accumulation, excretion of mucins (mucus);

7) modification of membranes synthesized in EPS and their transformation into plasmalemma membranes.

Lysosomes

Lysosomes - the smallest organelles of the cytoplasm, are bodies bounded by a bilipid membrane and containing an electron-dense matrix consisting of a set of hydrolytic enzyme proteins (more than thirty types of hydrolases) capable of splitting any polymeric compounds (proteins, fats, carbohydrates), their complexes into monomeric fragments.

The function of lysosomes is to ensure intracellular digestion, i.e., the breakdown of both exogenous and endogenous biopolymer substances.

Lysosome classification:

1) primary lysosomes - electron dense bodies;

2) secondary lysosomes - phagolysosomes, including autophagolysosomes;

3) tertiary lysosomes or residual bodies.

True lysosomes are called small electron-dense bodies that form in a lamellar complex. The digestive function of lysosomes begins only after fusion with a phagosome (a phagocytosed substance surrounded by a bilipid membrane) and the formation of a phagolysosome, in which phagocytosed material and lysosomal enzymes are mixed. After this, the splitting of the biopolymer compounds of the phagocytosed material into monomers - amino acids, sugars - begins. These molecules freely penetrate through the membrane of the phagolysosome into the hyaloplasm and are then utilized by the cell - they go to generate energy or build new intracellular macromolecular compounds.

Some compounds cannot be cleaved by lysosome enzymes and are therefore excreted unchanged from the cell by exocytosis (the reverse process of phagocytosis). Substances of a lipid nature are practically not broken down by enzymes, but accumulate and compact in the phagolysosome. These formations were called tertiary lysosomes (or residual bodies).

In the process of phagocytosis and exocytosis, membranes are recirculated in the cell: during phagocytosis, part of the plasmolemma is laced off and forms a phagosome shell; during exocytosis, this shell is again built into the plasmolemma.

Damaged, altered or obsolete cell organelles are utilized by it by the mechanism of intracellular phagocytosis with the help of lysosomes. Initially, these organelles are surrounded by a bilipid membrane, and a vacuole, an autophagosome, is formed. Then one or more lysosomes merge with it, and an autophagolysosome is formed, in which the hydrolytic cleavage of biopolymer substances is carried out, as in the phagolysosome.

Lysosomes are found in all cells, but in unequal numbers. Specialized cells - macrophages - contain a large number of primary and secondary lysosomes in the cytoplasm. They perform a protective function in tissues, absorb a significant number of exogenous substances - bacteria, viruses, other foreign agents and decay products of their own tissues.

Peroxisomes

Peroxisomes are microbodies of the cytoplasm (0,1 - 1,5 μm), similar in structure to lysosomes, but differ from them in that their matrix contains crystal-like structures, and among enzyme proteins there is catalase, which destroys hydrogen peroxide formed during oxidation amino acids.

Ribosomes

Ribosomes are the apparatus for the synthesis of protein and polypeptide molecules.

According to localization, they are divided into:

1) free, (located in the hyaloplasm);

2) non-free (or attached), - which are associated with EPS membranes.

Each ribosome consists of small and large subunits. Each subunit of the ribosome consists of ribosomal RNA and protein - ribonucleoprotein. Subunits are formed in the nucleolus, and assembly into a single ribosome is carried out in the cytoplasm. For protein synthesis, individual ribosomes with the help of matrix (information) RNA are combined into chains of ribosomes - polysomes. Free and attached ribosomes, in addition to differences in their localization, are characterized by a certain functional specificity: free ribosomes synthesize proteins.

Cell Center

Cell center - cytocenter, centrosome. In a nondividing cell, the cell center consists of two main structural components:

1) diplosomes;

2) centrosphere.

The diplosome consists of two centrioles (maternal and daughter) located at right angles to each other. Each centriole consists of microtubules forming a hollow cylinder, 0,2 µm in diameter and 0,3–0,5 µm long. Microtubules are combined into triplets (three tubes each), forming a total of nine triplets. The centrosphere is a structureless section of the hyaloplasm around the diplosome, from which microtubules extend radially (like a radiant sphere).

Functions of the cytocenter:

1) the formation of a fission spindle in the prophase of mitosis;

2) participation in the formation of microtubules of the cell scaffold;

3) playing the role of basic bodies of cilia in the ciliated epithelial cells of the centriole.

The position of centrioles in some epithelial cells determines their polar differentiation.

Microtubules

Microtubules - hollow cylinders (outer diameter - 24 mm, inner - 15 mm), are independent organelles, forming a cytoskeleton. They can also be part of other organelles - centrioles, cilia, flagella. The wall of microtubules consists of the globular protein tubulin, which is formed by separate rounded formations of a globule with a diameter of 5 nm. Globules can be in the hyaloplasm in a free state or connect with each other, resulting in the formation of microtubules. They can then again disintegrate into globules. Thus, spindle microtubules are formed and then disintegrate in different phases of mitosis. However, in the composition of centrioles, cilia and flagella, microtubules are stable formations. Most of the microtubules are involved in the formation of the intracellular scaffold, which maintains the shape of the cell, determining a certain position of the organelles in the cytoplasm, and also predetermines the direction of intracellular movements. Tubulin proteins do not have the ability to contract, therefore, microtubules do not contract. In the composition of cilia and flagella, microtubules interact with each other, they slide relative to each other, which ensures the movement of these organelles.

microfibrils

Microfibrils (intermediate filaments) are thin, non-branching filaments.

Basically, microfibrils are localized in the cortical (submembrane) layer of the cytoplasm. They consist of a protein that has a certain structure in cells of various classes (in epithelial cells it is a keratin protein, in muscle cells it is desmin).

The functional role of microfibrils is to participate, along with microtubules, in the formation of the cell scaffold, performing a supporting function.

Microtubules can combine into bundles and form tonofibrils, which are considered as independent organelles and perform a supporting function.

Microfilaments

Microfilaments are even thinner filamentous structures (5 - 7 nm), consisting of contractile proteins (actin, myosin, tropomyosin).

Microfilaments are localized mainly in the cortical layer of the cytoplasm.

Together, microfilaments make up the contractile apparatus of the cell, which provides various types of movements: the movement of organelles, the flow of hyaloplasm, the change in the cell surface, the formation of pseudopodia, and the movement of the cell.

The accumulation of microfilaments in muscle fibers forms special organelles of muscle tissue - myofibrils.

Inclusions

Inclusions are non-permanent structural components of the cytoplasm. Classification of inclusions:

1) trophic;

2) secretory;

3) excretory;

4) pigment.

During the life of cells, random inclusions can accumulate - medication, particles of various substances.

Trophic inclusions - lecithin in eggs, glycogen or lipids in various cells.

Secretory inclusions are secretory granules in secreting cells (eg, zymogenic granules in pancreatic acinar cells, secretory granules in various endocrine cells).

Excretory inclusions are substances that need to be removed from the cell (for example, granules of uric acid in the epithelium of the renal tubules).

Pigment inclusions - melanin, hemoglobin, lipofuscin, bilirubin. These inclusions give the cell that contains them a certain color: melanin stains the cell black or brown, hemoglobin yellow-red, bilirubin yellow. Pigment cells are found only in certain types of cells: melanin - in melanocytes, hemoglobin - in erythrocytes. Lipofuscin, unlike the other pigments mentioned, can be found in many cell types. The presence of lipofuscin in cells (especially in a significant amount) indicates aging and functional inferiority.

Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu.

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