What substances make up the cytoplasmic membrane. cytoplasmic membrane

basis The plasmalemma, like other membranes in cells (for example, mitochondria, plastids, etc.), is a layer of lipids that has two rows of molecules (Fig. 1). Since lipid molecules are polar (one pole is hydrophilic, that is, it is attracted by water, and the other is hydrophobic, that is, it is repelled by water), they are arranged in a certain order. The hydrophilic ends of the molecules of one layer are directed towards the aqueous medium - into the cytoplasm of the cell, and the other layer - outward from the cell - towards the intercellular substance (in multicellular organisms) or the aqueous medium (in unicellular organisms).

Rice. 1. The structure of the cell membrane according to the liquidmosaic model. Proteins and glycoproteins are immersed in a doublea layer of lipid molecules facing their hydrophilicends (circles) outward, and hydrophobic (wavy lines) -deep into the membrane

They secrete peripheral proteins (they are located only over the inner or outer surface of the membrane), the integral nye (they are firmly embedded in the membrane, immersed in it, able to change their position depending on the state cells). Functions of membrane proteins: receptor, structural(support the shape of the cell), enzymatic, adhesive, antigenic, transport.

Protein molecules are mosaically embedded in the bimolecular layer of lipids. From the outside of the animal cell, polysaccharide molecules attach to lipids and plasma membrane protein molecules, forming glycolipids and glycoproteins.

This aggregate forms the glycocalyx layer. The receptor function of the plasmalemma is associated with it (see below); it can also accumulate various substances used by the cell. In addition, the glycocalyx enhances the mechanical stability of the plasmalemma.

In the cells of plants and fungi, there is also a cell wall that plays a supporting and protective role. In plants, it is made up of cellulose, while in fungi, it is made up of chitin.

The structural scheme of the elementary membrane is liquid-mosaic: fats make up a liquid-crystalline frame, and proteins are mosaically embedded in it and can change their position.

The most important function of the membrane: promotes compartmentation - underdivision of the contents of the cell into separate cells, differing in the details of the chemical or enzymatic composition. This achieves a high orderliness of the internal contents of any eukaryotic cell. Compartmentation contributes spatial separation of processes occurring in the cell ke. A separate compartment (cell) is represented by some membrane organelle (for example, a lysosome) or part of it (cristae delimited by the inner membrane of mitochondria).

Other features:

1) barrier (delimitation of the internal contents of the cell);

2) structural (giving a certain shape to cells inresponsibility for the functions performed);

3) protective (due to selective permeability, receptionand antigenicity of the membrane);

4) regulatory (regulation of selective permeability for various substances (passive transport without energy expenditure according to the laws of diffusion or osmosis and active transport with energy expenditure by pinocytosis, endo- and exocytosis, the operation of the sodium-potassium pump, phagocytosis)). Whole cells or large particles are engulfed by phagocytosis (for example, remember feeding in amoebas or phagocytosis by protective blood cells of bacteria). In pinocytosis, small particles or droplets of a liquid substance are absorbed. Common to both processes is that absorbed substances are surrounded by an invaginating outer membrane with the formation of a vacuole, which then moves into the depths of the cell cytoplasm. Exocytosis is a process (being also active transport) opposite in direction to phagocytosis and pinocytosis (Fig. 13). With its help, undigested food residues in protozoa or biologically active substances formed in the secretory cell can be removed.

5) adhesive function (all cells are interconnected through specific contacts (tight and loose));

6) receptor (due to the work of peripheral membrane proteins). There are non-specific receptors that perceive several stimuli (for example, cold and heat thermoreceptors), and specific ones that perceive only one stimulus (receptors of the light-perceiving system of the eye);

7) electrogenic (change in the electrical potential of the cell surface due to the redistribution of potassium and sodium ions (the membrane potential of nerve cells is 90 mV));

8) antigenic: associated with glycoproteins and membrane polysaccharides. On the surface of each cell there are protein molecules that are specific only for this type of cell. With their help, the immune system is able to distinguish between self and foreign cells. The exchange of substances between the cell and the environment is carried out in different ways - passive and active.

The cytoplasmic cell membrane consists of three layers:

External - protein;

Middle - bimolecular layer of lipids;

Internal - protein.

The membrane thickness is 7.5-10 nm. The bimolecular layer of lipids is the matrix of the membrane. The lipid molecules of its both layers interact with the protein molecules immersed in them. From 60 to 75% of membrane lipids are phospholipids, 15-30% cholesterol. Proteins are represented mainly by glycoproteins. Distinguish integral proteins spanning the entire membrane, and peripheral located on the outer or inner surface.

integral proteins form ion channels that provide the exchange of certain ions between the extra- and intracellular fluid. They are also enzymes that carry out antigradient transport of ions across the membrane.

Peripheral proteins are chemoreceptors on the outer surface of the membrane, which can interact with various physiologically active substances.

Membrane functions:

1. Ensures the integrity of the cell as a structural unit of the tissue.

Carries out the exchange of ions between the cytoplasm and extracellular fluid.

Provides active transport of ions and other substances into and out of the cell.

Produces the perception and processing of information coming to the cell in the form of chemical and electrical signals.

Mechanisms of cell excitability. History of the study of bioelectric phenomena.

Basically, the information transmitted in the body is in the form of electrical signals (for example, nerve impulses). The presence of animal electricity was first established by the naturalist (physiologist) L. Galvani in 1786. In order to study atmospheric electricity, he hung neuromuscular preparations of frog legs on a copper hook. When these paws touched the iron railing of the balcony, the muscles contracted. This indicated the action of some kind of electricity on the nerve of the neuromuscular preparation. Galvani considered that this was due to the presence of electricity in the living tissues themselves. However, A. Volta found that the source of electricity is the place of contact of two dissimilar metals - copper and iron. In physiology Galvani's first classical experience it is considered to touch the nerve of the neuromuscular preparation with bimetallic tweezers made of copper and iron. To prove his case, Galvani produced second experience. He threw the end of the nerve innervating the neuromuscular preparation over the incision of his muscle. The result was a contraction. However, this experience did not convince Galvani's contemporaries. Therefore, another Italian Matteuchi made the following experiment. He superimposed the nerve of one neuromuscular frog preparation on the muscle of the second, which contracted under the influence of an irritating current. As a result, the first drug also began to decline. This indicated the transfer of electricity (action potential) from one muscle to another. The presence of a potential difference between damaged and undamaged parts of the muscle was first accurately established in the 19th century using a string galvanometer (ammeter) by Matteuchi. Moreover, the cut had a negative charge, and the surface of the muscle was positive.

The cytoplasmic membrane separating the cytoplasm from the cell wall is called the plasmalemma (plasma membrane), and the one separating it from the vacuole is called the tonoplast (elementary membrane).

Currently, the fluid mosaic model of the membrane is used (Fig. 1.9), according to which the membrane consists of a bilayer of lipid molecules (phospholipids) with hydrophilic heads and 2 hydrophobic tails facing the inside of the layer. In addition to lipids, membranes also contain proteins.

There are 3 types of membrane proteins "floating" in the bilayer: integral proteins penetrating the entire thickness of the bilayer; semi-integral, penetrating the bilayer incompletely; peripheral, attached from the outer or inner side of the membrane to other membrane proteins. Membrane proteins perform various functions: some of them are enzymes, others act as carriers of specific molecules across the membrane or form hydrophilic pores through which polar molecules can pass.

One of the main properties of cell membranes is their semipermeability: they pass water, but do not pass substances dissolved in it, i.e., they have selective permeability.

Rice. 1.9. Scheme of the structure of a biological membrane:

A - extracellular space; B - cytoplasm; 1 - bimolecular layer of lipids; 2 - peripheral protein; 3 - hydrophilic region of the integral protein; 4 - hydrophobic region of the integral protein; 5 - carbohydrate chain

Transport across membranes

Depending on the energy costs, the transport of substances and ions through the membrane is divided into passive, which does not require energy, and active, associated with energy consumption. Passive transport includes such processes as diffusion, facilitated diffusion, osmosis.

Diffusion- this is the process of penetration of molecules through the lipid bilayer along a concentration gradient (from an area of ​​\u200b\u200bhigher concentration to a lower one). The smaller the molecule and the more non-polar, the faster it diffuses through the membrane.

With facilitated diffusion, the passage of a substance through the membrane is assisted by a transport protein. Thus, various polar molecules enter the cell, such as sugars, amino acids, nucleotides, etc.

Osmosis is the diffusion of water through semi-permeable membranes. Osmosis causes water to move from a high water potential solution to a low water potential solution.

active transport- this is the transfer of molecules and ions through the membrane, accompanied by energy costs. Active transport goes against the concentration gradient and the electrochemical gradient and uses the energy of ATP. The mechanism of active transport of substances is based on the work of the proton pump (H + and K +) in plants and fungi, which maintain a high concentration of K + inside the cell and a low concentration of H + (Na + and K + - in animals). The energy needed to run this pump comes in the form of ATP, which is synthesized during cellular respiration.

Another type of active transport is known - endo- and exocytosis. These are 2 active processes by which various molecules are transported across the membrane into the cell ( endocytosis) or from it ( exocytosis).

During endocytosis, substances enter the cell as a result of invagination (invagination) of the plasma membrane. The resulting vesicles, or vacuoles, are transferred to the cytoplasm along with the substances contained in them. The ingestion of large particles, such as microorganisms or cell debris, is called phagocytosis. In this case, large bubbles are formed, called vacuoles. The absorption of liquids (suspensions, colloidal solutions) or solutes with the help of small bubbles is called pinocytosis.

The reverse process to endocytosis is called exocytosis. Many substances are removed from the cell in special vesicles or vacuoles. An example is the withdrawal from the secretory cells of their liquid secrets; another example is the participation of dictyosome vesicles in the formation of the cell wall.

PROTOPLAST DERIVATIVES

Vacuole

Vacuole is a reservoir bounded by a single membrane - the tonoplast. The vacuole contains cell sap - a concentrated solution of various substances, such as mineral salts, sugars, pigments, organic acids, enzymes. In mature cells, the vacuoles merge into one central vacuole.

Various substances are stored in vacuoles, including end products of metabolism. The osmotic properties of the cell depend to a large extent on the content of the vacuole.

Due to the fact that vacuoles contain strong solutions of salts and other substances, plant cells constantly absorb water osmotically and create hydrostatic pressure on the cell wall, called turgor pressure. Turgor pressure is opposed by an equal pressure of the cell wall, directed inside the cell. Most plant cells exist in a hypotonic environment. But if such a cell is placed in a hypertonic solution, water will begin to leave the cell according to the laws of osmosis (to equalize the water potential on both sides of the membrane). At the same time, the vacuole will shrink in volume, its pressure on the protoplast will decrease, and the membrane will begin to move away from the cell wall. The phenomenon of protoplast detachment from the cell wall is called plasmolysis. Under natural conditions, such a loss of turgor in the cells will lead to the withering of the plant, the lowering of leaves and stems. However, this process is reversible: if the cell is placed in water (for example, when watering a plant), a phenomenon occurs that is the opposite of plasmolysis - deplasmolysis (see Fig. 1.10).

Rice. 1.10. Plasmolysis scheme:

A - cell in a state of turgor (in isotonic solution); B - beginning of plasmolysis (cell placed in 6% KNO3 solution); C - complete plasmolysis (cell placed in 10% KNO3 solution); 1 - chloroplast; 2 - core; 3 - cell wall; 4 - protoplast; 5 - central vacuole

Inclusions

Cellular inclusions are reserve and excretory substances.

Spare substances (temporarily excluded from metabolism) and, together with them, waste products (excretory substances) are often called ergastic substances of the cell. Storage substances include storage proteins, fats and carbohydrates. These substances accumulate during the growing season in seeds, fruits, underground organs of the plant and in the core of the stem.

Spare substances

Spare proteins, related to simple proteins - proteins, are more often deposited in seeds. Settling proteins in vacuoles form round or elliptical grains called aleurone. If the aleurone grains do not have a noticeable internal structure and consist of an amorphous protein, they are called simple. If a crystal-like structure (crystalloid) and shiny colorless bodies of a rounded shape (globoids) are found in aleurone grains among amorphous protein, such aleurone grains are called complex (see Fig. 1.11). The amorphous protein of the aleurone grain is represented by a homogeneous, opaque yellowish protein that swells in water. Crystalloids have a rhombohedral shape characteristic of crystals, but unlike true crystals, their protein swells in water. Globoids consist of a calcium-magnesium salt, contain phosphorus, are insoluble in water and do not react to proteins.

Rice. 1.11. Complex aleurone grains:

1 - pores in the shell; 2 - globoids; 3 - amorphous protein mass; 4 - crystalloids immersed in amphora protein mass

Spare lipids are usually located in the hyaloplasm in the form of droplets and are found in almost all plant cells. This is the main type of reserve nutrients for most plants: seeds and fruits are the richest in them. Fats (lipids) are the most high-calorie reserve substance. The reagent for fat-like substances is Sudan III, which turns them orange.

Carbohydrates are part of each cell in the form of water-soluble sugars (glucose, fructose, sucrose) and water-insoluble polysaccharides (cellulose, starch). In the cell, carbohydrates play the role of an energy source for metabolic reactions. Sugars, binding with other biological substances of the cell, form glycosides, and polysaccharides with proteins form glycoproteins. The composition of plant cell carbohydrates is much more diverse than that of animal cells, due to the diverse composition of cell wall polysaccharides and vacuole cell sap sugars.

The main and most common storage carbohydrate is the polysaccharide starch. Primary assimilation starch is formed in chloroplasts. At night, when photosynthesis stops, starch is hydrolyzed to sugars and transported to storage tissues - tubers, bulbs, rhizomes. There, in special types of leukoplasts - amyloplasts - part of the sugars are deposited in the form of grains of secondary starch. For starch grains, lamination is characteristic, which is explained by the different water content due to the uneven supply of starch during the day. There is more water in dark layers than in light ones. A grain with one center of starch formation in the center of the amyloplast is called simple concentric, if the center is displaced - simple eccentric. Grain with several starch-forming centers is complex. In semi-complex grains, new layers are deposited around several starch centers, and then common layers form and cover the starch centers (see Fig. 1.12). The reagent for starch is a solution of iodine, which gives a blue color.

Rice. 1.12. Starch grains of potatoes (A):

1 - simple grain; 2 - semi-complex; 3 - complex; wheat (B), oats (C)

Excretory substances (secondary metabolic products)

Cellular inclusions also include excretory substances, such as calcium oxalate crystals ( single crystals, rafid - needle-shaped crystals, druses - intergrowths of crystals, crystalline sand - an accumulation of many small crystals) (see Fig. 1.13). More rarely, the crystals are composed of calcium carbonate or silica ( cystoliths; see fig. 1.14). Cystoliths are deposited on the cell wall, protruding into the cell in the form of bunches of grapes, and are typical, for example, for representatives of the nettle family, ficus leaves.

Unlike animals that excrete excess salts with urine, plants do not have developed excretory organs. Therefore, it is believed that calcium oxalate crystals are the end product of protoplast metabolism, which is formed as a device for removing excess calcium from metabolism. As a rule, these crystals accumulate in the organs that the plant periodically sheds (leaves, bark).

Rice. 1.13. Forms of calcium oxalate crystals in cells:

1, 2 - rafida (touchy; 1 - side view, 2 - in a cross section); 3 - druse (opuntia); 4 - crystalline sand (potatoes); 5 - single crystal (vanilla)

Rice. 1.14. Cystolith (on a cross section of a ficus leaf):

1 - leaf skin; 2 - cystolite

Essential oils accumulate in leaves (mint, lavender, sage), flowers (rose hips), fruits (citrus) and plant seeds (dill, anise). Essential oils do not take part in metabolism, but they are widely used in perfumery (rose, jasmine oils), food industry (anise, dill oils), medicine (mint, eucalyptus oils). Glands (mint), lysigenic receptacles (citrus), glandular hairs (geranium) can be reservoirs for the accumulation of essential oils.

resins- These are complex compounds formed in the course of normal life activity or as a result of tissue destruction. They are formed by the epithelial cells lining the resin ducts as a by-product of metabolism, often with essential oils. They can accumulate in cell sap, cytoplasm in the form of drops or in receptacles. They are insoluble in water, impermeable to microorganisms and, thanks to their antiseptic properties, increase the resistance of plants to diseases. Resins are used in medicine, as well as in the manufacture of paints, varnishes and lubricating oils. In modern industry, they are replaced by synthetic materials.

cell wall

The rigid cell wall surrounding the cell consists of cellulose microfibrils immersed in a matrix, which includes hemicelluloses and pectin substances. The cell wall provides mechanical support to the cell, protection of the protoplast and preservation of the shape of the cell. In this case, the cell wall is capable of stretching. Being a product of the vital activity of the protoplast, the wall can grow only in contact with it. Water and mineral salts move through the cell wall, but for macromolecular substances it is completely or partially impermeable. When the protoplast dies, the wall can continue to perform the function of conducting water. The presence of a cell wall, more than any other feature, distinguishes plant cells from animals. Cellulose largely determines the architecture of the cell wall. The monomer of cellulose is glucose. The bundles of cellulose molecules form micelles, which combine into larger bundles - microfibrils. The reagent for cellulose is chlorine-zinc-iodine (Cl-Zn-I), which gives a blue-violet color.

The cellulose scaffold of the cell wall is filled with non-cellulose matrix molecules. The matrix contains polysaccharides called hemicelluloses; pectin substances (pectin), very close to hemicelluloses, and glycoproteins. Pectic substances, merging between neighboring cells, form a median plate, which is located between the primary membranes of neighboring cells. When the middle plate is dissolved or destroyed (which occurs in the pulp of ripe fruits), maceration occurs (from Latin maceratio - softening). Natural maceration can be observed in many overripe fruits (watermelon, melon, peach). Artificial maceration (when treating tissues with alkali or acid) is used to prepare various anatomical and histological preparations.

The cell wall in the process of life can undergo various modifications - lignification, corking, sliming, cutinization, mineralization (see Table l.4).

Table 1.4.

Similar information.

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. In each cell membrane, at least two layers can be distinguished. 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).

Necessity 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 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 the structures delimited 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.

Go to lectures number 5"Cell Theory. Types of cellular organization»

Go to lectures number 7"Eukaryotic cell: structure and functions of organelles"

cytoplasmic membrane or plasmalemma(lat. membrana - skin, film) - the thinnest film ( 7– 10nm), delimiting the internal contents of the cell from the environment, is visible only in an electron microscope.

By chemical organization plasmalemma is a lipoprotein complex - molecules lipids And proteins.

It is based on a lipid bilayer consisting of phospholipids, in addition, glycolipids and cholesterol are present in the membranes. All of them have the property of amphipatricity, i.e. they have hydrophilic ("water-loving") and hydrophobic ("water-fearing") ends. Hydrophilic polar "heads" of lipid molecules (phosphate group) face the outside of the membrane, and hydrophobic non-polar "tails" (fatty acid residues) face each other, which creates a bipolar lipid layer. Lipid molecules are mobile and can move in their monolayer or rarely - from one monolayer to another. Lipid monolayers are asymmetric, i.e., they differ in lipid composition, which gives specificity to membranes even within the same cell. The lipid bilayer can be in the state of a liquid or solid crystal.

Proteins are the second essential component of the plasmalemma. Many membrane proteins are able to move in the plane of the membrane or rotate around their axis, but cannot move from one side of the lipid bilayer to the other.

Lipids provide the basic structural features of the membrane, while proteins provide its functions.

The functions of membrane proteins are different: maintaining the structure of membranes, receiving and converting signals from the environment, transporting certain substances, catalyzing reactions occurring on membranes.

There are several models of the structure of the cytoplasmic membrane.

①. SANDWICH MODEL(squirrels– lipids– proteins)

IN 1935 English scientists Danieli And Dawson expressed the idea of ​​a layer-by-layer arrangement in the membrane of protein molecules (dark layers in an electron microscope), which lie outside, and lipid molecules (light layer) - inside . For a long time there was an idea of ​​a single three-layer structure of all biological membranes.

A detailed study of the membrane using an electron microscope turned out that the light layer is actually represented by two layers of phospholipids - this lipid layer, and its water-soluble parts are hydrophilic heads directed to the protein layer, and insoluble (fatty acid residues) - hydrophobic tails facing each other.

②. LIQUID MOSAIC MODEL

IN 1972.Singer And Nicholson described a model of the membrane that has gained wide acceptance. According to this model, protein molecules do not form a continuous layer, but are immersed in the bipolar lipid layer at different depths in the form of a mosaic. Globules of protein molecules, like icebergs, are immersed in the "ocean"

lipids: some are located on the surface of the bilipid layer - peripheral proteins, others are half immersed in it - semi-integral proteins, third - integral proteins- penetrate it through and through, forming hydrophilic pores. Peripheral proteins, being on the surface of the lipid layer, are associated with the heads of lipid molecules by electrostatic interactions. But they never form a continuous layer and, in fact, are not the proteins of the membrane itself, but rather connect it with the supra-membrane or sub-membrane system of the cell surface apparatus.

The main role in the organization of the membrane itself is played by integral and semi-integral (transmembrane) proteins, which have a globular structure and are associated with the lipid phase by hydrophilic-hydrophobic interactions. Protein molecules, like lipids, are amphipatric and their hydrophobic regions interact with the hydrophobic tails of the bilipid layer, while the hydrophilic regions face the aquatic environment and form hydrogen bonds with water.

③. PROTEIN-CRYSTAL MODEL(lipoprotein mat model)

Membranes are formed by interweaving of lipid and protein molecules, which are combined with each other on the basis of hydrophilic

hydrophobic interactions.

Protein molecules, like pins, penetrate the lipid layer and perform the function of a framework in the membrane. After treatment of the membrane with fat-soluble substances, the protein framework is preserved, which proves the relationship between protein molecules in the membrane. Apparently, this model is implemented only in certain special areas of some membranes, where a rigid structure and close stable relationships between lipids and proteins are required (for example, in the region where the enzyme Na-K-ATP-ases).

The most universal model that meets thermodynamic principles (principles of hydrophilic-hydrophobic interactions), morpho-biochemical and experimental
Antal-cytological data is a fluid-mosaic model. However, all three models of membranes are not mutually exclusive and can occur in different regions of the same membrane, depending on the functional features of this region.

MEMBRANE PROPERTIES

1. Ability to self-assemble. After destructive influences, the membrane is able to restore its structure, because. lipid molecules, based on their physicochemical properties, are assembled into a bipolar layer, into which protein molecules are then embedded.

2. Fluidity. The membrane is not a rigid structure, most of its proteins and lipids can move in the plane of the membrane, they constantly fluctuate due to rotational and oscillatory movements. This determines the high rate of chemical reactions on the membrane.

3. Semipermeability. The membranes of living cells pass, in addition to water, only certain molecules and ions of dissolved substances. This ensures the maintenance of the ionic and molecular composition of the cell.

4. The membrane has no loose ends. It always closes in bubbles.

5. asymmetry. The composition of the outer and inner layers of both proteins and lipids is different.

6. Polarity. The outer side of the membrane carries a positive charge, while the inner side carries a negative charge.

MEMBRANE FUNCTIONS

1) Barrier - The plasmalemma separates the cytoplasm and nucleus from the external environment. In addition, the membrane divides the internal contents of the cell into sections (compartments), in which opposite biochemical reactions often occur.

2) Receptor(signal) - due to the important property of protein molecules - denaturation, the membrane is able to capture various changes in the environment. So, when a cell membrane is exposed to various environmental factors (physical, chemical, biological), the proteins that make up its composition change their spatial configuration, which serves as a kind of signal for the cell.

This provides communication with the external environment, cell recognition and their orientation during tissue formation, etc. This function is associated with the activity of various regulatory systems and the formation of an immune response.

3) exchange- the membrane contains not only structural proteins that form it, but also enzymatic proteins that are biological catalysts. They are located on the membrane in the form of a "catalytic conveyor" and determine the intensity and direction of metabolic reactions.

4) Transport– molecules of substances whose diameter does not exceed 50 nm can penetrate through passive and active transport through the pores in the membrane structure. Large substances enter the cell by endocytosis(transport in membrane packaging), requiring energy consumption. Its varieties are phage and pinocytosis.

Passive transport - a mode of transport in which the transfer of substances is carried out along a gradient of chemical or electrochemical concentration without the expenditure of ATP energy. There are two types of passive transport: simple and facilitated diffusion. Diffusion- this is the transfer of ions or molecules from a zone of their higher concentration to a zone of lower concentration, i.e. along the gradient.

simple diffusion- salt ions and water penetrate through transmembrane proteins or fat-soluble substances along a concentration gradient.

Facilitated diffusion- specific carrier proteins bind the substance and transfer it through the membrane according to the "ping-pong" principle. In this way, sugars and amino acids pass through the membrane. The rate of such transport is much higher than that of simple diffusion. In addition to carrier proteins, some antibiotics, such as gramitidin and vanomycin, are involved in facilitated diffusion.

Because they provide ion transport, they are called ionophores.

Active transport is a mode of transport in which the energy of ATP is consumed, it goes against the concentration gradient. It involves the enzymes ATPase. The outer cell membrane contains ATPases, which transport ions against a concentration gradient, a phenomenon called the ion pump. An example is the sodium-potassium pump. Normally, there are more potassium ions in the cell, and sodium ions in the external environment. Therefore, according to the laws of simple diffusion, potassium tends to leave the cell, and sodium enters the cell. In contrast, the sodium-potassium pump pumps potassium ions into the cell against a concentration gradient, and carries sodium ions into the external environment. This allows maintaining the constancy of the ionic composition in the cell and its viability. In an animal cell, one third of ATP is used to operate the sodium-potassium pump.

A type of active transport is membrane-packed transport. endocytosis. Large molecules of biopolymers cannot penetrate the membrane; they enter the cell in a membrane package. Distinguish between phagocytosis and pinocytosis. Phagocytosis- the capture of solid particles by the cell, pinocytosis- liquid particles. These processes are divided into stages:

1) recognition by membrane receptors of a substance; 2) invagination (invagination) of the membrane with the formation of a vesicle (vesicle); 3) detachment of the vesicle from the membrane, its fusion with the primary lysosome and restoration of the integrity of the membrane; 4) release of undigested material from the cell (exocytosis).

Endocytosis is a way of feeding for protozoa. Mammals and humans have a reticulo-histio-endothelial system of cells capable of endocytosis - these are leukocytes, macrophages, Kupffer cells in the liver.

OSMOTIC PROPERTIES OF THE CELL

Osmosis- one-way process of water penetration through a semi-permeable membrane from a region with a lower solution concentration to a region with a higher concentration. Osmosis determines osmotic pressure.

Dialysis– unilateral diffusion of dissolved substances.

A solution in which the osmotic pressure is the same as in cells is called isotonic. When a cell is immersed in an isotonic solution, its volume does not change. An isotonic solution is called physiological- This is a 0.9% sodium chloride solution, which is widely used in medicine for severe dehydration and loss of blood plasma.

A solution whose osmotic pressure is higher than in cells is called hypertonic.

Cells in a hypertonic solution lose water and shrivel. Hypertonic solutions are widely used in medicine. A gauze bandage soaked in a hypertonic solution absorbs pus well.

A solution where the concentration of salts is lower than in the cell is called hypotonic. When a cell is immersed in such a solution, water rushes into it. The cell swells, its turgor increases, and it can collapse. Hemolysis- destruction of blood cells in a hypotonic solution.

Osmotic pressure in the human body as a whole is regulated by the system of excretory organs.

Previous123456789Next

VIEW MORE:

cell membrane also called plasma (or cytoplasmic) membrane and plasmalemma. This structure not only separates the internal contents of the cell from the external environment, but also enters into the composition of most cell organelles and the nucleus, in turn separating them from the hyaloplasm (cytosol) - the viscous-liquid part of the cytoplasm. Let's agree to call cytoplasmic membrane one that separates the contents of the cell from the external environment. The remaining terms refer to all membranes.

The structure of the cell membrane

The basis of the structure of the cell (biological) membrane is a double layer of lipids (fats). The formation of such a layer is associated with the features of their molecules. Lipids do not dissolve in water, but condense in it in their own way. One part of a single lipid molecule is a polar head (it is attracted by water, i.e., hydrophilic), and the other is a pair of long non-polar tails (this part of the molecule is repelled by water, i.e., hydrophobic). This structure of the molecules makes them "hide" their tails from the water and turn their polar heads towards the water.

As a result, a lipid bilayer is formed, in which the non-polar tails are inside (facing each other), and the polar heads are facing out (to the external environment and cytoplasm). The surface of such a membrane is hydrophilic, but inside it is hydrophobic.

In cell membranes, phospholipids predominate among lipids (they are complex lipids). Their heads contain a residue of phosphoric acid. In addition to phospholipids, there are glycolipids (lipids + carbohydrates) and cholesterol (belongs to sterols). The latter gives the membrane rigidity, being located in its thickness between the tails of the remaining lipids (cholesterol is completely hydrophobic).

Due to electrostatic interaction, certain protein molecules are attached to the charged heads of lipids, which become surface membrane proteins. Other proteins interact with non-polar tails, partially sink into the bilayer, or penetrate it through and through.

Thus, the cell membrane consists of a bilayer of lipids, surface (peripheral), immersed (semi-integral), and penetrating (integral) proteins. In addition, some proteins and lipids on the outside of the membrane are associated with carbohydrate chains.

This fluid mosaic model of the membrane structure was put forward in the 70s of the XX century. Prior to this, a sandwich model of the structure was assumed, according to which the lipid bilayer is located inside, and on the inside and outside the membrane is covered with continuous layers of surface proteins. However, the accumulation of experimental data disproved this hypothesis.

The thickness of membranes in different cells is about 8 nm. Membranes (even different sides of one) differ from each other in the percentage of different types of lipids, proteins, enzymatic activity, etc. Some membranes are more liquid and more permeable, others are more dense.

Breaks in the cell membrane easily merge due to the physicochemical characteristics of the lipid bilayer. In the plane of the membrane, lipids and proteins (unless they are fixed by the cytoskeleton) move.

Functions of the cell membrane

Most of the proteins immersed in the cell membrane perform an enzymatic function (they are enzymes). Often (especially in the membranes of cell organelles) enzymes are arranged in a certain sequence so that the reaction products catalyzed by one enzyme pass to the second, then the third, etc. A conveyor is formed that stabilizes surface proteins, because they do not allow enzymes to swim along the lipid bilayer.

The cell membrane performs a delimiting (barrier) function from the environment and at the same time a transport function. It can be said that this is its most important purpose. The cytoplasmic membrane, having strength and selective permeability, maintains the constancy of the internal composition of the cell (its homeostasis and integrity).

In this case, the transport of substances occurs in various ways. Transport along a concentration gradient involves the movement of substances from an area with a higher concentration to an area with a lower one (diffusion). So, for example, gases diffuse (CO 2, O 2).

There is also transport against the concentration gradient, but with the expenditure of energy.

Transport is passive and lightweight (when it is helped by some kind of transference).
To). Passive diffusion across the cell membrane is possible for fat-soluble substances.

There are special proteins that make membranes permeable to sugars and other water-soluble substances. These carriers bind to transported molecules and drag them across the membrane.

3. Functions and structure of the cytoplasmic membrane

This is how glucose is transported into the red blood cells.

Spanning proteins, when combined, can form a pore for the movement of certain substances through the membrane. Such carriers do not move, but form a channel in the membrane and work similarly to enzymes, binding a specific substance. The transfer is carried out due to a change in the conformation of the protein, due to which channels are formed in the membrane. An example is the sodium-potassium pump.

The transport function of the eukaryotic cell membrane is also realized through endocytosis (and exocytosis). Thanks to these mechanisms, large molecules of biopolymers, even whole cells, enter the cell (and out of it). Endo- and exocytosis are not characteristic of all eukaryotic cells (prokaryotes do not have it at all). So endocytosis is observed in protozoa and lower invertebrates; in mammals, leukocytes and macrophages absorb harmful substances and bacteria, i.e., endocytosis performs a protective function for the body.

Endocytosis is divided into phagocytosis(cytoplasm envelops large particles) and pinocytosis(capture of liquid droplets with substances dissolved in it). The mechanism of these processes is approximately the same. Absorbed substances on the cell surface are surrounded by a membrane. A vesicle (phagocytic or pinocytic) is formed, which then moves into the cell.

Exocytosis is the removal of substances from the cell by the cytoplasmic membrane (hormones, polysaccharides, proteins, fats, etc.). These substances are enclosed in membrane vesicles that fit the cell membrane. Both membranes merge and the contents are outside the cell.

The cytoplasmic membrane performs a receptor function. To do this, on its outer side there are structures that can recognize a chemical or physical stimulus. Some of the proteins penetrating the plasmalemma are externally connected to polysaccharide chains (forming glycoproteins). These are peculiar molecular receptors that capture hormones. When a particular hormone binds to its receptor, it changes its structure. This, in turn, triggers the cellular response mechanism. At the same time, channels can open, and certain substances can begin to enter the cell or be removed from it.

The receptor function of cell membranes has been well studied based on the action of the hormone insulin. When insulin binds to its glycoprotein receptor, the catalytic intracellular part of this protein (the enzyme adenylate cyclase) is activated. The enzyme synthesizes cyclic AMP from ATP. Already it activates or inhibits various enzymes of cellular metabolism.

The receptor function of the cytoplasmic membrane also includes the recognition of neighboring cells of the same type. Such cells are attached to each other by various intercellular contacts.

In tissues, with the help of intercellular contacts, cells can exchange information with each other using specially synthesized low molecular weight substances. One example of such an interaction is contact inhibition, when cells stop growing after receiving information that the free space is occupied.

Intercellular contacts are simple (membranes of different cells are adjacent to each other), locking (invagination of the membrane of one cell into another), desmosomes (when the membranes are connected by bundles of transverse fibers penetrating into the cytoplasm). In addition, there is a variant of intercellular contacts due to mediators (intermediaries) - synapses. In them, the signal is transmitted not only chemically, but also electrically. Synapses transmit signals between nerve cells, as well as from nerve to muscle.

cell theory

In 1665, R. Hooke, examining a cut of a tree cork under a microscope, found empty cells, which he called "cells". He saw only the shells of plant cells, and for a long time the shell was considered the main structural component of the cell. In 1825 J. Purkinė described the protoplasm of cells, and in 1831 R. Brown described the nucleus. In 1837, M. Schleiden came to the conclusion that plant organisms consist of cells, and each cell contains a nucleus.

1.1. Using the data accumulated by that time, T.

The cytoplasmic membrane, its functions and structure

Schwann in 1839 formulated the main provisions of the cell theory:

1) the cell is the basic structural unit of plants and animals;

2) the process of cell formation determines the growth, development and differentiation of organisms.

In 1858, R. Virchow, the founder of pathological anatomy, supplemented the cell theory with the important position that a cell can only come from a cell (Omnis cellula e cellula) as a result of its division. He found that the basis of all diseases are changes in the structure and function of cells.

1.2. Modern cell theory includes the following provisions:

1) cell - the main structural, functional and genetic unit of living organisms, the smallest unit of living things;

2) the cells of all unicellular and multicellular organisms are similar in structure, chemical composition and the most important manifestations of life processes;

3) each new cell is formed as a result of the division of the original (mother) cell;

4) cells of multicellular organisms are specialized: they perform different functions and form tissues;

5) the cell is an open system through which flows of matter, energy and information pass and are transformed

The structure and functions of the cytoplasmic membrane

The cell is an open self-regulating system through which there is a constant flow of matter, energy and information. These flows are received special apparatus cells that include:

1) supramembranous component - glycocalyx;

2) elementary biological membrane or their complex;

3) submembrane support-contractile complex of hyaloplasm;

4) anabolic and catabolic systems.

The main component of this apparatus is the elementary membrane.

The cell contains various types of membranes, but the principle of their structure is the same.

In 1972, S. Singer and G. Nicholson proposed a fluid-mosaic model of the elementary membrane structure. According to this model, it is also based on the bilipid layer, but the proteins are located differently in relation to this layer. Some protein molecules lie on the surface of the lipid layers (peripheral proteins), some penetrate one lipid layer (semi-integral proteins), and some penetrate both lipid layers (integral proteins). The lipid layer is in the liquid phase ("lipid sea"). On the outer surface of the membranes there is a receptor apparatus - the glycocalyx, formed by branched molecules of glycoproteins, which "recognizes" certain substances and structures.

2.3. Membrane properties: 1) plasticity, 2) semi-permeability, 3) self-closing ability.

2.4. Functions of membranes: 1) structural - the membrane as a structural component is part of most organelles (membrane principle of the structure of organelles); 2) barrier and regulatory - maintains the constancy of the chemical composition and regulates all metabolic processes (metabolic reactions occur on membranes); 3) protective; 4) receptor.