By functional features The cell membrane can be divided into 9 functions it performs.
Cell membrane functions:
1. Transport. Produces the transport of substances from cell to cell;
2. Barrier. It has selective permeability, provides the necessary metabolism;
3. Receptor. Some proteins found in the membrane are receptors;
4. Mechanical. Ensures the autonomy of the cell and its mechanical structures;
5. Matrix. Provides optimal interaction and orientation of matrix proteins;
6. Energy. In membranes, energy transfer systems operate during cellular respiration in mitochondria;
7. Enzymatic. Membrane proteins are sometimes enzymes. For example, intestinal cell membranes;
8. Marking. There are antigens (glycoproteins) on the membrane that make it possible to identify the cell;
9. Generating. Carries out the generation and conduction of biopotentials.
You can see what the cell membrane looks like using the example of the structure of an animal cell or a plant cell.
The figure shows the structure of the cell membrane.
The components of the cell membrane include various proteins of the cell membrane (globular, peripheral, surface), as well as lipids of the cell membrane (glycolipid, phospholipid). Carbohydrates, cholesterol, glycoprotein and protein alpha helix are also present in the structure of the cell membrane.
Cell membrane composition
The main components of the cell membrane are:
1. Proteins - responsible for the various properties of the membrane;
2. Lipids three types(phospholipids, glycolipids and cholesterol) responsible for membrane rigidity.
Cell membrane proteins:
1. Globular protein;
2. Surface protein;
3. Peripheral protein.
The main purpose of the cell membrane
The main purpose of the cell membrane:
1. Regulate the exchange between the cell and the environment;
2. Separate the contents of any cell from the external environment, thereby ensuring its integrity;
3. Intracellular membranes divide the cell into specialized closed compartments - organelles or compartments, in which certain environmental conditions are maintained.
Cell membrane structure
The structure of the cell membrane is a two-dimensional solution of globular integral proteins dissolved in a liquid phospholipid matrix. This model of membrane structure was proposed by two scientists Nicholson and Singer in 1972. Thus, the basis of the membranes is a bimolecular lipid layer, with an ordered arrangement of molecules, which you could see on.
It's no secret to anyone that all living beings on our planet are composed of their cells, these countless "" organic matter. Cells, in turn, are surrounded by a special protective membrane - a membrane that plays a very important role in the life of the cell, and the functions of the cell membrane are not limited to protecting the cell, but represent the most complex mechanism involved in cell reproduction, nutrition, and regeneration.
What is a cell membrane
The word “membrane” itself is translated from Latin as “film”, although the membrane is not just a kind of film in which the cell is wrapped, but a combination of two films interconnected and having different properties. In fact, the cell membrane is a three-layer lipoprotein (fat-protein) shell that separates each cell from neighboring cells and the environment, and carries out a controlled exchange between cells and the environment, this is the academic definition of what a cell membrane is.
The value of the membrane is simply enormous, because it not only separates one cell from another, but also ensures the interaction of the cell, both with other cells and with the environment.
History of cell membrane research
An important contribution to the study of the cell membrane was made by two German scientists Gorter and Grendel back in 1925. It was then that they managed to conduct a complex biological experiment on red blood cells - erythrocytes, during which scientists received the so-called "shadows", empty shells of erythrocytes, which were folded into one pile and measured the surface area, and also calculated the amount of lipids in them. Based on the amount of lipids obtained, the scientists came to the conclusion that they are just enough for the double layer of the cell membrane.
In 1935, another pair of cell membrane researchers, this time the Americans Daniel and Dawson, after a series of long experiments, determined the protein content in the cell membrane. Otherwise, it was impossible to explain why the membrane has such high rate surface tension. Scientists cleverly presented a model of the cell membrane in the form of a sandwich, in which the role of bread is played by homogeneous lipid-protein layers, and between them instead of butter is emptiness.
In 1950, with the advent of the electronic theory of Daniel and Dawson, it was already possible to confirm practical observations - on micrographs of the cell membrane, layers of lipid and protein heads and also an empty space between them were clearly visible.
In 1960, the American biologist J. Robertson developed a theory about the three-layer structure of cell membranes, which for a long time was considered the only true one, but with the further development of science, doubts about its infallibility began to appear. So, for example, from the point of view of cells, it would be difficult and laborious to transport the necessary useful substances through the entire “sandwich”
And only in 1972, the American biologists S. Singer and G. Nicholson were able to explain the inconsistencies of Robertson's theory with the help of a new fluid-mosaic model of the cell membrane. In particular, they found that the cell membrane is not homogeneous in composition, moreover, it is asymmetric and filled with liquid. In addition, cells are in constant motion. And the notorious proteins that make up the cell membrane have different structures and functions.
Properties and functions of the cell membrane
Now let's look at what functions the cell membrane performs:
The barrier function of the cell membrane - the membrane, as a real border guard, stands guard over the boundaries of the cell, delaying, not letting through harmful or simply inappropriate molecules
The transport function of the cell membrane - the membrane is not only a border guard at the gates of the cell, but also a kind of customs checkpoint, through which the exchange of useful substances with other cells and the environment constantly passes.
Matrix function - it is the cell membrane that determines the location relative to each other, regulates the interaction between them.
Mechanical function - is responsible for the restriction of one cell from another and in parallel for the correct connection of cells with each other, for their formation into a homogeneous tissue.
The protective function of the cell membrane is the basis for building a protective shield of the cell. In nature, this function can be exemplified by hard wood, a dense skin, a protective shell, all due to the protective function of the membrane.
The enzymatic function is another important function performed by some cell proteins. For example, due to this function, the synthesis of digestive enzymes occurs in the intestinal epithelium.
Also, in addition to all this, cell metabolism is carried out through the cell membrane, which can take place by three different reactions:
- Phagocytosis is a cellular exchange in which phagocytic cells embedded in the membrane capture and digest various nutrients.
- Pinocytosis - is the process of capture by the cell membrane, fluid molecules in contact with it. To do this, special tendrils are formed on the surface of the membrane, which seem to surround a drop of liquid, forming a bubble, which is subsequently “swallowed” by the membrane.
- Exocytosis - is the reverse process, when the cell releases secretory functional fluid through the membrane to the surface.
The structure of the cell membrane
There are three classes of lipids in the cell membrane:
- phospholipids (they are a combination of fats and),
- glycolipids (combination of fats and carbohydrates),
- cholesterol.
Phospholipids and glycolipids, in turn, consist of a hydrophilic head, into which two long hydrophobic tails extend. Cholesterol, on the other hand, occupies the space between these tails, preventing them from bending, all this in some cases makes the membrane of certain cells very rigid. In addition to all this, cholesterol molecules regulate the structure of the cell membrane.
But be that as it may, the most important part of the structure of the cell membrane is protein, or rather different proteins that play various important roles. Despite the diversity of proteins contained in the membrane, there is something that unites them - annular lipids are located around all membrane proteins. Annular lipids are special structured fats that serve as a kind of protective shell for proteins, without which they simply would not work.
The structure of the cell membrane has three layers: the basis of the cell membrane is a homogeneous liquid lipid layer. Proteins cover it on both sides like a mosaic. It is proteins that, in addition to the functions described above, also play the role of peculiar channels through which substances pass through the membrane that are unable to penetrate the liquid layer of the membrane. These include, for example, potassium and sodium ions; for their penetration through the membrane, nature provides special ion channels of cell membranes. In other words, proteins provide the permeability of cell membranes.
If we look at the cell membrane through a microscope, we will see a layer of lipids formed by small spherical molecules on which proteins float like on the sea. Now you know what substances are part of the cell membrane.
Cell membrane, video
And finally, an educational video about the cell membrane.
This article is available at English language – .
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.
Basic Functions of the Cell Membrane
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 intercellular contacts, 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 improve metabolism, as well as to stronger bonds of individual cells with each other.
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.
Functions of the cell membrane (briefly)
| 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. |
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 plasmalemma is as follows: more than half of the chemical composition falls on proteins, a quarter is occupied by phospholipids, and a tenth is cholesterol.
Cell membrane and its types
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:
- mitochondria;
- core;
- endoplasmic reticulum;
- Golgi complex;
- lysosomes;
- chloroplasts (in plant cells).
There is also an inner and outer (plasmolemma) cell membrane.
The structure of the 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 required to build ion channels through which ions and hydrophilic substrates are transported. The latter are enzymes that catalyze intracellular reactions.
Basic properties of the plasma membrane
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. Importance has cholesterol, the content of the latter determines the viscosity of the plasma membrane.
Functions of the outer membrane of the cell
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. |
What is the importance of the cell membrane
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.
biological membranes- the general name of functionally active surface structures that limit cells (cellular or plasma membranes) and intracellular organelles (membranes of mitochondria, nuclei, lysosomes, endoplasmic reticulum, etc.). They contain lipids, proteins, heterogeneous molecules (glycoproteins, glycolipids) and, depending on the function performed, numerous minor components: coenzymes, nucleic acids, antioxidants, carotenoids, inorganic ions, etc.
The coordinated functioning of membrane systems - receptors, enzymes, transport mechanisms - helps to maintain cell homeostasis and at the same time quickly respond to changes in the external environment.
TO main functions of biological membranes can be attributed:
separation of the cell from the environment and the formation of intracellular compartments (compartments);
control and regulation of the transport of a huge variety of substances through membranes;
participation in providing intercellular interactions, transmission of signals inside the cell;
food energy conversion organic matter into energy chemical bonds ATP molecules.
Molecular organization The plasma (cell) membrane of all cells is approximately the same: it consists of two layers of lipid molecules with many specific proteins included in it. Some membrane proteins have enzymatic activity, while others bind nutrients from the environment and ensure their transport into the cell through membranes. Membrane proteins are distinguished by the nature of their association with membrane structures. Some proteins, called external or peripheral , loosely bound to the surface of the membrane, others, called internal or integral , are immersed inside the membrane. Peripheral proteins are easily extracted, while integral proteins can only be isolated using detergents or organic solvents. On fig. 4 shows the structure of the plasma membrane.
The outer, or plasma, membranes of many cells, as well as the membranes of intracellular organelles, such as mitochondria, chloroplasts, were isolated in a free form and their molecular composition was studied. All membranes contain polar lipids in an amount ranging from 20 to 80% of its mass, depending on the type of membranes, the rest is mainly accounted for by proteins. So, in the plasma membranes of animal cells, the amount of proteins and lipids, as a rule, is approximately the same; the inner mitochondrial membrane contains about 80% proteins and only 20% lipids, while the myelin membranes of brain cells, on the contrary, contain about 80% lipids and only 20% proteins.
Rice. 4. Structure of the plasma membrane
The lipid part of the membranes is a mixture of various kinds of polar lipids. Polar lipids, which include phosphoglycerolipids, sphingolipids, glycolipids, are not stored in fat cells, but are incorporated into cell membranes, and in strictly defined ratios.
All polar lipids in membranes are constantly renewed in the process of metabolism; under normal conditions, a dynamic stationary state is established in the cell, in which the rate of lipid synthesis is equal to the rate of their decay.
The membranes of animal cells contain mainly phosphoglycerolipids and, to a lesser extent, sphingolipids; triacylglycerols are found only in trace amounts. Some membranes of animal cells, especially the outer plasma membrane, contain significant amounts of cholesterol and its esters (Fig. 5).
Fig.5. Membrane lipids
Currently, the generally accepted model for the structure of membranes is the fluid mosaic model proposed in 1972 by S. Singer and J. Nicholson.
According to her, proteins can be likened to icebergs floating in a lipid sea. As mentioned above, there are 2 types of membrane proteins: integral and peripheral. Integral proteins penetrate the membrane through, they are amphipathic molecules. Peripheral proteins do not penetrate the membrane and are less strongly associated with it. The main continuous part of the membrane, that is, its matrix, is the polar lipid bilayer. At normal cell temperature, the matrix is in a liquid state, which is ensured by a certain ratio between saturated and unsaturated fatty acids in the hydrophobic tails of polar lipids.
The fluid mosaic model also suggests that on the surface of integral proteins located in the membrane there are R-groups of amino acid residues (mainly hydrophobic groups, due to which the proteins seem to “dissolve” in the central hydrophobic part of the bilayer). At the same time, on the surface of peripheral, or external proteins, there are mainly hydrophilic R-groups, which are attracted to the hydrophilic charged polar heads of lipids due to electrostatic forces. Integral proteins, and these include enzymes and transport proteins, are active only if they are located inside the hydrophobic part of the bilayer, where they acquire the spatial configuration necessary for the manifestation of activity (Fig. 6). It should be emphasized once again that no covalent bonds are formed between the molecules in the bilayer, nor between the proteins and lipids of the bilayer.
Fig.6. Membrane proteins
Membrane proteins can move freely in the lateral plane. Peripheral proteins literally float on the surface of the bilayer "sea", while integral proteins, like icebergs, are almost completely submerged in the hydrocarbon layer.
For the most part, the membranes are asymmetric, that is, they have unequal sides. This asymmetry is manifested in the following:
Firstly, the fact that the inner and outer sides of the plasma membranes of bacterial and animal cells differ in the composition of polar lipids. For example, the inner lipid layer of human erythrocyte membranes contains mainly phosphatidylethanolamine and phosphatidylserine, while the outer lipid layer contains phosphatidylcholine and sphingomyelin.
· secondly, some transport systems in membranes act only in one direction. For example, in the membranes of erythrocytes there is a transport system (“pump”) that pumps Na + ions from the cell to environment, and K + ions - inside the cell due to the energy released during ATP hydrolysis.
Thirdly, the outer surface of plasma membranes contains very big number oligosaccharide groups, which are the heads of glycolipids and oligosaccharide side chains of glycoproteins, while there are practically no oligosaccharide groups on the inner surface of the plasma membrane.
The asymmetry of biological membranes is preserved due to the fact that the transfer of individual phospholipid molecules from one side of the lipid bilayer to the other is very difficult for energy reasons. The polar lipid molecule is able to move freely on its side of the bilayer, but is limited in its ability to jump to the other side.
Lipid mobility depends on the relative content and type of unsaturated fatty acids present. The hydrocarbon nature of fatty acid chains gives the membrane properties of fluidity, mobility. In the presence of cis-unsaturated fatty acids, the cohesive forces between chains are weaker than in the case of saturated fatty acids alone, and lipids retain high mobility even at low temperatures.
On the outer side of the membranes there are specific recognition sites, the function of which is to recognize certain molecular signals. For example, it is through the membrane that some bacteria perceive slight changes in the concentration of a nutrient, which stimulates their movement towards the food source; this phenomenon is called chemotaxis.
The membranes of various cells and intracellular organelles have a certain specificity due to their structure, chemical composition and functions. The following main groups of membranes in eukaryotic organisms are distinguished:
plasma membrane (outer cell membrane, plasmalemma),
the nuclear membrane
The endoplasmic reticulum
membranes of the Golgi apparatus, mitochondria, chloroplasts, myelin sheaths,
excitable membranes.
In prokaryotic organisms, in addition to the plasma membrane, there are intracytoplasmic membrane formations; in heterotrophic prokaryotes, they are called mesosomes. The latter are formed by invagination into the outer cell membrane and in some cases remain in contact with it.
erythrocyte membrane consists of proteins (50%), lipids (40%) and carbohydrates (10%). The main part of carbohydrates (93%) is associated with proteins, the rest - with lipids. In the membrane, lipids are arranged asymmetrically in contrast to the symmetrical arrangement in micelles. For example, cephalin is found predominantly in inner layer lipids. This asymmetry is maintained, apparently, due to the transverse movement of phospholipids in the membrane, carried out with the help of membrane proteins and due to the energy of metabolism. In the inner layer of the erythrocyte membrane are mainly sphingomyelin, phosphatidylethanolamine, phosphatidylserine, in the outer layer - phosphatidylcholine. The erythrocyte membrane contains an integral glycoprotein glycophorin, consisting of 131 amino acid residues and penetrating the membrane, and the so-called band 3 protein, consisting of 900 amino acid residues. The carbohydrate components of glycophorin perform a receptor function for influenza viruses, phytohemagglutinins, and a number of hormones. Another integral protein containing few carbohydrates and penetrating the membrane was also found in the erythrocyte membrane. He is called tunnel protein(component a), as it is assumed that it forms a channel for anions. The peripheral protein associated with the inner side of the erythrocyte membrane is spectrin.
Myelin membranes , surrounding axons of neurons, are multilayered, they contain a large number of lipids (about 80%, half of them are phospholipids). The proteins of these membranes are important for the fixation of membrane salts lying one above the other.
chloroplast membranes. Chloroplasts are covered with a two-layer membrane. The outer membrane bears some resemblance to that of mitochondria. In addition to this surface membrane, chloroplasts have an internal membrane system - lamellae. Lamellae form or flattened vesicles - thylakoids, which, located one above the other, are collected in packs (grana) or form a membrane system of the stroma (stromal lamellae). Lamella gran and stroma on the outer side of the thylakoid membrane are concentrated hydrophilic groups, galacto- and sulfolipids. The phytolic part of the chlorophyll molecule is immersed in the globule and is in contact with the hydrophobic groups of proteins and lipids. The porphyrin nuclei of chlorophyll are mainly localized between the adjoining membranes of the thylakoids of the gran.
Inner (cytoplasmic) membrane of bacteria similar in structure to the inner membranes of chloroplasts and mitochondria. It contains enzymes of the respiratory chain, active transport; enzymes involved in the formation of membrane components. The predominant component of bacterial membranes are proteins: the protein/lipid ratio (by weight) is 3:1. The outer membrane of gram-negative bacteria, compared with the cytoplasmic one, contains a smaller amount of various phospholipids and proteins. Both membranes differ in lipid composition. The outer membrane contains proteins that form pores for the penetration of many low molecular weight substances. A characteristic component of the outer membrane is also a specific lipopolysaccharide. A number of outer membrane proteins serve as receptors for phages.
Virus membrane. Among viruses, membrane structures are characteristic of those containing a nucleocapsid, which consists of a protein and a nucleic acid. This "core" of viruses is surrounded by a membrane (envelope). It also consists of a bilayer of lipids with glycoproteins included in it, located mainly on the surface of the membrane. In a number of viruses (microviruses), 70-80% of all proteins enter the membranes, the remaining proteins are contained in the nucleocapsid.
Thus, cell membranes are very complex structures; their constituent molecular complexes form an ordered two-dimensional mosaic, which gives the membrane surface biological specificity.