Membranes are extremely viscous and at the same time plastic structures that surround all living cells. Functions cell membranes:

1. The plasma membrane is a barrier that maintains a different composition of the extra- and intracellular environment.

2. Membranes form specialized compartments inside the cell, i.e. numerous organelles - mitochondria, lysosomes, Golgi complex, endoplasmic reticulum, nuclear membranes.

3. Enzymes involved in energy conversion in processes such as oxidative phosphorylation and photosynthesis are localized in membranes.

Structure and composition of membranes

The basis of the membrane is a lipid bilayer, in the formation of which phospholipids and glycolipids participate. The lipid bilayer is formed by two rows of lipids, the hydrophobic radicals of which are hidden inside, and the hydrophilic groups are turned outward and are in contact with the aqueous medium. Protein molecules seem to be “dissolved” in the lipid bilayer.

Structure of membrane lipids

Membrane lipids are amphiphilic molecules, because the molecule has both a hydrophilic region (polar heads) and a hydrophobic region, represented by hydrocarbon radicals of fatty acids, spontaneously forming a bilayer. There are three main types of lipids in membranes: phospholipids, glycolipids, and cholesterol.

The lipid composition is different. The content of one or another lipid, apparently, is determined by the variety of functions performed by these lipids in membranes.

Phospholipids. All phospholipids can be divided into two groups - glycerophospholipids and sphingophospholipids. Glycerophospholipids are classified as derivatives of phosphatidic acid. The most common glycerophospholipids are phosphatidylcholines and phosphatidylethanolamines. Sphingophospholipids are based on the amino alcohol sphingosine.

Glycolipids. In glycolipids, the hydrophobic part is represented by alcohol ceramide, and the hydrophilic part is represented by a carbohydrate residue. Depending on the length and structure of the carbohydrate part, cerebrosides and gangliosides are distinguished. Polar "heads" of glycolipids are located on the outer surface of plasma membranes.

Cholesterol (CS). CS is present in all membranes of animal cells. Its molecule consists of a rigid hydrophobic core and a flexible hydrocarbon chain. The only hydroxyl group at the 3-position is the "polar head". For an animal cell, the average molar ratio of cholesterol / phospholipids is 0.3-0.4, but in the plasma membrane this ratio is much higher (0.8-0.9). The presence of cholesterol in membranes reduces the mobility of fatty acids, reduces the lateral diffusion of lipids, and therefore can affect the functions of membrane proteins.

Membrane Properties:

1. Selective permeability. The closed bilayer provides one of the main properties of the membrane: it is impermeable to most water-soluble molecules, since they do not dissolve in its hydrophobic core. Gases such as oxygen, CO 2 and nitrogen have the ability to easily penetrate into the cell due to the small size of the molecules and weak interaction with solvents. Also, molecules of a lipid nature, for example, steroid hormones, easily penetrate through the bilayer.

2. Liquidity. The membranes are characterized by fluidity (fluidity), the ability of lipids and proteins to move. Two types of phospholipid movements are possible: somersault (called “flip-flop” in the scientific literature) and lateral diffusion. In the first case, phospholipid molecules opposing each other in the bimolecular layer turn over (or somersault) towards each other and change places in the membrane, i.e. the outside becomes the inside and vice versa. Such jumps are associated with the expenditure of energy. More often, rotations around the axis (rotation) and lateral diffusion are observed - movement within the layer parallel to the membrane surface. The speed of movement of molecules depends on the microviscosity of membranes, which, in turn, is determined by the relative content of saturated and unsaturated fatty acids in the composition of lipids. Microviscosity is lower if unsaturated fatty acids predominate in the composition of lipids, and higher if the content of saturated fatty acids is high.

3. Asymmetry of membranes. The surfaces of the same membrane differ in the composition of lipids, proteins and carbohydrates (transverse asymmetry). For example, phosphatidylcholines predominate in the outer layer, while phosphatidylethanolamines and phosphatidylserines predominate in the inner layer. The carbohydrate components of glycoproteins and glycolipids come to the outer surface, forming a continuous pouch called the glycocalyx. There are no carbohydrates on the inner surface. Proteins - hormone receptors are located on the outer surface of the plasma membrane, and the enzymes regulated by them - adenylate cyclase, phospholipase C - on the inside, etc.

Membrane proteins

Membrane phospholipids act as a solvent for membrane proteins, creating a microenvironment in which the latter can function. Proteins account for 30 to 70% of the mass of membranes. The number of different proteins in the membrane varies from 6-8 in the sarcoplasmic reticulum to more than 100 in the plasma membrane. These are enzymes, transport proteins, structural proteins, antigens, including antigens of the main histocompatibility system, receptors for various molecules.

By localization in the membrane, proteins are divided into integral (partially or completely immersed in the membrane) and peripheral (located on its surface). Some integral proteins cross the membrane once (glycophorin), while others cross the membrane many times. For example, the retinal photoreceptor and β 2 -adrenergic receptor crosses the bilayer 7 times.

Peripheral proteins and domains of integral proteins located on the outer surface of all membranes are almost always glycosylated. Oligosaccharide residues protect the protein from proteolysis and are also involved in ligand recognition or adhesion.

It's no secret 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 a high 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 antennae 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 phosphorus),
• 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.

The cell membrane is an ultrathin film on the surface of a cell or cell organelle, consisting of a bimolecular layer of lipids with embedded proteins and polysaccharides.

Membrane functions:

• · Barrier - provides a regulated, selective, passive and active metabolism with the environment. For example, the peroxisome membrane protects the cytoplasm from peroxides that are dangerous for the cell. Selective permeability means that the permeability of a membrane to various atoms or molecules depends on their size, electrical charge, and chemical properties. Selective permeability ensures the separation of the cell and cellular compartments from the environment and supply them with the necessary substances.
• · Transport - through the membrane there is a transport of substances into the cell and out of the cell. Transport through membranes provides: the delivery of nutrients, the removal of end products of metabolism, the secretion of various substances, the creation of ionic gradients, the maintenance of optimal pH in the cell and the concentration of ions that are necessary for the functioning of cellular enzymes. Particles that for some reason are unable to cross the phospholipid bilayer (for example, due to hydrophilic properties, since the membrane inside is hydrophobic and does not allow hydrophilic substances to pass through, or because of their large size), but necessary for the cell, can penetrate the membrane through special carrier proteins (transporters) and channel proteins or by endocytosis. In passive transport, substances cross the lipid bilayer without energy expenditure along the concentration gradient by diffusion. A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance to pass through the membrane. This molecule may have a channel that allows only one type of substance to pass through. Active transport requires energy, as it occurs against a concentration gradient. There are special pump proteins on the membrane, including ATPase, which actively pumps potassium ions (K +) into the cell and pumps sodium ions (Na +) out of it.
• · matrix - provides a certain relative position and orientation of membrane proteins, their optimal interaction.
• Mechanical - ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). Cell walls play an important role in providing mechanical function, and in animals - intercellular substance.
• energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate;
• Receptor - some proteins located in the membrane are receptors (molecules with which the cell perceives certain signals). For example, hormones circulating in the blood only act on target cells that have receptors corresponding to these hormones. Neurotransmitters (chemicals that conduct nerve impulses) also bind to specific receptor proteins on target cells.
• Enzymatic - Membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.
• · Implementation of the generation and conduction of biopotentials. With the help of the membrane, a constant concentration of ions is maintained in the cell: the concentration of the K + ion inside the cell is much higher than outside, and the concentration of Na + is much lower, which is very important, since this maintains the potential difference across the membrane and generates a nerve impulse.
• Marking of the cell - there are antigens on the membrane that act as markers - "tags" that allow the cell to be identified. These are glycoproteins (that is, proteins with branched oligosaccharide side chains attached to them) that play the role of "antennas". Due to the myriad of side chain configurations, it is possible to make a specific marker for each cell type. With the help of markers, cells can recognize other cells and act in concert with them, for example, when forming organs and tissues. It also allows the immune system to recognize foreign antigens.

Some protein molecules diffuse freely in the plane of the lipid layer; in the normal state, parts of protein molecules that emerge on opposite sides of the cell membrane do not change their position.

The special morphology of cell membranes determines their electrical characteristics, among which the most important are capacitance and conductivity.

The capacitance properties are mainly determined by the phospholipid bilayer, which is impermeable to hydrated ions and at the same time thin enough (about 5 nm) to allow efficient separation and accumulation of charges, and electrostatic interaction of cations and anions. In addition, the capacitive properties of cell membranes are one of the reasons that determine the temporal characteristics of electrical processes occurring on cell membranes.

Conductivity (g) is the reciprocal of electrical resistance and equal to the ratio of the total transmembrane current for a given ion to the value that caused its transmembrane potential difference.

Various substances can diffuse through the phospholipid bilayer, and the degree of permeability (P), i.e., the ability of the cell membrane to pass these substances, depends on the difference in concentrations of the diffusing substance on both sides of the membrane, its solubility in lipids, and the properties of the cell membrane. The diffusion rate for charged ions in a constant field in the membrane is determined by the mobility of the ions, the thickness of the membrane, and the distribution of ions in the membrane. For non-electrolytes, the permeability of the membrane does not affect its conductivity, since non-electrolytes do not carry charges, that is, they cannot carry electric current.

The conductivity of a membrane is a measure of its ion permeability. An increase in conductivity indicates an increase in the number of ions passing through the membrane.

An important property of biological membranes is fluidity. All cell membranes are mobile fluid structures: most of the lipid and protein molecules that make up them are able to move quite quickly in the plane of the membrane

According to the 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 of three types (phospholipids, glycolipids and cholesterol) responsible for the rigidity of the membrane.
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.

Cell membranes: their structure and functions

Membranes are extremely viscous and at the same time plastic structures that surround all living cells. Functions of cell membranes:

1. The plasma membrane is a barrier that maintains a different composition of the extra- and intracellular environment.

2. Membranes form specialized compartments inside the cell, i.e. numerous organelles - mitochondria, lysosomes, Golgi complex, endoplasmic reticulum, nuclear membranes.

3. Enzymes involved in energy conversion in processes such as oxidative phosphorylation and photosynthesis are localized in membranes.

Membrane structure

In 1972, Singer and Nicholson proposed a fluid mosaic model of membrane structure. According to this model, functioning membranes are a two-dimensional solution of globular integral proteins dissolved in a liquid phospholipid matrix. Thus, the membranes are based on a bimolecular lipid layer, with an ordered arrangement of molecules.

In this case, the hydrophilic layer is formed by the polar head of phospholipids (a phosphate residue with choline, ethanolamine or serine attached to it) and also by the carbohydrate part of glycolipids. A hydrophobic layer - hydrocarbon radicals of fatty acids and sphingosine phospholipids and glycolipids.

Membrane properties:

1. Selective permeability. The closed bilayer provides one of the main properties of the membrane: it is impermeable to most water-soluble molecules, since they do not dissolve in its hydrophobic core. Gases such as oxygen, CO 2 and nitrogen have the ability to easily penetrate into the cell due to the small size of the molecules and weak interaction with solvents. Also, molecules of a lipid nature, for example, steroid hormones, easily penetrate through the bilayer.

2. Liquidity. The lipid bilayer has a liquid-crystalline structure, since the lipid layer is generally liquid, but there are areas of solidification in it, similar to crystalline structures. Although the position of lipid molecules is ordered, they retain the ability to move. Two types of phospholipid movements are possible: somersault (called “flip-flop” in the scientific literature) and lateral diffusion. In the first case, phospholipid molecules opposing each other in the bimolecular layer turn over (or somersault) towards each other and change places in the membrane, i.e. the outside becomes the inside and vice versa. Such jumps are associated with the expenditure of energy and are very rare. More often, rotations around the axis (rotation) and lateral diffusion are observed - movement within the layer parallel to the membrane surface.

3. Asymmetry of membranes. The surfaces of the same membrane differ in the composition of lipids, proteins and carbohydrates (transverse asymmetry). For example, phosphatidylcholines predominate in the outer layer, while phosphatidylethanolamines and phosphatidylserines predominate in the inner layer. The carbohydrate components of glycoproteins and glycolipids come to the outer surface, forming a continuous pouch called the glycocalyx. There are no carbohydrates on the inner surface. Proteins - hormone receptors are located on the outer surface of the plasma membrane, and the enzymes regulated by them - adenylate cyclase, phospholipase C - on the inside, etc.

Membrane proteins

Membrane phospholipids act as a solvent for membrane proteins, creating a microenvironment in which the latter can function. The number of different proteins in the membrane varies from 6-8 in the sarcoplasmic reticulum to more than 100 in the plasma membrane. These are enzymes, transport proteins, structural proteins, antigens, including antigens of the main histocompatibility system, receptors for various molecules.

By localization in the membrane, proteins are divided into integral (partially or completely immersed in the membrane) and peripheral (located on its surface). Some integral proteins pierce the membrane repeatedly. For example, the retinal photoreceptor and β 2 -adrenergic receptor crosses the bilayer 7 times.

Transfer of matter and information across membranes

Cell membranes are not tightly closed partitions. One of the main functions of membranes is the regulation of the transfer of substances and information. Transmembrane movement of small molecules is carried out 1) by diffusion, passive or facilitated, and 2) by active transport. Transmembrane movement of large molecules is carried out 1) by endocytosis and 2) by exocytosis. Signal transmission across membranes is carried out with the help of receptors localized on the outer surface of the plasma membrane. In this case, the signal either undergoes transformation (for example, glucagon cAMP), or it is internalized, associated with endocytosis (for example, LDL - LDL receptor).

Simple diffusion is the penetration of substances into the cell along an electrochemical gradient. In this case, no energy costs are required. The rate of simple diffusion is determined by 1) the transmembrane concentration gradient of the substance and 2) its solubility in the hydrophobic layer of the membrane.

With facilitated diffusion, substances are also transported through the membrane along a concentration gradient, without energy costs, but with the help of special membrane carrier proteins. Therefore, facilitated diffusion differs from passive diffusion in a number of parameters: 1) facilitated diffusion is characterized by high selectivity, since the carrier protein has an active center complementary to the transferred substance; 2) the rate of facilitated diffusion is capable of reaching a plateau, since the number of carrier molecules is limited.

Some transport proteins simply carry a substance from one side of the membrane to the other. Such a simple transfer is called a passive uniport. An example of a uniport is GLUT, a glucose transporter that transports glucose across cell membranes. Other proteins function as co-transport systems in which the transport of one substance depends on the simultaneous or sequential transport of another substance either in the same direction - such a transfer is called passive symport, or in the opposite direction - such a transfer is called passive antiport. Translocases of the mitochondrial inner membrane, in particular, ADP/ATP translocase, function according to the passive antiport mechanism.

With active transport, the transfer of a substance is carried out against a concentration gradient and, therefore, is associated with energy costs. If the transfer of ligands across the membrane is associated with the expenditure of ATP energy, then such a transfer is called primary active transport. An example is Na + K + -ATPase and Ca 2+ -ATPase localized in the plasma membrane of human cells and H + ,K + -ATPase of the gastric mucosa.

secondary active transport. The transport of some substances against the concentration gradient depends on the simultaneous or sequential transport of Na + (sodium ions) along the concentration gradient. In this case, if the ligand is transferred in the same direction as Na + , the process is called active symport. According to the mechanism of active symport, glucose is absorbed from the intestinal lumen, where its concentration is low. If the ligand is transferred in the opposite direction to sodium ions, then this process is called active antiport. An example is the Na + ,Ca 2+ exchanger of the plasma membrane.