What substances make up the cytoplasmic membrane. Cytoplasmic membrane

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

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

They secrete peripheral proteins (they are located only along the inner or outer surface of the membrane), integral ny (they are firmly built into the membrane, immersed in it, capable of changing their position depending on the state cells). Functions of membrane proteins: receptor, structural(maintain cell shape), enzymatic, adhesive, antigenic, transport.

Protein molecules are mosaically embedded in a bimolecular layer of lipids. On the outside of the animal cell, polysaccharide molecules are attached to the lipids and protein molecules of the plasmalemma, forming glycolipids and glycoproteins.

This aggregate forms the glycocalyx layer. The receptor function of the plasma membrane 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 consists of cellulose, and in fungi it is made of chitin.

The structure of the elementary membrane is liquid-mosaic: fats make up a liquid-crystalline frame, and proteins are mosaically built into it and can change their position.

The most important function of the membrane: promotes compartmentation - underdividing the contents of a cell into separate cells that differ in the details of their chemical or enzymatic composition. This achieves high orderliness of the internal contents of any eukaryotic cell. Compartmentation promotes 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 mitochondrial membrane).

Other features:

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

2) structural (giving a certain shape to cells in conjunction withresponsibility with the functions performed);

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

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

5) adhesive function (all cells are connected to each other through specific contacts (tight and loose));

6) receptor (due to the work of peripheral membrane proteins). There are nonspecific 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 polysaccharides of the membrane. On the surface of each cell there are protein molecules that are specific only to this type of cell. With their help, the immune system is able to distinguish between its own and foreign cells. Metabolism 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.

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

Integral proteins form ion channels that ensure the exchange of certain ions between extra- and intracellular fluid. They are also enzymes that carry out counter-gradient transport of ions across the membrane.

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

Membrane functions:

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

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

Mechanisms of cell excitability. History of research into bioelectric phenomena.

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

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

Currently, they use the liquid mosaic model of the membrane (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 bilipid layer: integral proteins that penetrate 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 semi-permeability: they allow water to pass through, but do not allow substances dissolved in it to pass through, 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 energy expenditure, 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 processes such as diffusion, facilitated diffusion, and osmosis.

Diffusion is the process of penetration of molecules through a lipid bilayer along a concentration gradient (from an area of ​​higher concentration to an area of ​​lower). The smaller the molecule and the more nonpolar, the faster it diffuses through the membrane.

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

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

Active transport- this is the transfer of molecules and ions through a 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+ and a low concentration of H+ inside the cell (Na+ and K+ in animals). The energy required to operate this pump is supplied in the form of ATP, 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 transported into the cytoplasm along with the substances contained in them. The absorption of large particles, such as microorganisms or cell debris, is called phagocytosis. In this case, large bubbles called vacuoles are formed. The absorption of liquids (suspensions, colloidal solutions) or solutes with the help of small bubbles is called pinocytosis.

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

PROTOPLAST DERIVATIVES

Vacuole

Vacuole- This 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 one.

Vacuoles store various substances, including metabolic end products. The osmotic properties of the cell strongly depend on the contents 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 from the cell wall, directed into 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). 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 cells will lead to withering of the plant, drooping of leaves and stems. However, this process is reversible: if a 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 an isotonic solution); B - beginning of plasmolysis (cell placed in a 6% KNO3 solution); B - complete plasmolysis (cell placed in a 10% KNO3 solution); 1 - chloroplast; 2 - core; 3 - cell wall; 4 - protoplast; 5 - central vacuole

Inclusions

Cellular inclusions are storage and excretory substances.

Reserve substances (temporarily excluded from metabolism) and with them waste (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 plant organs and in the core of the stem.

Spare substances

Storage proteins, related to simple proteins - proteins, are often deposited in seeds. Precipitated proteins in vacuoles form round or elliptical grains called aleurone. If aleurone grains have no discernible internal structure and are composed of amorphous protein, they are called simple. If in aleurone grains a crystal-like structure (crystalloid) and shiny, colorless, round-shaped bodies (globoids) are found among the amorphous protein, such aleurone grains are called complex (see Fig. 1.11). The amorphous protein of the aleurone grain is a homogeneous, opaque, yellowish protein that swells in water. Crystalloids have the characteristic rhombohedral shape of crystals, but unlike true crystals, their constituent protein swells in water. Globoids consist of a calcium-magnesium salt, contain phosphorus, are insoluble in water and do not react with proteins.

Rice. 1.11. Complex aleurone grains:

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

Storage lipids 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 in most plants: seeds and fruits are richest in them. Fats (lipids) are the most caloric reserve substance. The reagent for fat-like substances is Sudan III, which colors them orange.

Carbohydrates are included in the composition 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, when bound with other biological substances of the cell, form glycosides, and polysaccharides with proteins form glycoproteins. The composition of carbohydrates in a plant cell is much more diverse than in animal cells, due to the diverse composition of cell wall polysaccharides and sugars in the cell sap of vacuoles.

The main and most common storage carbohydrate is the polysaccharide starch. Primary assimilative starch is formed in chloroplasts. At night, when photosynthesis ceases, starch is hydrolyzed to sugars and transported to storage tissues - tubers, bulbs, rhizomes. There, in special types of leukoplasts - amyloplasts - some of the sugars are deposited in the form of grains of secondary starch. Starch grains are characterized by layering, which is explained by 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, it is called simple eccentric. A grain with several starch-forming centers is complex. In semi-compound grains, new layers are deposited around several starch-forming centers, and then common layers form and cover the starch-forming centers (see Fig. 1.12). The reagent for starch is an iodine solution, which gives a blue color.

Rice. 1.12. Potato starch grains (A):

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

Excretory substances (secondary metabolic products)

Cellular inclusions also include excretory substances, for example calcium oxalate crystals ( single crystals, raphids - needle-shaped crystals, druses - crystal intergrowths, crystalline sand - an accumulation of many small crystals) (see Fig. 1.13). Less commonly, 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 characteristic, for example, of representatives of the nettle family and ficus leaves.

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

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

1, 2 - raphida (impatiens; 1 - side view, 2 - cross section); 3 - druse (prickly pear); 4 - crystalline sand (potatoes); 5 - single crystal (vanilla)

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

1 - leaf skin; 2 - cystolitis

Essential oils accumulate in leaves (mint, lavender, sage), flowers (rose hips), fruits (citrus fruits) 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). Reservoirs for the accumulation of essential oils can be glands (mint), lysigenic receptacles (citrus fruits), glandular hairs (geranium).

Resins- these are complex compounds formed during normal life or as a result of tissue destruction. They are formed by 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 containers. They are insoluble in water, impermeable to microorganisms and, due to their antiseptic properties, increase plant resistance to disease. 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 embedded in a matrix containing hemicelluloses and pectin substances. The cell wall provides mechanical support to the cell, protects the protoplast, and maintains the cell's shape. In this case, the cell wall is capable of stretching. Being a product of the vital activity of the protoplast, the wall can only grow in contact with it. Water and mineral salts move through the cell wall, but it is completely or partially impermeable to high-molecular substances. When the protoplast dies, the wall can continue to perform the function of conducting water. The presence of a cell wall, more than all other characteristics, distinguishes plant cells from animal cells. The architecture of the cell wall is largely determined by cellulose. The monomer of cellulose is glucose. 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 framework of the cell wall is filled with non-cellulosic 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 ripened fruits), maceration occurs (from the Latin maceratio - softening). Natural maceration can be observed in many overripe fruits (watermelon, melon, peach). Artificial maceration (when tissue is treated 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, suberization, mucilage, cutinization, mineralization (see Table l.4).

Table 1.4.

Related information.

Cytoplasm- an obligatory part of the cell, enclosed between the plasma membrane and the nucleus; is divided into hyaloplasm (the main substance of the cytoplasm), organelles (permanent components of the cytoplasm) and inclusions (temporary components of the cytoplasm). 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 has an alkaline reaction. 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 by being in constant motion can it perform its functions.

Hyaloplasma ( cytosol) is a colorless, slimy, thick and transparent colloidal solution. It is in it that all metabolic processes take place, it ensures 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- thicker hyaloplasm. Mutual transitions are possible between them: the gel turns into a sol and vice versa.

Functions of the cytoplasm:

1. combining all cell components 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 membranes

Cell membranes 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. It is currently generally accepted fluid mosaic model of 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 containing the phosphoric acid residue is called the hydrophilic head, the sections containing the fatty acid residues are called the 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 outward, 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.). There are: 1) peripheral proteins(located on the outer or inner surface of the lipid bilayer), 2) semi-integral proteins(immersed in the lipid bilayer to varying depths), 3) integral, or transmembrane, proteins(pierce the membrane through, contacting both the external and internal environment of the cell). Integral proteins are in some cases called channel-forming or channel proteins, 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 phospholipid head; B - hydrophobic phospholipid tails; 1 - hydrophobic regions of proteins E and F; 2 — hydrophilic regions of protein F; 3 - 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). Carbohydrates are mainly located on the outer surface of the membrane. Carbohydrates provide receptor functions of the membrane. In animal cells, glycoproteins form a supra-membrane complex, the glycocalyx, which is several tens of nanometers thick. It contains many cell receptors, and with its help cell adhesion occurs.

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

Functions of membranes

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. dividing the cell into compartments (“compartments”),
4. place of localization of “enzymatic conveyors”,
5. ensuring 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, ensuring the exchange of substances between the cell and the external environment. The process of substances passing through the cell membrane is called transport of substances. There are: 1) passive transport- the process of passing substances without energy consumption; 2) active transport- the process of passage of substances that occurs with the expenditure of energy.

At passive transport substances move from an area of ​​higher concentration to an area of ​​lower, i.e. along the concentration gradient. In any solution there are solvent and solute molecules. The process of moving solute molecules is called diffusion, and the movement of solvent molecules is called osmosis. If the molecule is charged, then its transport is also affected by the electrical gradient. Therefore, people often talk about 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 the solvent is water).

Necessity active transport occurs when it is necessary to ensure the transport of molecules across a membrane against an 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.

Operation of 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 diffuse freely through the membrane pores. The Na + /K + pump counteracts the equalization of the concentrations of these ions 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, as a result of which it can attach both K + and Na +. The Na + /K + pump cycle can be divided into the following phases: 1) addition of Na + from the inside of the membrane, 2) phosphorylation of the pump protein, 3) release of Na + in the extracellular space, 4) addition of K + from the outside of the membrane , 5) dephosphorylation of the pump protein, 6) release of K + in the intracellular space. Almost a third of all energy required for cell functioning is spent on the operation of the sodium-potassium pump. In one cycle of operation, the pump pumps out 3Na + from the cell and pumps in 2K +.

Endocytosis- the process of absorption of large particles and macromolecules by the cell. There are two types of endocytosis: 1) phagocytosis- capture and absorption of large particles (cells, parts of cells, 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 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 intestinal epithelial cells and in the endothelium of blood capillaries.

Exocytosis- a process reverse to endocytosis: the removal of various substances from the cell. During exocytosis, the vesicle membrane merges 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 removed from the cells of the endocrine glands; in protozoa, undigested food remains are removed.

Go to lectures No. 5"Cell theory. Types of cellular organization"

Go to lectures No. 7“Eukaryotic cell: structure and functions of organelles”

Cytoplasmic membrane or plasmalemma(Latin membrana – skin, film) – the thinnest film ( 7– 10 nm), which delimits the internal contents of the cell from the environment, is visible only with an electron microscope.

By chemical organization The plasmalemma represents 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 being amphipatric, i.e. they have hydrophilic (“water loving”) and hydrophobic (“water fearing”) ends. The hydrophilic polar “heads” of lipid molecules (phosphate group) face outward of the membrane, and the hydrophobic nonpolar “tails” (fatty acid residues) face each other, which creates a bipolar lipid layer. Lipid molecules are mobile and can move within their monolayer or, rarely, from one monolayer to another. Lipid monolayers are asymmetric, that is, they differ in lipid composition, which gives specificity to membranes even within the same cell. The lipid bilayer can be in a liquid or solid crystal state.

The second essential component of the plasmalemma is proteins. 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 main structural features of the membrane, and 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 on the outside, and lipid molecules (light layer) on the 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 bilipid layer, and its water-soluble sections are hydrophilic heads directed towards the protein layer, and insoluble (fatty acid residues) - hydrophobic tails facing each other.

②. LIQUID MOSAIC MODEL

IN 1972.Singer And Nicholson described a membrane model that has received widespread acceptance. According to this model, protein molecules do not form a continuous layer, but are immersed in a bipolar lipid layer to 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, located on the surface of the bilipid layer, are associated with the heads of lipid molecules by electrostatic interactions. But they never form a continuous layer and, in fact, are not proteins of the membrane itself, but rather connect it with the supra-membrane or sub-membrane system of the cell’s 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, and the hydrophilic regions face the aqueous environment and form hydrogen bonds with water.

③. PROTEIN-CRYSTALLINE MODEL(lipoprotein mat model)

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

hydrophobic interactions.

Protein molecules, like pins, penetrate the lipid layer and act as a scaffold within the membrane. After treating 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 area where the enzyme is located Na-K –ATPase).

The most universal model that meets thermodynamic principles (principles of hydrophilic-hydrophobic interactions), morpho-biochemical and experimental
ntal-cytological data is a fluid-mosaic model. However, all three membrane models are not mutually exclusive and can be found in different areas of the same membrane depending on the functional characteristics of this area.

MEMBRANE PROPERTIES

1. Self-assembly ability. 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 the proteins and lipids included in its composition 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. Semi-permeability. The membranes of living cells allow, in addition to water, only certain molecules and ions of dissolved substances to pass through. This ensures the maintenance of the ionic and molecular composition of the cell.

4. The membrane has no free 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, and the inner side carries a negative charge.

MEMBRANE FUNCTIONS

1) Barrier – The plasmalemma delimits the cytoplasm and nucleus from the external environment. In addition, the membrane divides the internal contents of the cell into compartments, in which opposing biochemical reactions often occur.

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

This ensures communication with the external environment, recognition of cells 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, which 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 pores in the membrane structure. Large substances enter the cell by endocytosis(transport in membrane packaging), which requires energy. Its varieties are phago- and pinocytosis.

Passive transport – a type of transport in which the transfer of substances occurs 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– is the transfer of ions or molecules from a zone of higher concentration to a zone of lower concentration, i.e. by 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 transport it across the membrane according to the “ping-pong” principle. In this way, sugars and amino acids pass through the membrane. The speed of such transport is much higher than simple diffusion. In addition to carrier proteins, some antibiotics take part in facilitated diffusion, for example, gramitidine and vanomycin.

Because they provide ion transport, they are called ionophores.

Active transport is a type of transport in which ATP energy is consumed; it goes against the concentration gradient. ATPase enzymes take part in it. The outer cell membrane contains ATPases that 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 flows into the cell. In contrast, the sodium-potassium pump pumps potassium ions into the cell against the concentration gradient, and carries sodium ions into the external environment. This allows you to maintain 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 membrane packaging. There are phagocytosis and pinocytosis. Phagocytosis– capture of solid particles by the cell, pinocytosis– liquid particles. These processes include stages:

1) recognition of the substance by membrane receptors; 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 membrane integrity; 4) release of undigested material from the cell (exocytosis).

Endocytosis is a method of nutrition 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 CELLS

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

Dialysis– one-way diffusion of solutes.

A solution in which the osmotic pressure is the same as in the cells is called isotonic. When a cell is immersed in an isotonic solution, its volume does not change. An isotonic solution is called physiological 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 the cells is called hypertensive.

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

A solution where the salt concentration 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.

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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 is also part of most cellular 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 the one that separates the contents of the cell from the external environment. The remaining terms denote all membranes.

Structure of the cell membrane

The structure of the cellular (biological) membrane is based on a double layer of lipids (fats). The formation of such a layer is associated with the characteristics 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 to 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 molecules causes them to “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 nonpolar tails are inward (facing each other) and the polar heads are outward (toward the external environment and cytoplasm). The surface of such a membrane is hydrophilic, but inside it is hydrophobic.

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

Due to electrostatic interaction, some protein molecules are attached to the charged lipid heads, which become surface membrane proteins. Other proteins interact with nonpolar tails, are partially buried in the bilayer, or penetrate through it.

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

This fluid mosaic model of membrane structure was put forward in the 70s of the XX century. Previously, a sandwich model of 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 refuted 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.

Cell membrane breaks easily merge due to the physicochemical properties of the lipid bilayer. In the plane of the membrane, lipids and proteins (unless they are anchored by the cytoskeleton) move.

Functions of the cell membrane

Most proteins immersed in the cell membrane perform an enzymatic function (they are enzymes). Often (especially in the membranes of cell organelles) enzymes are located 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 the enzymes to float along the lipid bilayer.

The cell membrane performs a delimiting (barrier) function from the environment and at the same time transport functions. We can say 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). For example, gases (CO 2 , O 2 ) diffuse.

There is also transport against a concentration gradient, but with energy consumption.

Transport can be passive and lightweight (when it is helped by some kind of carrier)
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. Such carriers bind to transported molecules and pull them through the membrane.

3. Functions and structure of the cytoplasmic membrane

This is how glucose is transported inside red blood cells.

Threading proteins combine to form a pore for the movement of certain substances across the membrane. Such carriers do not move, but form a channel in the membrane and work similarly to enzymes, binding a specific substance. Transfer occurs due to a change in protein conformation, resulting in the formation of channels 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). Thus, 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(capturing droplets of liquid with substances dissolved in it). The mechanism of these processes is approximately the same. Absorbed substances on the surface of cells 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 (hormones, polysaccharides, proteins, fats, etc.) by the cytoplasmic membrane. These substances are contained in membrane vesicles that fit the cell membrane. Both membranes merge and the contents appear outside the cell.

The cytoplasmic membrane performs a receptor function. To do this, structures are located on its outer side that can recognize a chemical or physical stimulus. Some of the proteins that penetrate the plasmalemma are connected from the outside 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. In this case, channels can open, and certain substances can begin to enter or exit the cell.

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 (adenylate cyclase enzyme) is activated. The enzyme synthesizes cyclic AMP from ATP. Already it activates or suppresses various enzymes of cellular metabolism.

The receptor function of the cytoplasmic membrane also includes 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 substances. One example of such an interaction is contact inhibition, when cells stop growing after receiving information that free space is occupied.

Intercellular contacts can be simple (the membranes of different cells are adjacent to each other), locking (invaginations of the membrane of one cell into another), desmosomes (when the membranes are connected by bundles of transverse fibers that penetrate 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 cells.

Cell theory

In 1665, R. Hooke, examining a section of wood cork under a microscope, discovered empty cells, which he called “cells.” He saw only the membranes of plant cells, and for a long time the membrane was considered the main structural component of the cell. In 1825, J. Purkynė 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 this time, T.

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 originate from a cell (Omnis cellula e cellula) as a result of its division. He established that all diseases are based on changes in the structure and function of cells.

1.2. Modern cell theory includes the following provisions:

1) cell - the basic structural, functional and genetic unit of living organisms, the smallest unit of a living thing;

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

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

4) the 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

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 streams are accepted special apparatus cells, which include:

1) supra-membrane component – ​​glycocalyx;

2) an elementary biological membrane or their complex;

3) submembranous support-contractile complex of hyaloplasm;

4) anabolic and catabolic systems.

The main component of this device is an elementary membrane.

The cell contains different 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 structure of an elementary membrane. According to this model, it is also based on a 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 layer of lipids (semi-integral proteins), and some penetrate both layers of lipids (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. Properties of membranes: 1) plasticity, 2) semi-permeability, 3) ability to self-close.

2.4. Functions of membranes: 1) structural - the membrane as a structural component is part of most organelles (the 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.