Cell organelles and their functions. The structure of eukaryotic cells. The structure of the cell wall

Cell organelles (organelles) are permanent parts of the cell that have a specific structure and perform specific functions. Distinguish between membranous and non-membrane organelles. To membrane organelles include the cytoplasmic reticulum (endoplasmic reticulum), lamellar complex (Golgi apparatus), mitochondria, lysosomes, peroxisomes. Non-membrane organelles are represented by ribosomes (polyribosomes), the cell center and cytoskeletal elements: microtubules and fibrillar structures.

Rice. eight.Scheme of the ultramicroscopic structure of the cell:

1 - granular endoplasmic reticulum, on the membranes of which attached ribosomes are located; 2 - agranular endoplasmic reticulum; 3 - Golgi complex; 4 - mitochondria; 5 – developing phagosome; 6 - primary lysosome (accumulation granule); 7 - phagolysosome; 8 - endocytic vesicles; 9 - secondary lysosome; 10 - residual body; 11 - peroxisome; 12 - microtubules; 13 - microfilaments; 14 - centrioles; 15 - free ribosomes; 16 - transport bubbles; 17 - exocytotic vesicle; 18 - fatty inclusions (lipid drop); 19 - glycogen inclusions; 20 - karyolemma (nuclear membrane); 21 - nuclear pores; 22 - nucleolus; 23 - heterochromatin; 24 - euchromatin; 25 - basal body of the cilium; 26 - eyelash; 27 - special intercellular contact (desmosome); 28 - gap intercellular contact

2.5.2.1. Membrane organelles (organelles)

Endoplasmic reticulum (endoplasmic reticulum, cytoplasmic reticulum) - a set of tubules, vacuoles and "cisterns" that communicate with each other, the wall of which is formed by elementary biological membranes. Discovered by K.R. Porter in 1945. The discovery and description of the endoplasmic reticulum (ER) is due to the introduction into practice of cytological studies of the electron microscope. The membranes that form EPS differ from the cell plasmalemma with a smaller thickness (5-7 nm) and a higher concentration of proteins, primarily with enzymatic activity. . There are two types of EPS(Fig. 8): rough (granular) and smooth (agranular). Rough XPS It is represented by flattened tanks, on the surface of which ribosomes and polysomes are located. The membranes of the granular ER contain proteins that promote ribosome binding and cisterna flattening. Rough ER is especially well developed in cells specialized in protein synthesis. Smooth ER is formed by intertwining tubules, tubules and small bubbles. EPS channels and tanks of these two varieties are not distinguished: membranes of one type pass into membranes of another type, forming in the transition region the so-calledtransitional (transient) EPS.

Mainfunctions of granular ER are:

1) synthesis of proteins on attached ribosomes(secreted proteins, cell membrane proteins and specific proteins of the contents of membrane organelles); 2) hydroxylation, sulfation, phosphorylation and glycosylation of proteins; 3) transport of substances within the cytoplasm; 4) accumulation of both synthesized and transported substances; 5) regulation of biochemical reactions, associated with the orderliness of localization in the EPS structures of substances entering into reactions, as well as their catalysts - enzymes.

Smooth EPS characterized by the absence on the membranes of proteins (ribophorins) that bind the subunits of ribosomes. It is assumed that smooth ER is formed as a result of the formation of outgrowths of rough ER, the membrane of which loses ribosomes.

Functions of smooth EPS are: 1) lipid synthesis, including membrane lipids; 2) carbohydrate synthesis(glycogen, etc.); 3) cholesterol synthesis; 4) neutralization of toxic substances endogenous and exogenous origin; 5) accumulation of Ca ions 2+ ; 6) restoration of the karyolemma in the telophase of mitosis; 7) transport of substances; 8) accumulation of substances.

As a rule, smooth ER is less developed in cells than rough ER, however, it is much better developed in cells that produce steroids, triglycerides and cholesterol, as well as in liver cells that detoxify various substances.

Rice. 9. Golgi complex:

1 - a stack of flattened tanks; 2 - bubbles; 3 - secretory vesicles (vacuoles)

Transitional (transient) EPS - this is the site of the transition of granular ER to agranular ER, which is located at the emerging surface of the Golgi complex. The tubules and tubules of the transitional ER disintegrate into fragments, from which vesicles are formed, transporting material from the ER to the Golgi complex.

Lamellar complex (Golgi complex, Golgi apparatus) - a cell organelle involved in the final formation of its metabolic products(secrets, collagen, glycogen, lipids and other products),as well as in the synthesis of glycoproteins. The organoid is named after the Italian histologist C. Golgi who described it in 1898. Formed by three components(Fig. 9): 1) a stack of flattened tanks (bags); 2) bubbles; 3) secretory vesicles (vacuoles). The zone of accumulation of these elements is called dictyosomes. There can be several such zones in a cell (sometimes several tens or even hundreds). The Golgi complex is located near the cell nucleus, often near the centrioles, rarely scattered throughout the cytoplasm. In secretory cells, it is located in the apical part of the cell, through which secretion is secreted by exocytosis. From 3 to 30 tanks in the form of curved disks with a diameter of 0.5-5 microns form a stack. Adjacent tanks are separated by spaces of 15-30 nm. Separate groups of cisterns within the dictyosome are distinguished by a special composition of enzymes that determine the nature of biochemical reactions, in particular, protein processing, etc.

The second constituent element of the dictyosome is the vesicles are spherical formations with a diameter of 40-80 nm, moderately dense contents of which are surrounded by a membrane. Bubbles are formed by cleavage from cisterns.

The third element of the dictyosome is secretory vesicles (vacuoles) are relatively large (0.1-1.0 microns) spherical membrane formations containing a secret of moderate density, undergoing condensation and compaction (condensation vacuoles).

The Golgi complex is clearly polarized along the vertical. It distinguishes two surfaces (two poles):

1) cis-surface, or an immature surface, which has a convex shape, faces the endoplasmic reticulum (nucleus) and is associated with small transport vesicles that separate from it;

2) trans surface, or a surface facing a concave plasmolemma (Fig. 8), from the side of which vacuoles (secretory granules) are separated from the tanks of the Golgi complex.

Mainfunctions of the Golgi complex are: 1) the synthesis of glycoproteins and polysaccharides; 2) modification of the primary secret, its condensation and packaging into membrane vesicles (formation of secretory granules); 3) processing of molecules(phosphorylation, sulfation, acylation, etc.); 4) accumulation of substances secreted by the cell; 5) formation of lysosomes; 6) sorting of proteins synthesized by the cell at the trans surface before their final transport (produced by receptor proteins that recognize the signaling regions of macromolecules and direct them to various vesicles); 7) transport of substances: from the transport vesicles, substances penetrate into the stack of cisterns of the Golgi complex from the cis-surface, and leave it in the form of vacuoles from the trans-surface. The transport mechanism is explained by two models: a) a model for the movement of bubbles budding from the previous cistern and merging with the next cistern sequentially in the direction from the cis-surface to the trans-surface; b) a model of cisternae movement based on the concept of continuous neoformation of cisterns due to the fusion of bubbles on the cis-surface and subsequent disintegration into vacuoles of cisterns moving towards the trans-surface.

The above main functions allow us to state that the lamellar complex is the most important organelle of the eukaryotic cell, which ensures the organization and integration of intracellular metabolism. In this organoid, the final stages of formation, maturation, sorting and packaging of all products secreted by the cell, lysosome enzymes, as well as proteins and glycoproteins of the cell surface apparatus and other substances take place.

Organelles of intracellular digestion. Lysosomes are small vesicles bounded by an elementary membrane that contain hydrolytic enzymes. The lysosome membrane, about 6 nm thick, performs passive compartmentalization, temporarily separating hydrolytic enzymes (more than 30 varieties) from hyaloplasm. In an intact state, the membrane is resistant to the action of hydrolytic enzymes and prevents their leakage into the hyaloplasm. Corticosteroid hormones play an important role in membrane stabilization. Damage to lysosome membranes leads to self-digestion of the cell by hydrolytic enzymes.

The lysosome membrane contains an ATP-dependent proton pump, providing acidification of the environment inside the lysosomes. The latter contributes to the activation of lysosome enzymes - acid hydrolases. Along with the the membrane of lysosomes contains receptors that cause the binding of lysosomes to transport vesicles and phagosomes. The membrane also ensures the diffusion of substances from the lysosomes into the hyaloplasm. The binding of some hydrolase molecules to the lysosome membrane leads to their inactivation.

There are several types of lysosomes:primary lysosomes (hydrolase vesicles), secondary lysosomes (phagolysosomes or digestive vacuoles), endosomes, phagosomes, autophagolysosomes, residual bodies(Fig. 8).

Endosomes are membrane vesicles that carry macromolecules from the cell surface to lysosomes by endocytosis. In the process of transfer, the contents of endosomes may not change or undergo partial cleavage. In the latter case, hydrolases penetrate into endosomes or endosomes directly merge with hydrolase vesicles, as a result of which the medium is gradually acidified. Endosomes are divided into two groups: early (peripheral) and late (perinuclear) endosomes.

Early (peripheral) endosomes are formed in the early stages of endocytosis after the separation of vesicles with trapped contents from the plasmalemma. They are located in the peripheral layers of the cytoplasm and characterized by a neutral or slightly alkaline environment. In them, cleavage of ligands from receptors, sorting of ligands, and, possibly, return of receptors in special vesicles to the plasmalemma take place. Along with the in early endosomes, com-

Rice. 10(A). Scheme of the formation of lysosomes and their participation in intracellular digestion.(B)An electron micrograph of a section of secondary lysosomes (indicated by arrows):

1 - formation of small vesicles with enzymes from the granular endoplasmic reticulum; 2 - transfer of enzymes to the Golgi apparatus; 3 - formation of primary lysosomes; 4 - isolation and use (5) of hydrolases during extracellular cleavage; 6 - phagosomes; 7 - fusion of primary lysosomes with phagosomes; 8, 9 - formation of secondary lysosomes (phagolysosomes); 10 - excretion of residual bodies; 11 - fusion of primary lysosomes with collapsing cell structures; 12 - autophagolysosome

complexes "receptor-hormone", "antigen-antibody", limited cleavage of antigens, inactivation of individual molecules. Under conditions of acidification (рН=6.0) of the medium in early endosomes, partial cleavage of macromolecules can occur. Gradually, moving deep into the cytoplasm, early endosomes turn into late (perinuclear) endosomes, located in the deep layers of the cytoplasm, surrounding the core. They reach 0.6-0.8 microns in diameter and differ from early endosomes by more acidic (pH=5.5) contents and a higher level of enzymatic digestion of the contents.

Phagosomes (heterophagosomes) - membrane vesicles that contain material captured by the cell from the outside, subject to intracellular digestion.

Primary lysosomes (hydrolase vesicles) - vesicles with a diameter of 0.2-0.5 microns containing inactive enzymes (Fig. 10). Their movement in the cytoplasm is controlled by microtubules. Hydrolase vesicles carry out the transport of hydrolytic enzymes from the lamellar complex to the organelles of the endocytic pathway (phagosomes, endosomes, etc.).

Secondary lysosomes (phagolysosomes, digestive vacuoles) - vesicles in which intracellular digestion is actively carried out by hydrolases at pH≤5. Their diameter reaches 0.5-2 microns. Secondary lysosomes (phagolysosomes and autophagolysosomes) formed by the fusion of a phagosome with an endosome or primary lysosome (phagolysosome) or by the fusion of an autophagosome(membrane vesicle containing the cell's own components) with primary lysosome(Fig. 10) or late endosome (autophagolysosome). Autophagy provides digestion of cytoplasmic regions, mitochondria, ribosomes, membrane fragments, etc. The loss of the latter in the cell is compensated by their neoplasm, which leads to renewal ("rejuvenation") of cellular structures. So, in human nerve cells that have been functioning for many decades, most organelles are updated within 1 month.

A variety of lysosomes containing undigested substances (structures) is called residual bodies. The latter can stay in the cytoplasm for a long time or release their contents by exocytosis outside the cell.(Fig. 10). The most common type of residual bodies in animals are lipofuscin granules, which are membranous vesicles (0.3-3 μm) containing the sparingly soluble brown pigment lipofuscin.

Peroxisomes are membranous vesicles up to 1.5 µm in diameter, the matrix of which contains about 15 enzymes(Fig. 8). Among the latter, the most important catalase, which accounts for up to 40% of the total organoid protein, as well as peroxidase, amino acid oxidase, etc. Peroxisomes are formed in the endoplasmic reticulum and are renewed every 5-6 days. Along with mitochondria, Peroxisomes are an important oxygen utilization center in the cell. In particular, under the influence of catalase, hydrogen peroxide (H 2 O 2) decomposes, which is formed during the oxidation of amino acids, carbohydrates, and other cell substances. Thus, peroxisomes protect the cell from the damaging effect of hydrogen peroxide.

Organelles of energy metabolism. Mitochondria described for the first time by R. Kelliker in 1850 in the muscles of insects called sarcos. Later they were studied and described by R. Altman in 1894 as "bioplasts", and in 1897 K. Benda called them mitochondria. Mitochondria are membrane organelles that provide the cell (organism) with energy. The source of energy stored in the form of ATP phosphate bonds is oxidation processes. Along with the mitochondria are involved in the biosynthesis of steroids and nucleic acids, as well as in the oxidation of fatty acids.

M

Rice. eleven. Scheme of the structure of mitochondria:

1 - outer membrane; 2 - inner membrane; 3 - cristae; 4 - matrix

H

Rice. 12. Electronic photo of mitochondria (cross section)

The intermembrane space of mitochondria, 10–20 nm wide, contains a small amount of enzymes. It is limited from the inside by the inner mitochondrial membrane containing transport proteins, respiratory chain enzymes and succinate dehydrogenase, as well as the ATP synthetase complex. The inner membrane is characterized by low permeability to small ions. It forms folds 20 nm thick, which are most often perpendicular to the longitudinal axis of mitochondria, and in some cases (muscle and other cells) - longitudinally. With an increase in mitochondrial activity, the number of folds (their total area) increases. On the cristae areoxisomes - mushroom-shaped formations, consisting of a rounded head with a diameter of 9 nm and legs 3 nm thick. ATP synthesis occurs in the head region. The processes of ATP oxidation and synthesis in mitochondria are separated, which is why not all energy is accumulated in ATP, partially dissipating in the form of heat. This dissociation is most pronounced, for example, in brown adipose tissue used for spring “warming up” of animals that were in a state of “winter hibernation”.

The inner chamber of the mitochondria (the area between the inner membrane and the cristae) is fullmatrix (Fig. 11, 12), containing Krebs cycle enzymes, protein synthesis enzymes, fatty acid oxidation enzymes, mitochondrial DNA, ribosomes and mitochondrial granules.

Mitochondrial DNA is the genetic makeup of the mitochondria. It has the appearance of a circular double-stranded molecule, which contains about 37 genes. Mitochondrial DNA differs from nuclear DNA in its low content of non-coding sequences and the absence of histone bonds. Mitochondrial DNA encodes mRNA, tRNA and rRNA, however, it provides the synthesis of only 5-6% of mitochondrial proteins.(enzymes of the ion transport system and some enzymes of ATP synthesis). The synthesis of all other proteins, as well as the duplication of mitochondria, is controlled by nuclear DNA. Most of the mitochondrial ribosomal proteins are synthesized in the cytoplasm and then transported to the mitochondria. The inheritance of mitochondrial DNA in many eukaryotic species, including humans, occurs only through the maternal line: paternal mitochondrial DNA disappears during gametogenesis and fertilization.

Mitochondria have a relatively short life cycle (about 10 days). Their destruction occurs by autophagy, and neoplasm - by fission (ligation) previous mitochondria. The latter is preceded by mitochondrial DNA replication, which occurs independently of nuclear DNA replication at any phase of the cell cycle.

Prokaryotes do not have mitochondria, and their function is performed by the cell membrane. According to one hypothesis, mitochondria originated from aerobic bacteria as a result of symbiogenesis. There is an assumption about the participation of mitochondria in the transmission of hereditary information.

The elementary and functional unit of all life on our planet is the cell. In this article, you will learn in detail about its structure, the functions of organelles, and also find the answer to the question: “What is the difference between the structure of plant and animal cells?”.

Cell structure

The science that studies the structure of the cell and its functions is called cytology. Despite their small size, these parts of the body have a complex structure. Inside is a semi-liquid substance called the cytoplasm. All vital processes take place here and the constituent parts are located - organelles. Learn more about their features below.

Nucleus

The most important part is the core. It is separated from the cytoplasm by a membrane, which consists of two membranes. They have pores so that substances can get from the nucleus to the cytoplasm and vice versa. Inside is the nuclear juice (karyoplasm), which contains the nucleolus and chromatin.

Rice. 1. The structure of the nucleus.

It is the nucleus that controls the life of the cell and stores genetic information.

The functions of the internal contents of the nucleus are the synthesis of protein and RNA. They form special organelles - ribosomes.

Ribosomes

They are located around the endoplasmic reticulum, while making its surface rough. Sometimes ribosomes are freely located in the cytoplasm. Their functions include protein synthesis.

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Endoplasmic reticulum

EPS can have a rough or smooth surface. The rough surface is formed due to the presence of ribosomes on it.

The functions of EPS include protein synthesis and internal transport of substances. Part of the formed proteins, carbohydrates and fats through the channels of the endoplasmic reticulum enters special storage containers. These cavities are called the Golgi apparatus, they are presented in the form of stacks of "tanks", which are separated from the cytoplasm by a membrane.

golgi apparatus

Most often located near the nucleus. Its functions include protein conversion and the formation of lysosomes. This complex stores substances that were synthesized by the cell itself for the needs of the whole organism, and will later be removed from it.

Lysosomes are presented in the form of digestive enzymes, which are enclosed by a membrane in vesicles and carried through the cytoplasm.

Mitochondria

These organelles are covered with a double membrane:

• smooth - outer shell;
• cristae - the inner layer having folds and protrusions.

Rice. 2. The structure of mitochondria.

The functions of mitochondria are respiration and the conversion of nutrients into energy. The cristae contain an enzyme that synthesizes ATP molecules from nutrients. This substance is a universal source of energy for various processes.

The cell wall separates and protects the internal contents from the external environment. It maintains its shape, provides interconnection with other cells, and ensures the process of metabolism. The membrane consists of a double layer of lipids, between which are proteins.

Comparative characteristics

Plant and animal cells differ from each other in their structure, size and shape. Namely:

• the cell wall of a plant organism has a dense structure due to the presence of cellulose;
• a plant cell has plastids and vacuoles;
• the animal cell has centrioles, which are important in the process of division;
• The outer membrane of an animal organism is flexible and can take on various forms.

Rice. 3. Scheme of the structure of plant and animal cells.

The following table will help to summarize the knowledge about the main parts of the cellular organism:

Table "Cell structure"

What have we learned?

A living organism consists of cells that have a rather complex structure. Outside, it is covered with a dense shell that protects the internal contents from the effects of the external environment. Inside there is a nucleus that regulates all ongoing processes and stores the genetic code. Around the nucleus is the cytoplasm with organelles, each of which has its own characteristics and characteristics.

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Organelles are permanent components of the cell that perform certain functions.

Depending on the structural features, they are divided into membrane and non-membrane. Membrane organelles, in turn, are referred to as single-membrane (endoplasmic reticulum, Golgi complex and lysosomes) or double-membrane (mitochondria, plastids and nucleus). Non-membrane organelles are ribosomes, microtubules, microfilaments and the cell center. Of the listed organelles, only ribosomes are inherent in prokaryotes.

The structure and functions of the nucleus. Nucleus- a large two-membrane organelle lying in the center of the cell or on its periphery. The size of the nucleus can vary within 3-35 microns. The shape of the nucleus is more often spherical or ellipsoid, but there are also rod-shaped, spindle-shaped, bean-shaped, lobed and even segmented nuclei. Some researchers believe that the shape of the nucleus corresponds to the shape of the cell itself.

Most cells have one nucleus, but, for example, in liver and heart cells there can be two, and in a number of neurons - up to 15. Skeletal muscle fibers usually contain many nuclei, but they are not cells in the full sense of the word, since they are formed in the result of the fusion of several cells.

The core is surrounded nuclear envelope, and its inner space is filled nuclear juice, or nucleoplasm (karyoplasm)) into which are immersed chromatin and nucleolus. The nucleus performs such important functions as the storage and transmission of hereditary information, as well as the control of the life of the cell (Fig. 2.30).

The role of the nucleus in the transmission of hereditary information has been convincingly proven in experiments with the green algae acetabularia. In a single giant cell, reaching a length of 5 cm, a hat, a leg and a rhizoid are distinguished. Moreover, it contains only one nucleus located in the rhizoid. In the 1930s, I. Hemmerling transplanted the nucleus of one species of acetabularia with a green color into a rhizoid of another species, with a brown color, in which the nucleus was removed (Fig. 2.31). After some time, the plant with the transplanted nucleus grew a new hat, like the algae-donor of the nucleus. At the same time, the cap or stalk separated from the rhizoid, which did not contain a nucleus, died after some time.

nuclear envelope It is formed by two membranes - outer and inner, between which there is a space. The intermembrane space communicates with the cavity of the rough endoplasmic reticulum, and the outer membrane of the nucleus can carry ribosomes. The nuclear envelope is permeated with numerous pores, edged with special proteins. Substances are transported through the pores: the necessary proteins (including enzymes), ions, nucleotides and other substances enter the nucleus, and RNA molecules, waste proteins, and ribosome subunits leave it.

Thus, the functions of the nuclear envelope are the separation of the contents of the nucleus from the cytoplasm, as well as the regulation of the metabolism between the nucleus and the cytoplasm.

Nucleoplasm refers to the contents of the nucleus, in which chromatin and the nucleolus are immersed. It is a colloidal solution, chemically reminiscent of the cytoplasm. Enzymes of the nucleoplasm catalyze the exchange of amino acids, nucleotides, proteins, etc. The nucleoplasm is connected to the hyaloplasm through nuclear pores. The functions of the nucleoplasm, like the hyaloplasm, are to ensure the interconnection of all structural components of the nucleus and the implementation of a number of enzymatic reactions.

Chromatin is a collection of thin filaments and granules embedded in the nucleoplasm. It can only be detected by staining, since the refractive indices of chromatin and nucleoplasm are approximately the same. The filamentous component of chromatin is called euchromatin, and the granular component is called heterochromatin. Euchromatin is weakly compacted, since hereditary information is read from it, while more spiralized heterochromatin is genetically inactive.

Chromatin is a structural modification of chromosomes in a non-dividing nucleus. Thus, chromosomes are constantly present in the nucleus, only their state changes depending on the function that the nucleus performs at the moment.

Chromatin mainly consists of nucleoproteins (deoxyribonucleoproteins and ribonucleoproteins), as well as enzymes, the most important of which are associated with the synthesis of nucleic acids, and some other substances.

The functions of chromatin consist, firstly, in the synthesis of nucleic acids specific to a given organism, which direct the synthesis of specific proteins, and secondly, in the transfer of hereditary properties from the mother cell to daughter cells, for which chromatin threads are packed into chromosomes during division.

nucleolus- a spherical body, clearly visible under a microscope, with a diameter of 1-3 microns. It is formed in chromatin regions that encode information about the structure of rRNA and ribosome proteins. The nucleolus in the nucleus is often one, but in those cells where intensive vital processes take place, there may be two or more nucleoli. The functions of the nucleoli are the synthesis of rRNA and the assembly of ribosome subunits by combining rRNA with proteins coming from the cytoplasm.

Mitochondria- two-membrane organelles of a round, oval or rod-shaped shape, although spiral-shaped ones are also found (in spermatozoa). Mitochondria are up to 1 µm in diameter and up to 7 µm in length. The space inside the mitochondria is filled with matrix. The matrix is ​​the main substance of mitochondria. A circular DNA molecule and ribosomes are immersed in it. The outer membrane of mitochondria is smooth and impermeable to many substances. The inner membrane has outgrowths - cristae, which increase the surface area of ​​\u200b\u200bthe membranes for the occurrence of chemical reactions (Fig. 2.32). On the surface of the membrane are numerous protein complexes that make up the so-called respiratory chain, as well as mushroom-shaped enzymes of ATP synthetase. In mitochondria, the aerobic stage of respiration takes place, during which ATP is synthesized.

plastids- large two-membrane organelles, characteristic only for plant cells. The inner space of plastids is filled with stroma, or matrix. In the stroma there is a more or less developed system of membrane vesicles - thylakoids, which are collected in piles - grana, as well as its own circular DNA molecule and ribosomes. There are four main types of plastids: chloroplasts, chromoplasts, leucoplasts, and proplastids.

Chloroplasts- these are green plastids with a diameter of 3-10 microns, clearly visible under a microscope (Fig. 2.33). They are found only in the green parts of plants - leaves, young stems, flowers and fruits. Chloroplasts are mostly oval or ellipsoid in shape, but can also be cup-shaped, spiral-shaped, and even lobed. The number of chloroplasts in a cell averages from 10 to 100 pieces.

However, for example, in some algae it may be one, have a significant size and complex shape - then it is called chromatophore. In other cases, the number of chloroplasts can reach several hundred, while their size is small. The color of chloroplasts is due to the main pigment of photosynthesis - chlorophyll, although they contain additional pigments - carotenoids. Carotenoids become noticeable only in autumn, when chlorophyll in aging leaves is destroyed. The main function of chloroplasts is photosynthesis. Light reactions of photosynthesis occur on thylakoid membranes, on which chlorophyll molecules are fixed, and dark reactions occur in the stroma, which contains numerous enzymes.

Chromoplasts. are yellow, orange and red plastids containing carotenoid pigments. The shape of chromoplasts can also vary significantly: they are tubular, spherical, crystalline, etc. Chromoplasts give color to flowers and fruits of plants, attracting pollinators and dispersers of seeds and fruits.

Leucoplasts- These are white or colorless plastids, mostly round or oval in shape. They are common in non-photosynthetic parts of plants, such as leaf skins, potato tubers, etc. They store nutrients, most often starch, but in some plants it can be proteins or oil.

Plastids are formed in plant cells from proplastids, which are already present in the cells of the educational tissue and are small two-membrane bodies. At the early stages of development, different types of plastids are able to turn into each other: when exposed to light, the leukoplasts of a potato tuber and the chromoplasts of a carrot root turn green.

Plastids and mitochondria are called semi-autonomous cell organelles, since they have their own DNA molecules and ribosomes, carry out protein synthesis and divide independently of cell division. These features are explained by the origin from unicellular prokaryotic organisms. However, the “independence” of mitochondria and plastids is limited, since their DNA contains too few genes for free existence, while the rest of the information is encoded in the chromosomes of the nucleus, which allows it to control these organelles.

Endoplasmic reticulum(EPS), or endoplasmic reticulum(ER) is a single-membrane organelle, which is a network of membrane cavities and tubules, occupying up to 30% of the cytoplasm content. The diameter of ER tubules is about 25–30 nm. There are two types of EPS - rough and smooth. Rough XPS carries ribosomes, protein synthesis occurs on it (Fig. 2.34).

Smooth EPS devoid of ribosomes. Its function is the synthesis of lipids and carbohydrates, the formation of lysosomes, as well as the transport, storage and disposal of toxic substances. It is especially developed in those cells where intensive metabolic processes take place, for example, in liver cells - hepatocytes - and skeletal muscle fibers. Substances synthesized in the EPS are transported to the Golgi apparatus. In the ER, cell membranes are also assembled, but their formation is completed in the Golgi apparatus.

golgi apparatus, or golgi complex- a single-membrane organoid formed by a system of flat cisterns, tubules and vesicles that are laced off from them (Fig. 2.35).

The structural unit of the Golgi apparatus is dictyosome- a stack of tanks, at one pole of which substances from the ER come, and from the opposite pole, having undergone certain transformations, they are packed into bubbles and sent to other parts of the cell. The diameter of the tanks is about 2 microns, and that of small bubbles is about 20-30 microns. The main functions of the Golgi complex are the synthesis of certain substances and the modification (change) of proteins, lipids and carbohydrates coming from the EPS, the final formation of membranes, as well as the transport of substances through the cell, the renewal of its structures and the formation of lysosomes. The Golgi apparatus got its name in honor of the Italian scientist Camillo Golgi, who first discovered this organoid (1898).

Lysosomes- small single-membrane organelles up to 1 micron in diameter, which contain hydrolytic enzymes involved in intracellular digestion. The membranes of lysosomes are poorly permeable for these enzymes, so the performance of their functions by lysosomes is very accurate and targeted. So, they take an active part in the process of phagocytosis, forming digestive vacuoles, and in case of starvation or damage to certain parts of the cell, they digest them without affecting others. Recently, the role of lysosomes in cell death processes has been discovered.

Vacuole- this is a cavity in the cytoplasm of plant and animal cells, limited by a membrane and filled with liquid. Digestive and contractile vacuoles are found in protozoan cells. The former take part in the process of phagocytosis, as they break down nutrients. The latter ensure the maintenance of water-salt balance due to osmoregulation. In multicellular animals, digestive vacuoles are mainly found.

In plant cells, vacuoles are always present, they are surrounded by a special membrane and filled with cell sap. The membrane surrounding the vacuole is similar in chemical composition, structure and functions to the plasma membrane. cell sap represents an aqueous solution of various inorganic and organic substances, including mineral salts, organic acids, carbohydrates, proteins, glycosides, alkaloids, etc. The vacuole can occupy up to 90% of the cell volume and push the nucleus to the periphery. This part of the cell performs storage, excretory, osmotic, protective, lysosomal and other functions, since it accumulates nutrients and waste products, it provides water supply and maintains the shape and volume of the cell, and also contains enzymes for the breakdown of many cell components. In addition, the biologically active substances of vacuoles can prevent many animals from eating these plants. In a number of plants, due to the swelling of vacuoles, cell growth occurs by stretching.

Vacuoles are also present in the cells of some fungi and bacteria, but in fungi they perform only the function of osmoregulation, while in cyanobacteria they maintain buoyancy and participate in the processes of nitrogen assimilation from the air.

Ribosomes- small non-membrane organelles with a diameter of 15-20 microns, consisting of two subunits - large and small (Fig. 2.36).

Eukaryotic ribosome subunits are assembled in the nucleolus and then transported to the cytoplasm. The ribosomes of prokaryotes, mitochondria, and plastids are smaller than those of eukaryotes. Ribosome subunits include rRNA and proteins.

The number of ribosomes per cell can reach several tens of millions: in the cytoplasm, mitochondria and plastids they are in a free state, and on the rough ER they are in a bound state. They take part in protein synthesis, in particular, they carry out the process of translation - the biosynthesis of a polypeptide chain on an mRNA molecule. Proteins of hyaloplasm, mitochondria, plastids and own proteins of ribosomes are synthesized on free ribosomes, while proteins are translated on ribosomes attached to the rough ER for excretion from cells, assembly of membranes, formation of lysosomes and vacuoles.

Ribosomes can be located in the hyaloplasm singly or assembled in groups with simultaneous synthesis of several polypeptide chains on one mRNA. These groups of ribosomes are called polyribosomes, or polysomes(Fig. 2.37).

microtubules- These are cylindrical hollow non-membrane organelles that penetrate the entire cytoplasm of the cell. Their diameter is about 25 nm, the wall thickness is 6-8 nm. They are made up of numerous protein molecules. tubulin, which first form 13 strands resembling beads and then assemble into a microtubule. Microtubules form a cytoplasmic reticulum that gives the cell shape and volume, connects the plasma membrane with other parts of the cell, provides transport of substances through the cell, takes part in the movement of the cell and intracellular components, as well as in the division of genetic material. They are part of the cell center and organelles of movement - flagella and cilia.

microfilaments, or microfilament, are also non-membrane organelles, however, they have a filamentous shape and are formed not by tubulin, but actin. They take part in the processes of membrane transport, intercellular recognition, division of the cell cytoplasm and in its movement. In muscle cells, the interaction of actin microfilaments with myosin filaments provides contraction.

Microtubules and microfilaments form the inner skeleton of the cell - cytoskeleton. It is a complex network of fibers that provide mechanical support for the plasma membrane, determines the shape of the cell, the location of cellular organelles and their movement during cell division (Fig. 2.38).

Cell Center- non-membrane organelle located in animal cells near the nucleus; it is absent in plant cells (Fig. 2.39). Its length is about 0.2-0.3 microns, and its diameter is 0.1-0.15 microns. The cell center is made up of two centrioles, lying in mutually perpendicular planes, and radiant sphere from microtubules. Each centriole is formed by nine groups of microtubules, collected in threes, i.e. triplets. The cell center takes part in the assembly of microtubules, the division of the hereditary material of the cell, as well as in the formation of flagella and cilia.

Organelles of movement. Flagella and cilia are outgrowths of cells covered with plasmalemma. These organelles are based on nine pairs of microtubules located along the periphery and two free microtubules in the center (Fig. 2.40). Microtubules are interconnected by various proteins, which ensure their coordinated deviation from the axis - oscillation. Fluctuations are energy-dependent, that is, the energy of macroergic bonds of ATP is spent on this process. ATP breakdown is a function basal bodies, or kinetosomes, located at the base of the flagella and cilia.

The length of the cilia is about 10-15 nm, and the length of the flagella is 20-50 microns. Due to the strictly directed movements of the flagella and cilia, not only the movement of unicellular animals, spermatozoa, etc. is carried out, but also the airways are cleared, the egg moves through the fallopian tubes, since all these parts of the human body are lined with ciliated epithelium.

Animal and plant cells, both multicellular and unicellular, are in principle similar in structure. Differences in the details of the structure of cells are associated with their functional specialization.

The main elements of all cells are the nucleus and cytoplasm. The nucleus has a complex structure that changes at different phases of cell division, or cycle. The nucleus of a nondividing cell occupies approximately 10–20% of its total volume. It consists of a karyoplasm (nucleoplasm), one or more nucleoli (nucleolus) and a nuclear envelope. Karyoplasm is a nuclear juice, or karyolymph, in which there are chromatin threads that form chromosomes.

The main properties of the cell:

• metabolism
• sensitivity
• ability to reproduce

The cell lives in the internal environment of the body - blood, lymph and tissue fluid. The main processes in the cell are oxidation, glycolysis - the breakdown of carbohydrates without oxygen. Cell permeability is selective. It is determined by the reaction to high or low salt concentration, phago- and pinocytosis. Secretion - the formation and secretion by cells of mucus-like substances (mucin and mucoids), which protect against damage and participate in the formation of intercellular substance.

Types of cell movements:

1. amoeboid (false legs) - leukocytes and macrophages.
2. sliding - fibroblasts
3. flagellate type - spermatozoa (cilia and flagella)

Cell division:

1. indirect (mitosis, karyokinesis, meiosis)
2. direct (amitosis)

During mitosis, the nuclear substance is distributed evenly between the daughter cells, because The chromatin of the nucleus is concentrated in chromosomes, which split into two chromatids, diverging into daughter cells.

Structures of a living cell

Chromosomes

Mandatory elements of the nucleus are chromosomes that have a specific chemical and morphological structure. They take an active part in the metabolism in the cell and are directly related to the hereditary transmission of properties from one generation to another. However, it should be borne in mind that, although heredity is provided by the whole cell as a single system, nuclear structures, namely chromosomes, occupy a special place in this. Chromosomes, unlike cell organelles, are unique structures characterized by a constant qualitative and quantitative composition. They cannot interchange each other. An imbalance in the chromosome set of a cell ultimately leads to its death.

Cytoplasm

The cytoplasm of a cell exhibits a very complex structure. The introduction of the technique of thin sections and electron microscopy made it possible to see the fine structure of the underlying cytoplasm. It has been established that the latter consists of parallel complex structures in the form of plates and tubules, on the surface of which there are the smallest granules with a diameter of 100–120 Å. These formations are called the endoplasmic complex. This complex includes various differentiated organelles: mitochondria, ribosomes, the Golgi apparatus, in the cells of lower animals and plants - the centrosome, in animals - lysosomes, in plants - plastids. In addition, a number of inclusions are found in the cytoplasm that take part in the metabolism of the cell: starch, fat droplets, urea crystals, etc.

Membrane

The cell is surrounded by a plasma membrane (from Latin "membrane" - skin, film). Its functions are very diverse, but the main one is protective: it protects the internal contents of the cell from the effects of the external environment. Due to various outgrowths, folds on the surface of the membrane, the cells are firmly interconnected. The membrane is permeated with special proteins through which certain substances necessary for the cell or to be removed from it can move. Thus, the exchange of substances is carried out through the membrane. Moreover, what is very important, substances are passed through the membrane selectively, due to which the required set of substances is maintained in the cell.

In plants, the plasma membrane is covered on the outside with a dense membrane consisting of cellulose (fiber). The shell performs protective and supporting functions. It serves as the outer frame of the cell, giving it a certain shape and size, preventing excessive swelling.

Nucleus

Located in the center of the cell and separated by a two-layer membrane. It has a spherical or elongated shape. The shell - the karyolemma - has pores necessary for the exchange of substances between the nucleus and the cytoplasm. The contents of the nucleus are liquid - karyoplasm, which contains dense bodies - nucleoli. They are granular - ribosomes. The bulk of the nucleus - nuclear proteins - nucleoproteins, in the nucleoli - ribonucleoproteins, and in the karyoplasm - deoxyribonucleoproteins. The cell is covered with a cell membrane, which consists of protein and lipid molecules having a mosaic structure. The membrane ensures the exchange of substances between the cell and the intercellular fluid.

EPS

This is a system of tubules and cavities, on the walls of which there are ribosomes that provide protein synthesis. Ribosomes can also be freely located in the cytoplasm. There are two types of ER - rough and smooth: on the rough ER (or granular) there are many ribosomes that carry out protein synthesis. Ribosomes give membranes a rough appearance. Smooth ER membranes do not carry ribosomes on their surface; they contain enzymes for the synthesis and breakdown of carbohydrates and lipids. Smooth EPS looks like a system of thin tubes and tanks.

Ribosomes

Small bodies with a diameter of 15–20 mm. Carry out the synthesis of protein molecules, their assembly from amino acids.

Mitochondria

These are two-membrane organelles, the inner membrane of which has outgrowths - cristae. The contents of the cavities is the matrix. Mitochondria contain a large number of lipoproteins and enzymes. These are the energy stations of the cell.

Plastids (peculiar to plant cells only!)

Their content in the cell is the main feature of the plant organism. There are three main types of plastids: leucoplasts, chromoplasts, and chloroplasts. They have different colors. Colorless leukoplasts are found in the cytoplasm of the cells of the unstained parts of plants: stems, roots, tubers. For example, there are many of them in potato tubers, in which starch grains accumulate. Chromoplasts are found in the cytoplasm of flowers, fruits, stems, and leaves. Chromoplasts provide the yellow, red, orange color of plants. Green chloroplasts are found in the cells of leaves, stems, and other plant parts, as well as in a variety of algae. Chloroplasts are 4-6 µm in size and often have an oval shape. In higher plants, one cell contains several dozen chloroplasts.

Green chloroplasts are able to pass into chromoplasts - therefore, in autumn, the leaves turn yellow, and green tomatoes turn red when ripe. Leukoplasts can turn into chloroplasts (greening of potato tubers in the light). Thus, chloroplasts, chromoplasts and leukoplasts are capable of mutual transition.

The main function of chloroplasts is photosynthesis, i.e. in chloroplasts in the light, organic substances are synthesized from inorganic ones by converting solar energy into the energy of ATP molecules. Chloroplasts of higher plants are 5-10 microns in size and resemble a biconvex lens in shape. Each chloroplast is surrounded by a double membrane with selective permeability. Outside, there is a smooth membrane, and the inside has a folded structure. The main structural unit of the chloroplast is the thylakoid, a flat two-membrane sac that plays a leading role in the process of photosynthesis. The thylakoid membrane contains proteins similar to mitochondrial proteins that are involved in the electron transfer chain. The thylakoids are arranged in stacks resembling stacks of coins (from 10 to 150) and called grana. Grana has a complex structure: in the center is chlorophyll, surrounded by a layer of protein; then there is a layer of lipoids, again protein and chlorophyll.

Golgi complex

This system of cavities delimited from the cytoplasm by a membrane can have a different shape. The accumulation of proteins, fats and carbohydrates in them. Implementation of the synthesis of fats and carbohydrates on membranes. Forms lysosomes.

The main structural element of the Golgi apparatus is a membrane that forms packages of flattened cisterns, large and small vesicles. The cisterns of the Golgi apparatus are connected to the channels of the endoplasmic reticulum. Proteins, polysaccharides, fats produced on the membranes of the endoplasmic reticulum are transferred to the Golgi apparatus, accumulated inside its structures and “packed” in the form of a substance ready either for release or for use in the cell itself during its life. Lysosomes are formed in the Golgi apparatus. In addition, it is involved in the growth of the cytoplasmic membrane, for example, during cell division.

Lysosomes

Bodies separated from the cytoplasm by a single membrane. The enzymes contained in them accelerate the reaction of splitting complex molecules into simple ones: proteins to amino acids, complex carbohydrates to simple ones, lipids to glycerol and fatty acids, and also destroy dead parts of the cell, whole cells. Lysosomes contain more than 30 types of enzymes (substances of a protein nature that increase the rate of a chemical reaction by tens and hundreds of thousands of times) that can break down proteins, nucleic acids, polysaccharides, fats and other substances. The breakdown of substances with the help of enzymes is called lysis, hence the name of the organoid. Lysosomes are formed either from the structures of the Golgi complex, or from the endoplasmic reticulum. One of the main functions of lysosomes is participation in the intracellular digestion of nutrients. In addition, lysosomes can destroy the structures of the cell itself when it dies, during embryonic development, and in a number of other cases.

Vacuoles

They are cavities in the cytoplasm filled with cell sap, a place of accumulation of reserve nutrients, harmful substances; they regulate the water content in the cell.

Cell Center

It consists of two small bodies - centrioles and centrosphere - a compacted area of ​​​​the cytoplasm. Plays an important role in cell division

Organelles of cell movement

1. Flagella and cilia, which are cell outgrowths and have the same structure in animals and plants
2. Myofibrils - thin threads more than 1 cm long with a diameter of 1 micron, arranged in bundles along the muscle fiber
3. Pseudopodia (perform the function of movement; due to them, muscle contraction occurs)

Similarities between plant and animal cells

The features that plant and animal cells are similar to include the following:

1. A similar structure of the structure system, i.e. the presence of a nucleus and cytoplasm.
2. The exchange process of substances and energy is similar in principle of implementation.
3. Both animal and plant cells have a membrane structure.
4. The chemical composition of cells is very similar.
5. In plant and animal cells, there is a similar process of cell division.
6. The plant cell and the animal have the same principle of transmitting the code of heredity.

Significant differences between plant and animal cells

In addition to the general features of the structure and vital activity of plant and animal cells, there are special distinctive features of each of them.

Thus, we can say that plant and animal cells are similar to each other in the content of some important elements and some life processes, and also have significant differences in structure and metabolic processes.

The smallest units of life. However, many highly differentiated cells have lost this ability. Cytology as a science At the end of the 19th century. The main attention of cytologists was directed to a detailed study of the structure of cells, the process of their division, and the elucidation of their role as the most important units that provide the physical basis of heredity and the process of development. Development of new methods. At first at...

As "beautiful May, which blooms only once, and never again" (I. Goethe), exhausted itself and was displaced by the Christian Middle Ages. 2. Cell as a structural and functional unit of the living. The composition and structure of the cell Modern cell theory includes the following provisions: 1. All living organisms are composed of cells. A cell is a structural, functional unit of a living, ...

0.05 - 0.10 Calcium Magnesium Sodium Iron Zinc Copper Iodine Fluorine 0.04 - 2.00 0.02 - 0.03 0.02 - 0.03 0.01 - 0.015 0.0003 0.0002 0.0001 0.0001 Cell content of chemical compounds Compounds (in %) Inorganic Organic Water Inorganic substances 70 - 80 1.0 - 1.5 Proteins Carbohydrates Fats Nucleic acids 10 - 20 0.2 ...

And these two organoids, as noted above, represent a single apparatus for the synthesis and transportation of proteins formed in the cell. Golgi complex. The Golgi complex is a cell organoid, named after the Italian scientist C. Golgi, who first saw it in the cytoplasm of nerve cells (1898) and designated it as a mesh apparatus. Now the Golgi complex is found in all plant cells and ...