Cell membrane structure. What functions does the outer cell membrane perform? The structure of the outer cell membrane

Cell membrane - molecular structure that consists of lipids and proteins. Its main properties and functions:

separation of the contents of any cell from the external environment, ensuring its integrity;
control and establishment of exchange between the environment and the cell;
intracellular membranes divide the cell into special compartments: organelles or compartments.

The word "membrane" in Latin means "film". If we talk about the cell membrane, then it is a combination of two films that have different properties.

The biological membrane includes three types of proteins:

Peripheral – located on the surface of the film;
Integral – completely penetrate the membrane;
Semi-integral - one end penetrates into the bilipid layer.

What functions does the cell membrane perform?

1. The cell wall is a durable cell membrane that is located outside the cytoplasmic membrane. It performs protective, transport and structural functions. Present in many plants, bacteria, fungi and archaea.

2. Provides a barrier function, that is, selective, regulated, active and passive metabolism with the external environment.

3. Capable of transmitting and storing information, and also takes part in the reproduction process.

4. Performs a transport function that can transport substances into and out of the cell through the membrane.

5. The cell membrane has one-way conductivity. Thanks to this, water molecules can pass through the cell membrane without delay, and molecules of other substances penetrate selectively.

6. With the help of the cell membrane, water, oxygen and nutrients are obtained, and through it the products of cellular metabolism are removed.

7. Performs cellular metabolism through membranes, and can perform them using 3 main types of reactions: pinocytosis, phagocytosis, exocytosis.

8. The membrane ensures the specificity of intercellular contacts.

9. The membrane contains numerous receptors that are capable of perceiving chemical signals - mediators, hormones and many other biological active substances. So it has the power to change the metabolic activity of the cell.

10. Basic properties and functions of the cell membrane:

Matrix
Barrier
Transport
Energy
Mechanical
Enzymatic
Receptor
Protective
Marking
Biopotential

What function does the plasma membrane perform in a cell?

Delimits the contents of the cell;
Carries out the entry of substances into the cell;
Provides removal of a number of substances from the cell.

Cell membrane structure

Cell membranes include lipids of 3 classes:

Glycolipids;
Phospholipids;
Cholesterol.

Basically, the cell membrane consists of proteins and lipids, and has a thickness of no more than 11 nm. From 40 to 90% of all lipids are phospholipids. It is also important to note glycolipids, which are one of the main components of the membrane.

The structure of the cell membrane is three-layered. In the center there is a homogeneous liquid bilipid layer, and proteins cover it on both sides (like a mosaic), partially penetrating into the thickness. Proteins are also necessary for the membrane to allow special substances into and out of cells that cannot penetrate the fat layer. For example, sodium and potassium ions.

This is interesting -

Cell structure - video

Cytoplasm- an obligatory part of the cell, enclosed between the plasma membrane and the nucleus; is divided into hyaloplasm (the main substance of the cytoplasm), organelles (permanent components of the cytoplasm) and inclusions (temporary components of the cytoplasm). Chemical composition of the cytoplasm: the basis is water (60-90% of the total mass of the cytoplasm), various organic and inorganic compounds. The cytoplasm has an alkaline reaction. A characteristic feature of the cytoplasm of a eukaryotic cell is constant movement ( cyclosis). It is detected primarily by the movement of cell organelles, such as chloroplasts. If the movement of the cytoplasm stops, the cell dies, since only by being in constant motion can it perform its functions.

Hyaloplasma ( cytosol) is a colorless, slimy, thick and transparent colloidal solution. It is in it that all metabolic processes take place, it ensures the interconnection of the nucleus and all organelles. Depending on the predominance of the liquid part or large molecules in the hyaloplasm, two forms of hyaloplasm are distinguished: sol- more liquid hyaloplasm and gel- thicker hyaloplasm. Mutual transitions are possible between them: the gel turns into a sol and vice versa.

Functions of the cytoplasm:

combining all cell components into a single system,
environment for the passage of many biochemical and physiological processes,
environment for the existence and functioning of organelles.

Cell membranes

Cell membranes limit eukaryotic cells. In each cell membrane, at least two layers can be distinguished. The inner layer is adjacent to the cytoplasm and is represented by plasma membrane(synonyms - plasmalemma, cell membrane, cytoplasmic membrane), over which the outer layer is formed. In an animal cell it is thin and is called glycocalyx(formed by glycoproteins, glycolipids, lipoproteins), in a plant cell - thick, called cell wall(formed by cellulose).

All biological membranes have common structural features and properties. It is currently generally accepted fluid mosaic model of membrane structure. The basis of the membrane is a lipid bilayer formed mainly by phospholipids. Phospholipids are triglycerides in which one fatty acid residue is replaced by a phosphoric acid residue; the section of the molecule containing the phosphoric acid residue is called the hydrophilic head, the sections containing the fatty acid residues are called the hydrophobic tails. In the membrane, phospholipids are arranged in a strictly ordered manner: the hydrophobic tails of the molecules face each other, and the hydrophilic heads face outward, towards the water.

In addition to lipids, the membrane contains proteins (on average ≈ 60%). They determine most of the specific functions of the membrane (transport of certain molecules, catalysis of reactions, receiving and converting signals from the environment, etc.). There are: 1) peripheral proteins(located on the outer or inner surface of the lipid bilayer), 2) semi-integral proteins(immersed in the lipid bilayer to varying depths), 3) integral, or transmembrane, proteins(pierce the membrane through, contacting both the external and internal environment of the cell). Integral proteins are in some cases called channel-forming or channel proteins, since they can be considered as hydrophilic channels through which polar molecules pass into the cell (the lipid component of the membrane would not let them through).

A - hydrophilic phospholipid head; B - hydrophobic phospholipid tails; 1 - hydrophobic regions of proteins E and F; 2 — hydrophilic regions of protein F; 3 - branched oligosaccharide chain attached to a lipid in a glycolipid molecule (glycolipids are less common than glycoproteins); 4 - branched oligosaccharide chain attached to a protein in a glycoprotein molecule; 5 - hydrophilic channel (functions as a pore through which ions and some polar molecules can pass).

The membrane may contain carbohydrates (up to 10%). The carbohydrate component of membranes is represented by oligosaccharide or polysaccharide chains associated with protein molecules (glycoproteins) or lipids (glycolipids). Carbohydrates are mainly located on the outer surface of the membrane. Carbohydrates provide receptor functions of the membrane. In animal cells, glycoproteins form a supra-membrane complex, the glycocalyx, which is several tens of nanometers thick. It contains many cell receptors, and with its help cell adhesion occurs.

Molecules of proteins, carbohydrates and lipids are mobile, capable of moving in the plane of the membrane. The thickness of the plasma membrane is approximately 7.5 nm.

Functions of membranes

Membranes perform the following functions:

separation of cellular contents from the external environment,
regulation of metabolism between the cell and the environment,
dividing the cell into compartments (“compartments”),
place of localization of “enzymatic conveyors”,
ensuring communication between cells in the tissues of multicellular organisms (adhesion),
signal recognition.

The most important membrane property— selective permeability, i.e. membranes are highly permeable to some substances or molecules and poorly permeable (or completely impermeable) to others. This property underlies the regulatory function of membranes, ensuring the exchange of substances between the cell and the external environment. The process of substances passing through the cell membrane is called transport of substances. There are: 1) passive transport- the process of passing substances without energy consumption; 2) active transport- the process of passage of substances that occurs with the expenditure of energy.

At passive transport substances move from an area of higher concentration to an area of lower, i.e. along the concentration gradient. In any solution there are solvent and solute molecules. The process of moving solute molecules is called diffusion, and the movement of solvent molecules is called osmosis. If the molecule is charged, then its transport is also affected by the electrical gradient. Therefore, people often talk about an electrochemical gradient, combining both gradients together. The speed of transport depends on the magnitude of the gradient.

The following types of passive transport can be distinguished: 1) simple diffusion— transport of substances directly through the lipid bilayer (oxygen, carbon dioxide); 2) diffusion through membrane channels— transport through channel-forming proteins (Na +, K +, Ca 2+, Cl -); 3) facilitated diffusion- transport of substances using special transport proteins, each of which is responsible for the movement of certain molecules or groups of related molecules (glucose, amino acids, nucleotides); 4) osmosis— transport of water molecules (in all biological systems the solvent is water).

Necessity active transport occurs when it is necessary to ensure the transport of molecules across a membrane against an electrochemical gradient. This transport is carried out by special carrier proteins, the activity of which requires energy expenditure. The energy source is ATP molecules. Active transport includes: 1) Na + /K + pump (sodium-potassium pump), 2) endocytosis, 3) exocytosis.

Operation of Na + /K + pump. For normal functioning, the cell must maintain a certain ratio of K + and Na + ions in the cytoplasm and in the external environment. The concentration of K + inside the cell should be significantly higher than outside it, and Na + - vice versa. It should be noted that Na + and K + can diffuse freely through the membrane pores. The Na + /K + pump counteracts the equalization of the concentrations of these ions and actively pumps Na + out of the cell and K + into the cell. The Na + /K + pump is a transmembrane protein capable of conformational changes, as a result of which it can attach both K + and Na +. The Na + /K + pump cycle can be divided into the following phases: 1) addition of Na + from the inside of the membrane, 2) phosphorylation of the pump protein, 3) release of Na + in the extracellular space, 4) addition of K + from the outside of the membrane , 5) dephosphorylation of the pump protein, 6) release of K + in the intracellular space. Almost a third of all energy required for cell functioning is spent on the operation of the sodium-potassium pump. In one cycle of operation, the pump pumps out 3Na + from the cell and pumps in 2K +.

Endocytosis- the process of absorption of large particles and macromolecules by the cell. There are two types of endocytosis: 1) phagocytosis- capture and absorption of large particles (cells, parts of cells, macromolecules) and 2) pinocytosis— capture and absorption of liquid material (solution, colloidal solution, suspension). The phenomenon of phagocytosis was discovered by I.I. Mechnikov in 1882. During endocytosis, the plasma membrane forms an invagination, its edges merge, and structures delimited from the cytoplasm by a single membrane are laced into the cytoplasm. Many protozoa and some leukocytes are capable of phagocytosis. Pinocytosis is observed in intestinal epithelial cells and in the endothelium of blood capillaries.

Exocytosis- a process reverse to endocytosis: the removal of various substances from the cell. During exocytosis, the vesicle membrane merges with the outer cytoplasmic membrane, the contents of the vesicle are removed outside the cell, and its membrane is included in the outer cytoplasmic membrane. In this way, hormones are removed from the cells of the endocrine glands; in protozoa, undigested food remains are removed.

Go to lectures No. 5"Cell theory. Types of cellular organization"

Go to lectures No. 7“Eukaryotic cell: structure and functions of organelles”

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Cells are separated from the internal environment of the body by a cell or plasma membrane.

The membrane provides:

1) Selective penetration into and out of the cell of molecules and ions necessary to perform specific cell functions;
2) Selective transport of ions across the membrane, maintaining a transmembrane electrical potential difference;
3) Specificity of intercellular contacts.

Due to the presence in the membrane of numerous receptors that perceive chemical signals - hormones, mediators and other biologically active substances, it is capable of changing the metabolic activity of the cell. Membranes provide the specificity of immune manifestations due to the presence of antigens on them - structures that cause the formation of antibodies that can specifically bind to these antigens.
The nucleus and organelles of the cell are also separated from the cytoplasm by membranes, which prevent the free movement of water and substances dissolved in it from the cytoplasm into them and vice versa. This creates conditions for the separation of biochemical processes occurring in different compartments inside the cell.

Cell membrane structure

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The cell membrane is an elastic structure, with a thickness of 7 to 11 nm (Fig. 1.1). It consists mainly of lipids and proteins. From 40 to 90% of all lipids are phospholipids - phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, sphingomyelin and phosphatidylinositol. An important component of the membrane are glycolipids, represented by cerebrosides, sulfatides, gangliosides and cholesterol.

Rice. 1.1 Organization of the membrane.

Basic structure of the cell membrane is a double layer of phospholipid molecules. Due to hydrophobic interactions, the carbohydrate chains of lipid molecules are held near each other in an elongated state. Groups of phospholipid molecules of both layers interact with protein molecules immersed in the lipid membrane. Due to the fact that most of the lipid components of the bilayer are in a liquid state, the membrane has mobility and makes wave-like movements. Its sections, as well as proteins immersed in the lipid bilayer, are mixed from one part to another. The mobility (fluidity) of cell membranes facilitates the processes of transport of substances across the membrane.

Cell membrane proteins are represented mainly by glycoproteins. There are:

integral proteins, penetrating through the entire thickness of the membrane and
peripheral proteins, attached only to the surface of the membrane, mainly to its inner part.

Peripheral proteins almost all function as enzymes (acetylcholinesterase, acid and silk phosphatases, etc.). But some enzymes are also represented by integral proteins - ATPase.

Integral proteins provide selective exchange of ions through membrane channels between extracellular and intracellular fluid, and also act as proteins that transport large molecules.

Membrane receptors and antigens can be represented by both integral and peripheral proteins.

Proteins adjacent to the membrane from the cytoplasmic side are classified as cell cytoskeleton . They can attach to membrane proteins.

So, protein band 3 (band number during protein electrophoresis) of erythrocyte membranes is combined into an ensemble with other cytoskeletal molecules - spectrin through the low molecular weight protein ankyrin (Fig. 1.2).

Rice. 1.2 Scheme of the arrangement of proteins in the near-membrane cytoskeleton of erythrocytes.
1 - spectrin; 2 - ankyrin; 3 - protein of band 3; 4 - protein band 4.1; 5 - band protein 4.9; 6 - actin oligomer; 7 - protein 6; 8 - gpicophorin A; 9 - membrane.

Spectrin is a major cytoskeletal protein constituting a two-dimensional network to which actin is attached.

Actin forms microfilaments, which are the contractile apparatus of the cytoskeleton.

Cytoskeleton allows the cell to exhibit flexible-elastic properties and provides additional strength to the membrane.

Most integral proteins are glycoproteins. Their carbohydrate part protrudes from the cell membrane to the outside. Many glycoproteins have a large negative charge due to their significant sialic acid content (for example, the glycophorin molecule). This provides the surfaces of most cells with a negative charge, helping to repel other negatively charged objects. Carbohydrate protrusions of glycoproteins are carriers of blood group antigens, other antigenic determinants of the cell, and they act as receptors that bind hormones. Glycoproteins form adhesive molecules that cause cells to attach to one another, i.e. close intercellular contacts.

Features of metabolism in the membrane

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Membrane components are subject to many metabolic transformations under the influence of enzymes located on or within their membrane. These include oxidative enzymes, which play an important role in the modification of hydrophobic elements of membranes - cholesterol, etc. In membranes, when enzymes - phospholipases are activated - biologically active compounds - prostaglandins and their derivatives - are formed from arachidonic acid. As a result of activation of phospholipid metabolism, thromboxanes and leukotrienes are formed in the membrane, which have a powerful effect on platelet adhesion, the process of inflammation, etc.

The processes of renewal of its components continuously occur in the membrane . Thus, the lifetime of membrane proteins ranges from 2 to 5 days. However, there are mechanisms in the cell that ensure the delivery of newly synthesized protein molecules to membrane receptors, which facilitate the incorporation of the protein into the membrane. “Recognition” of this receptor by the newly synthesized protein is facilitated by the formation of a signal peptide, which helps to find the receptor on the membrane.

Membrane lipids are also characterized by a significant rate of exchange, which requires large amounts of fatty acids for the synthesis of these membrane components.
The specificity of the lipid composition of cell membranes is influenced by changes in the human environment and the nature of his diet.

For example, an increase in dietary fatty acids with unsaturated bonds increases the liquid state of lipids in cell membranes of various tissues, leading to a favorable change in the ratio of phospholipids to sphingomyelins and lipids to proteins for the function of the cell membrane.

Excess cholesterol in membranes, on the contrary, increases the microviscosity of their bilayer of phospholipid molecules, reducing the rate of diffusion of certain substances through cell membranes.

Food enriched with vitamins A, E, C, P improves lipid metabolism in erythrocyte membranes and reduces membrane microviscosity. This increases the deformability of red blood cells and facilitates their transport function (Chapter 6).

Deficiency of fatty acids and cholesterol in food disrupts the lipid composition and functions of cell membranes.

For example, fat deficiency disrupts the functions of the neutrophil membrane, which inhibits their ability to move and phagocytosis (the active capture and absorption of microscopic foreign living objects and particulate matter by single-celled organisms or some cells).

In the regulation of the lipid composition of membranes and their permeability, regulation of cell proliferation an important role is played by reactive oxygen species formed in the cell in conjunction with normally occurring metabolic reactions (microsomal oxidation, etc.).

Generated reactive oxygen species- superoxide radical (O 2), hydrogen peroxide (H 2 O 2), etc. are extremely reactive substances. Their main substrate in free radical oxidation reactions are unsaturated fatty acids that are part of the phospholipids of cell membranes (the so-called lipid peroxidation reactions). The intensification of these reactions can cause damage to the cell membrane, its barrier, receptor and metabolic functions, modification of nucleic acid molecules and proteins, which leads to mutations and inactivation of enzymes.

Under physiological conditions, the intensification of lipid peroxidation is regulated by the antioxidant system of cells, represented by enzymes that inactivate reactive oxygen species - superoxide dismutase, catalase, peroxidase and substances with antioxidant activity - tocopherol (vitamin E), ubiquinone, etc. A pronounced protective effect on cell membranes (cytoprotective effect) with various damaging effects on the body, prostaglandins E and J2 have, “quenching” the activation of free radical oxidation. Prostaglandins protect the gastric mucosa and hepatocytes from chemical damage, neurons, neuroglial cells, cardiomyocytes - from hypoxic damage, skeletal muscles - during heavy physical activity. Prostaglandins, by binding to specific receptors on cell membranes, stabilize the bilayer of the latter and reduce the loss of phospholipids by the membranes.

Functions of membrane receptors

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A chemical or mechanical signal is first perceived by cell membrane receptors. The consequence of this is a chemical modification of membrane proteins, leading to the activation of “second messengers” that ensure rapid propagation of the signal in the cell to its genome, enzymes, contractile elements, etc.

Transmembrane signal transmission in a cell can be schematically represented as follows:

1) The receptor, excited by the received signal, activates the γ-proteins of the cell membrane. This occurs when they bind guanosine triphosphate (GTP).

2) The interaction of the GTP-γ-protein complex, in turn, activates the enzyme - the precursor of secondary messengers, located on the inner side of the membrane.

The precursor of one second messenger, cAMP, formed from ATP, is the enzyme adenylate cyclase;
The precursor of other secondary messengers - inositol triphosphate and diacylglycerol, formed from membrane phosphatidylinositol-4,5-diphosphate, is the enzyme phospholipase C. In addition, inositol triphosphate mobilizes another secondary messenger in the cell - calcium ions, which are involved in almost all regulatory processes in the cell. For example, the resulting inositol triphosphate causes the release of calcium from the endoplasmic reticulum and an increase in its concentration in the cytoplasm, thereby turning on various forms of cellular response. With the help of inositol triphosphate and diacylglycerol, the function of smooth muscles and B cells of the pancreas is regulated by acetylcholine, the anterior lobe of the pituitary gland by thyrogropin-releasing factor, the response of lymphocytes to antigen, etc.
In some cells, the role of a second messenger is played by cGMP, formed from GTP with the help of the enzyme guanylate cyclase. It serves, for example, as a second messenger for natriuretic hormone in the smooth muscle of the walls of blood vessels. cAMP serves as a secondary messenger for many hormones - adrenaline, erythropoietin, etc. (Chapter 3).

All living organisms, depending on the structure of the cell, are divided into three groups (see Fig. 1):

1. Prokaryotes (non-nuclear)

2. Eukaryotes (nuclear)

3. Viruses (non-cellular)

Rice. 1. Living organisms

In this lesson we will begin to study the structure of cells of eukaryotic organisms, which include plants, fungi and animals. Their cells are the largest and more complex in structure compared to the cells of prokaryotes.

As is known, cells are capable of independent activity. They can exchange matter and energy with the environment, as well as grow and reproduce, therefore the internal structure of the cell is very complex and primarily depends on the function that the cell performs in a multicellular organism.

The principles of constructing all cells are the same. The following main parts can be distinguished in each eukaryotic cell (see Fig. 2):

1. The outer membrane that separates the contents of the cell from the external environment.

2. Cytoplasm with organelles.

Rice. 2. Main parts of a eukaryotic cell

The term "membrane" was proposed about a hundred years ago to refer to the boundaries of the cell, but with the development of electron microscopy it became clear that the cell membrane is part of the structural elements of the cell.

In 1959, J.D. Robertson formulated a hypothesis about the structure of the elementary membrane, according to which the cell membranes of animals and plants are built according to the same type.

In 1972, Singer and Nicholson proposed it, which is now generally accepted. According to this model, the basis of any membrane is a bilayer of phospholipids.

Phospholipids (compounds containing a phosphate group) have molecules consisting of a polar head and two non-polar tails (see Figure 3).

Rice. 3. Phospholipid

In the phospholipid bilayer, the hydrophobic fatty acid residues face inward, and the hydrophilic heads, including the phosphoric acid residue, face outward (see Fig. 4).

Rice. 4. Phospholipid bilayer

The phospholipid bilayer is presented as a dynamic structure; lipids can move, changing their position.

A double layer of lipids provides the barrier function of the membrane, preventing the contents of the cell from spreading, and prevents toxic substances from entering the cell.

The presence of a boundary membrane between the cell and the environment was known long before the advent of the electron microscope. Physical chemists denied the existence of the plasma membrane and believed that there was an interface between living colloidal contents and the environment, but Pfeffer (a German botanist and plant physiologist) confirmed its existence in 1890.

At the beginning of the last century, Overton (a British physiologist and biologist) discovered that the rate of penetration of many substances into red blood cells is directly proportional to their solubility in lipids. In this regard, the scientist suggested that the membrane contains a large amount of lipids and substances, dissolving in it, pass through it and end up on the other side of the membrane.

In 1925, Gorter and Grendel (American biologists) isolated lipids from the cell membrane of red blood cells. They distributed the resulting lipids over the surface of the water, one molecule thick. It turned out that the surface area occupied by the lipid layer is twice the area of the red blood cell itself. Therefore, these scientists concluded that the cell membrane consists of not one, but two layers of lipids.

Dawson and Danielli (English biologists) in 1935 suggested that in cell membranes the lipid bimolecular layer is sandwiched between two layers of protein molecules (see Fig. 5).

Rice. 5. Membrane model proposed by Dawson and Danielli

With the advent of the electron microscope, the opportunity opened up to get acquainted with the structure of the membrane, and then it was discovered that the membranes of animal and plant cells look like a three-layer structure (see Fig. 6).

Rice. 6. Cell membrane under a microscope

In 1959, biologist J.D. Robertson, combining the data available at that time, put forward a hypothesis about the structure of the “elementary membrane”, in which he postulated a structure common to all biological membranes.

Robertson's postulates on the structure of the “elementary membrane”

1. All membranes have a thickness of about 7.5 nm.

2. In an electron microscope, they all appear three-layered.

3. The three-layer appearance of the membrane is the result of exactly the arrangement of proteins and polar lipids that was provided for by the Dawson and Danielli model - the central lipid bilayer is sandwiched between two layers of protein.

This hypothesis about the structure of the “elementary membrane” underwent various changes, and in 1972 it was put forward fluid mosaic membrane model(see Fig. 7), which is now generally accepted.

Rice. 7. Liquid-mosaic membrane model

Protein molecules are immersed in the lipid bilayer of the membrane; they form a mobile mosaic. Based on their location in the membrane and the method of interaction with the lipid bilayer, proteins can be divided into:

- superficial (or peripheral) membrane proteins associated with the hydrophilic surface of the lipid bilayer;

- integral (membrane) proteins embedded in the hydrophobic region of the bilayer.

Integral proteins differ in the degree to which they are embedded in the hydrophobic region of the bilayer. They can be completely submerged ( integral) or partially submerged ( semi-integral), and can also penetrate the membrane through ( transmembrane).

Membrane proteins can be divided into two groups according to their functions:

- structural proteins. They are part of cell membranes and participate in maintaining their structure.

- dynamic proteins. They are located on membranes and participate in the processes occurring on it.

There are three classes of dynamic proteins.

1. Receptor. With the help of these proteins, the cell perceives various influences on its surface. That is, they specifically bind compounds such as hormones, neurotransmitters, and toxins on the outside of the membrane, which serves as a signal to change various processes inside the cell or the membrane itself.

2. Transport. These proteins transport certain substances across the membrane, and they also form channels through which various ions are transported into and out of the cell.

3. Enzymatic. These are enzyme proteins that are located in the membrane and participate in various chemical processes.

Transport of substances across the membrane

Lipid bilayers are largely impermeable to many substances, so a large amount of energy is required to transport substances across the membrane, and the formation of various structures is also required.

There are two types of transport: passive and active.

Passive transport

Passive transport is the transfer of molecules along a concentration gradient. That is, it is determined only by the difference in the concentration of the transferred substance on opposite sides of the membrane and is carried out without energy expenditure.

There are two types of passive transport:

- simple diffusion(see Fig. 8), which occurs without the participation of a membrane protein. The mechanism of simple diffusion carries out the transmembrane transfer of gases (oxygen and carbon dioxide), water and some simple organic ions. Simple diffusion has a low rate.

Rice. 8. Simple diffusion

- facilitated diffusion(see Fig. 9) differs from simple one in that it occurs with the participation of carrier proteins. This process is specific and occurs at a higher rate than simple diffusion.

Rice. 9. Facilitated diffusion

Two types of membrane transport proteins are known: carrier proteins (translocases) and channel-forming proteins. Transport proteins bind specific substances and transport them across the membrane along their concentration gradient, and, therefore, this process, as with simple diffusion, does not require the expenditure of ATP energy.

Food particles cannot pass through the membrane; they enter the cell by endocytosis (see Fig. 10). During endocytosis, the plasma membrane forms invaginations and projections and captures solid food particles. A vacuole (or vesicle) is formed around the food bolus, which is then detached from the plasma membrane, and the solid particle in the vacuole ends up inside the cell.

Rice. 10. Endocytosis

There are two types of endocytosis.

1. Phagocytosis- absorption of solid particles. Specialized cells that carry out phagocytosis are called phagocytes.

2. Pinocytosis- absorption of liquid material (solution, colloidal solution, suspension).

Exocytosis(see Fig. 11) is the reverse process of endocytosis. Substances synthesized in the cell, such as hormones, are packaged in membrane vesicles that fit into the cell membrane, are embedded in it, and the contents of the vesicle are released from the cell. In the same way, the cell can get rid of waste products it does not need.

Rice. 11. Exocytosis

Active transport

Unlike facilitated diffusion, active transport is the movement of substances against a concentration gradient. In this case, substances move from an area with a lower concentration to an area with a higher concentration. Since this movement occurs in the opposite direction to normal diffusion, the cell must expend energy in the process.

Among examples of active transport, the best studied is the so-called sodium-potassium pump. This pump pumps sodium ions out of the cell and pumps potassium ions into the cell, using the energy of ATP.

1. Structural (the cell membrane separates the cell from the environment).

2. Transport (substances are transported through the cell membrane, and the cell membrane is a highly selective filter).

3. Receptor (receptors located on the surface of the membrane perceive external influences and transmit this information inside the cell, allowing it to quickly respond to changes in the environment).

In addition to the above, the membrane also performs metabolic and energy-transforming functions.

Metabolic function

Biological membranes directly or indirectly participate in the processes of metabolic transformations of substances in the cell, since most enzymes are associated with membranes.

The lipid environment of enzymes in the membrane creates certain conditions for their functioning, imposes restrictions on the activity of membrane proteins and thus has a regulatory effect on metabolic processes.

Energy conversion function

The most important function of many biomembranes is the conversion of one form of energy into another.

Energy-converting membranes include the inner membranes of mitochondria and the thylakoids of chloroplasts (see Fig. 12).

Rice. 12. Mitochondria and chloroplast

Bibliography

Kamensky A.A., Kriksunov E.A., Pasechnik V.V. General biology 10-11 grade Bustard, 2005.
Biology. Grade 10. General biology. Basic level / P.V. Izhevsky, O.A. Kornilova, T.E. Loshchilina and others - 2nd ed., revised. - Ventana-Graf, 2010. - 224 pp.
Belyaev D.K. Biology 10-11 grade. General biology. A basic level of. - 11th ed., stereotype. - M.: Education, 2012. - 304 p.
Agafonova I.B., Zakharova E.T., Sivoglazov V.I. Biology 10-11 grade. General biology. A basic level of. - 6th ed., add. - Bustard, 2010. - 384 p.

Ayzdorov.ru ().
Youtube.com().
Doctor-v.ru ().
Animals-world.ru ().

Homework

What is the structure of the cell membrane?
Due to what properties are lipids capable of forming membranes?
Due to what functions are proteins able to participate in the transport of substances across the membrane?
List the functions of the plasma membrane.
How does passive transport across a membrane occur?
How does active transport across a membrane occur?
What is the function of the sodium-potassium pump?
What is phagocytosis, pinocytosis?

9.5.1. One of the main functions of membranes is participation in the transfer of substances. This process is achieved through three main mechanisms: simple diffusion, facilitated diffusion and active transport (Figure 9.10). Remember the most important features of these mechanisms and examples of the substances transported in each case.

Figure 9.10. Mechanisms of transport of molecules across the membrane

Simple diffusion- transfer of substances through the membrane without the participation of special mechanisms. Transport occurs along a concentration gradient without energy consumption. By simple diffusion, small biomolecules are transported - H2O, CO2, O2, urea, hydrophobic low-molecular substances. The rate of simple diffusion is proportional to the concentration gradient.

Facilitated diffusion- transfer of substances across the membrane using protein channels or special carrier proteins. It is carried out along a concentration gradient without energy consumption. Monosaccharides, amino acids, nucleotides, glycerol, and some ions are transported. Saturation kinetics is characteristic - at a certain (saturating) concentration of the transported substance, all molecules of the carrier take part in the transfer and the transport speed reaches a maximum value.

Active transport- also requires the participation of special transport proteins, but transport occurs against the concentration gradient and therefore requires energy expenditure. Using this mechanism, Na+, K+, Ca2+, Mg2+ ions are transported through the cell membrane, and protons are transported through the mitochondrial membrane. Active transport of substances is characterized by saturation kinetics.

9.5.2. An example of a transport system that carries out active transport of ions is Na+,K+-adenosine triphosphatase (Na+,K+-ATPase or Na+,K+-pump). This protein is located deep in the plasma membrane and is capable of catalyzing the reaction of ATP hydrolysis. The energy released during the hydrolysis of 1 ATP molecule is used to transfer 3 Na+ ions from the cell to the extracellular space and 2 K+ ions in the opposite direction (Figure 9.11). As a result of the action of Na+,K+-ATPase, a concentration difference is created between the cell cytosol and the extracellular fluid. Since the transfer of ions is not equivalent, an electrical potential difference occurs. Thus, an electrochemical potential arises, which consists of the energy of the difference in electrical potentials Δφ and the energy of the difference in the concentrations of substances ΔC on both sides of the membrane.

Figure 9.11. Na+, K+ pump diagram.

9.5.3. Transport of particles and high molecular weight compounds across membranes

Along with the transport of organic substances and ions carried out by carriers, there is a very special mechanism in the cell designed to absorb high-molecular compounds into the cell and remove high-molecular compounds from it by changing the shape of the biomembrane. This mechanism is called vesicular transport.

Figure 9.12. Types of vesicular transport: 1 - endocytosis; 2 - exocytosis.

During the transfer of macromolecules, sequential formation and fusion of membrane-surrounded vesicles (vesicles) occurs. Based on the direction of transport and the nature of the substances transported, the following types of vesicular transport are distinguished:

Endocytosis(Figure 9.12, 1) - transfer of substances into the cell. Depending on the size of the resulting vesicles, they are distinguished:

A) pinocytosis — absorption of liquid and dissolved macromolecules (proteins, polysaccharides, nucleic acids) using small bubbles (150 nm in diameter);

b) phagocytosis — absorption of large particles, such as microorganisms or cell debris. In this case, large vesicles called phagosomes with a diameter of more than 250 nm are formed.

Pinocytosis is characteristic of most eukaryotic cells, while large particles are absorbed by specialized cells - leukocytes and macrophages. At the first stage of endocytosis, substances or particles are adsorbed on the surface of the membrane; this process occurs without energy consumption. At the next stage, the membrane with the adsorbed substance deepens into the cytoplasm; the resulting local invaginations of the plasma membrane are detached from the cell surface, forming vesicles, which then migrate into the cell. This process is connected by a system of microfilaments and is energy dependent. The vesicles and phagosomes that enter the cell can merge with lysosomes. Enzymes contained in lysosomes break down substances contained in vesicles and phagosomes into low molecular weight products (amino acids, monosaccharides, nucleotides), which are transported into the cytosol, where they can be used by the cell.

Exocytosis(Figure 9.12, 2) - transfer of particles and large compounds from the cell. This process, like endocytosis, occurs with the absorption of energy. The main types of exocytosis are:

A) secretion - removal from the cell of water-soluble compounds that are used or affect other cells of the body. It can be carried out both by unspecialized cells and by cells of the endocrine glands, the mucous membrane of the gastrointestinal tract, adapted for the secretion of the substances they produce (hormones, neurotransmitters, proenzymes) depending on the specific needs of the body.

Secreted proteins are synthesized on ribosomes associated with the membranes of the rough endoplasmic reticulum. These proteins are then transported to the Golgi apparatus, where they are modified, concentrated, sorted, and then packaged into vesicles, which are released into the cytosol and subsequently fuse with the plasma membrane so that the contents of the vesicles are outside the cell.

Unlike macromolecules, small secreted particles, such as protons, are transported out of the cell using the mechanisms of facilitated diffusion and active transport.

b) excretion - removal from the cell of substances that cannot be used (for example, during erythropoiesis, removal from reticulocytes of the mesh substance, which is aggregated remains of organelles). The mechanism of excretion appears to be that the excreted particles are initially trapped in a cytoplasmic vesicle, which then fuses with the plasma membrane.