Cytoplasmic membrane, its structure and functions. The structure and functions of the cytoplasmic membrane of cells The structure of the cytoplasmic membrane includes

06.04.2024

General information about the eukaryotic cell

Each eukaryotic cell has a separate nucleus, which contains genetic material delimited from the matrix by the nuclear membrane (this is the main difference from prokaryotic cells). The genetic material is concentrated mainly in the form of chromosomes, which have a complex structure and consist of strands of DNA and protein molecules. Cell division occurs through mitosis (and for germ cells, meiosis). Eukaryotes include both unicellular and multicellular organisms.

There are several theories of the origin of eukaryotic cells, one of them is endosymbiontic. An aerobic, bacteria-like cell penetrated into a heterotrophic anaerobic cell, which served as the basis for the appearance of mitochondria. Spirochete-like cells began to penetrate into these cells, which gave rise to the formation of centrioles. The hereditary material was separated from the cytoplasm, a nucleus appeared, and mitosis appeared. Some eukaryotic cells were invaded by cells such as blue-green algae, which gave rise to chloroplasts. This is how the plant kingdom subsequently arose.

The sizes of human body cells vary from 2-7 microns (for platelets) to gigantic sizes (up to 140 microns for an egg).

The shape of the cells is determined by the function they perform: nerve cells are stellate due to the large number of processes (axons and dendrites), muscle cells are elongated because they must contract, red blood cells can change their shape as they move through small capillaries.

The structure of eukaryotic cells of animal and plant organisms is largely similar. Each cell is bounded on the outside by a cell membrane, or plasmalemma. It consists of a cytoplasmic membrane and a layer glycocalyx(10-20 nm thick), which covers it from the outside. The components of the glycocalyx are complexes of polysaccharides with proteins (glycoproteins) and fats (glycolipids).

The cytoplasmic membrane is a complex of a bilayer of phospholipids with proteins and polysaccharides.

In the cell they secrete nucleus and cytoplasm. The cell nucleus consists of a membrane, nuclear sap, nucleolus and chromatin. The nuclear envelope consists of two membranes separated by a perinuclear space and is permeated with pores.

The basis of the nuclear juice (matrix) is made up of proteins: filamentous, fibrillar (supporting function), globular, heteronuclear RNA and mRNA (result of processing).

Nucleolus is the structure where the formation and maturation of ribosomal RNA (r-RNA) occurs.

Chromatin in the form of clumps, it is scattered in the nucleoplasm and is a nitrogen-phase form of chromosome existence.

The cytoplasm contains the main substance (matrix, hyaloplasm), organelles and inclusions.

Organelles can be of general importance and special (in cells that perform specific functions: microvilli of the intestinal absorptive epithelium, myofibrils of muscle cells, etc.).

Organelles of general importance are the endoplasmic reticulum (smooth and rough), the Golgi complex, mitochondria, ribosomes and polysomes, lysosomes, peroxisomes, microfibrils and microtubules, centrioles of the cell center.

Plant cells also contain chloroplasts, in which photosynthesis occurs.

The elementary membrane consists of a bilayer of lipids in complex with proteins (glycoproteins: proteins + carbohydrates, lipoproteins: fats + proteins). Lipids include phospholipids, cholesterol, glycolipids (carbohydrates + fats), and lipoproteins. Each fat molecule has a polar hydrophilic head and a non-polar hydrophobic tail. In this case, the molecules are oriented so that the heads face outward and inside the cell, and the non-polar tails face inside the membrane itself. This achieves selective permeability for substances entering the cell.

There are peripheral proteins (they are located only on the inner or outer surface of the membrane), integral (they are firmly embedded in the membrane, immersed in it, and are able to change their position depending on the state of the cell). Functions of membrane proteins: receptor, structural (maintain the shape of the cell), enzymatic, adhesive, antigenic, transport.

The structure of the elementary membrane is liquid-mosaic: fats make up a liquid-crystalline frame, and proteins are mosaically built into it and can change their position.

The most important function: promotes compartmentation - the division of cell contents into separate cells that differ in the details of their chemical or enzymatic composition. This achieves high orderliness of the internal contents of any eukaryotic cell. Compartmentation promotes spatial separation of processes occurring in the cell. A separate compartment (cell) is represented by some membrane organelle (for example, a lysosome) or its part (cristae delimited by the inner membrane of mitochondria).

Other features:

1) barrier (delimitation of the internal contents of the cell);

2) structural (giving a certain shape to cells in accordance with the functions they perform);

3) protective (due to selective permeability, reception and antigenicity of the membrane);

4) regulatory (regulation of selective permeability for various substances (passive transport without energy consumption according to the laws of diffusion or osmosis and active transport with energy consumption by pinocytosis, endo- and exocytosis, sodium-potassium pump, phagocytosis));

5) adhesive function (all cells are connected to each other through specific contacts (tight and loose));

6) receptor (due to the work of peripheral membrane proteins). There are nonspecific receptors that perceive several stimuli (for example, cold and heat thermoreceptors), and specific ones that perceive only one stimulus (receptors of the light-perceiving system of the eye);

7) electrogenic (change in the electrical potential of the cell surface due to the redistribution of potassium and sodium ions (the membrane potential of nerve cells is 90 mV));

8) antigenic: associated with glycoproteins and polysaccharides of the membrane. On the surface of each cell there are protein molecules that are specific only to this type of cell. With their help, the immune system is able to distinguish between its own and foreign cells.

Cytoplasmic membrane (plasmalemma)- the main part of the surface apparatus, universal for all cells. Its thickness is about 10 nm. The plasmalemma limits the cytoplasm and protects it from external influences, and takes part in metabolic processes between the cell and the extracellular environment.

The main components of the membrane are lipids and proteins. Lipids make up about 40% of the mass of membranes. Phospholipids predominate among them.

Phospholipid molecules are arranged in a double layer (lipid bilayer). As you already know, each phospholipid molecule is formed by a polar hydrophilic head and non-polar hydrophobic tails. In the cytoplasmic membrane, the hydrophilic heads face the outer and inner sides of the membrane, and the hydrophobic tails face the inside of the membrane (Fig. 30).

In addition to lipids, membranes contain two types of proteins: integral and peripheral. Integral proteins are more or less deeply immersed in the membrane or penetrate through it. Peripheral proteins are located on the outer and inner surfaces of the membrane, and many of them ensure the interaction of the plasmalemma with supramembrane and intracellular structures.

Oligo- and polysaccharide molecules can be located on the outer surface of the cytoplasmic membrane. They covalently bind to membrane lipids and proteins, forming glycolipids and glycoproteins. In animal cells, such a carbohydrate layer covers the entire surface of the plasma membrane, forming a supramembrane complex. It is called glycocalyx(from lat. glycis — sweet, kalyum- thick skin).

Functions of the cytoplasmic membrane. The plasma membrane performs a number of functions, the most important of which are barrier, receptor and transport.

Barrier function. The cytoplasmic membrane surrounds the cell on all sides, playing the role of a barrier - an obstacle between the complexly organized intracellular contents and the extracellular environment. The barrier function is provided, first of all, by the lipid bilayer, which does not allow the cell contents to spread and prevents the penetration of foreign substances into the cell.

Receptor function. The cytoplasmic membrane contains proteins that are capable of changing their spatial structure in response to various environmental factors and thus transmitting signals into the cell. Consequently, the cytoplasmic membrane provides cell irritability (the ability to perceive stimuli and respond to them in a certain way), exchanging information between the cell and the environment.

Some receptor proteins of the cytoplasmic membrane are able to recognize certain substances and specifically bind to them. Such proteins may be involved in the selection of necessary molecules entering cells.

Receptor proteins include, for example, antigen recognition receptors of lymphocytes, hormone and neurotransmitter receptors, etc. In the implementation of receptor function, in addition to membrane proteins, elements of the glycocalyx play an important role.

The diversity and specificity of sets of receptors on the surface of cells leads to the creation of a complex system of markers that make it possible to distinguish s.self:/ cells (of the same individual or the same species) from s.foreign:/ cells. Thanks to this, cells can interact with each other (for example, conjugation in bacteria, tissue formation in animals).

Specific receptors that respond to various physical factors can be localized in the cytoplasmic membrane. For example, in the plasmalemma of light-sensitive animal cells there is a special photoreceptor system, the key role in the functioning of which is played by the visual pigment rhodopsin. With the help of photoreceptors, the light signal is converted into a chemical signal, which, in turn, leads to the emergence of a nerve impulse.

Transport function. One of the main functions of the plasmalemma is to ensure the transport of substances both into the cell and out of it into the extracellular environment. There are several main methods of transport of substances across the cytoplasmic membrane: simple diffusion, facilitated diffusion, active transport and transport in membrane packaging (Fig. 31).

With simple diffusion, spontaneous movement of substances across a membrane is observed from an area where the concentration of these substances is higher to an area where their concentration is lower. By simple diffusion, small molecules (for example, H 2 0, 0 2, CO 2, urea) and ions can pass through the plasmalemma. As a rule, nonpolar substances are transported directly through the lipid bilayer, and polar molecules and ions are transported through channels formed by special membrane proteins. Simple diffusion occurs relatively slowly. To accelerate diffuse transport, there are membrane carrier proteins. They selectively bind to one or another ion or molecule and transport them across the membrane. This type of transport is called facilitated diffusion. The rate of substance transfer during facilitated diffusion is many times higher than during simple diffusion.

Diffusion (simple and facilitated) are types of passive transport. It is characterized by the fact that substances are transported through the membrane without energy expenditure and only in the direction where there is a lower concentration of these substances.

Active transport is the transfer of substances across a membrane from an area of low concentration of these substances to an area of higher concentration. For this purpose, the membrane contains special pumps that operate using energy (see Fig. 31). Most often, ATP energy is used to operate membrane pumps.

One of the most common membrane pumps is the sodium-potassium AT Phase (Na + /K + - AT Phase). It removes Na + ions from the cell and pumps K + ions into it. To work, Na + /K + -ATPase uses the energy released during the hydrolysis of ATP. Thanks to this pump, the difference between the concentrations of Na + and K + in the cell and the extracellular environment is maintained, which underlies many bioelectrical and transport processes.

As a result of active transport with the help of membrane pumps, the content of Mgr +, Ca 2+ and other ions in the cell is also regulated.

By active transport, not only ions, but also monosaccharides, amino acids, and other low-molecular substances can move across the cytoplasmic membrane.

A unique and relatively well-studied type of membrane transport is membrane-packed transport. Depending on the direction in which substances are transported (into or out of the cell), two types of this transport are distinguished - endocytosis and exocytosis.

Endocytosis (Greek. endon- inside, kitos- cell, cell) - absorption of external particles by a cell through the formation of membrane vesicles. During endocytosis, a certain area of the plasmalemma envelops extracellular material and captures it, enclosing it in a membrane package (Fig. 32).

There are such types of endocytosis as phagocytosis (capture and absorption of solid particles) and pinocytosis (absorption of liquid).

Through endocytosis, heterotrophic protists feed, the body’s defense reactions (absorption of foreign particles by leukocytes), etc.

Exocytosis (from Greek. exo- outside) - transportation of substances enclosed in membrane packaging from the cell to the external environment. For example, the Golgi complex vesicle moves to the cytoplasmic membrane and fuses with it, and the contents of the vesicle are released into the extracellular environment. In this way, cells secrete digestive enzymes, hormones and other substances.

1. Is it possible to see the plasmalemma with a light microscope? What are the chemical composition and structure of the cytoplasmic membrane?

2. What is a glycocalyx? What cells is it characteristic of?

3. List and explain the main functions of the plasmalemma.

4. In what ways can substances be transported across a membrane? What is the fundamental difference between passive transport and active transport?

5. How do the processes of phagocytosis and pinocytosis differ? What are the similarities between these processes?

6. Compare the different types of transport of substances into the cell. Indicate their similarities and differences.

7. What functions could not be performed by the cytoplasmic membrane if it did not contain proteins? Justify your answer.

8. Some substances (for example, diethyl ether, chloroform) penetrate biological membranes even faster than water, although their molecules are much larger than water molecules. What is this connected with?

Chapter 1. Chemical components of living organisms

§ 1. Content of chemical elements in the body. Macro- and microelements
§ 2. Chemical compounds in living organisms. Inorganic substances

Chapter 2. Cell - structural and functional unit of living organisms

§ 10. History of the discovery of the cell. Creation of cell theory
§ 15. Endoplasmic reticulum. Golgi complex. Lysosomes

Chapter 3. Metabolism and energy conversion in the body

§ 24. General characteristics of metabolism and energy conversion

Chapter 4. Structural organization and regulation of functions in living organisms

cell cytoplasm membrane organoid

The basis of the plasma membrane, like other membranes in cells (for example, mitochondria, plastids, etc.), is a layer of lipids, which has two rows of molecules. Since lipid molecules are polar (one pole is hydrophilic, i.e. attracted by water, and the other is hydrophobic, i.e. repelled by water), they are arranged in a certain order. The hydrophilic ends of the molecules of one layer are directed towards the aqueous environment - into the cytoplasm of the cell, and the other layer - outwards from the cell - towards the intercellular substance (in multicellular organisms) or the aqueous environment (in unicellular organisms).

Protein molecules are mosaically embedded in a bimolecular layer of lipids. On the outside of the animal cell, polysaccharide molecules are attached to the lipids and protein molecules of the plasmalemma, forming glycolipids and glycoproteins.

This aggregate forms the glycocalyx layer. The receptor function of the plasma membrane is associated with it (see below); it can also accumulate various substances used by the cell. In addition, the glycocalyx enhances the mechanical stability of the plasmalemma.

In the cells of plants and fungi there is also a cell wall that plays a supporting and protective role. In plants it consists of cellulose, and in fungi it is made of chitin.

The structure of the elementary membrane is liquid-mosaic: fats make up a liquid-crystalline frame, and proteins are mosaically built into it and can change their position.

The most important function of the membrane: promotes compartmentation - the division of cell contents into separate cells that differ in the details of their chemical or enzymatic composition. This achieves high orderliness of the internal contents of any eukaryotic cell. Compartmentation promotes spatial separation of processes occurring in the cell. A separate compartment (cell) is represented by some membrane organelle (for example, a lysosome) or its part (cristae delimited by the inner membrane of mitochondria).

Other features:

1) barrier (delimitation of the internal contents of the cell);
2) structural (giving a certain shape to cells in accordance with the functions they perform);
3) protective (due to selective permeability, reception and antigenicity of the membrane);
4) regulatory (regulation of selective permeability for various substances (passive transport without energy consumption according to the laws of diffusion or osmosis and active transport with energy consumption by pinocytosis, endo- and exocytosis, sodium-potassium pump, phagocytosis)). By phagocytosis, whole cells or large particles are engulfed (for example, think about nutrition in amoebas or phagocytosis by protective blood cells of bacteria). During pinocytosis, small particles or droplets of a liquid substance are absorbed. Common to both processes is that the absorbed substances are surrounded by an invaginating outer membrane to form a vacuole, which then moves deep into the cytoplasm of the cell. Exocytosis is a process (being also an active transport) opposite in direction to phagocytosis and pinocytosis. With its help, undigested food remains in protozoa or biologically active substances formed in the secretory cell can be removed.
5) adhesive function (all cells are connected to each other through specific contacts (tight and loose));
6) receptor (due to the work of peripheral membrane proteins). There are nonspecific receptors that perceive several stimuli (for example, cold and heat thermoreceptors), and specific ones that perceive only one stimulus (receptors of the light-perceiving system of the eye);
7) electrogenic (change in the electrical potential of the cell surface due to the redistribution of potassium and sodium ions (the membrane potential of nerve cells is 90 mV));
8) antigenic: associated with glycoproteins and polysaccharides of the membrane. On the surface of each cell there are protein molecules that are specific only to this type of cell. With their help, the immune system is able to distinguish between its own and foreign cells. Metabolism between the cell and the environment is carried out in different ways - passive and active.

Cell structure

The modern definition of a cell is: cell is an open, limited by an active membrane, structured system of biopolymers (proteins and nucleic acids) and their macromolecular complexes participating in a single set of metabolic and energy processes that maintain and reproduce the entire system as a whole.

There is another definition of cell. Cell is an open biological system that has arisen as a result of evolution, bounded by a semi-permeable membrane, consisting of a nucleus and cytoplasm, capable of self-regulation and self-reproduction.

As we can see from the definitions, the cell structure is quite complex. In addition, when speaking about cells, we can mean cells of different organisms and organ tissues. Thus, each type of cell has its own unique characteristics. Let's try to select from this diversity those features and characteristics that unite cells of different types. An ideal cell consists of three parts: cell membrane, nucleus, cytoplasm with organelles and organelles.

Cytoplasmic membrane (CPM)

The structure of the membrane remains largely mysterious. There were several theories regarding the structure of PM. Back in the 30s of the twentieth century, a hypothesis was put forward, named after its authors Dawson–Daneeli model(sandwich model or sandwich model). According to this model, the membrane is based on a double hydrophobic layer of fats. This layer is surrounded by two layers of proteins.

However, by the beginning of the 70s of the 20th century, data contradicting this hypothesis had accumulated. As a result, a model was put forward called the Singer–Nicholson model. This is a dynamic membrane model. This model is based on the same double layer of fats, but proteins, according to this model, are mobile islands in a sea of fats.

The cell (or plasma) membrane is a thin, flexible and elastic structure with a thickness of only 7.5-10 nm. It consists mainly of proteins and lipids. The approximate ratio of its components is as follows: proteins - 55%, phospholipids - 25%, cholesterol - 13%, other lipids - 4%, carbohydrates - 3%.

The lipid layer of the cell membrane prevents the penetration of water. The basis of the membrane is lipid bilayer- a thin lipid film consisting of two monolayers and completely covering the cell. Proteins are located throughout the membrane in the form of large globules.

The lipid bilayer consists mainly of molecules phospholipids. One end of such a molecule is hydrophilic, i.e. soluble in water (a phosphate group is located on it), the other is hydrophobic, i.e. soluble only in fats (it contains a fatty acid).

Due to the fact that the hydrophobic part of the phospholipid molecule repels water, but is attracted to similar parts of the same molecules, phospholipids have the natural property of attaching to each other in the thickness of the membrane. The hydrophilic part with the phosphate group forms two membrane surfaces: the outer one, which is in contact with the extracellular fluid, and the inner one, which is in contact with the intracellular fluid.

The middle of the lipid layer is impermeable to ions and aqueous solutions of glucose and urea. Fat-soluble substances, including oxygen, carbon dioxide, and alcohol, on the contrary, easily penetrate this area of the membrane.

Cholesterol molecules, which is part of the membrane, are also lipids in nature, since their steroid group is highly soluble in fats. These molecules seem to be dissolved in the lipid bilayer. Their main purpose is to regulate the permeability (or impermeability) of membranes for water-soluble components of body fluids. In addition, cholesterol is the main regulator of membrane viscosity.

Cell membrane proteins. In the figure, globular particles are visible in the lipid bilayer - these are membrane proteins, most of which are glycoproteins. There are two types of membrane proteins: (1) integral, which penetrate the membrane through; (2) peripheral, which protrude only above one of its surfaces, without reaching the other.

Many integral proteins form channels (or pores) through which water and water-soluble substances, especially ions, can diffuse into the intra- and extracellular fluid. Due to the selectivity of the channels, some substances diffuse better than others.

Other integral proteins function as carrier proteins, transporting substances for which the lipid bilayer is impermeable. Sometimes carrier proteins act in the direction opposite to diffusion; such transport is called active transport. Some integral proteins are enzymes.

Integral membrane proteins can also serve as receptors for water-soluble substances, including peptide hormones, since the membrane is impermeable to them. Thus, integral proteins embedded in the cell membrane involve it in the process of transmitting information about the external environment into the cell.

The plasma membrane, or plasma membrane, limits the outside of the cell, acting as a mechanical barrier. Through it, substances are transported into and out of the cell. The membrane has the property of selective permeability. Molecules pass through it at different speeds: the larger the size of the molecules, the slower the speed at which they pass through the membrane.

On the outer surface of the plasma membrane in an animal cell, protein and lipid molecules are associated with carbohydrate chains, forming glycocalyx. Carbohydrate chains act as receptors. Thanks to them, intercellular recognition occurs. The cell acquires the ability to specifically respond to external influences.

Under the plasma membrane on the cytoplasmic side there is a cortical layer and intracellular fibrillar structures that provide mechanical stability of the plasma membrane

In plant cells, outside the membrane there is a dense structure - the cell membrane or cell wall, consisting of polysaccharides (cellulose)

Scheme of the structure of the plant cell wall. O - middle plate, / - primary shell (two layers on either side of 0), 2 - layers of the secondary shell, 3 - tertiary shell, PM -
plasma membrane, B - vacuole, R - nucleus.

Cell wall components are synthesized by the cell, released from the cytoplasm, and assembled outside the cell, near the plasma membrane, to form complex complexes. The cell wall in plants performs a protective function, forms an external frame, and ensures the turgor properties of cells. The presence of a cell wall regulates the flow of water into the cell. As a result, internal pressure, turgor, arises, preventing further flow of water.

The structure of the elementary membrane is liquid-mosaic: fats make up a liquid-crystalline frame, and proteins are mosaically built into it and can change their position.

Other features:

1) barrier (delimitation of the internal contents of the cell);

2) structural (giving a certain shape to cells in accordance with the functions they perform);

3) protective (due to selective permeability, reception and antigenicity of the membrane);

5) adhesive function (all cells are connected to each other through specific contacts (tight and loose));

7) electrogenic (change in the electrical potential of the cell surface due to the redistribution of potassium and sodium ions (the membrane potential of nerve cells is 90 mV));