What cellular organisms have a membrane structure. What functions does the outer cell membrane perform? The structure of the outer cell membrane

Short description:

Sazonov V.F. 1_1 Structure of the cell membrane [Electronic resource] // Kinesiologist, 2009-2018: [website]. Update date: 02/06/2018..__.201_). _The structure and functioning of the cell membrane is described (synonyms: plasmalemma, plasmalemma, biomembrane, cell membrane, outer cell membrane, cell membrane, cytoplasmic membrane). This initial information is necessary both for cytology and for understanding the processes of nervous activity: nervous excitation, inhibition, the functioning of synapses and sensory receptors.

Cell membrane (plasma) A lemma or plasma O lemma)

Definition of the concept

The cell membrane (synonyms: plasmalemma, plasmalemma, cytoplasmic membrane, biomembrane) is a triple lipoprotein (i.e., “fat-protein”) membrane that separates the cell from the environment and carries out controlled exchange and communication between the cell and its environment.

The main thing in this definition is not that the membrane separates the cell from the environment, but precisely that it connects cell with the environment. The membrane is active the structure of the cell, it is constantly working.

A biological membrane is an ultrathin bimolecular film of phospholipids encrusted with proteins and polysaccharides. This cellular structure underlies the barrier, mechanical and matrix properties of a living organism (Antonov V.F., 1996).

A figurative representation of a membrane

To me, the cell membrane looks like a lattice fence with many doors in it, which surrounds a certain territory. Any small living creature can move freely back and forth through this fence. But larger visitors can only enter through doors, and even then not all doors. Different visitors have keys only to their own doors, and they cannot go through other people's doors. So, through this fence there are constantly flows of visitors back and forth, because the main function of the membrane fence is twofold: to separate the territory from the surrounding space and at the same time connect it with the surrounding space. This is why there are many holes and doors in the fence - !

Membrane properties

1. Permeability.

2. Semi-permeability (partial permeability).

3. Selective (synonym: selective) permeability.

4. Active permeability (synonym: active transport).

5. Controlled permeability.

As you can see, the main property of a membrane is its permeability to various substances.

6. Phagocytosis and pinocytosis.

7. Exocytosis.

8. The presence of electrical and chemical potentials, or rather the potential difference between the inner and outer sides of the membrane. Figuratively we can say that “the membrane turns the cell into an “electric battery” by controlling ionic flows”. Details: .

9. Changes in electrical and chemical potential.

10. Irritability. Special molecular receptors located on the membrane can connect with signaling (control) substances, as a result of which the state of the membrane and the entire cell can change. Molecular receptors trigger biochemical reactions in response to the connection of ligands (control substances) with them. It is important to note that the signaling substance acts on the receptor from the outside, and the changes continue inside the cell. It turns out that the membrane transferred information from the environment to the internal environment of the cell.

11. Catalytic enzymatic activity. Enzymes can be embedded in the membrane or associated with its surface (both inside and outside the cell), and there they carry out their enzymatic activities.

12. Changing the shape of the surface and its area. This allows the membrane to form outgrowths outward or, conversely, invaginations into the cell.

13. The ability to form contacts with other cell membranes.

14. Adhesion - the ability to stick to hard surfaces.

Brief list of membrane properties

• Permeability.
• Endocytosis, exocytosis, transcytosis.
• Potentials.
• Irritability.
• Enzyme activity.
• Contacts.
• Adhesion.

Membrane functions

1. Incomplete isolation of internal contents from the external environment.

2. The main thing in the functioning of the cell membrane is exchange various substances between the cell and the intercellular environment. This is due to the membrane property of permeability. In addition, the membrane regulates this exchange by regulating its permeability.

3. Another important function of the membrane is creating a difference in chemical and electrical potentials between its inner and outer sides. Due to this, the inside of the cell has a negative electrical potential - .

4. The membrane also carries out information exchange between the cell and its environment. Special molecular receptors located on the membrane can bind to control substances (hormones, mediators, modulators) and trigger biochemical reactions in the cell, leading to various changes in the functioning of the cell or in its structures.

Video:Cell membrane structure

Video lecture:Details about membrane structure and transport

Membrane structure

The cell membrane has a universal three-layer structure. Its middle fat layer is continuous, and the upper and lower protein layers cover it in the form of a mosaic of separate protein areas. The fat layer is the basis that ensures the isolation of the cell from the environment, isolating it from the environment. By itself, it allows water-soluble substances to pass through very poorly, but easily allows fat-soluble substances to pass through. Therefore, the permeability of the membrane for water-soluble substances (for example, ions) must be ensured by special protein structures - and.

Below are micrographs of real cell membranes of contacting cells obtained using an electron microscope, as well as a schematic drawing showing the three-layer structure of the membrane and the mosaic nature of its protein layers. To enlarge the image, click on it.

A separate image of the inner lipid (fat) layer of the cell membrane, permeated with integral embedded proteins. The top and bottom protein layers have been removed so as not to interfere with viewing the lipid bilayer

Figure above: Partial schematic representation of a cell membrane (cell membrane), given on Wikipedia.

Please note that the outer and inner protein layers have been removed from the membrane here so that we can better see the central fatty lipid bilayer. In a real cell membrane, large protein “islands” float above and below the fatty film (small balls in the figure), and the membrane turns out to be thicker, three-layered: protein-fat-protein . So it's actually like a sandwich of two protein "pieces of bread" with a fatty layer of "butter" in the middle, i.e. has a three-layer structure, not a two-layer one.

In this picture, the small blue and white balls correspond to the hydrophilic (wettable) “heads” of the lipids, and the “strings” attached to them correspond to the hydrophobic (non-wettable) “tails”. Of the proteins, only integral end-to-end membrane proteins (red globules and yellow helices) are shown. The yellow oval dots inside the membrane are cholesterol molecules. The yellow-green chains of beads on the outside of the membrane are chains of oligosaccharides that form the glycocalyx. A glycocalyx is a kind of carbohydrate (“sugar”) “fluff” on a membrane, formed by long carbohydrate-protein molecules sticking out of it.

Living is a small “protein-fat sac” filled with semi-liquid jelly-like contents, which are permeated with films and tubes.

The walls of this sac are formed by a double fatty (lipid) film, covered inside and outside with proteins - the cell membrane. Therefore they say that the membrane has three-layer structure : proteins-fat-proteins. Inside the cell there are also many similar fatty membranes that divide its internal space into compartments. The same membranes surround cellular organelles: nucleus, mitochondria, chloroplasts. So the membrane is a universal molecular structure common to all cells and all living organisms.

On the left is no longer a real, but an artificial model of a piece of a biological membrane: this is an instantaneous snapshot of a fatty phospholipid bilayer (i.e., a double layer) in the process of its molecular dynamics simulation. The calculation cell of the model is shown - 96 PC molecules ( f osphatidyl X olina) and 2304 water molecules, for a total of 20544 atoms.

On the right is a visual model of a single molecule of the same lipid from which the membrane lipid bilayer is assembled. At the top it has a hydrophilic (water-loving) head, and at the bottom there are two hydrophobic (water-afraid) tails. This lipid has a simple name: 1-steroyl-2-docosahexaenoyl-Sn-glycero-3-phosphatidylcholine (18:0/22:6(n-3)cis PC), but you don't need to remember it unless you you plan to make your teacher faint with the depth of your knowledge.

A more precise scientific definition of a cell can be given:

is an ordered, structured, heterogeneous system of biopolymers bounded by an active membrane, participating in a single set of metabolic, energy and information processes, and also maintaining and reproducing the entire system as a whole.

Inside the cell is also permeated with membranes, and between the membranes there is not water, but a viscous gel/sol of variable density. Therefore, interacting molecules in a cell do not float freely, as in a test tube with an aqueous solution, but mostly sit (immobilized) on the polymer structures of the cytoskeleton or intracellular membranes. And chemical reactions therefore take place inside the cell almost as if in a solid rather than in a liquid. The outer membrane surrounding the cell is also lined with enzymes and molecular receptors, making it a very active part of the cell.

The cell membrane (plasmalemma, plasmolemma) is an active membrane that separates the cell from the environment and connects it with the environment. © Sazonov V.F., 2016.

From this definition of a membrane it follows that it not only limits the cell, but actively working, connecting it with its environment.

The fat that makes up the membranes is special, so its molecules are usually called not just fat, but "lipids", "phospholipids", "sphingolipids". The membrane film is double, that is, it consists of two films stuck together. Therefore, in textbooks they write that the basis of the cell membrane consists of two lipid layers (or " bilayer", i.e. a double layer). For each individual lipid layer, one side can be wetted with water, but the other cannot. So, these films stick to each other precisely with their non-wettable sides.

Bacteria membrane

The prokaryotic cell wall of gram-negative bacteria consists of several layers, shown in the figure below.
Layers of the shell of gram-negative bacteria:
1. Internal three-layer cytoplasmic membrane, which is in contact with the cytoplasm.
2. Cell wall, which consists of murein.
3. The outer three-layer cytoplasmic membrane, which has the same system of lipids with protein complexes as the inner membrane.
The communication of gram-negative bacterial cells with the outside world through such a complex three-stage structure does not give them an advantage in survival in harsh conditions compared to gram-positive bacteria that have a less powerful membrane. They also do not tolerate high temperatures, increased acidity and pressure changes.

Video lecture:Plasma membrane. E.V. Cheval, Ph.D.

Video lecture:Membrane as a cell boundary. A. Ilyaskin

Importance of Membrane Ion Channels

It is easy to understand that only fat-soluble substances can penetrate the cell through the membrane fat film. These are fats, alcohols, gases. For example, in red blood cells, oxygen and carbon dioxide easily pass in and out directly through the membrane. But water and water-soluble substances (for example, ions) simply cannot pass through the membrane into any cell. This means that they require special holes. But if you just make a hole in the fatty film, it will immediately close back. What to do? A solution was found in nature: it is necessary to make special protein transport structures and stretch them through the membrane. This is exactly how channels are formed for the passage of fat-insoluble substances - ion channels of the cell membrane.

So, to give its membrane additional properties of permeability to polar molecules (ions and water), the cell synthesizes special proteins in the cytoplasm, which are then integrated into the membrane. They come in two types: transport proteins (for example, transport ATPases) and channel-forming proteins (channel builders). These proteins are embedded in the fatty double layer of the membrane and form transport structures in the form of transporters or in the form of ion channels. Various water-soluble substances that cannot otherwise pass through the fatty membrane film can now pass through these transport structures.

In general, proteins embedded in the membrane are also called integral, precisely because they seem to be included in the membrane and penetrate it through. Other proteins, not integral, form islands, as it were, “floating” on the surface of the membrane: either on its outer surface or on its inner surface. After all, everyone knows that fat is a good lubricant and it’s easy to glide over it!

conclusions

1. In general, the membrane turns out to be three-layer:

1) outer layer of protein “islands”,

2) fatty two-layer “sea” (lipid bilayer), i.e. double lipid film,

3) an inner layer of protein “islands”.

But there is also a loose outer layer - the glycocalyx, which is formed by glycoproteins protruding from the membrane. They are molecular receptors to which signaling control substances bind.

2. Special protein structures are built into the membrane, ensuring its permeability to ions or other substances. We must not forget that in some places the sea of ​​fat is permeated through and through with integral proteins. And it is the integral proteins that form special transport structures cell membrane (see section 1_2 Membrane transport mechanisms). Through them, substances enter the cell and are also removed from the cell to the outside.

3. On any side of the membrane (outer and inner), as well as inside the membrane, enzyme proteins can be located, which affect both the state of the membrane itself and the life of the entire cell.

So the cell membrane is an active, variable structure that actively works in the interests of the entire cell and connects it with the outside world, and is not just a “protective shell”. This is the most important thing you need to know about the cell membrane.

In medicine, membrane proteins are often used as “targets” for drugs. Such targets include receptors, ion channels, enzymes, and transport systems. Recently, in addition to the membrane, genes hidden in the cell nucleus have also become targets for drugs.

Video:Introduction to the biophysics of the cell membrane: Membrane structure 1 (Vladimirov Yu.A.)

Video:History, structure and functions of the cell membrane: Membrane structure 2 (Vladimirov Yu.A.)

© 2010-2018 Sazonov V.F., © 2010-2016 kineziolog.bodhy.

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:

1. Peripheral – located on the surface of the film;
2. Integral – completely penetrate the membrane;
3. 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?

1. Delimits the contents of the cell;
2. Carries out the entry of substances into the cell;
3. 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

The vast majority of organisms living on Earth consists of cells that are largely similar in their chemical composition, structure and vital functions. Metabolism and energy conversion occur in every cell. Cell division underlies the processes of growth and reproduction of organisms. Thus, the cell is a unit of structure, development and reproduction of organisms.

A cell can only exist as an integral system, indivisible into parts. Cell integrity is ensured by biological membranes. A cell is an element of a system of a higher rank - an organism. Cell parts and organelles, consisting of complex molecules, represent integral systems of a lower rank.

The cell is an open system connected with the environment by the exchange of substances and energy. It is a functional system in which each molecule performs specific functions. The cell has stability, the ability to self-regulate and self-reproduce.

The cell is a self-governing system. The control genetic system of a cell is represented by complex macromolecules - nucleic acids (DNA and RNA).

In 1838-1839 German biologists M. Schleiden and T. Schwann summarized knowledge about the cell and formulated the main position of the cell theory, the essence of which is that all organisms, both plant and animal, consist of cells.

In 1859, R. Virchow described the process of cell division and formulated one of the most important provisions of cell theory: “Every cell comes from another cell.” New cells are formed as a result of division of the mother cell, and not from non-cellular substance, as was previously thought.

The discovery of mammalian eggs by the Russian scientist K. Baer in 1826 led to the conclusion that the cell underlies the development of multicellular organisms.

Modern cell theory includes the following provisions:

1) cell - the unit of structure and development of all organisms;

2) cells of organisms from different kingdoms of living nature are similar in structure, chemical composition, metabolism, and basic manifestations of life activity;

3) new cells are formed as a result of division of the mother cell;

4) in a multicellular organism, cells form tissues;

5) organs are made up of tissues.

With the introduction of modern biological, physical and chemical research methods into biology, it has become possible to study the structure and functioning of various components of the cell. One of the methods for studying cells is microscopy. A modern light microscope magnifies objects 3000 times and allows you to see the largest cell organelles, observe the movement of the cytoplasm, and cell division.

Invented in the 40s. XX century An electron microscope gives magnification of tens and hundreds of thousands of times. An electron microscope uses a stream of electrons instead of light, and electromagnetic fields instead of lenses. Therefore, an electron microscope produces clear images at much higher magnifications. Using such a microscope, it was possible to study the structure of cell organelles.

The structure and composition of cell organelles is studied using the method centrifugation. Chopped tissues with destroyed cell membranes are placed in test tubes and rotated in a centrifuge at high speed. The method is based on the fact that different cellular organoids have different mass and density. More dense organelles are deposited in a test tube at low centrifugation speeds, less dense ones - at high speeds. These layers are studied separately.

Widely used cell and tissue culture method, which consists in the fact that from one or several cells on a special nutrient medium one can obtain a group of the same type of animal or plant cells and even grow a whole plant. Using this method, you can get an answer to the question of how various tissues and organs of the body are formed from one cell.

The basic principles of cell theory were first formulated by M. Schleiden and T. Schwann. A cell is a unit of structure, vital activity, reproduction and development of all living organisms. To study cells, methods of microscopy, centrifugation, cell and tissue culture, etc. are used.

The cells of fungi, plants and animals have much in common not only in chemical composition, but also in structure. When examining a cell under a microscope, various structures are visible in it - organoids. Each organelle performs specific functions. There are three main parts in a cell: the plasma membrane, the nucleus and the cytoplasm (Figure 1).

Plasma membrane separates the cell and its contents from the environment. In Figure 2 you see: the membrane is formed by two layers of lipids, and protein molecules penetrate the thickness of the membrane.

Main function of the plasma membrane transport. It ensures the flow of nutrients into the cell and the removal of metabolic products from it.

An important property of the membrane is selective permeability, or semi-permeability, allows the cell to interact with the environment: only certain substances enter and are removed from it. Small molecules of water and some other substances penetrate the cell by diffusion, partly through pores in the membrane.

Sugars, organic acids, and salts are dissolved in the cytoplasm, the cell sap of the vacuoles of a plant cell. Moreover, their concentration in the cell is much higher than in the environment. The higher the concentration of these substances in the cell, the more water it absorbs. It is known that water is constantly consumed by the cell, due to which the concentration of cell sap increases and water again enters the cell.

The entry of larger molecules (glucose, amino acids) into the cell is ensured by membrane transport proteins, which, combining with the molecules of transported substances, transport them across the membrane. This process involves enzymes that break down ATP.

Figure 1. Generalized diagram of the structure of a eukaryotic cell.
(to enlarge the image, click on the picture)

Figure 2. Structure of the plasma membrane.
1 - piercing proteins, 2 - submerged proteins, 3 - external proteins

Figure 3. Diagram of pinocytosis and phagocytosis.

Even larger molecules of proteins and polysaccharides enter the cell by phagocytosis (from the Greek. phagos- devouring and kitos- vessel, cell), and drops of liquid - by pinocytosis (from the Greek. pinot- I drink and kitos) (Figure 3).

Animal cells, unlike plant cells, are surrounded by a soft and flexible “coat” formed mainly by polysaccharide molecules, which, joining some membrane proteins and lipids, surround the cell from the outside. The composition of polysaccharides is specific to different tissues, due to which cells “recognize” each other and connect with each other.

Plant cells do not have such a “coat”. They have a pore-ridden plasma membrane above them. cell membrane, consisting predominantly of cellulose. Through the pores, threads of cytoplasm stretch from cell to cell, connecting the cells to each other. This is how communication between cells is achieved and the integrity of the body is achieved.

The cell membrane in plants plays the role of a strong skeleton and protects the cell from damage.

Most bacteria and all fungi have a cell membrane, only its chemical composition is different. In fungi it consists of a chitin-like substance.

The cells of fungi, plants and animals have a similar structure. A cell has three main parts: the nucleus, the cytoplasm, and the plasma membrane. The plasma membrane is composed of lipids and proteins. It ensures the entry of substances into the cell and their release from the cell. In the cells of plants, fungi and most bacteria there is a cell membrane above the plasma membrane. It performs a protective function and plays the role of a skeleton. In plants, the cell wall consists of cellulose, and in fungi, it is made of a chitin-like substance. Animal cells are covered with polysaccharides that provide contacts between cells of the same tissue.

Do you know that the main part of the cell is cytoplasm. It consists of water, amino acids, proteins, carbohydrates, ATP, and ions of inorganic substances. The cytoplasm contains the nucleus and organelles of the cell. In it, substances move from one part of the cell to another. Cytoplasm ensures the interaction of all organelles. Chemical reactions take place here.

The entire cytoplasm is permeated with thin protein microtubules that form cell cytoskeleton, thanks to which it maintains a constant shape. The cell cytoskeleton is flexible, since microtubules are able to change their position, move from one end and shorten from the other. Various substances enter the cell. What happens to them in the cage?

In lysosomes - small round membrane vesicles (see Fig. 1) molecules of complex organic substances are broken down into simpler molecules with the help of hydrolytic enzymes. For example, proteins are broken down into amino acids, polysaccharides into monosaccharides, fats into glycyrin and fatty acids. For this function, lysosomes are often called the “digestive stations” of the cell.

If the membrane of lysosomes is destroyed, the enzymes contained in them can digest the cell itself. Therefore, lysosomes are sometimes called “cell killing weapons.”

The enzymatic oxidation of small molecules of amino acids, monosaccharides, fatty acids and alcohols formed in lysosomes to carbon dioxide and water begins in the cytoplasm and ends in other organelles - mitochondria. Mitochondria are rod-shaped, thread-like or spherical organelles, delimited from the cytoplasm by two membranes (Fig. 4). The outer membrane is smooth, and the inner one forms folds - cristas, which increase its surface. The inner membrane contains enzymes that participate in the oxidation of organic substances to carbon dioxide and water. This releases energy that is stored by the cell in ATP molecules. Therefore, mitochondria are called the “power stations” of the cell.

In the cell, organic substances are not only oxidized, but also synthesized. The synthesis of lipids and carbohydrates is carried out on the endoplasmic reticulum - EPS (Fig. 5), and proteins - on ribosomes. What is EPS? This is a system of tubules and cisterns, the walls of which are formed by a membrane. They permeate the entire cytoplasm. Substances move through the ER channels to different parts of the cell.

There is smooth and rough EPS. On the surface of the smooth ER, carbohydrates and lipids are synthesized with the participation of enzymes. The roughness of the ER is given by the small round bodies located on it - ribosomes(see Fig. 1), which are involved in protein synthesis.

The synthesis of organic substances also occurs in plastids, which are found only in plant cells.

Rice. 4. Scheme of the structure of mitochondria.
1.- outer membrane; 2.- inner membrane; 3.- folds of the inner membrane - cristae.

Rice. 5. Scheme of the structure of rough EPS.

Rice. 6. Diagram of the structure of a chloroplast.
1.- outer membrane; 2.- inner membrane; 3.- internal contents of the chloroplast; 4.- folds of the inner membrane, collected in “stacks” and forming grana.

In colorless plastids - leucoplasts(from Greek leukos- white and plastos- created) starch accumulates. Potato tubers are very rich in leucoplasts. Yellow, orange, and red colors are given to fruits and flowers. chromoplasts(from Greek chromium- color and plastos). They synthesize pigments involved in photosynthesis - carotenoids. In plant life, it is especially important chloroplasts(from Greek chloros- greenish and plastos) - green plastids. In Figure 6 you see that chloroplasts are covered with two membranes: an outer and an inner. The inner membrane forms folds; between the folds there are bubbles arranged in stacks - grains. Granas contain chlorophyll molecules, which are involved in photosynthesis. Each chloroplast has about 50 grains arranged in a checkerboard pattern. This arrangement ensures maximum illumination of each face.

In the cytoplasm, proteins, lipids, and carbohydrates can accumulate in the form of grains, crystals, and droplets. These inclusion- reserve nutrients that are consumed by the cell as needed.

In plant cells, some of the reserve nutrients, as well as breakdown products, accumulate in the cell sap of vacuoles (see Fig. 1). They can account for up to 90% of the volume of a plant cell. Animal cells have temporary vacuoles that occupy no more than 5% of their volume.

Rice. 7. Scheme of the structure of the Golgi complex.

In Figure 7 you see a system of cavities surrounded by a membrane. This Golgi complex, which performs various functions in the cell: participates in the accumulation and transportation of substances, their removal from the cell, the formation of lysosomes and the cell membrane. For example, cellulose molecules enter the cavity of the Golgi complex, which, using vesicles, move to the cell surface and are included in the cell membrane.

Most cells reproduce by division. Participating in this process cell center. It consists of two centrioles surrounded by dense cytoplasm (see Fig. 1). At the beginning of division, the centrioles move towards the poles of the cell. Protein threads emanate from them, which connect to the chromosomes and ensure their uniform distribution between the two daughter cells.

All cell organelles are closely interconnected. For example, protein molecules are synthesized in ribosomes, they are transported through ER channels to different parts of the cell, and proteins are destroyed in lysosomes. Newly synthesized molecules are used to build cell structures or accumulate in the cytoplasm and vacuoles as reserve nutrients.

The cell is filled with cytoplasm. The cytoplasm contains the nucleus and various organelles: lysosomes, mitochondria, plastids, vacuoles, ER, cell center, Golgi complex. They differ in their structure and functions. All organelles of the cytoplasm interact with each other, ensuring the normal functioning of the cell.

Table 1. CELL STRUCTURE

The most important role in the life activity and division of cells of fungi, plants and animals belongs to the nucleus and the chromosomes located in it. Most cells of these organisms have a single nucleus, but there are also multinucleated cells, such as muscle cells. The nucleus is located in the cytoplasm and has a round or oval shape. It is covered with a shell consisting of two membranes. The nuclear envelope has pores through which the exchange of substances occurs between the nucleus and the cytoplasm. The nucleus is filled with nuclear juice, in which nucleoli and chromosomes are located.

Nucleoli- these are “workshops for the production” of ribosomes, which are formed from ribosomal RNA produced in the nucleus and proteins synthesized in the cytoplasm.

The main function of the nucleus - storage and transmission of hereditary information - is associated with chromosomes. Each type of organism has its own set of chromosomes: a certain number, shape and size.

All cells of the body, except the sex cells, are called somatic(from Greek soma- body). Cells of an organism of the same species contain the same set of chromosomes. For example, in humans, each cell of the body contains 46 chromosomes, in the fruit fly Drosophila - 8 chromosomes.

Somatic cells, as a rule, have a double set of chromosomes. It is called diploid and is denoted by 2 n. So, a person has 23 pairs of chromosomes, that is, 2 n= 46. Sex cells contain half as many chromosomes. Is it single, or haploid, kit. Person has 1 n = 23.

All chromosomes in somatic cells, unlike chromosomes in germ cells, are paired. The chromosomes that make up one pair are identical to each other. Paired chromosomes are called homologous. Chromosomes that belong to different pairs and differ in shape and size are called non-homologous(Fig. 8).

In some species the number of chromosomes may be the same. For example, red clover and peas have 2 n= 14. However, their chromosomes differ in shape, size, and nucleotide composition of DNA molecules.

Rice. 8. Set of chromosomes in Drosophila cells.

Rice. 9. Chromosome structure.

To understand the role of chromosomes in the transmission of hereditary information, it is necessary to become familiar with their structure and chemical composition.

The chromosomes of a non-dividing cell look like long thin threads. Before cell division, each chromosome consists of two identical strands - chromatid, which are connected between the waists of the waist - (Fig. 9).

Chromosomes are made up of DNA and proteins. Because the nucleotide composition of DNA varies among species, the composition of chromosomes is unique to each species.

Every cell, except bacterial cells, has a nucleus in which nucleoli and chromosomes are located. Each species is characterized by a certain set of chromosomes: number, shape and size. In the somatic cells of most organisms the set of chromosomes is diploid, in the sex cells it is haploid. Paired chromosomes are called homologous. Chromosomes are made up of DNA and proteins. DNA molecules ensure the storage and transmission of hereditary information from cell to cell and from organism to organism.

Having worked through these topics, you should be able to:

1. Explain in what cases a light microscope (structure) or a transmission electron microscope should be used.
2. Describe the structure of the cell membrane and explain the relationship between the structure of the membrane and its ability to exchange substances between the cell and its environment.
3. Define the processes: diffusion, facilitated diffusion, active transport, endocytosis, exocytosis and osmosis. Indicate the differences between these processes.
4. Name the functions of the structures and indicate in which cells (plant, animal or prokaryotic) they are located: nucleus, nuclear membrane, nucleoplasm, chromosomes, plasma membrane, ribosome, mitochondrion, cell wall, chloroplast, vacuole, lysosome, smooth endoplasmic reticulum (agranular) and rough (granular), cell center, Golgi apparatus, cilium, flagellum, mesosoma, pili or fimbriae.
5. Name at least three signs by which a plant cell can be distinguished from an animal cell.
6. List the most important differences between prokaryotic and eukaryotic cells.

Ivanova T.V., Kalinova G.S., Myagkova A.N. "General Biology". Moscow, "Enlightenment", 2000

• Topic 1. "Plasma membrane." §1, §8 pp. 5;20
• Topic 2. "Cage." §8-10 pp. 20-30
• Topic 3. "Prokaryotic cell. Viruses." §11 pp. 31-34

The membrane is an ultra-fine structure that forms the surfaces of organelles and the cell as a whole. All membranes have a similar structure and are connected into one system.

Chemical composition

Cell membranes are chemically homogeneous and consist of proteins and lipids of various groups:

• phospholipids;
• galactolipids;
• sulfolipids.

They also contain nucleic acids, polysaccharides and other substances.

Physical properties

At normal temperatures, the membranes are in a liquid crystalline state and constantly fluctuate. Their viscosity is close to that of vegetable oil.

The membrane is recoverable, durable, elastic and porous. Membrane thickness is 7 - 14 nm.

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The membrane is impermeable to large molecules. Small molecules and ions can pass through the pores and the membrane itself under the influence of concentration differences on different sides of the membrane, as well as with the help of transport proteins.

Model

Typically, the structure of membranes is described using a fluid mosaic model. The membrane has a framework - two rows of lipid molecules, tightly adjacent to each other, like bricks.

Rice. 1. Sandwich-type biological membrane.

On both sides the surface of lipids is covered with proteins. The mosaic pattern is formed by protein molecules unevenly distributed on the surface of the membrane.

According to the degree of immersion in the bilipid layer, protein molecules are divided into three groups:

• transmembrane;
• submerged;
• superficial.

Proteins provide the main property of the membrane - its selective permeability to various substances.

Membrane types

All cell membranes according to localization can be divided into the following types:

• external;
• nuclear;
• organelle membranes.

The outer cytoplasmic membrane, or plasmolemma, is the boundary of the cell. Connecting with the elements of the cytoskeleton, it maintains its shape and size.

Rice. 2. Cytoskeleton.

The nuclear membrane, or karyolemma, is the boundary of the nuclear contents. It is constructed of two membranes, very similar to the outer one. The outer membrane of the nucleus is connected to the membranes of the endoplasmic reticulum (ER) and, through pores, to the inner membrane.

ER membranes penetrate the entire cytoplasm, forming surfaces on which the synthesis of various substances, including membrane proteins, takes place.

Organelle membranes

Most organelles have a membrane structure.

The walls are built from one membrane:

• Golgi complex;
• vacuoles;
• lysosomes

Plastids and mitochondria are built from two layers of membranes. Their outer membrane is smooth, and the inner one forms many folds.

Features of photosynthetic membranes of chloroplasts are built-in chlorophyll molecules.

Animal cells have a carbohydrate layer on the surface of their outer membrane called the glycocalyx.

Rice. 3. Glycocalyx.

The glycocalyx is most developed in the cells of the intestinal epithelium, where it creates conditions for digestion and protects the plasmalemma.

Table "Structure of the cell membrane"

What have we learned?

We looked at the structure and functions of the cell membrane. The membrane is a selective (selective) barrier of the cell, nucleus and organelles. The structure of the cell membrane is described by the fluid mosaic model. According to this model, protein molecules are built into the bilayer of viscous lipids.

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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:

1. combining all cell components into a single system,
2. environment for the passage of many biochemical and physiological processes,
3. 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:

1. separation of cellular contents from the external environment,
2. regulation of metabolism between the cell and the environment,
3. dividing the cell into compartments (“compartments”),
4. place of localization of “enzymatic conveyors”,
5. ensuring communication between cells in the tissues of multicellular organisms (adhesion),
6. 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”