Do plants have cytoplasm? The main functions and role of the cytoplasm in cells. plant cell vacuoles

Cytoplasm - obligatory part of the cell, enclosed between the plasma membrane and the nucleus and representing hyaloplasm - basic substance of the cytoplasm organelles- constant components of the cytoplasm and inclusion- temporary components of the cytoplasm. The chemical composition of the cytoplasm is diverse. Its basis is water (60-90% of the total mass of the cytoplasm). The cytoplasm is rich in proteins, it may include fats and fat-like substances, various organic and inorganic compounds. The cytoplasm is alkaline. One of the characteristic features of the cytoplasm is the constant movement (cyclosis). It is detected primarily by the movement of cell organelles, such as chloroplasts. If the movement of the cytoplasm stops, the cell dies, since only being in constant motion can it perform its functions.

The main substance of the cytoplasm is hyaloplasm (cytosol)- is a colorless, slimy, thick and transparent colloidal solution. It is in it that all metabolic processes take place, it provides the interconnection of the nucleus and all organelles. Depending on the predominance of the liquid part or large molecules in the hyaloplasm, two forms of hyaloplasm are distinguished: sol - more liquid hyaloplasm and gel- denser hyaloplasm. Mutual transitions are possible between them: the gel easily turns into a sol and vice versa.

Cell walls eukaryotic organisms have a different structure, but the plasma membrane always adjoins the cytoplasm, an outer layer forms on its surface. In animals it is called glycocalyx(formed by glycoproteins, glycolipids, lipoproteins), in plants - cell wall from a powerful layer of fiber fibers.

The structure of membranes. All biological membranes have common structural features and properties. Currently generally accepted fluid mosaic model membrane structure (sandwich model). The basis of the membrane is a lipid bilayer, formed mainly phospholipids. In the bilayer, the tails of the molecules in the membrane face each other, and the polar heads face outward, towards the water. In addition to lipids, the membrane contains proteins (60% on average). They determine most of the specific functions of the membrane. Protein molecules do not form a continuous layer, they distinguish peripheral proteins- proteins located on the outer or inner surface of the lipid bilayer, semi-integral proteins- proteins immersed in the lipid bilayer to different depths, integral, or transmembrane proteins- proteins penetrating the membrane through, while in contact with the external and internal environment of the cell.

Membrane proteins can perform various functions: the transport of certain molecules, the catalysis of reactions occurring on membranes, the maintenance of membrane structure, and the receipt and conversion of signals from the environment.

The membrane may contain from 2 to 10% carbohydrates. The carbohydrate component of membranes is usually represented by oligosaccharide or polysaccharide chains associated with protein molecules (glycoproteins) or lipids (glycolipids). Basically, carbohydrates are located on the outer surface of the membrane. Carbohydrates provide receptor functions of the membrane. In animal cells, glycoproteins form an epimembrane complex - glycocalyx, having a thickness of several tens of nanometers. Extracellular digestion takes place in it, many cell receptors are located, and with its help, apparently, cell adhesion occurs.

Molecules of proteins and lipids are mobile, able to move mainly in the plane of the membrane. The thickness of the plasma membrane is on average 7.5 nm.

Membrane functions.

1. They separate the cellular contents from the external environment.

2. Regulate the metabolism between the cell and the environment.

3. Cells are divided into compartments designed for various reactions to occur.

4. Many chemical reactions take place on enzyme conveyors located on the membranes themselves.

5. Provide communication between cells in the tissues of multicellular organisms.

6. There are receptor sites on the membranes for recognizing external stimuli.

One of the main functions of the membrane is transport, ensuring the exchange of substances between the cell and the external environment. The membranes have the property selective permeability, that is, they are well permeable to some substances or molecules and poorly permeable (or completely impermeable) to others. There are various mechanisms for the transport of substances across the membrane. Depending on the need to use energy for the transport of substances, there are : passive transport- transport of substances without energy consumption; active transport - transport that uses energy.

Passive transport is based on the difference in concentrations and charges. In passive transport, substances always move from an area of ​​higher concentration to an area of ​​lower concentration, that is, along a concentration gradient.

Distinguish three main passive transport mechanisms:simple diffusion- transport of substances directly through the lipid bilayer. Gases, non-polar or small uncharged polar molecules easily pass through it. The smaller the molecule and the more fat soluble it is, the faster it will cross the membrane. Interestingly, polar water molecules penetrate the lipid bilayer very quickly. This is because its molecules are small and electrically neutral. Diffusion of water across membranes is called osmosis.

Diffusion through membrane channels. Charged molecules and ions (Na +, K +, Ca 2+, C1~) are not able to pass through the lipid bilayer by simple diffusion, however, they penetrate the membrane due to the presence in it of special channel-forming proteins that form pores. Most of the water passes through the membrane through channels formed by aquaporins.

Facilitated diffusion- transport of substances with the help of special transport proteins, each of which is responsible for the transport of certain molecules or groups of related molecules. They interact with the molecule of the transferred substance and in some way move it through the membrane. This is how sugars, amino acids, nucleotides and many other polar molecules are transported into the cell.

Need active transport occurs when it is necessary to ensure the transfer of molecules through the membrane against the electrochemical gradient. This transport is carried out by carrier proteins, the activity of which requires energy expenditure. The energy source is ATP molecules. One of the most studied active transport systems is the sodium-potassium pump. The concentration of K + inside the cell is much higher than outside it, and Na + - vice versa. Therefore, K + passively diffuses out of the cell through the water pores of the membrane, and Na + - into the cell. At the same time, for the normal functioning of the cell, it is important to maintain a certain ratio of K + and Na + ions in the cytoplasm and in the external environment. This is possible because the membrane, due to the presence of a sodium-potassium pump, actively pumps Na + out of the cell, and K + into the cell. The sodium-potassium pump consumes almost a third of all the energy necessary for the life of the cell. For one cycle of operation, the pump pumps out 3 Na + ions from the cell and pumps 2 K + ions. K + diffuses passively out of the cell faster than Na + into the cell.

The cell has mechanisms by which it can transport large particles and macromolecules across the membrane. The process of absorption of macromolecules by the cell is called endocytosis. During endocytosis, the plasma membrane forms an invagination, its edges merge, and the structures delimited from the cytoplasm by a single membrane, which is part of the outer cytoplasmic membrane, are laced into the cytoplasm. There are two types of endocytosis: phagocytosis- capture and absorption of large particles (for example, phagocytosis of lymphocytes, protozoa, etc.) and pinocytosis - the process of capturing and absorbing droplets of a liquid with substances dissolved in it.

Exocytosis- the process of removing various substances from the cell. During exocytosis, the vesicle membrane fuses with the outer cytoplasmic membrane, the contents of the vesicle are removed outside the cell, and its membrane is included in the outer cytoplasmic membrane.

cell organelles

Organelles (organelles)- permanent cellular structures that ensure the performance of specific functions by the cell. Each organelle has a specific structure and performs specific functions.

There are: membrane organelles - having a membrane structure, and they can be single-membrane (endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles of plant cells) and two-membrane (mitochondria, plastids, nucleus).

In addition to membrane organelles, there can also be non-membrane organelles - not having a membrane structure (chromosomes, ribosomes, cell center and centrioles, cilia and flagella with basal bodies, microtubules, microfilaments).

Single membrane organelles:

1. Endoplasmic reticulum (ER). It is a system of membranes that form tanks and channels, connected to each other and limiting a single internal space - EPR cavity. On the one hand, the membranes are connected to the outer cytoplasmic membrane, on the other hand, to the outer shell of the nuclear membrane. There are two types of EPR: rough (granular), containing ribosomes on its surface and representing a set of flattened sacs, and smooth (agranular), whose membranes do not carry ribosomes.

Functions: divides the cytoplasm of the cell into isolated compartments, thereby providing a spatial demarcation from each other of many different reactions running in parallel, Carries out the synthesis and breakdown of carbohydrates and lipids (smooth EPR) and provides protein synthesis (rough EPR), accumulates in channels and cavities, and then transports the products of biosynthesis to the cell organelles.

2. Golgi apparatus. An organoid usually located near the cell nucleus (often near the cell center in animal cells). It is a stack of flattened cisterns with expanded edges, with which a system of small single-membrane vesicles (Golgi vesicles) is connected. Each stack usually consists of 4-6 tanks. The number of Golgi stacks in a cell ranges from one to several hundred.

The most important function of the Golgi complex is the removal of various secrets (enzymes, hormones) from the cell, therefore it is well developed in secretory cells. Here is the synthesis of complex carbohydrates from simple sugars, the maturation of proteins, the formation of lysosomes.

3. Lysosomes. The smallest single-membrane cell organelles, which are vesicles with a diameter of 0.2-0.8 microns, containing up to 60 hydrolytic enzymes active in a slightly acidic environment.

The formation of lysosomes occurs in the Golgi apparatus, where the enzymes synthesized in it come from the EPR. The breakdown of substances with the help of enzymes is called lysis, hence the name of the organoid.

There are: primary lysosomes - lysosomes that have detached from the Golgi apparatus and contain enzymes in an inactive form, and secondary lysosomes - lysosomes formed as a result of the fusion of primary lysosomes with pinocytic or phagocytic vacuoles; digestion and lysis of substances entering the cell occurs in them (therefore, they are often called digestive vacuoles).

Digestion products are absorbed by the cytoplasm of the cell, but some of the material remains undigested. The secondary lysosome containing this undigested material is called the residual body. By exocytosis, undigested particles are removed from the cell.

Sometimes with the participation of lysosomes, self-destruction of the cell occurs. This process is called autolysis. This usually occurs during some processes of differentiation (for example, the replacement of cartilage with bone tissue, the disappearance of the tail in the frog tadpole).

4. Cilia and flagella. Formed by nine double microtubules that form the wall of a cylinder covered with a membrane; in its center are two single microtubules. This 9+2 type structure is characteristic of the cilia and flagella of almost all eukaryotic organisms, from protozoa to humans.

Cilia and flagella are reinforced in the cytoplasm by basal bodies that lie at the base of these organelles. Each basal body consists of nine triplets of microtubules; there are no microtubules in its center.

5. Single-membrane organelles also include vacuoles, surrounded by a membrane - tonoplast. In plant cells, they can occupy up to 90% of the cell volume and ensure the entry of water into the cell due to the high osmotic potential and turgor (intracellular pressure). In animal cells, vacuoles are small, formed by endocytosis (phagocytosis and pinocytosis), after fusion with primary lysosomes, they are called digestive vacuoles.

Double membrane organelles:

1. Mitochondria. Two-membrane organelles of a eukaryotic cell that provide the body with energy. The number of mitochondria in a cell varies widely, from 1 to 100 thousand, and depends on its metabolic activity. The number of mitochondria can increase by dividing, since these organelles have their own DNA.

The outer membrane of mitochondria is smooth, the inner membrane forms numerous invaginations or tubular outgrowths - cristae. The number of cristae can vary from several tens to several hundreds and even thousands, depending on the functions of the cell. They increase the surface of the inner membrane, on which the enzyme systems involved in the synthesis of ATP molecules are located.

The inner space of mitochondria is filled matrix. The matrix contains a circular molecule of mitochondrial DNA, specific mRNA, tRNA and ribosomes (prokaryotic type) that carry out autonomous biosynthesis of part of the proteins that make up the inner membrane. These facts testify in favor of the origin of mitochondria from oxidizing bacteria (according to the symbiogenesis hypothesis). But most of the mitochondrial genes have moved into the nucleus, and the synthesis of many mitochondrial proteins occurs in the cytoplasm. In addition, there are enzymes that form ATP molecules. Mitochondria are capable of reproducing by fission.

The functions of mitochondria are the oxygen breakdown of carbohydrates, amino acids, glycerol and fatty acids with the formation of ATP, the synthesis of mitochondrial proteins.

2. Plastids. There are three main types of plastids: leucoplasts- colorless plastids in the cells of unstained parts of plants, chromoplasts- colored plastids, usually yellow, red and orange, chloroplasts- green plastids. Plastids are formed from proplastids - two-membrane vesicles up to 1 micron in size.

Since plastids have a common origin, interconversions are possible between them. The transformation of leukoplasts into chloroplasts most often occurs (greening of potato tubers in the light), the reverse process occurs in the dark. When the leaves turn yellow and the fruits turn red, chloroplasts turn into chromoplasts. Only the transformation of chromoplasts into leukoplasts or chloroplasts is considered impossible.

Chloroplasts. The main function 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. The chloroplasts of higher plants are shaped like a biconvex lens. The outer membrane is smooth, while the inner membrane has a folded structure. As a result of the formation of protrusions of the inner membrane, a system of lamellae and thylakoids arises. The internal environment of chloroplasts - stroma contains circular DNA and prokaryotic-type ribosomes. Plastids are capable of autonomous division, as are mitochondria. The facts, according to the symbiogenesis hypothesis, also testify in favor of the origin of plastids from cyanobacteria.

Rice. Modern (generalized) scheme of the plant cell structure, compiled according to the data of electron microscopic examination of various plant cells: 1 - Golgi apparatus; 2 - freely located ribosomes; 3 - chloroplasts; 4 - intercellular spaces; 5 - polyribosomes (several interconnected ribosomes); 6 - mitochondria; 7 - lysosomes; 8 - granular endoplasmic reticulum; 9 - smooth endoplasmic reticulum; 10 - microtubules; 11 - plastids; 12 - plasmodesmata passing through the shell; 13 - cell membrane; 14 - nucleolus; 15, 18 - nuclear envelope; 16 - pores in the nuclear envelope; 17 - plasmalemma; 19 - hyaloplasm; 20 - tonoplast; 21 - vacuoles; 22 - core.

Rice. Membrane structure

Rice. The structure of the mitochondria. Above and in the middle - a view of the longitudinal section through the mitochondria (above - mitochondria from the embryonic cell of the root tip; in the middle - from the cell of an adult leaf of elodea). Below is a three-dimensional diagram in which part of the mitochondria is cut away, which allows you to see its internal structure. 1 - outer membrane; 2 - inner membrane; 3 - cristae; 4 - matrix.

Rice. The structure of the chloroplast. Left - longitudinal section through the chloroplast: 1 - grana formed by stacked lamellae; 2 - shell; 3 - stroma (matrix); 4 - lamellae; 5 - drops of fat formed in the chloroplast. On the right - a three-dimensional diagram of the location and relationship of lamellae and grana inside the chloroplast: 1 - grana; 2 - lamellae.

Cytoplasm - the contents of the cell outside the nucleus, enclosed in the plasma membrane. It has a transparent color and a gel-like consistency. The cytoplasm consists mainly of water, and also contains enzymes, salts, and various organic molecules.

Function of the cytoplasm

The cytoplasm functions to support and suspend organelles and cellular molecules. Many cellular processes also take place in the cytoplasm.

Some of these processes include protein synthesis, the first step known as glycolysis, and . In addition, the cytoplasm helps move substances such as hormones around the cell and also dissolves cellular waste.

Components of the cytoplasm

Organelles

Organelles are tiny cellular structures that perform specific functions within the cell. Examples of organelles include: , and .

Also inside the cytoplasm is a network of fibers that help the cell maintain its shape and provide support for the organelles.

Cytoplasmic inclusions

Cytoplasmic inclusions are particles temporarily suspended in the cytoplasm. Inclusions consist of macromolecules and granules.

The three types of inclusions found in the cytoplasm are secretory and nutrient inclusions, and pigment granules. Examples of secretory inclusions are proteins, enzymes and acids. Glycogen (storage of glucose molecules) and lipids are examples of nutrient inclusions. The melanin present in skin cells is an example of the incorporation of pigment granules.

Cytoplasmic divisions

Cytoplasm can be divided into two main parts: endoplasm and ectoplasm. Endoplasm is the central region of the cytoplasm that contains organelles. Ectoplasm is the more gel-like peripheral portion of the cell's cytoplasm.

cell membrane

The cell or plasma membrane is a structure that prevents the cytoplasm from spilling out of the cell. This membrane is composed of phospholipids that form a lipid bilayer that separates the contents of the cell from the extracellular fluid. The lipid bilayer is semi-permeable, meaning that only some molecules are able to diffuse across the membrane to enter or exit the cell. Extracellular fluid, proteins, lipids and other molecules can be added to the cytoplasm of the cell with the help of. In this process, molecules and extracellular fluid are internalized as the membrane forms a vesicle.

The vesicle separates fluid, molecules, and kidneys from the cell membrane, forming an endosome. The endosome moves within the cell to deliver its contents to the appropriate destinations. Substances are removed from the cytoplasm by. In this process, vesicles budded from the Golgi bodies fuse with the cell membrane, forcing their contents out of the cell. The plasma membrane also provides structural support to the cell, acting as a stable platform for the attachment of the cytoskeleton and .

The structure of the cytoplasm

The interior of the cell is divided into cytoplasm and nucleus. The cytoplasm is the bulk of the cell.

Definition 1

Cytoplasm- this is the internal semi-liquid colloidal environment of the cell, separated from the external environment by the cell membrane, in which the nucleus, all organelles of the membrane and non-membrane structure are located.

The entire space between the organelles in the cell is filled with the soluble contents of the cytoplasm ( cytosol). The aggregate state of the cytoplasm can be different: rare - sol and viscous gel. The chemical composition of the cytoplasm is quite complex. This is a semi-liquid mucous colorless mass of a complex physico-chemical structure (biological colloid).

Animal cells and very young plant cells are completely filled with cytoplasm. In plant cells, during differentiation, small vacuoles are formed, in the process of merging of which a central vacuole is formed, and the cytoplasm moves to the membrane and lines it with a continuous layer.

The cytoplasm contains:

• salt (1%),
• sugar (4-6%),
• amino acids and proteins (10-12%),
• fats and lipids (2-3%) enzymes,
• up to 80% water.

All these substances form a colloidal solution that does not mix with water or vacuolar content.

The cytoplasm contains:

• matrix (hyaloplasm),
• cytoskeleton,
• organelles,
• inclusions.

Hyaloplasm- colloidal colorless structure of the cell. It consists of soluble proteins, RNA, polysaccharides, lipids and cellular structures arranged in a certain way: membranes, organelles, inclusions.

cytoskeleton, or intracellular skeleton, - a system of protein formations - microtubules and microfilaments - performs a supporting function in the cell, participates in changing the shape of the cell and its movement, provides a certain arrangement of enzymes in the cell.

Organelles- these are stable cellular structures that perform certain functions that ensure all the processes of the cell's vital activity (movement, respiration, nutrition, synthesis of organic compounds, their transport, preservation and transmission of hereditary information).

Eukaryotic organelles are divided into:

1. two-membrane (mitochondria, plastids);
2. single-membrane (endoplasmic reticulum, Golgi apparatus (complex), lysosomes, vacuoles);
3. non-membrane (flagella, cilia, pseudopodia, myofibrils).

Inclusions- temporary structures of the cell. These include reserve compounds and metabolic end products: grains of starch and glycogen, fat drops, salt crystals.

Functions and properties of the cytoplasm

The cytoplasmic content of the cell is able to move, which favors the optimal placement of organelles and, as a result, biochemical reactions proceed better, the release of metabolic products, etc.

In protozoa (amoeba), due to the movement of the cytoplasm, the main movement of cells in space is carried out.

Cytoplasm formed various external formations of the cell - flagella, cilia, surface outgrowths, which play an important role in the movement of cells and contribute to the connection of cells in tissues.

The cytoplasm is the matrix for all cellular elements, ensuring the interaction of all cellular structures, various chemical reactions take place in it, substances move through the cytoplasm in the cell, as well as from cell to cell.

Hyaloplasm (cytoplasm matrix) is a transparent colloidal solution of organic and inorganic
connections. Of the inorganic compounds in the hyaloplasm, water predominates (from 50 to 90%),
there are Ca2+, K+ cations, anions of carbonic and phosphoric acids, dissolved oxygen,
carbon dioxide and other gases. Organic compounds are proteins, amino acids,
lipids, carbohydrates, different types of RNA, individual nucleotides. In the cytoplasm there are ground substance, organelles and inclusions. The main substance of the cytoplasm is hyaloplasm fills the space between the plasmalemma, nuclear membrane and other intracellular structures. The main substance of the cytoplasm forms the true internal environment of the cell, which unites all intracellular structures and provides interaction with each other. The fulfillment by the matrix of the unifying, as well as the scaffolding, functions can be associated with the help of a super-powerful electron microscope of the microtrabecular network formed by thin fibrils. Also functionally, the cytoplasmic matrix is ​​the site of intracellular metabolism. Through the hyaloplasm, a significant amount of intracellular movements of substances and structures is carried out. Hyaloplasm should be considered as a complex colloidal system capable of changing from liquid to gel state.

Hyaloplasm functions:
1. It is an internal environment in which many chemical processes of energy
and plastic exchange, and in particular:
- processes of oxygen-free energy metabolism with the formation of a small amount
ATP;
- processes of protein synthesis on ribosomes with the participation of mRNA, tRNA.
2. It unites all cellular structures and provides interaction between them.
The cytoplasm of a living cell is characterized by the constant movement of its colloidal particles and other
components (cyclosis). Cyclose provides the transport of substances and the movement of organelles
(for example, the movement of chloroplasts, digestive vacuoles), optimization of metabolic processes
substances, removal of metabolic products from the cell.
Organelles are permanent specialized components of the cytoplasm that have
a certain structure and perform certain functions in the cell.

7. General purpose organelles. Their structure and functions .

General purpose organelles are divided into membrane and non-membrane. Membrane, in turn, are divided into single membrane and two-membrane.
To single membrane include:

Endoplasmic reticulum (EPR)). It is a system of membranes that form tanks and channels, connected to each other and limiting a single internal space - EPR cavities. There are two types of EPR: rough containing ribosomes on its surface smooth, whose membranes do not carry ribosomes.
Functions: divides the cytoplasm of the cell into isolated compartments, thereby providing a spatial delimitation from each other of many different reactions running in parallel. Carries out the synthesis and breakdown of carbohydrates and lipids (smooth ER) and provides protein synthesis (rough ER), accumulates in channels and cavities, and then transports biosynthesis products to cell organelles.

Golgi apparatus. An organoid usually located near the cell nucleus (often near the cell center in animal cells). It is a stack of flattened tanks with widened edges, consists of 4-6 tanks. The number of Golgi stacks in a cell ranges from one to several hundred.
The most important function of the Golgi complex is the removal of various secrets (enzymes, hormones) from the cell, therefore it is well developed in secretory cells. Here is the synthesis of complex carbohydrates from simple sugars, the maturation of proteins, the formation of lysosomes.

Lysosomes. The smallest single-membrane cell organelles, which are vesicles with a diameter of 0.2-0.8 microns, containing up to 60 hydrolytic enzymes. Lysosomes are formed in the Golgi apparatus. The breakdown of substances with the help of enzymes is called lysis, hence the name of the organoid.
There are: primary secondary lysosomes - lysosomes formed as a result of the fusion of primary lysosomes with pinocytic or phagocytic vacuoles; digestion and lysis of substances entering the cell occur in them (therefore, they are often called digestive vacuoles):
Sometimes with the participation of lysosomes, self-destruction of the cell occurs. This process is called autolysis.

Vacuoles- large membranous vesicles or cavities in the cytoplasm filled with cell sap. Vacuoles are formed in the cells of plants and fungi from vesicular extensions of the endoplasmic reticulum or from the vesicles of the Golgi complex. vacuoles absorb excess water and then bring it out through contractions.

To double membrane organelles include:

plastids - plastids- organelles characteristic only for plant cells and found in all living cells of green plants. The inner membrane of the chloroplast forms invaginations into the stroma - thylakoids. Leucoplasts- small colorless plastids of various shapes are mainly found in the cells of organs hidden from sunlight (roots, rhizomes, tubers, seeds). They carry out secondary synthesis and accumulation of reserve nutrients.

Mitochondria are integral components of all eukaryotic cells. 0.5 µm thick and up to 7-10 µm long. Mitochondria are limited by two membranes - outer and inner. The outer membrane separates it from the hyaloplasm. The inner membrane forms many protrusions inside the mitochondria - the so-called cristae. Enzymes are located on the cristae membrane or inside it, which are involved in oxygen respiration. The internal content of the mitochondria (matrix) limited by it is close in composition to the cytoplasm. The matrix contains various proteins, including enzymes, DNA (circular molecule), all types of RNA, amino acids, ribosomes, and a number of vitamins. DNA provides some genetic autonomy for mitochondria, although in general their work is coordinated by nuclear DNA. Mitochondria are the powerhouse of the cell.

Non-membrane organelles:

Cell center. In the cells of most animals, as well as some fungi, algae, mosses and ferns, there are centrioles. They are usually located in the center of the cell, which determined their name. Centrioles are hollow cylinders no more than 0.5 µm long. They are arranged in pairs perpendicular to each other. Each centriole is built from nine triplets of microtubules. The main function of centrioles is the organization of microtubules of the cell division spindle.

Ribosomes- these are the smallest spherical granules, which are the site of protein synthesis from amino acids. They are found in the cells of all organisms. 2 subunits- large and small, formed from
ribosomal RNA molecules and proteins.

cytoskeleton-Elements of the cytoskeleton, closely associated with the outer cytoplasmic membrane and the nuclear membrane, form complex weaves in the cytoplasm. The cytoskeleton is formed microtubules and microfilaments, determines the shape of the cell, participates in its movements, in the division and movements of the cell itself, in the intracellular transport of organelles and individual compounds.

8. Organelles for special purposes. Their structure and functions.
Special-purpose organelles are present in cells specialized for a specific function, but can be found in small numbers in other cell types. They include, for example, microvilli of the absorptive surface of the intestinal epithelial cell, cilia of the epithelium of the trachea and bronchi, synaptic vesicles, transporting carriers of nervous excitation from one nerve cell to another or a cell of the working organ, myofibrils on which muscle contraction depends.

Cytoplasm- an obligatory part of the cell, enclosed between the plasma membrane and the nucleus; It is subdivided into hyaloplasm (the main substance of the cytoplasm), organelles (permanent components of the cytoplasm) and inclusions (temporary components of the cytoplasm). The chemical composition of the cytoplasm: the basis is water (60-90% of the total mass of the cytoplasm), various organic and inorganic compounds. The cytoplasm is alkaline. A characteristic feature of the cytoplasm of a eukaryotic cell is constant movement ( cyclosis). It is detected primarily by the movement of cell organelles, such as chloroplasts. If the movement of the cytoplasm stops, the cell dies, since only being in constant motion can it perform its functions.

Hyaloplasm ( cytosol) is a colorless, slimy, thick and transparent colloidal solution. It is in it that all metabolic processes take place, it provides the interconnection of the nucleus and all organelles. Depending on the predominance of the liquid part or large molecules in the hyaloplasm, two forms of hyaloplasm are distinguished: sol- more liquid hyaloplasm and gel- denser hyaloplasm. Mutual transitions are possible between them: the gel turns into a sol and vice versa.

Functions of the cytoplasm:

1. integration of all components of the cell into a single system,
2. environment for the passage of many biochemical and physiological processes,
3. environment for the existence and functioning of organelles.

Cell walls

Cell walls limit eukaryotic cells. At least two layers can be distinguished in each cell membrane. The inner layer is adjacent to the cytoplasm and is represented by plasma membrane(synonyms - plasmalemma, cell membrane, cytoplasmic membrane), over which the outer layer is formed. In an animal cell, it is thin and is called glycocalyx(formed by glycoproteins, glycolipids, lipoproteins), in a plant cell - thick, called cell wall(formed by cellulose).

All biological membranes have common structural features and properties. Currently generally accepted fluid mosaic model of the membrane structure. The basis of the membrane is a lipid bilayer, formed mainly by phospholipids. Phospholipids are triglycerides in which one fatty acid residue is replaced by a phosphoric acid residue; the section of the molecule in which the residue of phosphoric acid is located is called the hydrophilic head, the sections in which fatty acid residues are located are called hydrophobic tails. In the membrane, phospholipids are arranged in a strictly ordered manner: the hydrophobic tails of the molecules face each other, and the hydrophilic heads face outwards, towards the water.

In addition to lipids, the membrane contains proteins (on average ≈ 60%). They determine most of the specific functions of the membrane (transport of certain molecules, catalysis of reactions, receiving and converting signals from the environment, etc.). Distinguish: 1) peripheral proteins(located on the outer or inner surface of the lipid bilayer), 2) semi-integral proteins(immersed in the lipid bilayer to different depths), 3) integral or transmembrane proteins(permeate the membrane through and through, while in contact with both the external and internal environment of the cell). Integral proteins in some cases are called channel-forming, or channel, since they can be considered as hydrophilic channels through which polar molecules pass into the cell (the lipid component of the membrane would not let them through).

A - hydrophilic head of the phospholipid; C, hydrophobic tails of the phospholipid; 1 - hydrophobic regions of proteins E and F; 2, hydrophilic regions of protein F; 3 - a branched oligosaccharide chain attached to a lipid in a glycolipid molecule (glycolipids are less common than glycoproteins); 4 - branched oligosaccharide chain attached to a protein in a glycoprotein molecule; 5 - hydrophilic channel (functions as a pore through which ions and some polar molecules can pass).

The membrane may contain carbohydrates (up to 10%). The carbohydrate component of membranes is represented by oligosaccharide or polysaccharide chains associated with protein molecules (glycoproteins) or lipids (glycolipids). Basically, carbohydrates are located on the outer surface of the membrane. Carbohydrates provide receptor functions of the membrane. In animal cells, glycoproteins form an epimembrane complex, the glycocalyx, several tens of nanometers thick. Many cell receptors are located in it, with its help cell adhesion occurs.

Molecules of proteins, carbohydrates and lipids are mobile, able to move in the plane of the membrane. The thickness of the plasma membrane is approximately 7.5 nm.

Membrane functions

The membranes perform the following functions:

1. separation of cellular contents from the external environment,
2. regulation of metabolism between the cell and the environment,
3. division of the cell into compartments ("compartments"),
4. location of "enzymatic conveyors",
5. providing communication between cells in the tissues of multicellular organisms (adhesion),
6. signal recognition.

The most important membrane property- selective permeability, i.e. membranes are highly permeable to some substances or molecules and poorly permeable (or completely impermeable) to others. This property underlies the regulatory function of membranes, which ensures the exchange of substances between the cell and the external environment. The process by which substances pass through the cell membrane is called transport of substances. Distinguish: 1) passive transport- the process of passing substances, going without energy; 2) active transport- the process of passing substances, going with the cost of energy.

At passive transport substances move from an area with a higher concentration to an area with a lower one, i.e. along the concentration gradient. In any solution there are molecules of the solvent and the solute. The process of movement of solute molecules is called diffusion, the movement of solvent molecules is called osmosis. If the molecule is charged, then its transport is affected by the electrical gradient. Therefore, one often speaks of an electrochemical gradient, combining both gradients together. The speed of transport depends on the magnitude of the gradient.

The following types of passive transport can be distinguished: 1) simple diffusion- transport of substances directly through the lipid bilayer (oxygen, carbon dioxide); 2) diffusion through membrane channels- transport through channel-forming proteins (Na +, K +, Ca 2+, Cl -); 3) facilitated diffusion- transport of substances using special transport proteins, each of which is responsible for the movement of certain molecules or groups of related molecules (glucose, amino acids, nucleotides); 4) osmosis- transport of water molecules (in all biological systems, water is the solvent).

Need active transport occurs when it is necessary to ensure the transfer of molecules through the membrane against the electrochemical gradient. This transport is carried out by special carrier proteins, the activity of which requires energy expenditure. The energy source is ATP molecules. Active transport includes: 1) Na + /K + -pump (sodium-potassium pump), 2) endocytosis, 3) exocytosis.

Work Na + /K + -pump. For normal functioning, the cell must maintain a certain ratio of K + and Na + ions in the cytoplasm and in the external environment. The concentration of K + inside the cell should be significantly higher than outside it, and Na + - vice versa. It should be noted that Na + and K + can freely diffuse through the membrane pores. The Na+/K+ pump counteracts the equalization of these ion concentrations and actively pumps Na+ out of the cell and K+ into the cell. The Na + /K + -pump is a transmembrane protein capable of conformational changes, so that it can attach both K + and Na + . The cycle of operation of the Na + /K + -pump can be divided into the following phases: 1) attachment of Na + from the inside of the membrane, 2) phosphorylation of the pump protein, 3) release of Na + in the extracellular space, 4) attachment of K + from the outside of the membrane , 5) dephosphorylation of the pump protein, 6) release of K + in the intracellular space. The sodium-potassium pump consumes almost a third of all the energy necessary for the life of the cell. During one cycle of operation, the pump pumps out 3Na + from the cell and pumps in 2K +.

Endocytosis- the process of absorption by the cell of large particles and macromolecules. There are two types of endocytosis: 1) phagocytosis- capture and absorption of large particles (cells, cell parts, macromolecules) and 2) pinocytosis- capture and absorption of liquid material (solution, colloidal solution, suspension). The phenomenon of phagocytosis was discovered by I.I. Mechnikov in 1882. During endocytosis, the plasma membrane forms an invagination, its edges merge, and structures separated from the cytoplasm by a single membrane are laced into the cytoplasm. Many protozoa and some leukocytes are capable of phagocytosis. Pinocytosis is observed in the epithelial cells of the intestine, in the endothelium of blood capillaries.

Exocytosis- the reverse process of endocytosis: the removal of various substances from the cell. During exocytosis, the vesicle membrane fuses with the outer cytoplasmic membrane, the contents of the vesicle are removed outside the cell, and its membrane is included in the outer cytoplasmic membrane. In this way, hormones are excreted from the cells of the endocrine glands, and in protozoa, undigested food remains.

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