Delimitative ( barrier) - separate cellular contents from external environment;
Regulate the exchange between the cell and the environment;
They divide cells into compartments, or compartments, intended for certain specialized metabolic pathways ( dividing);
It is the site of some chemical reactions (light reactions of photosynthesis in chloroplasts, oxidative phosphorylation during respiration in mitochondria);
Provide communication between cells in the tissues of multicellular organisms;
Transport- carries out transmembrane transport.
Receptor- are the location of receptor sites that recognize external stimuli.
Transport of substances through the membrane - one of the leading functions of the membrane, ensuring the exchange of substances between the cell and the external environment. Depending on the energy consumption for the transfer of substances, they are distinguished:
passive transport, or facilitated diffusion;
active (selective) transport with the participation of ATP and enzymes.
transport in membrane packaging. There are endocytosis (into the cell) and exocytosis (out of the cell) - mechanisms that transport large particles and macromolecules through the membrane. During endocytosis, the plasma membrane forms an invagination, its edges merge, and a vesicle is released into the cytoplasm. The vesicle is delimited from the cytoplasm by a single membrane, which is part of the outer cytoplasmic membrane. There are phagocytosis and pinocytosis. Phagocytosis is the absorption of large particles that are quite hard. For example, phagocytosis of lymphocytes, protozoa, etc. Pinocytosis is the process of capturing and absorbing droplets of liquid with substances dissolved in it.
Exocytosis is the process of removing various substances from the cell. During exocytosis, the membrane of the vesicle, or vacuole, fuses with the outer cytoplasmic membrane. The contents of the vesicle are removed beyond the cell surface, and the membrane is included in the outer cytoplasmic membrane.
At the core passive transport of uncharged molecules lies in the difference between the concentrations of hydrogen and charges, i.e. electrochemical gradient. Substances will move from an area with a higher gradient to an area with a lower one. The speed of transport depends on the difference in gradients.
Simple diffusion is the transport of substances directly through the lipid bilayer. Characteristic of gases, non-polar or small uncharged polar molecules, soluble in fats. Water quickly penetrates the bilayer because its molecule is small and electrically neutral. The diffusion of water through membranes is called osmosis.
Diffusion through membrane channels is the transport of charged molecules and ions (Na, K, Ca, Cl) penetrating through the membrane due to the presence of special channel-forming proteins that form water pores.
Facilitated diffusion is the transport of substances using special transport proteins. Each protein is responsible for a strictly defined molecule or group of related molecules, interacts with it and moves through the membrane. For example, sugars, amino acids, nucleotides and other polar molecules.
Active transport carried out by carrier proteins (ATPase) against an electrochemical gradient, with energy consumption. Its source is ATP molecules. For example, sodium is a potassium pump.
The concentration of potassium inside the cell is much higher than outside it, and sodium - vice versa. Therefore, potassium and sodium cations passively diffuse through the water pores of the membrane along a concentration gradient. This is explained by the fact that the permeability of the membrane for potassium ions is higher than for sodium ions. Accordingly, potassium diffuses out of the cell faster than sodium into the cell. However, for normal cell functioning a certain ratio of 3 potassium and 2 sodium ions is necessary. Therefore, there is a sodium-potassium pump in the membrane that actively pumps sodium out of the cell and potassium into the cell. This pump is a transmembrane membrane protein capable of conformational rearrangements. Therefore, it can attach to itself both potassium and sodium ions (antiport). The process is energy intensive:
WITH inside membranes, sodium ions and an ATP molecule enter the pump protein, and potassium ions come from the outside membrane.
Sodium ions combine with protein molecule, and the protein acquires ATPase activity, i.e. the ability to cause ATP hydrolysis, which is accompanied by the release of energy that drives the pump.
The phosphate released during ATP hydrolysis attaches to the protein, i.e. phosphorylates the protein.
Phosphorylation causes conformational changes in the protein; it becomes unable to retain sodium ions. They are released and move outside the cell.
The new conformation of the protein promotes the attachment of potassium ions to it.
The addition of potassium ions causes dephosphorylation of the protein. It changes its conformation again.
A change in protein conformation leads to the release of potassium ions inside the cell.
The protein is again ready to attach sodium ions to itself.
In one cycle of operation, the pump pumps out 3 sodium ions from the cell and pumps in 2 potassium ions.
Cytoplasm– an obligatory component of the cell, located between the surface apparatus of the cell and the nucleus. This is a complex heterogeneous structural complex consisting of:
hyaloplasma
organelles (permanent components of the cytoplasm)
inclusions are temporary components of the cytoplasm.
Cytoplasmic matrix(hyaloplasm) is the internal contents of the cell - a colorless, thick and transparent colloidal solution. The components of the cytoplasmic matrix carry out biosynthesis processes in the cell and contain enzymes necessary for energy production, mainly due to anaerobic glycolysis.
Basic properties of the cytoplasmic matrix.
Determines the colloidal properties of the cell. Together with the intracellular membranes of the vacuolar system, it can be considered a highly heterogeneous or multiphase colloidal system.
Provides a change in the viscosity of the cytoplasm, a transition from a gel (thicker) to a sol (more liquid), which occurs under the influence of external and internal factors.
Provides cyclosis, amoeboid movement, cell division and movement of pigment in chromatophores.
Determines the polarity of the location of intracellular components.
Provides mechanical properties of cells - elasticity, ability to merge, rigidity.
Organelles– permanent cellular structures that ensure the cell performs specific functions. Depending on the structural features, they are distinguished:
membrane organelles - have a membrane structure. They can be single-membrane (ER, Golgi apparatus, lysosomes, vacuoles of plant cells). Double-membrane (mitochondria, plastids, nucleus).
Non-membrane organelles - do not have a membrane structure (chromosomes, ribosomes, cell center, cytoskeleton).
General-purpose organelles are characteristic of all cells: nucleus, mitochondria, cell center, Golgi apparatus, ribosomes, EPS, lysosomes. When organelles are characteristic of certain cell types, they are called specialty organelles (for example, myofibrils that contract a muscle fiber).
Endoplasmic reticulum- a single continuous structure, the membrane of which forms many invaginations and folds that look like tubules, microvacuoles and large cisterns. EPS membranes are connected, on the one hand, to the cell cytoplasmic membrane, and on the other, to outer shell nuclear membrane.
There are two types of EPS - rough and smooth.
In rough or granular ER, cisterns and tubules are associated with ribosomes. is the outer side of the membrane. Smooth or agranular ER has no connection with ribosomes. This is the inner side of the membrane.
Biological membranes.
The term “membrane” (Latin membrana - skin, film) began to be used more than 100 years ago to designate a cell boundary that serves, on the one hand, as a barrier between the contents of the cell and the external environment, and on the other, as a semi-permeable partition through which water can pass. and some substances. However, the functions of the membrane are not limited to this, since biological membranes form the basis of the structural organization of the cell.
Membrane structure. According to this model, the main membrane is a lipid bilayer in which the hydrophobic tails of the molecules face inward and the hydrophilic heads face outward. Lipids are represented by phospholipids - derivatives of glycerol or sphingosine. Proteins are associated with the lipid layer. Integral (transmembrane) proteins penetrate the membrane through and are firmly associated with it; peripheral ones do not penetrate and are less firmly connected to the membrane. Functions of membrane proteins: maintaining membrane structure, receiving and converting signals from the environment. environment, transport of certain substances, catalysis of reactions occurring on membranes. The membrane thickness ranges from 6 to 10 nm.
Membrane properties:
1. Fluidity. The membrane is not a rigid structure; most of its constituent proteins and lipids can move in the plane of the membrane.
2. Asymmetry. Composition of external and inner layers both proteins and lipids are different. Besides, plasma membranes animal cells have a layer of glycoproteins on the outside (glycocalyx, which performs signaling and receptor functions, and is also important for uniting cells into tissues)
3. Polarity. The outer side of the membrane carries a positive charge, while the inner side carries a negative charge.
4. Selective permeability. The membranes of living cells, in addition to water, allow only certain molecules and ions of dissolved substances to pass through. (The use of the term “semi-permeability” in relation to cell membranes is not entirely correct, since this concept implies that the membrane allows only solvent molecules to pass through, while retaining all molecules and ions of dissolved substances.)
The outer cell membrane (plasmalemma) is an ultramicroscopic film 7.5 nm thick, consisting of proteins, phospholipids and water. An elastic film that is well wetted by water and quickly restores its integrity after damage. It has a universal structure, typical of all biological membranes. The borderline position of this membrane, its participation in the processes of selective permeability, pinocytosis, phagocytosis, excretion of excretory products and synthesis, in interaction with neighboring cells and protection of the cell from damage makes its role extremely important. Animal cells outside the membrane are sometimes covered with a thin layer consisting of polysaccharides and proteins - the glycocalyx. On the outside of plant cells cell membrane there is a strong cell wall that creates external support and maintains the shape of the cell. It consists of fiber (cellulose), a water-insoluble polysaccharide.
Plasma membrane , or plasmalemma,- the most permanent, basic, universal membrane for all cells. It is a thin (about 10 nm) film covering the entire cell. The plasmalemma consists of protein molecules and phospholipids (Fig. 1.6).
Phospholipid molecules are arranged in two rows - with hydrophobic ends inward, hydrophilic heads towards the internal and external aqueous environment. In some places, the bilayer (double layer) of phospholipids is penetrated through and through by protein molecules (integral proteins). Inside such protein molecules there are channels - pores through which water-soluble substances pass. Other protein molecules penetrate the lipid bilayer halfway on one side or the other (semi-integral proteins). There are peripheral proteins on the surface of the membranes of eukaryotic cells. Lipid and protein molecules are held together due to hydrophilic-hydrophobic interactions.
Properties and functions of membranes. All cell membranes are mobile fluid structures, since lipid and protein molecules are not interconnected by covalent bonds and are able to move quite quickly in the plane of the membrane. Thanks to this, membranes can change their configuration, i.e. they have fluidity.
Membranes are very dynamic structures. They quickly recover from damage and also stretch and contract with cellular movements.
Membranes of different types of cells differ significantly both in chemical composition and in the relative content of proteins, glycoproteins, lipids in them, and, consequently, in the nature of the receptors they contain. Each cell type is therefore characterized by an individuality, which is determined mainly glycoproteins. Branched chain glycoproteins protruding from the cell membrane are involved in factor recognition external environment, as well as in mutual recognition of related cells. For example, an egg and a sperm recognize each other by cell surface glycoproteins, which fit together as separate elements of a whole structure. Such mutual recognition is a necessary stage preceding fertilization.
A similar phenomenon is observed in the process of tissue differentiation. In this case, cells similar in structure, with the help of recognition areas of the plasmalemma, are correctly oriented relative to each other, thereby ensuring their adhesion and tissue formation. Associated with recognition transport regulation molecules and ions through the membrane, as well as an immunological response in which glycoproteins play the role of antigens. Sugars can thus function as information molecules (like proteins and nucleic acids). The membranes also contain specific receptors, electron carriers, energy converters, and enzyme proteins. Proteins are involved in ensuring the transport of certain molecules into or out of the cell, provide a structural connection between the cytoskeleton and cell membranes, or serve as receptors for receiving and converting chemical signals from the environment.
The most important property of the membrane is also selective permeability. This means that molecules and ions pass through it with at different speeds, and the larger the size of the molecules, the lower the speed of their passage through the membrane. This property defines the plasma membrane as osmotic barrier. Water and gases dissolved in it have the maximum penetrating ability; Ions pass through the membrane much more slowly. The diffusion of water through a membrane is called by osmosis.
There are several mechanisms for transporting substances across the membrane.
Diffusion- penetration of substances through a membrane along a concentration gradient (from an area where their concentration is higher to an area where their concentration is lower). Diffuse transport of substances (water, ions) is carried out with the participation of membrane proteins, which have molecular pores, or with the participation of the lipid phase (for fat-soluble substances).
With facilitated diffusion special membrane transport proteins selectively bind to one or another ion or molecule and transport them across the membrane along a concentration gradient.
Active transport involves energy costs and serves to transport substances against their concentration gradient. He carried out by special carrier proteins that form the so-called ion pumps. The most studied is the Na - / K - pump in animal cells, which actively pumps Na + ions out while absorbing K - ions. Due to this, a higher concentration of K - and a lower concentration of Na + is maintained in the cell compared to the environment. This process requires ATP energy.
As a result of active transport using a membrane pump in the cell, the concentration of Mg 2- and Ca 2+ is also regulated.
During the process of active transport of ions into the cell, various sugars, nucleotides, and amino acids penetrate through the cytoplasmic membrane.
Macromolecules of proteins, nucleic acids, polysaccharides, lipoprotein complexes, etc. do not pass through cell membranes, unlike ions and monomers. Transport of macromolecules, their complexes and particles into the cell occurs in a completely different way - through endocytosis. At endocytosis (endo...- inward) a certain area of the plasmalemma captures and, as it were, envelops extracellular material, enclosing it in a membrane vacuole that arises as a result of invagination of the membrane. Subsequently, such a vacuole connects with a lysosome, the enzymes of which break down macromolecules into monomers.
The reverse process of endocytosis is exocytosis (exo...- out). Thanks to it, the cell removes intracellular products or undigested residues enclosed in vacuoles or pu-
zyryki. The vesicle approaches the cytoplasmic membrane, merges with it, and its contents are released into the environment. This is how digestive enzymes, hormones, hemicellulose, etc. are removed.
Thus, biological membranes, as the main structural elements of a cell, serve not just as physical boundaries, but are dynamic functional surfaces. Numerous biochemical processes take place on the membranes of organelles, such as active absorption of substances, energy conversion, ATP synthesis, etc.
Functions of biological membranes the following:
They delimit the contents of the cell from the external environment and the contents of organelles from the cytoplasm.
They ensure the transport of substances into and out of the cell, from the cytoplasm to organelles and vice versa.
Act as receptors (receiving and converting chemicals from the environment, recognizing cell substances, etc.).
They are catalysts (providing for near-membrane chemical processes).
Participate in energy conversion.
Main structural unit living organism - a cell, which is a differentiated section of the cytoplasm surrounded by a cell membrane. Due to the fact that the cell performs many important functions, such as reproduction, nutrition, movement, the membrane must be plastic and dense.
History of the discovery and research of the cell membrane
In 1925, Grendel and Gorder staged successful experiment to identify “shadows” of red blood cells, or empty membranes. Despite several serious mistakes, scientists discovered the lipid bilayer. Their work was continued by Danielli, Dawson in 1935, and Robertson in 1960. As a result of many years of work and accumulation of arguments, in 1972 Singer and Nicholson created a fluid-mosaic model of the membrane structure. Further experiments and studies confirmed the works of scientists.
Meaning
What is a cell membrane? This word began to be used more than a hundred years ago; translated from Latin it means “film”, “skin”. This is how the cell boundary is designated, which is a natural barrier between the internal contents and the external environment. The structure of the cell membrane implies semi-permeability, due to which moisture and nutrients and decomposition products can freely pass through it. This shell can be called the main structural component of the cell organization.
Let's consider the main functions of the cell membrane
1. Separates the internal contents of the cell and components of the external environment.
2. Helps maintain a constant chemical composition of the cell.
3. Regulates proper metabolism.
4. Provides communication between cells.
5. Recognizes signals.
6. Protection function.
"Plasma Shell"
The outer cell membrane, also called the plasma membrane, is an ultramicroscopic film whose thickness ranges from five to seven nanomillimeters. It consists mainly of protein compounds, phospholides, and water. The film is elastic, easily absorbs water, and quickly restores its integrity after damage.
It has a universal structure. This membrane occupies a border position, participates in the process of selective permeability, removal of decay products, and synthesizes them. Relationship with neighbors and reliable protection internal contents from damage makes it an important component in such a matter as the structure of the cell. The cell membrane of animal organisms is sometimes covered with a thin layer - the glycocalyx, which includes proteins and polysaccharides. Plant cells outside the membrane are protected by a cell wall, which serves as support and maintains shape. The main component of its composition is fiber (cellulose) - a polysaccharide that is insoluble in water.
Thus, the outer cell membrane has the function of repair, protection and interaction with other cells.
Structure of the cell membrane
The thickness of this movable shell varies from six to ten nanomillimeters. The cell membrane of a cell has special composition, the basis of which is a lipid bilayer. Hydrophobic tails, inert to water, are located on the inside, while hydrophilic heads, interacting with water, face outward. Each lipid is a phospholipid, which is the result of the interaction of substances such as glycerol and sphingosine. The lipid framework is closely surrounded by proteins, which are arranged in a non-continuous layer. Some of them are immersed in the lipid layer, the rest pass through it. As a result, areas permeable to water are formed. The functions performed by these proteins are different. Some of them are enzymes, the rest are transport proteins that transfer various substances from the external environment to the cytoplasm and back.
The cell membrane is permeated through and closely connected by integral proteins, and the connection with peripheral ones is less strong. These proteins perform an important function, which is to maintain the structure of the membrane, receive and convert signals from the environment, transport substances, and catalyze reactions that occur on membranes.
Compound
The basis of the cell membrane is a bimolecular layer. Thanks to its continuity, the cell has barrier and mechanical properties. At different stages of life, this bilayer can be disrupted. As a result, structural defects of through hydrophilic pores are formed. In this case, absolutely all functions of such a component as the cell membrane can change. The core may suffer from external influences.
Properties
The cell membrane of a cell has interesting features. Due to its fluidity, this membrane is not a rigid structure, and the bulk of the proteins and lipids that make up it move freely on the plane of the membrane.
In general, the cell membrane is asymmetrical, so the composition of the protein and lipid layers differs. Plasma membranes in animal cells, on their outer side, have a glycoprotein layer that performs receptor and signaling functions, and also plays a large role in the process of combining cells into tissue. The cell membrane is polar, that is, the charge on the outside is positive and the charge on the inside is negative. In addition to all of the above, the cell membrane has selective insight.
This means that, in addition to water, only a certain group of molecules and ions of dissolved substances are allowed into the cell. The concentration of a substance such as sodium in most cells is much lower than in the external environment. Potassium ions have a different ratio: their amount in the cell is much higher than in environment. In this regard, sodium ions tend to penetrate the cell membrane, and potassium ions tend to be released outside. Under these circumstances, the membrane activates a special system that plays a “pumping” role, leveling the concentration of substances: sodium ions are pumped to the surface of the cell, and potassium ions are pumped inside. This feature included in essential functions cell membrane.
This tendency of sodium and potassium ions to move inward from the surface plays a big role in the transport of sugar and amino acids into the cell. In the process of actively removing sodium ions from the cell, the membrane creates conditions for new intakes of glucose and amino acids inside. On the contrary, in the process of transferring potassium ions into the cell, the number of “transporters” of decay products from inside the cell to the external environment is replenished.
How does cell nutrition occur through the cell membrane?
Many cells take up substances through processes such as phagocytosis and pinocytosis. In the first option, a flexible outer membrane creates a small depression in which the captured particle ends up. The diameter of the recess then becomes larger until the enclosed particle enters the cell cytoplasm. Through phagocytosis, some protozoa, such as amoebas, are fed, as well as blood cells - leukocytes and phagocytes. Similarly, cells absorb fluid, which contains the necessary nutrients. This phenomenon is called pinocytosis.
The outer membrane is closely connected to the endoplasmic reticulum of the cell.
Many types of main tissue components have protrusions, folds, and microvilli on the surface of the membrane. Plant cells the outside of this shell is covered with another, thick and clearly visible under a microscope. The fiber from which they are composed helps to form tissue support plant origin, for example, wood. Animal cells also have a number of external structures, which are located on top of the cell membrane. They are exclusively protective in nature, an example of this is chitin contained in cover cells insects
In addition to the cellular membrane, there is an intracellular membrane. Its function is to divide the cell into several specialized closed compartments - compartments or organelles, where a certain environment must be maintained.
Thus, it is impossible to overestimate the role of such a component of the basic unit of a living organism as the cell membrane. The structure and functions suggest a significant expansion of the total cell surface area, improvement metabolic processes. This molecular structure consists of proteins and lipids. Separating the cell from the external environment, the membrane ensures its integrity. With its help, intercellular connections are maintained at a fairly strong level, forming tissues. In this regard, we can conclude that one of the critical roles The cell membrane plays a role in the cell. The structure and functions performed by it differ radically in different cells, depending on their purpose. Through these features, a variety of physiological activities of cell membranes and their roles in the existence of cells and tissues is achieved.
The membrane is an ultra-fine structure that forms the surfaces of organelles and the cell as a whole. All membranes have a similar structure and are connected into one system.
Chemical composition
Cell membranes are chemically homogeneous and consist of proteins and lipids of various groups:
- phospholipids;
- galactolipids;
- sulfolipids.
They also contain nucleic acids, polysaccharides and other substances.
Physical properties
At normal temperature The membranes are in a liquid crystalline state and constantly oscillate. Their viscosity is close to that of vegetable oil.
The membrane is recoverable, durable, elastic and porous. Membrane thickness is 7 - 14 nm.
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The membrane is impermeable to large molecules. Small molecules and ions can pass through the pores and the membrane itself under the influence of concentration differences on different sides of the membrane, as well as with the help of transport proteins.
Model
Typically, the structure of membranes is described using a fluid mosaic model. The membrane has a framework - two rows of lipid molecules, tightly adjacent to each other, like bricks.
Rice. 1. Sandwich-type biological membrane.
On both sides the surface of lipids is covered with proteins. The mosaic pattern is formed by protein molecules unevenly distributed on the surface of the membrane.
According to the degree of immersion in the bilipid layer, protein molecules are divided into three groups:
- transmembrane;
- submerged;
- superficial.
Proteins provide the main property of the membrane - its selective permeability to various substances.
Membrane types
All cell membranes according to localization can be divided into the following types:
- external;
- nuclear;
- organelle membranes.
The outer cytoplasmic membrane, or plasmolemma, is the boundary of the cell. Connecting with the elements of the cytoskeleton, it maintains its shape and size.
Rice. 2. Cytoskeleton.
The nuclear membrane, or karyolemma, is the boundary of the nuclear contents. It is constructed of two membranes, very similar to the outer one. The outer membrane of the nucleus is associated with membranes endoplasmic reticulum(EPS) and, through the pores, with the inner membrane.
ER membranes penetrate the entire cytoplasm, forming surfaces on which the synthesis of various substances, including membrane proteins, takes place.
Organelle membranes
Most organelles have a membrane structure.
The walls are built from one membrane:
- Golgi complex;
- vacuoles;
- lysosomes
Plastids and mitochondria are built from two layers of membranes. Their outer membrane is smooth, and the inner one forms many folds.
Features of photosynthetic membranes of chloroplasts are built-in chlorophyll molecules.
Animal cells have a carbohydrate layer on the surface of their outer membrane called the glycocalyx.
Rice. 3. Glycocalyx.
The glycocalyx is most developed in the cells of the intestinal epithelium, where it creates conditions for digestion and protects the plasmalemma.
Table "Structure of the cell membrane"
What have we learned?
We looked at the structure and functions of the cell membrane. The membrane is a selective (selective) barrier of the cell, nucleus and organelles. The structure of the cell membrane is described by the fluid mosaic model. According to this model, protein molecules are built into the bilayer of viscous lipids.
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