The cell membrane is the structure that covers the outside of the cell. It is also called cytolemma or plasmolemma.
This formation is built from a bilipid layer (bilayer) with proteins embedded in it. The carbohydrates that make up the plasmalemma are in a bound state.
The distribution of the main components of the plasma membrane is as follows: more than half of the chemical composition is proteins, a quarter is occupied by phospholipids, and a tenth is cholesterol.
The cell membrane is a thin film, which is based on layers of lipoproteins and proteins.
By localization, membrane organelles are distinguished, which have some features in plant and animal cells:
There is also an inner and outer (plasmolemma) cell membrane.
The cell membrane contains carbohydrates that cover it in the form of a glycocalyx. This is a supra-membrane structure that performs a barrier function. The proteins located here are in a free state. Unbound proteins are involved in enzymatic reactions, providing extracellular breakdown of substances.
Proteins of the cytoplasmic membrane are represented by glycoproteins. According to the chemical composition, proteins are isolated that are completely included in the lipid layer (throughout) - integral proteins. Also peripheral, not reaching one of the surfaces of the plasmalemma.
The former function as receptors, binding to neurotransmitters, hormones, and other substances. Insertion proteins are necessary for the construction of ion channels through which ions and hydrophilic substrates are transported. The latter are enzymes that catalyze intracellular reactions.
The lipid bilayer prevents the penetration of water. Lipids are hydrophobic compounds present in the cell as phospholipids. The phosphate group is turned outward and consists of two layers: the outer one, directed to the extracellular environment, and the inner one, delimiting the intracellular contents.
Water-soluble areas are called hydrophilic heads. The fatty acid sites are directed inside the cell, in the form of hydrophobic tails. The hydrophobic part interacts with neighboring lipids, which ensures their attachment to each other. The double layer has selective permeability in different areas.
So, in the middle, the membrane is impermeable to glucose and urea, hydrophobic substances pass freely here: carbon dioxide, oxygen, alcohol. Cholesterol is important, the content of the latter determines the viscosity of the plasma membrane.
The characteristics of the functions are briefly listed in the table:
| Membrane function | Description |
| barrier role | The plasmalemma performs a protective function, protecting the contents of the cell from the effects of foreign agents. Due to the special organization of proteins, lipids, carbohydrates, the semi-permeability of the plasma membrane is ensured. |
| Receptor function | Through the cell membrane, biologically active substances are activated in the process of binding to receptors. Thus, immune reactions are mediated through the recognition of foreign agents by the receptor apparatus of cells localized on the cell membrane. |
| transport function | The presence of pores in the plasmalemma allows you to regulate the flow of substances into the cell. The transfer process proceeds passively (without energy consumption) for compounds with low molecular weight. Active transfer is associated with the expenditure of energy released during the breakdown of adenosine triphosphate (ATP). This method takes place for the transfer of organic compounds. |
| Participation in the processes of digestion | Substances are deposited on the cell membrane (sorption). Receptors bind to the substrate, moving it inside the cell. A vesicle is formed, lying freely inside the cell. Merging, such vesicles form lysosomes with hydrolytic enzymes. |
| Enzymatic function | Enzymes, necessary components of intracellular digestion. Reactions that require the participation of catalysts proceed with the participation of enzymes. |
The cell membrane is involved in maintaining homeostasis due to the high selectivity of substances entering and leaving the cell (in biology this is called selective permeability).
Outgrowths of the plasmolemma divide the cell into compartments (compartments) responsible for performing certain functions. Specifically arranged membranes, corresponding to the fluid-mosaic scheme, ensure the integrity of the cell.
Outside, the cell is covered with a plasma membrane (or outer cell membrane) about 6-10 nm thick.
The cell membrane is a dense film of proteins and lipids (mainly phospholipids). Lipid molecules are arranged in an orderly manner - perpendicular to the surface, in two layers, so that their parts that interact intensively with water (hydrophilic) are directed outward, and the parts that are inert to water (hydrophobic) are directed inward.
Protein molecules are located in a non-continuous layer on the surface of the lipid framework on both sides. Some of them are immersed in the lipid layer, and some pass through it, forming areas permeable to water. These proteins perform various functions - some of them are enzymes, others are transport proteins involved in the transfer of certain substances from the environment to the cytoplasm and vice versa.
One of the main properties of biological membranes is selective permeability (semipermeability)- some substances pass through them with difficulty, others easily and even towards a higher concentration. Thus, for most cells, the concentration of Na ions inside is much lower than in the environment. For K ions, the reverse ratio is characteristic: their concentration inside the cell is higher than outside. Therefore, Na ions always tend to enter the cell, and K ions - to go outside. The equalization of the concentrations of these ions is prevented by the presence in the membrane of a special system that plays the role of a pump that pumps Na ions out of the cell and simultaneously pumps K ions inside.
The desire of Na ions to move from outside to inside is used to transport sugars and amino acids into the cell. With the active removal of Na ions from the cell, conditions are created for the entry of glucose and amino acids into it.
In many cells, absorption of substances also occurs by phagocytosis and pinocytosis. At phagocytosis the flexible outer membrane forms a small depression where the captured particle enters. This recess increases, and, surrounded by a portion of the outer membrane, the particle is immersed in the cytoplasm of the cell. The phenomenon of phagocytosis is characteristic of amoeba and some other protozoa, as well as leukocytes (phagocytes). Similarly, the cells absorb liquids containing the substances necessary for the cell. This phenomenon has been called pinocytosis.
The outer membranes of various cells differ significantly both in the chemical composition of their proteins and lipids, and in their relative content. It is these features that determine the diversity in the physiological activity of the membranes of various cells and their role in the life of cells and tissues.
The endoplasmic reticulum of the cell is connected to the outer membrane. With the help of outer membranes, various types of intercellular contacts are carried out, i.e. communication between individual cells.
Many types of cells are characterized by the presence on their surface of a large number of protrusions, folds, microvilli. They contribute both to a significant increase in the surface area of cells and an improvement in metabolism, as well as to stronger bonds between individual cells.
On the outside of the cell membrane, plant cells have thick membranes that are clearly visible in an optical microscope, consisting of cellulose (cellulose). They create a strong support for plant tissues (wood).
Some cells of animal origin also have a number of external structures that are located on top of the cell membrane and have a protective character. An example is the chitin of the integumentary cells of insects.
| Function | Description |
|---|---|
| protective barrier | Separates the internal organelles of the cell from the external environment |
| Regulatory | It regulates the exchange of substances between the internal contents of the cell and the external environment. |
| Delimiting (compartmentalization) | Separation of the internal space of the cell into independent blocks (compartments) |
| Energy | - Accumulation and transformation of energy; - light reactions of photosynthesis in chloroplasts; - Absorption and secretion. |
| Receptor (information) | Participates in the formation of excitation and its conduct. |
| Motor | Carries out the movement of the cell or its individual parts. |
cell membrane also called plasma (or cytoplasmic) membrane and plasmalemma. This structure not only separates the internal contents of the cell from the external environment, but also enters into the composition of most cell organelles and the nucleus, in turn separating them from the hyaloplasm (cytosol) - the viscous-liquid part of the cytoplasm. Let's agree to call cytoplasmic membrane one that separates the contents of the cell from the external environment. The remaining terms refer to all membranes.
The basis of the structure of the cell (biological) membrane is a double layer of lipids (fats). The formation of such a layer is associated with the features of their molecules. Lipids do not dissolve in water, but condense in it in their own way. One part of a single lipid molecule is a polar head (it is attracted by water, i.e., hydrophilic), and the other is a pair of long non-polar tails (this part of the molecule is repelled by water, i.e., hydrophobic). This structure of the molecules makes them "hide" their tails from the water and turn their polar heads towards the water.
As a result, a lipid bilayer is formed, in which the non-polar tails are inside (facing each other), and the polar heads are facing out (to the external environment and cytoplasm). The surface of such a membrane is hydrophilic, but inside it is hydrophobic.
In cell membranes, phospholipids predominate among lipids (they are complex lipids). Their heads contain a residue of phosphoric acid. In addition to phospholipids, there are glycolipids (lipids + carbohydrates) and cholesterol (belongs to sterols). The latter gives the membrane rigidity, being located in its thickness between the tails of the remaining lipids (cholesterol is completely hydrophobic).
Due to electrostatic interaction, certain protein molecules are attached to the charged heads of lipids, which become surface membrane proteins. Other proteins interact with non-polar tails, partially sink into the bilayer, or penetrate it through and through.
Thus, the cell membrane consists of a bilayer of lipids, surface (peripheral), immersed (semi-integral), and penetrating (integral) proteins. In addition, some proteins and lipids on the outside of the membrane are associated with carbohydrate chains.
This fluid mosaic model of the membrane structure was put forward in the 70s of the XX century. Prior to this, a sandwich model of the structure was assumed, according to which the lipid bilayer is located inside, and on the inside and outside the membrane is covered with continuous layers of surface proteins. However, the accumulation of experimental data disproved this hypothesis.
The thickness of membranes in different cells is about 8 nm. Membranes (even different sides of one) differ from each other in the percentage of different types of lipids, proteins, enzymatic activity, etc. Some membranes are more liquid and more permeable, others are more dense.
Breaks in the cell membrane easily merge due to the physicochemical characteristics of the lipid bilayer. In the plane of the membrane, lipids and proteins (unless they are fixed by the cytoskeleton) move.
Most of the proteins immersed in the cell membrane perform an enzymatic function (they are enzymes). Often (especially in the membranes of cell organelles) enzymes are arranged in a certain sequence so that the reaction products catalyzed by one enzyme pass to the second, then the third, etc. A conveyor is formed that stabilizes surface proteins, because they do not allow enzymes to swim along the lipid bilayer.
The cell membrane performs a delimiting (barrier) function from the environment and at the same time a transport function. It can be said that this is its most important purpose. The cytoplasmic membrane, having strength and selective permeability, maintains the constancy of the internal composition of the cell (its homeostasis and integrity).
In this case, the transport of substances occurs in various ways. Transport along a concentration gradient involves the movement of substances from an area with a higher concentration to an area with a lower one (diffusion). So, for example, gases diffuse (CO 2, O 2).
There is also transport against the concentration gradient, but with the expenditure of energy.
Transport is passive and lightweight (when some carrier helps him). Passive diffusion across the cell membrane is possible for fat-soluble substances.
There are special proteins that make membranes permeable to sugars and other water-soluble substances. These carriers bind to transported molecules and drag them across the membrane. This is how glucose is transported into the red blood cells.
Spanning proteins, when combined, can form a pore for the movement of certain substances through the membrane. Such carriers do not move, but form a channel in the membrane and work similarly to enzymes, binding a specific substance. The transfer is carried out due to a change in the conformation of the protein, due to which channels are formed in the membrane. An example is the sodium-potassium pump.
The transport function of the eukaryotic cell membrane is also realized through endocytosis (and exocytosis). Through these mechanisms, large molecules of biopolymers, even whole cells, enter the cell (and out of it). Endo- and exocytosis are not characteristic of all eukaryotic cells (prokaryotes do not have it at all). So endocytosis is observed in protozoa and lower invertebrates; in mammals, leukocytes and macrophages absorb harmful substances and bacteria, i.e., endocytosis performs a protective function for the body.
Endocytosis is divided into phagocytosis(cytoplasm envelops large particles) and pinocytosis(capture of liquid droplets with substances dissolved in it). The mechanism of these processes is approximately the same. Absorbed substances on the cell surface are surrounded by a membrane. A vesicle (phagocytic or pinocytic) is formed, which then moves into the cell.
Exocytosis is the removal of substances from the cell by the cytoplasmic membrane (hormones, polysaccharides, proteins, fats, etc.). These substances are enclosed in membrane vesicles that fit the cell membrane. Both membranes merge and the contents are outside the cell.
The cytoplasmic membrane performs a receptor function. To do this, on its outer side there are structures that can recognize a chemical or physical stimulus. Some of the proteins penetrating the plasmalemma are 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. At the same time, channels can open, and certain substances can begin to enter the cell or be removed from it.
The receptor function of cell membranes has been well studied based on the action of the hormone insulin. When insulin binds to its glycoprotein receptor, the catalytic intracellular part of this protein (the enzyme adenylate cyclase) is activated. The enzyme synthesizes cyclic AMP from ATP. Already it activates or inhibits various enzymes of cellular metabolism.
The receptor function of the cytoplasmic membrane also includes the recognition of neighboring cells of the same type. Such cells are attached to each other by various intercellular contacts.
In tissues, with the help of intercellular contacts, cells can exchange information with each other using specially synthesized low molecular weight substances. One example of such an interaction is contact inhibition, when cells stop growing after receiving information that the free space is occupied.
Intercellular contacts are simple (membranes of different cells are adjacent to each other), locking (invagination of the membrane of one cell into another), desmosomes (when the membranes are connected by bundles of transverse fibers penetrating into the cytoplasm). In addition, there is a variant of intercellular contacts due to mediators (intermediaries) - synapses. In them, the signal is transmitted not only chemically, but also electrically. Synapses transmit signals between nerve cells, as well as from nerve to muscle.
The basic structural unit of a living organism is a cell, which is a differentiated section of the cytoplasm surrounded by a cell membrane. In view of the fact that the cell performs many important functions, such as reproduction, nutrition, movement, the shell must be plastic and dense.
In 1925, Grendel and Gorder made a successful experiment to identify the "shadows" of erythrocytes, or empty shells. Despite several gross mistakes made, scientists discovered the lipid bilayer. Their work was continued by Danielli, Dawson in 1935, Robertson in 1960. As a result of many years of work and the accumulation of arguments in 1972, Singer and Nicholson created a fluid mosaic model of the structure of the membrane. Further experiments and studies confirmed the works of scientists.
What is a cell membrane? This word began to be used more than a hundred years ago, translated from Latin it means "film", "skin". So designate the border of the cell, which is a natural barrier between the internal contents and the external environment. The structure of the cell membrane suggests semi-permeability, due to which moisture and nutrients and decay products can freely pass through it. This shell can be called the main structural component of the organization of the cell.
1. Separates the internal contents of the cell and the components of the external environment.
2. Helps maintain a constant chemical composition of the cell.
3. Regulates the correct metabolism.
4. Provides interconnection between cells.
5. Recognizes signals.
6. Protection function.
The outer cell membrane, also called the plasma membrane, is an ultramicroscopic film that is five to seven nanometers thick. It consists mainly of protein compounds, phospholide, water. The film is elastic, easily absorbs water, and also quickly restores its integrity after damage.
Differs in a universal structure. This membrane occupies a boundary position, participates in the process of selective permeability, excretion of decay products, synthesizes them. The relationship with the "neighbors" and the reliable protection of the 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 sometimes turns out to be covered with the thinnest layer - glycocalyx, which includes proteins and polysaccharides. Plant cells outside the membrane are protected by a cell wall that acts as a 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 performs the function of repair, protection and interaction with other cells.
The thickness of this movable shell varies from six to ten nanometers. The cell membrane of a cell has a special composition, the basis of which is the lipid bilayer. The hydrophobic tails, which are inert to water, are located on the inside, while the hydrophilic heads, which interact with water, are turned outward. Each lipid is a phospholipid, which is the result of the interaction of substances such as glycerol and sphingosine. The lipid scaffold is closely surrounded by proteins, which are located in a non-continuous layer. Some of them are immersed in the lipid layer, the rest pass through it. As a result, water-permeable areas are formed. The functions performed by these proteins are different. Some of them are enzymes, the rest are transport proteins that carry various substances from the external environment to the cytoplasm and vice versa.
The cell membrane is permeated through and closely connected with integral proteins, while 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.
The basis of the cell membrane is a bimolecular layer. Due 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 a cell membrane can change. In this case, the nucleus may suffer from external influences.
The cell membrane of a cell has interesting features. Due to its fluidity, this shell is not a rigid structure, and the main part of the proteins and lipids that make up its composition freely moves on the plane of the membrane.
In general, the cell membrane is asymmetric, so the composition of the protein and lipid layers is different. Plasma membranes in animal cells have a glycoprotein layer on their outer side, which performs receptor and signal functions, and also plays an important role in the process of combining cells into tissue. The cell membrane is polar, that is, the charge on the outside is positive, and on the inside it 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. For potassium ions, a different ratio is characteristic: their number in the cell is much higher than in the 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 performs a “pumping” role, leveling the concentration of substances: sodium ions are pumped out to the cell surface, and potassium ions are pumped inward. This feature is included in the most important functions of the cell membrane.
This tendency of sodium and potassium ions to move inward from the surface plays a large 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 inflows 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.
Many cells take in substances through processes such as phagocytosis and pinocytosis. In the first variant, a small recess is created by a flexible outer membrane, in which the captured particle is located. Then the diameter of the recess becomes larger until the surrounded particle enters the cell cytoplasm. Through phagocytosis, some protozoa, such as amoeba, as well as blood cells - leukocytes and phagocytes, are fed. Similarly, cells absorb fluid that contains the necessary nutrients. This phenomenon is called pinocytosis.
The outer membrane is closely connected to the endoplasmic reticulum of the cell.
In many types of basic tissue components, protrusions, folds, and microvilli are located on the surface of the membrane. Plant cells on the outside of this shell are covered with another one, thick and clearly visible under a microscope. The fiber they are made of helps form the support for plant tissues such as wood. Animal cells also have a number of external structures that sit on top of the cell membrane. They are exclusively protective in nature, an example of this is the chitin contained in the integumentary cells of insects.
In addition to the cell 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 a cell membrane. The structure and functions imply a significant expansion of the total cell surface area, improvement of 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 bonds are maintained at a sufficiently strong level, forming tissues. In this regard, we can conclude that one of the most important roles in the cell is played by the cell membrane. The structure and functions performed by it are radically different in different cells, depending on their purpose. Through these features, a variety of physiological activity of cell membranes and their roles in the existence of cells and tissues is achieved.
Short description:
Sazonov V.F. 1_1 The structure of the cell membrane [Electronic resource] // Kinesiologist, 2009-2018: [website]. Date of update: 06.02.2018..__.201_). _The structure and functioning of the cell membrane is described (synonyms: plasmalemma, plasmolemma, biomembrane, cell membrane, outer cell membrane, cell membrane, cytoplasmic membrane). This initial information is necessary both for cytology and for understanding the processes of nervous activity: nervous excitation, inhibition, the work of synapses and sensory receptors.
Concept definition
The cell membrane (synonyms: plasmalemma, plasmolemma, cytoplasmic membrane, biomembrane) is a triple lipoprotein (i.e. "fat-protein") membrane that separates the cell from the environment and carries out a controlled exchange and communication between the cell and its environment.
The main thing in this definition is not that the membrane separates the cell from the environment, but just that it connects cell with the environment. The membrane is active structure of the cell, it is constantly working.
A biological membrane is an ultrathin bimolecular film of phospholipids encrusted with proteins and polysaccharides. This cellular structure underlies the barrier, mechanical and matrix properties of a living organism (Antonov VF, 1996).
Figurative representation of the membrane
To me, the cell membrane appears as a lattice fence with many doors in it, which surrounds a certain territory. Any small living creatures can freely move back and forth through this fence. But larger visitors can only enter through the doors, and even then not all. Different visitors have keys only to their own doors, and they cannot pass through other people's doors. So, through this fence there are constantly flows of visitors back and forth, because the main function of the membrane-fence is twofold: to separate the territory from the surrounding space and at the same time connect it with the surrounding space. For this, there are many holes and doors in the fence - !
Membrane properties
1. Permeability.
2. Semi-permeability (partial permeability).
3. Selective (synonym: selective) permeability.
4. Active permeability (synonym: active transport).
5. Controlled permeability.
As you can see, the main property of the membrane is its permeability with respect to various substances.
6. Phagocytosis and pinocytosis.
7. Exocytosis.
8. The presence of electrical and chemical potentials, more precisely, the potential difference between the inner and outer sides of the membrane. Figuratively, one can say that "the membrane turns the cell into an "electric battery" by controlling ion flows". Details: .
9. Changes in electrical and chemical potential.
10. Irritability. Special molecular receptors located on the membrane can connect with signal (control) substances, as a result of which the state of the membrane and the entire cell can change. Molecular receptors trigger biochemical reactions in response to the combination of ligands (control substances) with them. It is important to note that the signaling substance acts on the receptor from the outside, while the changes continue inside the cell. It turns out that the membrane transmitted information from the environment to the internal environment of the cell.
11. Catalytic enzymatic activity. Enzymes can be embedded in the membrane or associated with its surface (both inside and outside the cell), and there they carry out their enzymatic activity.
12. Changing the shape of the surface and its area. This allows the membrane to form outgrowths outward or, conversely, invaginations into the cell.
13. The ability to form contacts with other cell membranes.
14. Adhesion - the ability to stick to solid surfaces.
Membrane functions
1. Incomplete isolation of internal content from the external environment.
2. The main thing in the work of the cell membrane is exchange various substances between the cell and the extracellular environment. This is due to such property of the membrane as permeability. In addition, the membrane regulates this exchange by regulating its permeability.
3. Another important function of the membrane is creating a difference in chemical and electrical potentials between its inner and outer sides. Due to this, inside the cell has a negative electrical potential -.
4. Through the membrane is also carried out information exchange between the cell and its environment. Special molecular receptors located on the membrane can bind to control substances (hormones, mediators, modulators) and trigger biochemical reactions in the cell, leading to various changes in the cell or in its structures.
Video:The structure of the cell membrane
Video lecture:Details about the structure of the membrane and transport
Membrane structure
The cell membrane has a universal three-layer structure. Its middle fat layer is continuous, and the upper and lower protein layers cover it in the form of a mosaic of separate protein areas. The fat layer is the basis that ensures the isolation of the cell from the environment, isolating it from the environment. By itself, it passes water-soluble substances very poorly, but easily passes fat-soluble ones. Therefore, the permeability of the membrane for water-soluble substances (for example, ions) has to be provided with special protein structures - and.
Below are microphotographs of real cell membranes of contacting cells, obtained using an electron microscope, as well as a schematic drawing showing the three-layered membrane and the mosaic nature of its protein layers. To enlarge an image, click on it.
Separate image of the inner lipid (fatty) layer of the cell membrane, permeated with integral embedded proteins. The upper and lower protein layers are removed so as not to interfere with the consideration of the lipid bilayer
Figure above: An incomplete schematic representation of the cell membrane (cell wall) from Wikipedia.
Note that the outer and inner protein layers have been removed from the membrane here so that we can better see the central fatty double lipid layer. In a real cell membrane, large protein "islands" float above and below along the fatty film (small balls in the figure), and the membrane turns out to be thicker, three-layered: protein-fat-protein . So it's actually like a sandwich of two protein "slices of bread" with a thick layer of "butter" in the middle, ie. has a three-layer structure, not a two-layer one.
In this figure, small blue and white balls correspond to the hydrophilic (wettable) "heads" of lipids, and the "strings" attached to them correspond to the hydrophobic (non-wettable) "tails". Of the proteins, only integral end-to-end membrane proteins (red globules and yellow helices) are shown. Yellow oval dots inside the membrane are cholesterol molecules Yellow-green chains of beads on the outside of the membrane are oligosaccharide chains that form the glycocalyx. Glycocalyx is like a carbohydrate ("sugar") "fluff" on the membrane, formed by long carbohydrate-protein molecules protruding from it.
Living is a small "protein-fat bag" filled with semi-liquid jelly-like contents, which is penetrated by films and tubes.
The walls of this sac are formed by a double fatty (lipid) film, covered inside and out with proteins - the cell membrane. Therefore, the membrane is said to have three-layer structure : proteins-fats-proteins. Inside the cell there are also many similar fatty membranes that divide its internal space into compartments. Cellular organelles are surrounded by the same membranes: nucleus, mitochondria, chloroplasts. So the membrane is a universal molecular structure inherent in all cells and all living organisms.
On the left - no longer a real, but an artificial model of a piece of a biological membrane: this is an instant snapshot of an adipose phospholipid bilayer (i.e. a double layer) in the process of its molecular dynamics modeling. The calculation cell of the model is shown - 96 PQ molecules ( f osphatidil X oline) and 2304 water molecules, total 20544 atoms.
On the right is a visual model of a single molecule of the same lipid, from which the membrane lipid bilayer is assembled. It has a hydrophilic (water-loving) head at the top, and two hydrophobic (water-fearing) tails at the bottom. This lipid has a simple name: 1-steroyl-2-docosahexaenoyl-Sn-glycero-3-phosphatidylcholine (18:0/22:6(n-3)cis PC), but you don't need to memorize it unless you plan to make your teacher swoon with the depth of your knowledge.
You can give a more precise scientific definition of a cell:
is an ordered, structured heterogeneous system of biopolymers limited by an active membrane, participating in a single set of metabolic, energy and information processes, and also maintaining and reproducing the entire system as a whole.
Inside the cell is also penetrated by membranes, and between the membranes is not water, but a viscous gel / sol of variable density. Therefore, the interacting molecules in the cell do not float freely, as in a test tube with an aqueous solution, but mostly sit (immobilized) on the polymer structures of the cytoskeleton or intracellular membranes. And therefore, chemical reactions take place inside the cell almost like in a solid body, and not in a liquid. The outer membrane that surrounds the cell is also covered in enzymes and molecular receptors, making it a very active part of the cell.
The cell membrane (plasmalemma, plasmolemma) is an active shell that separates the cell from the environment and connects it with the environment. © Sazonov V.F., 2016.
From this definition of a membrane, it follows that it does not simply limit the cell, but actively working linking it to its environment.
The fat that makes up the membranes is special, so its molecules are usually called not just fat, but lipids, phospholipids, sphingolipids. The membrane film is double, i.e. it consists of two films stuck together. Therefore, textbooks write that the base of the cell membrane consists of two lipid layers (or " bilayer", i.e. double layer). For each individual lipid layer, one side can be wetted by water, and the other cannot. So, these films stick together with each other precisely by their non-wetting sides.
bacteria membrane
The shell of a prokaryotic cell of gram-negative bacteria consists of several layers, shown in the figure below.
Layers of the shell of gram-negative bacteria:
1. The inner three-layer cytoplasmic membrane, which is in contact with the cytoplasm.
2. Cell wall, which consists of murein.
3. The outer three-layer cytoplasmic membrane, which has the same system of lipids with protein complexes as the inner membrane.
Communication of gram-negative bacterial cells with the outside world through such a complex three-step structure does not give them an advantage in surviving in harsh conditions compared to gram-positive bacteria that have a less powerful shell. They just as badly tolerate high temperatures, high acidity and pressure drops.
Video lecture:Plasma membrane. E.V. Cheval, Ph.D.
Video lecture:The membrane as a cell boundary. A. Ilyaskin
Importance of Membrane Ion Channels
It is easy to understand that only fat-soluble substances can enter the cell through the membrane fatty film. These are fats, alcohols, gases. For example, in erythrocytes, oxygen and carbon dioxide easily pass in and out directly through the membrane. But water and water-soluble substances (for example, ions) simply cannot pass through the membrane into any cell. This means that they need special holes. But if you just make a hole in the fatty film, then it will immediately tighten back. What to do? A solution was found in nature: it is necessary to make special protein transport structures and stretch them through the membrane. This is how the channels for the passage of fat-insoluble substances are obtained - the ion channels of the cell membrane.
So, in order to give its membrane additional properties of permeability for polar molecules (ions and water), the cell synthesizes special proteins in the cytoplasm, which are then integrated into the membrane. They are of two types: transporter proteins (for example, transport ATPases) and channel-forming proteins (channel formers). These proteins are embedded in the double fatty layer of the membrane and form transport structures in the form of transporters or in the form of ion channels. Various water-soluble substances can now pass through these transport structures, which otherwise cannot pass through the fatty membrane film.
In general, proteins embedded in the membrane are also called integral, precisely because they are, as it were, included in the composition of the membrane and penetrate it through and through. Other proteins, not integral, form, as it were, islands that "float" on the surface of the membrane: either along its outer surface or along its inner one. After all, everyone knows that fat is a good lubricant and it is easy to slide on it!
conclusions
1. In general, the membrane is three-layered:
1) the outer layer of protein "islands",
2) fatty two-layer "sea" (lipid bilayer), i.e. double lipid film
3) the inner layer of protein "islands".
But there is also a loose outer layer - the glycocalyx, which is formed by glycoproteins sticking out of the membrane. They are molecular receptors to which signaling controls bind.
2. Special protein structures are built into the membrane, ensuring its permeability to ions or other substances. We must not forget that in some places the sea of fat is permeated through with integral proteins. And it is integral proteins that form special transport structures cell membrane (see section 1_2 Membrane transport mechanisms). Through them, substances enter the cell, and are also removed from the cell to the outside.
3. Enzyme proteins can be located on any side of the membrane (outer and inner), as well as inside the membrane, which affect both the state of the membrane itself and the life of the entire cell.
So the cell membrane is an active variable structure that actively works in the interests of the whole cell and connects it with the outside world, and is not just a "protective shell". This is the most important thing to know about the cell membrane.
In medicine, membrane proteins are often used as “targets” for drugs. Receptors, ion channels, enzymes, transport systems act as such targets. Recently, in addition to the membrane, genes hidden in the cell nucleus have also become targets for drugs.
Video:Introduction to Cell Membrane Biophysics: Structure of Membrane 1 (Vladimirov Yu.A.)
Video:History, structure and functions of the cell membrane: Structure of membranes 2 (Vladimirov Yu.A.)
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