The structure of the plasma membrane in detail. Functions of the plasma membrane
Membrane properties: formation of a compartment (enclosed space), selective permeability, asymmetry of the structure, fluidity.
Membrane functions:
. transport of substances into and out of the cell, gas exchange;
. receptor; contacts between cells in a multicellular organism (single-membrane structures, outer
membrane in mitochondria, outer and inner membrane of the nucleus);
. the boundary between the external and internal environment of the cell;
. modified membrane folds form many cell organelles (mesosome).
The basis of membranes is a lipid bilayer (see Fig. 1). Lipid molecules have a dual nature, manifested in the way they behave in relation to water. Lipids are composed of a polar (i.e., hydrophilic, affinity for water) head and two non-polar (hydrophobic) tails. All molecules are oriented in the same way: the heads of the molecules are in the water, and the hydrocarbon tails are above its surface.
Rice. 1. The structure of the plasma membrane
Protein molecules are, as it were, "dissolved" in the lipid bilayer of the membrane. They can be located only on the outer or only on the inner surface of the membrane, or only partially immersed in the lipid bilayer.
Functions of proteins in membranes:
. differentiation of cells into tissues (glycoproteins);
. transport of large molecules (pores and channels, pumps);
. promoting the restoration of membrane damage by delivering phospholipids;
. catalysis of reactions occurring on membranes;
. mutual connection of the internal parts of the cell with the surrounding space;
. maintaining the structure of membranes;
. receiving and converting chemical signals from the environment (receptors).
Transport of substances across the membrane
Depending on the need to use energy to carry out the transport of substances, passive transport is distinguished, which occurs without the consumption of ATP, and active transport, during which ATP is consumed.Passive transport is based on the difference in concentrations and charges. In this case, substances move from an area with a higher concentration to an area with a lower one, i.e. along the concentration gradient. If the molecule is charged, then its transport is affected by the electrical gradient. The speed of transport depends on the magnitude of the gradient. Methods of passive transport across the membrane:
. simple diffusion - directly through the lipid layer (gases, non-polar or small uncharged polar molecules). Diffusion of water through membranes - osmosis;
. diffusion through membrane channels - transport of charged molecules and ions;
. facilitated diffusion - the transport of substances with the help of special transport proteins (sugars, amino acids, nucleotides).
Active transport occurs against an electrochemical gradient with the help of carrier proteins. One of these systems is called the sodium-potassium pump, or sodium-potassium ATPase (Fig. 8). This protein is remarkable in that a colossal amount of ATP is spent on it - about a third of the ATP synthesized in the cell. It is a protein that transports potassium ions inward through the membrane and sodium ions outward. As a result, it turns out that sodium accumulates outside the cells.
Rice. 8. Potassium sodium pump
Pump phases:
. sodium ions and an ATP molecule enter the pump protein from the inside of the membrane, and potassium ions from the outside;
. sodium ions combine with a protein molecule and the protein acquires ATPase activity, i.e. the ability to cause ATP hydrolysis, accompanied by the release of energy that drives the pump;
. the phosphate released during the hydrolysis of ATP is attached to the protein;
. conformational changes in the protein, it is unable to retain sodium ions, and they are released and go outside the cell;
. protein attaches potassium ions;
. phosphate is cleaved off from the protein and the conformation of the protein changes again;
. release of potassium ions into the cell;
. protein resumes the ability to attach sodium ions.
In one cycle of operation, the pump pumps 3 sodium ions out of the cell and 2 potassium ions are pumped in. A positive charge builds up outside. In this case, the charge inside the cell is negative. As a result, any positive ion can be transported through the membrane relatively easily simply due to the fact that there is a charge difference. So, through a sodium-dependent protein for glucose transport, it attaches a sodium ion and a glucose molecule from the outside, and then, due to the fact that the sodium ion is attracted inward, the protein easily transfers both sodium and glucose inside. Based on the same principle, nerve cells have the same distribution of charges, and this will allow sodium to pass in and very quickly create a change in charge, called a nerve impulse.
Large molecules enter through the membrane during endocytosis. In this case, the membrane forms an invagination, its edges merge, and vesicles, single-membrane sacs, are laced into the cytoplasm. There are two types of endocytosis: phagocytosis (absorption of large solid particles) and pinocytosis (absorption of solutions).
Exocytosis is the process of removing various substances from the cell. In this case, the vesicles merge with the plasma membrane, and their contents are excreted outside the cell.
What is the structure of the plasma membrane? What are its functions?
basis structural organization Cells make up biological membranes. The plasma membrane (plasmalemma) is the membrane that surrounds the cytoplasm of a living cell. Membranes are made up of lipids and proteins. Lipids (mainly phospholipids) form a double layer in which the hydrophobic "tails" of the molecules face inside the membrane, and the hydrophilic tails - to its surfaces. Protein molecules can be located on the outer and inner surface of the membrane, can be partially immersed in the lipid layer or penetrate it through. Most of the immersed membrane proteins are enzymes. This is a fluid mosaic model of the plasma membrane structure. Protein and lipid molecules are mobile, which ensures the dynamism of the membrane. The membranes also contain carbohydrates in the form of glycolipids and glycoproteins (glycocalix) located on the outer surface of the membrane. The set of proteins and carbohydrates on the membrane surface of each cell is specific and is a kind of cell type indicator.
Membrane functions:
- Dividing. It consists in the formation of a barrier between the internal contents of the cell and external environment.
- Ensuring the exchange of substances between the cytoplasm and the external environment. Water, ions, inorganic and organic molecules enter the cell (transport function). Products formed in the cell are excreted into the external environment (secretory function).
- Transport. Transport across the membrane can take place in different ways. Passive transport is carried out without energy expenditure, by simple diffusion, osmosis or facilitated diffusion with the help of carrier proteins. Active transport is by carrier proteins and requires energy input (eg sodium-potassium pump).
Large molecules of biopolymers enter the cell as a result of endocytosis. It is divided into phagocytosis and pinocytosis. Phagocytosis is the capture and absorption of large particles by the cell. The phenomenon was first described by I.I. Mechnikov. First, substances adhere to the plasma membrane, to specific receptor proteins, then the membrane flexes, forming a depression.
A digestive vacuole is formed. It digests the substances that enter the cell. In humans and animals, leukocytes are capable of phagocytosis. Leukocytes engulf bacteria and other solid particles.
Pinocytosis is the process of capturing and absorbing liquid droplets with substances dissolved in it. Substances adhere to membrane proteins (receptors), and a drop of solution is surrounded by a membrane, forming a vacuole. Pinocytosis and phagocytosis occur with the expenditure of ATP energy.
- Secretory. Secretion - release by the cell of substances synthesized in the cell into the external environment. Hormones, polysaccharides, proteins, fat droplets are enclosed in membrane-bound vesicles and approach the plasmalemma. The membranes fuse and the contents of the vesicle are released into the environment surrounding the cell.
- The connection of cells in the tissue (due to folded outgrowths).
- Receptor. There are a large number of receptors in membranes - special proteins, the role of which is to transmit signals from the outside to the inside of the cell.
The cell membrane (plasma membrane) is a thin, semi-permeable membrane that surrounds cells.
Function and role of the cell membrane
Its function is to protect the integrity of the interior by letting some essential substances into the cell and preventing others from entering.
It also serves as the basis for attachment to some organisms and to others. Thus, the plasma membrane also provides the shape of the cell. Another function of the membrane is to regulate cell growth through balance and.
In endocytosis, lipids and proteins are removed from the cell membrane as substances are absorbed. In exocytosis, vesicles containing lipids and proteins fuse with the cell membrane, increasing cell size. , and fungal cells have plasma membranes. Internal, for example, are also enclosed in protective membranes.
Cell membrane structure
The plasma membrane is mainly composed of a mixture of proteins and lipids. Depending on the location and role of the membrane in the body, lipids can make up 20 to 80 percent of the membrane, with the rest being proteins. While lipids help make the membrane flexible, proteins control and maintain chemical composition cells and also help in the transport of molecules across the membrane.
Membrane lipids
Phospholipids are the main component of plasma membranes. They form a lipid bilayer in which the hydrophilic (water-attracted) "head" regions spontaneously organize to resist the aqueous cytosol and extracellular fluid, while the hydrophobic (water-repellent) "tail" regions face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only some molecules to diffuse across the membrane.
Cholesterol is another lipid component of animal cell membranes. Cholesterol molecules are selectively dispersed between membrane phospholipids. This helps keep cell membranes rigid by preventing phospholipids from being too tightly packed. Cholesterol is absent in plant cell membranes.
Glycolipids are located on the outer surface of cell membranes and are connected to them by a carbohydrate chain. They help the cell recognize other cells in the body.
Membrane proteins
The cell membrane contains two types of associated proteins. Peripheral membrane proteins are external and associated with it by interacting with other proteins. Integral membrane proteins are introduced into the membrane and most pass through it. Parts of these transmembrane proteins are located on both sides of it.
Plasma membrane proteins have a number of different functions. Structural proteins provide support and shape to cells. Membrane receptor proteins help cells communicate with their external environment through the use of hormones, neurotransmitters, and other signaling molecules. Transport proteins, such as globular proteins, carry molecules across cell membranes by facilitated diffusion. Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane, helping in the exchange and transport of molecules.
Organelle membranes
Some cell organelles are also surrounded by protective membranes. Core,
The plasma membrane performs a number of important functions:
1) Barrier. The barrier function of the plasma membrane is to limit the free diffusion of substances from cell to cell, to prevent the leakage of water-soluble contents of the cell. But since the cell must receive the necessary nutrients, allocate final products metabolism, regulate intracellular concentrations of ions, then it formed special mechanisms for the transfer of substances through the cell membrane.
2) Transport. The transport function is Ensuring the entry and exit of various substances into and out of the cell. An important property of the membrane is selective permeability, or semipermeability. It easily passes water and water-soluble gases and repels polar molecules such as glucose or amino acids.
There are several mechanisms for the transport of substances across the membrane:
passive transport;
active transport;
transport in membrane packaging.
Passive transport.Diffusion - This is the movement of particles of the medium, leading to the transfer of a substance from an area where its concentration is high to an area with a low concentration. During diffusion transport, the membrane functions as an osmotic barrier. The rate of diffusion depends on the size of the molecules and their relative solubility in fats. How smaller sizes molecules and the more they are fat-soluble (lipophilic), the faster they will move through the lipid bilayer. Diffusion can be neutral(transfer of uncharged molecules) and lightweight(with the help of special carrier proteins). Facilitated diffusion is faster than neutral diffusion. Water has the maximum penetrating power, since its molecules are small and uncharged. Diffusion of water across a cell membrane is called osmosis. It is assumed that special "pores" exist in the cell membrane for the penetration of water and some ions. Their number is small, and the diameter is about 0.3-0.8 nm. Easily soluble molecules in the lipid bilayer, such as O, and uncharged polar molecules of small diameter (CO, urea) diffuse most rapidly through the membrane.
The transfer of polar molecules (sugars, amino acids) carried out with the help of special membrane transport proteins is called facilitated diffusion. Such proteins are found in all types of biological membranes, and each specific protein is designed to carry molecules of a certain class. Transport proteins are transmembrane; their polypeptide chain crosses the lipid bilayer several times, forming through passages in it. This ensures the transfer of specific substances through the membrane without direct contact with it. There are two main classes of transport proteins: carrier proteins (transporters) And channel-forming proteins (protein channels). Carrier proteins carry molecules across the membrane by first changing their configuration. Channel-forming proteins form water-filled pores in the membrane. When the pores are open, molecules of specific substances (usually inorganic ions right size and charge) pass through them. If the molecule of the transported substance has no charge, then the direction of transport is determined by the concentration gradient. If the molecule is charged, then its transport, in addition to the concentration gradient, is also affected by the electric charge of the membrane (membrane potential). Inner side The plasmalemma is usually negatively charged relative to the outer one. The membrane potential facilitates the penetration of positively charged ions into the cell and prevents the passage of negatively charged ions.
active transport. Active transport is the movement of substances against an electrochemical gradient. It is always carried out by transporter proteins and is closely associated with an energy source. Carrier proteins have binding sites with the transported substance. The more such sites associated with the substance, the higher the rate of transport. The selective transfer of one substance is called uniport. The transfer of several substances is carried out cotransport systems. If the transfer goes in one direction, it is symport, if in opposite antiport. For example, glucose is transported from the extracellular fluid into the cell in a uniportal manner. The transfer of glucose and Na 4 from the intestinal cavity or tubules of the kidneys, respectively, to the cells of the intestine or blood is carried out symportally, and the transfer of C1 ~ and HCO "is antiport. .
An example of a carrier protein that uses the energy released during ATP hydrolysis to transport substances is Na + -TO + pump, found in the plasma membrane of all cells. The Na + -K pump works on the antiport principle, pumping Na "out of the cell and K t into the cell against their electrochemical gradients. The Na + gradient creates osmotic pressure, maintains cell volume and ensures the transport of sugars and amino acids. A third of all energy is spent on this pump necessary for the vital activity of cells.When studying the mechanism of action of the Na + -K + pump, it was found that it is an ATPase enzyme and a transmembrane integral protein.In the presence of Na + and ATP, under the action of ATPase, terminal phosphate is separated from ATP and attached to the residue of aspartic acid on the ATPase molecule. The ATPase molecule is phosphorylated, changes its configuration and Na + is excreted from the cell. Following the excretion of Na from the cell, transport of K "into the cell always occurs. For this, the previously attached phosphate is cleaved from ATPase in the presence of K. The enzyme is dephosphorylated, restores its configuration, and K 1 is "pumped" into the cell.
ATPase is formed by two subunits, large and small. The large subunit consists of thousands of amino acid residues that cross the bilayer several times. It has catalytic activity and can be reversibly phosphorylated and dephosphorylated. The large subunit on the cytoplasmic side has sites for binding Na + and ATP, and on the outside - sites for binding K + and ouabain. The small subunit is a glycoprotein and its function is not yet known.
Na + -K pump has an electrogenic effect. It removes three positively charged Na f ions from the cell and introduces two K ions into it. As a result, a current flows through the membrane, forming an electric potential with negative value in the inner part of the cell in relation to its outer surface. Na "-K + pump regulates cell volume, controls the concentration of substances inside the cell, maintains osmotic pressure, and participates in the creation of membrane potential.
Transport in membrane packaging. The transfer of macromolecules (proteins, nucleic acids, polysaccharides, lipoproteins) and other particles through the membrane is carried out through the sequential formation and fusion of vesicles (vesicles) surrounded by the membrane. The process of vesicular transport occurs in two stages. Initially, the vesicle membrane and plasmalemma stick together and then merge. For the course of stage 2, it is necessary that water molecules be displaced by interacting lipid bilayers, which approach each other up to a distance of 1-5 nm. It is believed that this process is activated by special fusion proteins(they have been isolated so far only in viruses). Vesicular transport has important feature- absorbed or secreted macromolecules in the vesicles usually do not mix with other macromolecules or organelles of the cell. Bubbles can fuse with specific membranes, which ensures the exchange of macromolecules between the extracellular space and the contents of the cell. Similarly, macromolecules are transferred from one cell compartment to another.
The transport of macromolecules and particles into a cell is called endocytosis. In this case, the transported substances are enveloped by a part of the plasma membrane, a bubble (vacuole) is formed, which moves inside the cell. Depending on the size of the formed vesicles, two types of endocytosis are distinguished - pinocytosis and phagocytosis.
pinocytosis provides absorption of liquid and dissolved substances in the form of small bubbles (d=150 nm). Phagocytosis - this is the absorption of large particles, microorganisms or fragments of organelles, cells. In this case, large vesicles, phagosomes or vacuoles (d-250 nm or more) are formed. In protozoa, the phagocytic function is a form of nutrition. In mammals, the phagocytic function is carried out by macrophages and neutrophils, which protect the body from infection by engulfing invading microbes. Macrophages are also involved in the disposal of old or damaged cells and their fragments (in the human body, macrophages absorb more than 100 old red blood cells daily). Phagocytosis begins only when the absorbed particle binds to the surface of the phagocyte and activates specialized receptor cells. The binding of particles to specific membrane receptors causes the formation of pseudopodia, which envelop the particle and, merging at the edges, form a bubble - phagosome. The formation of a phagosome and proper phagocytosis occurs only if, during the enveloping process, the particle is in constant contact with the plasmalemma receptors, as if "zipping up".
A significant part of the material absorbed by the cell by endocytosis ends up in lysosomes. Large particles are included in phagosomes which then fuse with lysosomes to form phagolysosomes. Fluid and macromolecules taken up during pinocytosis are initially transferred to endosomes, which also fuse with lysosomes to form endolysosomes. Various hydrolytic enzymes present in lysosomes quickly destroy macromolecules. Hydrolysis products (amino acids, sugars, nucleotides) are transported from lysosomes to the cytosol, where they are used by the cell. Most of the membrane components of endocytic vesicles from phagosomes and endosomes are returned by exocytosis to the plasma membrane and reutilized there. Main biological significance Endocytosis is the acquisition of building blocks through intracellular digestion of macromolecules in lysosomes.
The absorption of substances in eukaryotic cells begins in specialized areas of the plasma membrane, the so-called bordered pits. On electron micrographs, the pits look like invaginations of the plasma membrane, the cytoplasmic side of which is covered with a fibrous layer. The layer, as it were, borders small pits of the plasmalemma. The pits occupy about 2% of the total surface of the eukaryotic cell membrane. Within a minute, the pits grow, invaginate deeper and deeper, are drawn into the cell and then, narrowing at the base, split off, forming bordered vesicles. It has been established that approximately a quarter of the membrane in the form of bordered vesicles is split off from the plasma membrane of fibroblasts within one minute. The vesicles quickly lose their border and acquire the ability to merge with the lysosome.
Endocytosis may be non-specific(constitutive) and specific(receptor). At nonspecific endocytosis the cell captures and absorbs substances that are completely alien to it, for example, soot particles, dyes. Initially, particles are deposited on the glycocalyx of the plasmalemma. Positively charged protein groups are especially well precipitated (adsorbed), since the glycocalyx carries a negative charge. Then the morphology of the cell membrane changes. It can either sink, forming invaginations (invaginations), or, conversely, form outgrowths that seem to fold, separating small volumes of the liquid medium. The formation of invaginations is more typical for cells of the intestinal epithelium, amoebae, and outgrowths - for phagocytes and fibroblasts. These processes can be blocked by respiratory inhibitors. The resulting vesicles - primary endosomes - can merge with each other, increasing in size. Subsequently, they combine with lysosomes, turning into an endolysosome - a digestive vacuole. The intensity of liquid-phase nonspecific pinocytosis is quite high. Macrophages form up to 125, and epithelial cells of the small intestine up to a thousand pinosomes per minute. The abundance of pinosomes leads to the fact that the plasmalemma is quickly spent on the formation of many small vacuoles. Restoration of the membrane proceeds quite rapidly during recyclization during exocytosis due to the return of vacuoles and their incorporation into the plasmalemma. In macrophages, the entire plasma membrane is replaced in 30 minutes, and in fibroblasts in 2 hours.
More effective way absorption from the extracellular fluid of specific macromolecules is specific endocytosis(mediated by receptors). In this case, macromolecules bind to complementary receptors on the cell surface, accumulate in the bordered fossa, and then, forming an endosome, are immersed in the cytosol. Receptor endocytosis ensures the accumulation of specific macromolecules at its receptor. Molecules that bind to a receptor on the surface of the plasmalemma are called ligands. With the help of receptor endocytosis in many animal cells, cholesterol is absorbed from the extracellular environment.
The plasma membrane takes part in the removal of substances from the cell (exocytosis). In this case, the vacuoles approach the plasmalemma. At the points of contact, the plasmolemma and the vacuole membrane merge and the contents of the vacuole enter the environment. In some protozoa, sites on the cell membrane for exocytosis are predetermined. So, in the plasma membrane of some ciliary ciliates there are certain areas with correct location large globules of integral proteins. Mucocysts and trichocysts of ciliates that are completely ready for secretion have a halo of integral protein globules on the upper part of the plasmalemma. These sections of the membrane of the mucocysts and trichocysts are in contact with the surface of the cell. A peculiar exocytosis is observed in neutrophils. They are able, under certain conditions, to release their lysosomes into the environment. In some cases, small outgrowths of the plasmalemma containing lysosomes are formed, which then break off and pass into the environment. In other cases, there is invagination of the plasmalemma deep into the cell and its capture of lysosomes located far from the cell surface.
The processes of endocytosis and exocytosis are carried out with the participation of the system of fibrillar components of the cytoplasm associated with the plasmolemma.
Receptor function of the plasmalemma. This is one of the main, universal for all cells, is the receptor function of the plasmalemma. It determines the interaction of cells with each other and with the external environment.
The whole variety of informational intercellular interactions can be schematically represented as a chain of successive reactions signal-receptor-secondary messenger-response (signal-response concept). The transfer of information from cell to cell is carried out by signaling molecules that are produced in some cells and specifically affect others that are sensitive to the signal (target cells). Signal molecule - primary intermediary binds to receptors located on target cells that respond only to certain signals. Signal molecules - ligands - approach their receptor like a key to a lock. Ligands for membrane receptors (plasmalemma receptors) are hydrophilic molecules, peptide hormones, neurotransmitters, cytokines, antibodies, and for nuclear receptors - fat-soluble molecules, steroid and thyroid hormones, vitamin D. Membrane proteins or glycocalyx elements can act as receptors on the cell surface - polysaccharides and glycoproteins. It is believed that areas sensitive to individual substances are scattered over the surface of the cell or collected in small zones. So, on the surface of prokaryotic cells and animal cells there are a limited number of places with which viral particles can bind. Membrane proteins (carriers and channels) recognize, interact and carry only certain substances. Cell receptors are involved in the transmission of signals from the surface of the cell into it. The diversity and specificity of the sets of receptors on the cell surface leads to the creation of a very complex system of markers that make it possible to distinguish one's own cells from those of others. Similar cells interact with each other, their surfaces can stick together (conjugation in protozoa, tissue formation in multicellular). Cells that do not perceive markers, as well as those that differ in the set of determinant markers, are destroyed or rejected. When the receptor-ligand complex is formed, transmembrane proteins are activated: converter protein, amplifier protein. As a result, the receptor changes its conformation and interacts with the precursor of the second messenger located in the cell - messenger. Messengers can be ionized calcium, phospholipase C, adenylate cyclase, guanylate cyclase. Under the influence of the messenger, the activation of enzymes involved in the synthesis cyclic monophosphates - AMP or HMF. The latter alter the activity of two types of protein kinase enzymes in the cell cytoplasm, leading to the phosphorylation of numerous intracellular proteins.
The most common formation of cAMP, under the influence of which the secretion of a number of hormones - thyroxine, cortisone, progesterone, increases, the breakdown of glycogen in the liver and muscles, the frequency and strength of heart contractions, osteodestruction, and reverse absorption of water in the nephron tubules increase.
The activity of the adenylate cyclase system is very high - the synthesis of cAMP leads to a ten thousandth increase in the signal.
Under the action of cGMP, the secretion of insulin by the pancreas, histamine by mast cells, serotonin by platelets increases, and smooth muscle tissue is reduced.
In many cases, the formation of a receptor-ligand complex results in a change in the membrane potential, which in turn leads to a change in the permeability of the plasmalemma and metabolic processes in the cell.
The plasma membrane contains specific receptors that respond to physical factors. So, in photosynthetic bacteria, chlorophylls are located on the cell surface that react to light. In light-sensitive animals, the plasma membrane contains a whole system of phogoreceptor proteins-rhodopsins, with the help of which the light stimulus is transformed into a chemical signal, and then an electrical impulse.
Lecture number 4.
Number of hours: 2
plasma membrane
1.
2.
3. Intercellular contacts.
1. The structure of the plasma membrane
The plasma membrane, or plasmalemma, is a surface peripheral structure that limitsthe cell from the outside and providing its connection with other cells and the extracellular environment. It has a thicknessabout 10 nm. Among other cell membranes, the plasmalemma is the thickest. Chemically, the plasma membrane is lipoprotein complex. The main components are lipids (about 40%), proteins (more than 60%) and carbohydrates (about 2-10%).
The lipids are large group organic matter having poor solubility in water (hydrophobicity) and good solubility in organic solvents and fats (lipophilicity).Representative lipids found in the plasma membrane are phospholipids, sphingomyelins, and cholesterol. In plant cells, cholesterol is replaced by phytosterol. According to their biological role, plasmalemma proteins can be divided into enzyme proteins, receptor and structural proteins. Plasmalemma carbohydrates are part of the plasmalemma in a bound state (glycolipids and glycoproteins).
It is currently generally accepted fluid-mosaic model of the structure of a biological membrane. According to this model structural framework membrane forms a double layer of phospholipids encrusted with proteins. The tails of the molecules face each other in a double layer, while the polar heads remain outside, forming hydrophilic surfaces. Protein molecules do not form a continuous layer, they are located in the lipid layer, plunging to different depths (there are peripheral proteins, some proteins penetrate the membrane through, some are immersed in the lipid layer). Most proteins are not associated with membrane lipids; they seem to float in a "lipid lake". Therefore, protein molecules are able to move along the membrane, gather in groups, or, conversely, disperse on the membrane surface. This suggests that the plasma membrane is not a static, frozen formation.
Outside of the plasmalemma is the epimembrane layer - glycocalyx. The thickness of this layer is about 3-4 nm. Glycocalyx is found in almost all animal cells. It is associated with the plasma membrane glycoprotein complex. Carbohydrates form long, branching chains of polysaccharides associated with proteins and lipids of the plasma membrane. The glycocalyx can contain enzyme proteins involved in the extracellular breakdown of various substances. Products of enzymatic activity (amino acids, nucleotides, fatty acids, etc.) are transported through the plasma membrane and absorbed by cells.
The plasma membrane is constantly being renewed. This occurs by lacing off small bubbles from its surface into the cell and embedding vacuoles from inside the cell into the membrane. Thus, in the cell there is a constant flow of membrane elements: from the plasma membrane into the cytoplasm (endocytosis) and the flow of membrane structures from the cytoplasm to the cell surface (exocytosis). In the circulation of membranes, the leading role is assigned to the system of membrane vacuoles of the Golgi complex.
4. Functions of the plasma membrane. Mechanisms of transport of substances through the plasmalemma. Receptor function of the plasmalemma
The plasma membrane performs a number of important functions:
1) Barrier.The barrier function of the plasma membrane is tolimiting the free diffusion of substances from cell to cell, preventingrotation leakage of the water-soluble contents of the cell. But sincehow the cell must receive the necessary nutrients, youdivide metabolic end products, regulate intracellularion concentrations, then it formed special mechanisms for the transfer of substances through the cell membrane.
2) Transport.The transport function is Ensuring the entry and exit of various substances into and out of the cell. An important property of the membrane is selective permeability, or semipermeability. It easily passes water and water-solublegases and repels polar molecules such as glucose or amino acids.
There are several mechanisms for the transport of substances across the membrane:
passive transport;
active transport;
transport in membrane packaging.
Passive transport. Diffusion -this is the movement of particles of the medium, leading to the transfer ofsubstances from an area where its concentration is high to an area with a low concentrationtion. During diffusion transport, the membrane functions as an osmotic barrier. The diffusion rate depends on the valuemolecules and their relative solubility in fats. The less timesmeasures of molecules and the more they are fat-soluble (lipophilic), the faster they will move through the lipid bilayer.Diffusion can be neutral(transfer of unchargedmolecules) and lightweight(with the help of special proteinsnoses). Facilitated diffusion is faster than neutral diffusion.Maximum penetratingwater has the abilityhow its molecules are small and uncharged. Diffusion of water through cellsmembrane is called osmo catfishIt is assumed that in the cellmembrane for penetrationwater and some ions essentiallythere are special "pores". Their numbersmall, and the diameter isabout 0.3-0.8 nm. Diffuse most rapidly through the membrane well, easily soluble in lipid bilayer of a molecule, for example O, and uncharged polar moleculesly of small diameter (SO, mo chevin).
Transport of polar molecules (withsugars, amino acids), implementeddelivered with the help of special membrane transportproteins is called facilitated diffusion. Such proteins areare found in all types of biological membranes, and each specific ny protein is designed to transfer molecules of a certain class sa. Transport proteins are transmembrane, their polypeptide chain crosses the lipid bilayer several times, forming it has through passages. This ensures the transfer of specificsubstances through the membrane without direct contact with it.There are two main classes of transport proteins: squirrels- carriers (transporters) And channel-forming proteins (whiteki channels). Carrier proteins carry molecules across the membrane by first changing their configuration. Channel-forming proteins form filled membranes pore water. When the pores are open, molecules of specific substances(usually inorganic ions of a suitable size and charge) pass through them. If the molecule of the transported substance has no charge, then the direction of transport is determined by the concentration gradient. If the molecule is charged, then on its transport, in addition to the gradient, con concentration, the electric charge of the membrane also affects (membranepotential). The inner side of the plasmalemma is usually charged from negative in relation to the outside. The membrane potential facilitates the penetration of positively charged ions into the cell and prevents the passage of negatively charged ions.
active transport. Active transport is the movement of substances against an electrochemical gradient. It is always carried out by trans proteins.porters and closely related zan with energy sourcegee. In proteins-transfer chiki have plots binding with transporttiable substance. The more such tkov communicates with thingsthe higher the speedtransport growth. The selective transfer of one substance is called uniport. The transfer of several substances is carried out kotran sports systems. If the transfer goes in one direction -This symport, if in opposite antiport. So,for example, glucose is transported from the extracellular fluid into the cell in a uniportal manner. The transfer of glucose and Na 4 from the intestinal tract ortubules of the kidneys, respectively, into the cells of the intestine or blood is carried out symportally, and the transfer of C1 ~ and HCO "is antiport. Presumably It is assumed that during the transfer, reversible conformational changes in the conveyor, which allows the substances connected to it to move.
An example of a carrier protein used for transportsubstances, the energy released during the hydrolysis of ATP isNa + -K + pump, found in the plasma membrane of all cells. Na+-K The pump works on the principle of anti-port, pumping wai Na "from the cell and to the inside of the cell against their electrochemical gradients. Gradient Na+ creates osmotic pressure, maintains cell volume and ensures the transport of sugars and amino acidsnoacids. The operation of this pump consumes a third of all the energy necessary for the vital activity of cells.When studying the mechanism of action Na + - K + pump has been installedit is known that it is an ATPase enzyme and a transmembrane enzyme tegral protein. In the presence Na+ and ATP under the action of ATP-terminal phosphate is separated from ATP and attached to the residueaspartic acid on the ATPase molecule. Phos ATPase moleculephorylated, changes its configuration and Na+ is derived from cells. Following the withdrawal Na K" is always transported from the cell into the cell. To do this, the previously attached phosphate is cleaved from ATPase in the presence of K. The enzyme is dephosphorylated, restores its configuration, and K 1 is "pumped" into the cell.
ATPase is formed by two subunits, large and small.The large subunit consists of thousands of amino acid residues,crossing the bilayer several times. It has a catalytic activity and is capable of being reversibly phosphorylated and dephosphorilled. Large subunit on the cytoplasmic sidedoes not have binding sites Na+ and ATP, and on the outside -sites for binding K + and ouabain. The small subunit isglycoprotein and its function is not yet known.
Na+-K the pump has an electrogenic effect. He removes threepositively charged ion Na f out of the cell and introduces twoion K As a result, a current flows through the membrane, forming an electronric potential with a negative value in the inner part of the cell in relation to its outer surface. Na "-K + the pump regulates cell volume, controls the concentration of substancesinside the cell, maintains osmotic pressure, participates in the creation of membrane potential.
Transport in membrane packaging. Transfer across the membrane of macromolecules (proteins, nucleic acids)lot, polysaccharides, lipoproteins) and other particles is carried out through the sequential formation and fusion of surroundedmembrane-bound vesicles (vesicles). Vesicular transport processit goes through two stages. At the beginningvesicle membrane and plasmalemmastick together and then merge.For the passage of stage 2, it is necessarydimo so that the water molecules are youare crowded by interacting lipid bilayers, which approach each other up to a distance of 1-5 nm. Considers that this process is activatedspecial fusion proteins(They isolated so far only from viruses). Vesicular transport hasimportant feature - absorbed or secreted macromolecules,contained in vesicles, usually notmixable with other macromolesculae or cell organelles. Pu vesicles can merge with specific membranes, which providechivaet exchange of macromolecules betweendu extracellular space andthe contents of the cell. Similarlymacromolecules are transferred from one cell compartment to another.
The transport of macromolecules and particles into a cell is called endo cytosis.In this case, the transported substances are enveloped in teathe plasma membrane, a vesicle (vacuole) is formed, whichwhich moves into the cell. Depending on the image sizebubbles, there are two types of endocytosis - pinocytosis and phagocytosis.
pinocytosisprovides absorption of liquid and dissolvedsubstances in the form of small bubbles ( d =150 nm). Phagocytosis -is the absorption of large particles, microorganismcall or fragments of organelles, cells. At the same time, they formXia large vesicles, phagosomes or vacuoles ( d -250 nm or more). At protozoan phagocytic function - a form of nutrition. In mammals, the phagocytic function is carried out by macrophages and neutprofiles that protect the body from infection by absorbing invading microbes. Macrophages are also involved in the utilizationtion of old or damaged cells and their fragments (in the bodyhuman macrophages ingest more than 100 old erythritides dailyrocytes). Phagocytosis begins only when the ingested particlebinds to the surface of the phagocyte and activates specializednye receptor cells. Associating particles with specific remembrane receptors causes the formation of pseudopodia, whichrye envelop the particle and, merging at the edges, form a bubble -phagosome.Phagosome formation and phagocytosis properwalks only if in the process of enveloping the particleis constantly in contact with plasmalemma receptors, as if "stagnant flashing lightning."
A significant part of the material absorbed by the cell by endocytosis, ends up in lysosomes. Large particles includinghope in phagosomes which then fuse with lysosomes to form phagolysosomes. Liquid and macromolecules absorbed duringpinocytosis, are initially transferred to endosomes, whichfuse with lysosomes to form endolysosomes. I am present various hydrolytic enzymes present in lysosomesro destroy macromolecules. hydrolysis products (amino acidlots, sugars, nucleotides) are transported from lysosomes to the cytosol, where they are used by the cell. Most membrane components endocytic vesicles from phagosomes and endosomes return by exocytosis to the plasma membrane and there re-disappearlyse. The main biological significance of endocytosis is there is a receipt of building blocks due to intracellular digestion of macromolecules in lysosomes.
Absorption of substances in eukaryotic cells begins in thecialized regions of the plasma membrane, the so-calledwe are X bordered pits. On electron micrographspits look like invaginations of the plasma membrane, cytoplasmthe mat side of which is covered with a fibrous layer. layer likewould border small pits plaz malemmas. The pits occupy about 2% of theover the surface of the cell membranewe are eukaryotes. Within a minute the fossae grow deeper and deeper Xia, are drawn into the cage and then, narrowing at the base, split off,forming fringed vesicles.It has been established that frommatic membrane fibroblasttov within one minute flakeabout a quarter ofmembranes in the form of bordered pu zyrkov. Bubbles lose fast their border and acquire a wayability to fuse with the lysosome.
Endocytosis may be non-specific(constitutive)And specific(receptor).At nonspecific endocytosis the cell takes overabsorbs substances completely alien to it, for example, soot particles,dyes. First, particles are deposited on the glycocalyx plasmalemma. Especially well precipitated (adsorbed) on positively charged groups of proteins, since the glycocalyx carries negative charge. Then the cell morphology changesmembranes. It can either sink, forming invaginations(invagination), or, conversely, form outgrowths, which seem to add up, separating small volumes liquid environment. The formation of invaginations is more characteristic for cells of the intestinal epithelium, amoebae, and outgrowths - for phagocytes and fibroblasts. These processes can be blocked by inhibitorsbreathing. The resulting vesicles are primary endosomes that can merge each other, increasing in size. In the future, they will join mingle with lysosomes, turning into an endolysosome - a digester new vacuole. The intensity of liquid-phase nonspecific pinocytosis up towildly high. Macrophages form up to 125, and epithelial cells finelyth intestine up to a thousand pinosom per minute. The abundance of pinosomes leads to the fact that the plasmalemma is quickly spent on the formation of multiplethe presence of small vacuoles. Membrane recovery is quite fast.tro during recycling in the process of exocytosis due to the return of wakuoles and their incorporation into the plasmalemma. In macrophages, all plasmaThe classical membrane is replaced in 30 minutes, and in fibroblasts in 2 hours.
A more efficient way to absorb from extracellular fluidbone-specific macromolecules is specific en docytosis(mediated by receptors). The macromolecules in thisbind to complementary receptors on the surfacecells accumulate in the bordered fossa, and then, forming an endosome, are immersed in the cytosol. Receptor endocytosis ensures the accumulation of specific macromolecules at its receptor.Molecules that bind on the surface of the plasmalemma to the receptortorus are called ligands. With the help of the receptor endocytosis in many animal cells is the absorptionextracellular cholesterol environment.
The plasma membrane takes part in the removal of substances from the cell (exocytosis). In this case, the vacuoles approach the plasmalemma. At the points of contact, the plasmolemma and the vacuole membrane merge and the contents of the vacuole enter the environment.In some protozoa, sites on the cell membrane for exocytosis are predetermined. So, in the plasma membrane some ciliary ciliates have certain areas with the correct arrangement of large globules of integral proteins. Atmucocysts and trichocysts of ciliates completely ready for secretion, on the upper part of the plasmalemma there is a corolla of integral globulesproteins. These sections of the membrane of mucocysts and trichocysts of soprikaadhere to the surface of the cell.A peculiar exocytosis is observed in neutrophils. They spocapable of being released into the environment under certain conditionsblow your lysosomes. In some cases, small outgrowths of the plasmalemma containing lysosomes are formed, which then break off and pass into the environment. In other cases, there is invagination of the plasmalemma deep into the cell and its capture of lysosomes, located nyh far from the cell surface.
The processes of endocytosis and exocytosis are carried out with the participation of the system of fibrillar components of the cytoplasm associated with the plasmolemma.
Receptor function of the plasmalemma. This one of the main, universal for all cells, is rereceptor function of the plasmalemma. It defines interactioncells with each other and with the environment.
The whole variety of informational intercellular interactions can be schematically represented as a chain of successivesignal-receptor-second messenger-response reactions (concept signal-response).Signals transmit information from cell to cellnye molecules that are produced in some cells and specialphysically affect others sensitive to the signal (cells-mi sheni). Signal molecule - primary intermediary binding etsya with receptors located on target cells, react only for certain signals. Signal molecules - ligands- approach their receptor like a key to a lock. Ligand-mi for membrane receptors (plasmalemma receptors) ishydrophilic molecules, peptide hormones, neuromedia tori, cytokines, antibodies, and for nuclear receptors - fat soluble molecules, steroid and thyroid hormones, vitamin DAs receptors on topcells can act as proteinsmembranes or glycocalytic elementsca - polysaccharides and glycoproteins.It is believed that sensitive toto separate substances areas, scattersan along the surface of the cell orbranes into small bands. Yes, onsurface of prokaryotic cellsand animal cells there are limitsa fixed number of places with which they canbind viral particles. memesquirrels (carriers and canaly) learn, interact and transferwear only certain substances.Cell receptors are involved intransmission of signals from the surface of the cell to the inside.Diversity and specificitya ditch of receptors on the cell surfaceleads to a very complex systemwe are markers to distinguishown cells from strangers. Similar cellsinteract with each other, their surfaces can stick together (conjugation atprotozoa, tissue formation in multicellular organisms). Cells do not perceivecommon markers, as well as those differing inboron of determinant markersare torn apart or rejected.Upon formation of the receptor-ligand complex,transmembrane proteins: protein converter, protein enhancer.As a result, the receptor changes its conformation and interactionno with cellular precursor of the second messenger ka - messenger.Messengers can be ionized calcium, phospholipfor C, adenylate cyclase, guanylate cyclase. Influenced by messengeractivation of enzymes involved in the synthesis cyclic monophosphates - AMP or HMF. The latter change the assetThe presence of two types of protein kinase enzymes in the cytoplasm of the cell, leading to the phosphorylation of numerous intracellular proteins.
The most common formation of cAMP, under the action ofthe secretion of a number of hormones - thyroxine, cortisone, progesterone, increases, the breakdown of glycogen in the liver and muscles increases,heart rate and force, osteodestruction, reverse absorption of water in the tubules of the nephron.
The activity of the adenylate cyclase system is very high - the synthesis of cAMP leads to a ten thousandth increase in the signal.
Under the action of cGMP, the secretion of insulin by the pancreas, histamine by mast cells, serotoninbocytes, smooth muscle tissue is reduced.
In many cases, upon formation of the receptor-ligand complexthere is a change in the membrane potential, which in turn leads to a change in the permeability of the plasmalemma and metabolicsome processes in the cell.
The plasma membrane contains specific receptors tori responding to physical factors. So, in photosynthetic bacteria, chlorophylls are located on the surface of the cell,reacting to light. In photosensitive animals in plasmaThere is a whole system of phogoreceptor proteins in the caustic membrane -rhodopsins, with the help of which the light stimulus transforms is converted into a chemical signal and then an electrical impulse.
3. Intercellular contacts
In multicellular animals, the plasmolemma takes part in the formation intercellular connections providing intercellular interactions. There are several types of such structures.
§ Simple contact.A simple contact is found among the majority of cells of various origins adjacent to each other. Represents the convergence of the plasma membranes of neighboring cells at a distance of 15-20 nm. In this case, the interaction of the glycocalyx layers of neighboring cells occurs.
§ Tight (closing) contact. With such a connection, the outer layers of the two plasma membranes are as close as possible. The rapprochement is so dense that there is a kind of merging of sections of the plasma membranes of two neighboring cells. The fusion of membranes does not occur over the entire area of tight contact, but is a series of point convergence of membranes. The role of tight contact is to mechanically connect cells to each other. This area is impenetrable for macromolecules and ions and, therefore, it locks, delimits intercellular gaps (and together with them actually internal environment organism) from the external environment.
§ The patch of adhesion, or desmosome. The desmosome is a small area up to 0.5 µm in diameter. In the zone of the desmosome on the side of the cytoplasm, there is an area of thin fibrils. The functional role of desmosomes is mainly in the mechanical connection between cells.
§ Gap contact, or nexus. With this type of contact, the plasma membranes of neighboring cells are separated by a gap of 2-3 nm over a distance of 0.5–3 µm. In the structure of plasmolemms, special protein complexes (connexons) are located. One connexon on the cell's plasma membrane is precisely opposed by a connexon on the plasma membrane of the neighboring cell. As a result, a channel is formed from one cell to another. Connexons can contract, changing the diameter of the internal channel, and thereby participate in the regulation of the transport of molecules between cells. This type of connection is found in all tissue groups. The functional role of the gap junction is to carry ions and small molecules from cell to cell. So, in the heart muscle, excitation, which is based on the process of changing ion permeability, is transmitted from cell to cell through the nexus.
§ Synaptic contact, or synapse. Synapses are areas of contact between two cells specialized for one-way transmission of excitation or inhibition from one element to another. This type of connection is characteristic of nervous tissue and occurs both between two neurons and between a neuron and some other element. The membranes of these cells are separated by an intercellular space - a synaptic cleft about 20-30 nm wide. The membrane in the area of synaptic contact of one cell is called presynaptic, the other - postsynaptic. Near the presynaptic membrane great amount small vacuoles (synaptic vesicles) containing a neurotransmitter. At the time of passage of the nerve impulse, synaptic vesicles eject the neurotransmitter into the synaptic cleft. The mediator interacts with the receptor sites of the postsynaptic membrane, which ultimately leads to the transmission of a nerve impulse. In addition to transmitting a nerve impulse, synapses provide a rigid connection between the surfaces of two interacting cells.
§ Plasmodesma.This type of intercellular communication is found in plants. Plasmodesmata are thin tubular channels that connect two adjacent cells. The diameter of these channels is usually 40-50 nm. Plasmodesmata pass through the cell wall that separates the cells. In young cells, the number of plasmodesmata can be very high (up to 1000 per cell). With aging of cells, their number decreases due to ruptures with increasing thickness. cell wall. The functional role of plasmodesmata is to ensure intercellular circulation of solutions containing nutrients, ions and other compounds. Plasmodesmata infect cells with plant viruses.
Specialized Structures plasma membrane
The plasmalemma of many animal cells forms outgrowths of various structures (microvilli, cilia, flagella). Most often found on the surface of many animal cells microvilli. These outgrowths of the cytoplasm, bounded by the plasmalemma, have the shape of a cylinder with a rounded top. Microvilli are characteristic of epithelial cells, but are also found in cells of other tissues. The microvilli are about 100 nm in diameter. Their number and length are different for different types cells. The significance of microvilli lies in a significant increase in the area of the cell surface. This is especially important for cells involved in absorption. So, in the intestinal epithelium, there are up to 2x10 8 microvilli per 1 mm 2 of the surface.
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