What layers does the cell membrane consist of? Cell membranes, their structure. Functions of the cell membrane

Cell— self-regulating structural and functional unit of tissues and organs. cell theory structure of organs and tissues was developed by Schleiden and Schwann in 1839. Subsequently, using electron microscopy and ultracentrifugation, it was possible to elucidate the structure of all the main organelles of animal and plant cells (Fig. 1).

Rice. 1. Scheme of the structure of the cell of animal organisms

The main parts of the cell are the cytoplasm and the nucleus. Each cell is surrounded by a very thin membrane that limits its contents.

The cell membrane is called plasma membrane and is characterized by selective permeability. This property allows essential nutrients and chemical elements get inside the cell, and excess products come out of it. The plasma membrane consists of two layers of lipid molecules with the inclusion of specific proteins in it. The main membrane lipids are phospholipids. They contain phosphorus, a polar head, and two non-polar long-chain fatty acid tails. Membrane lipids include cholesterol and cholesterol esters. In accordance with the fluid mosaic model of the structure, membranes contain inclusions of protein and lipid molecules that can mix relative to the bilayer. For each type of membrane, any animal cell characterized by its relatively constant lipid composition.

Membrane proteins are divided into two types according to their structure: integral and peripheral. Peripheral proteins can be removed from the membrane without destroying it. There are four types membrane proteins: transport proteins, enzymes, receptors and structural proteins. Some membrane proteins have enzymatic activity, while others bind certain substances and facilitate their transfer into the cell. Proteins provide several pathways for the movement of substances across membranes: they form large pores consisting of several protein subunits that allow water molecules and ions to move between cells; form ion channels specialized for the movement of certain types of ions across the membrane under certain conditions. Structural proteins are associated with the inner lipid layer and provide the cytoskeleton of the cell. The cytoskeleton provides mechanical strength cell membrane. In various membranes, proteins account for 20 to 80% of the mass. Membrane proteins can move freely in the lateral plane.

Carbohydrates are also present in the membrane, which can covalently bind to lipids or proteins. There are three types of membrane carbohydrates: glycolipids (gangliosides), glycoproteins and proteoglycans. Most membrane lipids are in a liquid state and have a certain fluidity, i.e. the ability to move from one area to another. On the outer side of the membrane there are receptor sites that bind various hormones. Other specific sections of the membrane can> t recognize and bind some proteins alien to these cells and various biologically active compounds.

The inner space of the cell is filled with cytoplasm, in which most enzyme-catalyzed reactions of cellular metabolism take place. The cytoplasm consists of two layers: the inner, called the endoplasm, and the peripheral, the ectoplasm, which has a high viscosity and is devoid of granules. The cytoplasm contains all the components of a cell or organelle. The most important of the cell organelles are the endoplasmic reticulum, ribosomes, mitochondria, the Golgi apparatus, lysosomes, microfilaments and microtubules, peroxisomes.

Endoplasmic reticulum is a system of interconnected channels and cavities penetrating the entire cytoplasm. It provides transport of substances from environment and inside cells. The endoplasmic reticulum also serves as a depot for intracellular Ca 2+ ions and serves as the main site for lipid synthesis in the cell.

Ribosomes - microscopic spherical particles with a diameter of 10-25 nm. Ribosomes are freely located in the cytoplasm or attached to outer surface membranes of the endoplasmic reticulum and nuclear membrane. They interact with informational and transport RNA, and protein synthesis is carried out in them. They synthesize proteins that enter the cisterns or the Golgi apparatus and are then released outside. Ribosomes that are free in the cytoplasm synthesize protein for use by the cell itself, and ribosomes associated with the endoplasmic reticulum produce protein that is excreted from the cell. Various functional proteins are synthesized in ribosomes: carrier proteins, enzymes, receptors, cytoskeletal proteins.

golgi apparatus formed by a system of tubules, cisterns and vesicles. It is associated with the endoplasmic reticulum, and the biologically active substances that have entered here are stored in a compacted form in secretory vesicles. The latter are constantly separated from the Golgi apparatus, transported to the cell membrane and merge with it, and the substances contained in the vesicles are removed from the cell in the process of exocytosis.

Lysosomes - particles surrounded by a membrane with a size of 0.25-0.8 microns. They contain numerous enzymes involved in the breakdown of proteins, polysaccharides, fats, nucleic acids, bacteria and cells.

Peroxisomes formed from a smooth endoplasmic reticulum, resemble lysosomes and contain enzymes that catalyze the decomposition of hydrogen peroxide, which is cleaved under the influence of peroxidases and catalase.

Mitochondria contain outer and inner membranes and are the "energy station" of the cell. Mitochondria are round or elongated structures with a double membrane. The inner membrane forms folds protruding into the mitochondria - cristae. ATP is synthesized in them, the substrates of the Krebs cycle are oxidized, and many biochemical reactions are carried out. ATP molecules formed in mitochondria diffuse into all parts of the cell. Mitochondria contain a small amount of DNA, RNA, ribosomes, and with their participation, renewal and synthesis of new mitochondria takes place.

Microfilaments are thin protein filaments, consisting of myosin and actin, and form the contractile apparatus of the cell. Microfilaments are involved in the formation of folds or protrusions of the cell membrane, as well as in the movement of various structures inside cells.

microtubules form the basis of the cytoskeleton and provide its strength. The cytoskeleton gives the cells a characteristic appearance and shape, serves as a site for attachment of intracellular organelles and various bodies. AT nerve cells bundles of microtubules are involved in the transport of substances from the cell body to the ends of axons. With their participation, the functioning of the mitotic spindle during cell division is carried out. They play the role of motor elements in the villi and flagella in eukaryotes.

Nucleus is the main structure of the cell, is involved in the transmission of hereditary traits and in the synthesis of proteins. The nucleus is surrounded by a nuclear membrane containing many nuclear pores through which various substances are exchanged between the nucleus and the cytoplasm. Inside it is the nucleolus. The important role of the nucleolus in the synthesis of ribosomal RNA and histone proteins has been established. The rest of the nucleus contains chromatin, consisting of DNA, RNA, and a number of specific proteins.

Functions of the cell membrane

Cell membranes play an important role in the regulation of intracellular and intercellular metabolism. They are selective. Their specific structure makes it possible to provide barrier, transport and regulatory functions.

barrier function It manifests itself in limiting the penetration of compounds dissolved in water through the membrane. The membrane is impermeable to large protein molecules and organic anions.

Regulatory function membrane is the regulation of intracellular metabolism in response to chemical, biological and mechanical influences. Various influences are perceived by special membrane receptors with a subsequent change in the activity of enzymes.

transport function through biological membranes can be carried out passively (diffusion, filtration, osmosis) or with the help of active transport.

Diffusion - the movement of a gas or solute along a concentration and electrochemical gradient. The diffusion rate depends on the permeability of the cell membrane, as well as the concentration gradient for uncharged particles, electric and concentration gradients for charged particles. simple diffusion occurs through the lipid bilayer or through channels. Charged particles move along the electrochemical gradient, while uncharged particles follow the chemical gradient. For example, simple diffusion through the lipid layer of the membrane permeates oxygen, steroid hormones, urea, alcohol, etc. Various ions and particles move through the channels. Ion channels are formed by proteins and are divided into gated and uncontrolled channels. Depending on the selectivity, there are ion-selective ropes that allow only one ion to pass through, and channels that do not have selectivity. Channels have a mouth and a selective filter, and controlled channels have a gate mechanism.

Facilitated diffusion - a process in which substances are transported across a membrane by special membrane carrier proteins. In this way, amino acids and monosugars enter the cell. This mode of transport is very fast.

Osmosis - movement of water across a membrane from a solution with a lower osmotic pressure to a solution with a higher osmotic pressure.

Active transport - transfer of substances against a concentration gradient using transport ATPases (ion pumps). This transfer occurs with the expenditure of energy.

Na + /K + -, Ca 2+ - and H + pumps have been studied to a greater extent. Pumps are located on cell membranes.

A type of active transport is endocytosis and exocytosis. With the help of these mechanisms, larger substances (proteins, polysaccharides, nucleic acids) that cannot be transported through the channels are transported. This transport is more common in the epithelial cells of the intestine, renal tubules, and vascular endothelium.

At In endocytosis, cell membranes form invaginations into the cell, which, when laced, turn into vesicles. During exocytosis, vesicles with contents are transferred to the cell membrane and merge with it, and the contents of the vesicles are released into the extracellular environment.

The structure and functions of the cell membrane

To understand the processes that ensure the existence of electrical potentials in living cells, it is first of all necessary to understand the structure of the cell membrane and its properties.

At present, the fluid-mosaic model of the membrane, proposed by S. Singer and G. Nicholson in 1972, enjoys the greatest recognition. The basis of the membrane is a double layer of phospholipids (bilayer), the hydrophobic fragments of the molecule of which are immersed in the thickness of the membrane, and the polar hydrophilic groups are oriented outward, those. into the environment aquatic environment(Fig. 2).

Membrane proteins are localized on the membrane surface or can be embedded at different depths in the hydrophobic zone. Some proteins penetrate the membrane through and through, and different hydrophilic groups of the same protein are found on both sides of the cell membrane. Proteins found in the plasma membrane play a very important role: they participate in the formation of ion channels, play the role of membrane pumps and carriers of various substances, and can also perform a receptor function.

The main functions of the cell membrane: barrier, transport, regulatory, catalytic.

The barrier function is to limit the diffusion of water-soluble compounds through the membrane, which is necessary to protect cells from foreign, toxic substances and maintaining a relatively constant content of various substances inside the cells. So, the cell membrane can slow down the diffusion of various substances by 100,000-10,000,000 times.

Rice. 2. Three-dimensional scheme of the fluid-mosaic model of the Singer-Nicolson membrane

Globular integral proteins embedded in a lipid bilayer are shown. Some proteins are ion channels, others (glycoproteins) contain oligosaccharide side chains involved in the recognition of each other by cells and in the intercellular tissue. Cholesterol molecules are closely adjacent to the phospholipid heads and fix the adjacent areas of the "tails". The inner regions of the tails of the phospholipid molecule are not limited in their movement and are responsible for the fluidity of the membrane (Bretscher, 1985)

There are channels in the membrane through which ions penetrate. Channels are potential dependent and potential independent. Potential-gated channels open when the potential difference changes, and potential-independent(hormone-regulated) open when the receptors interact with substances. Channels can be opened or closed thanks to gates. Two types of gates are built into the membrane: activation(in the depth of the channel) and inactivation(on the surface of the channel). The gate can be in one of three states:

• open state (both types of gate are open);
• closed state (activation gate closed);
• inactivation state (inactivation gates are closed).

Another characteristic feature membranes is the ability to selectively transfer inorganic ions, nutrients, and various metabolic products. There are systems of passive and active transfer (transport) of substances. Passive transport is carried out through ion channels with or without the help of carrier proteins, and its driving force is the difference electrochemical potentials ions between intra- and extracellular space. The selectivity of ion channels is determined by its geometric parameters and the chemical nature of the groups lining the channel walls and mouth.

At present, channels with selective permeability for Na + , K + , Ca 2+ ions and also for water (the so-called aquaporins) are the most well studied. The diameter of ion channels, according to various studies, is 0.5-0.7 nm. The throughput of the channels can be changed; 10 7 - 10 8 ions per second can pass through one ion channel.

Active transport occurs with the expenditure of energy and is carried out by the so-called ion pumps. Ion pumps are molecular protein structures embedded in the membrane and carrying out the transfer of ions towards a higher electrochemical potential.

The operation of the pumps is carried out due to the energy of ATP hydrolysis. Na + / K + - ATPase, Ca 2+ - ATPase, H + - ATPase, H + / K + - ATPase, Mg 2+ - ATPase, which ensure the movement of Na +, K +, Ca 2+ ions, respectively, are well studied. , H+, Mg 2+ isolated or conjugated (Na+ and K+; H+ and K+). Molecular mechanism active transport has not been fully elucidated.

Cell membrane.

The cell membrane separates the contents of any cell from the external environment, ensuring its integrity; regulates the exchange between the cell and the environment; intracellular membranes divide the cell into specialized closed compartments - compartments or organelles, in which certain environmental conditions are maintained.

Structure.

The cell membrane is a double layer (bilayer) of molecules of the class of lipids (fats), most of which are the so-called complex lipids - phospholipids. Lipid molecules have a hydrophilic (“head”) and a hydrophobic (“tail”) part. During the formation of membranes, the hydrophobic portions of the molecules turn inward, while the hydrophilic portions turn outward. Membranes are very similar structures in different organisms. The membrane thickness is 7-8 nm. (10-9 meters)

hydrophilicity- the ability of a substance to be wetted by water.
hydrophobicity- the inability of a substance to be wetted by water.

The biological membrane also includes various proteins:
- integral (penetrating the membrane through)
- semi-integral (immersed at one end into the outer or inner lipid layer)
- superficial (located on the outer or adjacent to inner sides membranes).
Some proteins are the points of contact of the cell membrane with the cytoskeleton inside the cell, and the cell wall (if any) outside.

cytoskeleton- cell scaffold inside the cell.

Functions.

1) Barrier- provides a regulated, selective, passive and active metabolism with the environment.

2) Transport- substances are transported through the membrane into and out of the cell. matrix - provides a certain relative position and orientation of membrane proteins, their optimal interaction.

3) Mechanical- ensures the autonomy of the cell, its intracellular structures, as well as the connection with other cells (in tissues). The intercellular substance plays an important role in ensuring the mechanical function.

4) Receptor- some proteins in the membrane are receptors (molecules by which the cell perceives certain signals).

For example, hormones circulating in the blood only act on target cells that have receptors corresponding to those hormones. Neurotransmitters ( chemical substances, which ensure the conduction of nerve impulses) also bind to specific receptor proteins of target cells.

Hormones- biologically active signaling chemicals.

5) Enzymatic Membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.

6) 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.

nerve impulse – a wave of excitation transmitted along a nerve fiber.

7) Cell labeling- there are antigens on the membrane that act as markers - "labels" that allow you to identify the cell. 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. This also allows immune system recognize foreign antigens.

permeability features.

Cell membranes have selective permeability: they slowly penetrate through them in different ways:

• Glucose is the main source of energy.
• Amino acids are the building blocks that make up all the proteins in the body.
• Fatty acids - structural, energy and other functions.
• Glycerol - makes the body retain water and reduces the production of urine.
• Ions are enzymes for reactions.

Moreover, the membranes themselves actively regulate this process to a certain extent - some substances pass through, while others do not. There are four main mechanisms for the entry of substances into the cell or their removal from the cell to the outside:

Passive permeability mechanisms:

1) 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.

Diffusion- the process of mutual penetration of molecules of one substance between the molecules of another.

Osmosis– the process of one-way diffusion through a semipermeable membrane of solvent molecules towards a higher concentration of a solute.

The membrane surrounding a normal blood cell is permeable only to water molecules, oxygen, some of the nutrients dissolved in the blood, and cellular waste products.

Active permeability mechanisms:

1) Active transport.

active transport– the transfer of a substance from an area of ​​low concentration to an area of ​​high concentration.

Active transport requires energy, as it moves from an area of ​​low concentration to an area of ​​high concentration. There are special pump proteins on the membrane that actively pump potassium ions (K +) into the cell and pump sodium ions (Na +) out of it, ATP serves as energy.

ATP– universal source of energy for all biochemical processes. .(more later)

2) Endocytosis.

Particles that for some reason are not able to cross the cell membrane, but are necessary for the cell, can penetrate the membrane by endocytosis.

Endocytosis– the process of uptake of external material by the cell.

The selective permeability of the membrane during passive transport is due to special channels - integral proteins. They penetrate the membrane through and through, forming a kind of passage. The elements K, Na and Cl have their own channels. With respect to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, the sodium ion channels open, and there is a sharp influx of sodium ions into the cell. This results in an imbalance in the membrane potential. After that, the membrane potential is restored. Potassium channels are always open, through which potassium ions slowly enter the cell.

Membrane structure

Permeability

active transport

Osmosis

Endocytosis

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, electric charge and chemical properties. Selective permeability ensures the separation of the cell and cellular compartments from the environment and supply them with essential 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 operation 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 due to large sizes), 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 those 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 - "labels" 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 exit along different sides cell membrane does not change its position.

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

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 ensure 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 value of the total transmembrane current for a given ion to the value that caused its transmembrane potential difference.

Can diffuse through the phospholipid bilayer various substances, 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. Diffusion rate for charged ions under conditions constant field in the membrane is determined by the mobility of ions, the thickness of the membrane, 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.

Important property biological membranes- fluidity. All cell membranes are mobile fluid structures: most of their constituent lipid and protein molecules are able to move quickly enough in the plane of the membrane

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.

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;
• nucleus;
• 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. By chemical composition secrete proteins included in the lipid layer completely (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.

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. Cholesterol is important, 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	Biologically activated through the cell membrane active substances 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 has a 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.

The cell membrane is one of the most important organelles, serving as a kind of barrier between a given cell and the external environment. Scientific names are plasmalemma, cytolemma or plasma membrane. It is through it that the interaction of the cell with the external environment takes place, through it nutrients, and what has already been processed is highlighted outside. The plasmalemma has a rather complex structure, and also performs many functions in the body. This article will discuss in detail the cell membrane and its structure.

This organoid was discovered relatively recently, only at the beginning of the twentieth century. The discovery was made by German scientists - Gorter and Grendel. During the entire previous century, scientists actively studied the cytolemma, various theories about its structure were put forward, which were refuted over time, and new ones took their place. And only by the seventies, scientists were able to reliably determine its structure.

So what is the cell membrane made of? Through numerous studies, it was found that it has three layers in its composition. The upper and lower layers are non-continuous sections of associations of protein molecules, and the inner layer, on the contrary, is continuous, consisting of fats, it is he who is the main one, due to which isolation from the external environment is provided. The fat layer includes two rows of lipids (otherwise it is called bilipid).

The following types of lipids are present in the cytolemma:

• phospholipids (fats and phosphorus);
• glycolipids (fats and carbohydrates);
• cholesterol.

The protein outer and inner layers serve to ensure that substances that cannot penetrate through the fat layer can get there through these layers, that is, they are "crossings" for water-soluble substances.

So, the cell membrane is formed by three levels, two of which are a kind of transporters for substances that cannot penetrate the third level, which is the main one, this is the barrier that isolates the internal contents, but also provides a connection with other cells, because it is through it that the main amount of nutrients enters.

It is also important to understand that the cell membrane and cell wall are different organelles. There are many differences, and they are significant, the wall is located above the cytolemma, it serves as protection against mechanical damage and pressure. The functions of the cytolemma, in turn, are different.

Watch a video about the cell membrane and its functions.

The functions of the cell membrane include:

1. Barrier. Serves as a natural filter for molecules that are about to get inside, it allows only those that meet certain parameters to pass through.
2. Protective. Since most animals do not have a cell wall, the plasmalemma also serves as protection against mechanical stress and prevents damage. The cell membrane in a plant cell does not perform such a function, since plant cells have a complex wall that can protect them.
3. Matrix. Responsible for the arrangement of internal organelles relative to each other to maintain the internal balance necessary for full-fledged activity.
4. Transport. It completely controls the exchange of necessary substances with the external environment, helps, thanks to special features, those of them that are necessary for life, but at the same time, they cannot penetrate inside on their own.
5. Enzymatic. Necessary for the production of enzymes needed, for example, for the digestion of food.
6. Receptor. It is necessary to receive signals that speak about what is happening in the external environment.
7. Marking. Each cell is unique, and the cells are able to recognize each other, this is necessary in order to interact with each other. Recognition occurs due to the structure of the cytolemma, which is not repeated.

The cytolemmas of any living creatures perform essentially the same series of functions, with only slight variations, regardless of whether the cytolemma of whom is considered: an animal, a person, an insect, or a plant cell membrane.

Conclusions about the plasmalemma

Having considered the structure and functions of this organoid, one can notice that the cell membrane has features that are not characteristic of other components of the cell. Its discovery at the beginning of the last century contributed to further development medicine, served as the key to understanding many human diseases, as well as ways to treat them.

The cell membrane is characteristic of the cells of every organism. It serves as protection, and also performs very important features, because through it various substances penetrate inside. In order for this organoid to function normally, and, consequently, for the cell as a whole to function normally, it is necessary that conditions be maintained in the body that do not interfere with its activity.

As you know, the plasma membrane, its structure is a set of channels, through which exchange with the external environment is ensured. Scientists have found that for normal functioning, in particular, so that the cell does not begin to degenerate into a cancer cell, it is necessary that the plasmalemma channels work properly, do not become clogged, and do not let inappropriate molecules through.

• proper nutrition;
• regular walks on the fresh air;
• maintaining the body's water balance.

This is amazing, but it is precisely such a seemingly insignificant organoid that can greatly affect a person’s well-being and health. Therefore, the discovery of the plasmalemma was a huge step forward for biological science.

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