Which of the following functions is performed by the cell membrane? Cell membrane: its structure and functions

It's no secret to anyone that all living beings on our planet are composed of their cells, these countless "" organic matter. The cells, in turn, are surrounded by a special protective membrane - a membrane that plays a very important role in the life of the cell, and the functions cell membrane are not limited only to the protection of the cell, but represent the most complex mechanism involved in the reproduction, nutrition, and regeneration of the cell.

What is a cell membrane

The word “membrane” itself is translated from Latin as “film”, although the membrane is not just a kind of film in which the cell is wrapped, but a combination of two films interconnected and having different properties. In fact, the cell membrane is a three-layer lipoprotein (fat-protein) shell that separates each cell from neighboring cells and the environment, and carries out a controlled exchange between cells and the environment, this is the academic definition of what a cell membrane is.

The value of the membrane is simply enormous, because it not only separates one cell from another, but also ensures the interaction of the cell, both with other cells and with the environment.

History of cell membrane research

An important contribution to the study of the cell membrane was made by two German scientists Gorter and Grendel back in 1925. It was then that they managed to conduct a complex biological experiment on red blood cells - erythrocytes, during which scientists received the so-called "shadows", empty shells of erythrocytes, which were folded into one pile and measured the surface area, and also calculated the amount of lipids in them. Based on the amount of lipids obtained, the scientists came to the conclusion that they are just enough for the double layer of the cell membrane.

In 1935, another pair of cell membrane researchers, this time the Americans Daniel and Dawson, after a series of long experiments, determined the protein content in the cell membrane. Otherwise, it was impossible to explain why the membrane has such a high surface tension. Scientists cleverly presented a model of the cell membrane in the form of a sandwich, in which the role of bread is played by homogeneous lipid-protein layers, and between them instead of butter is emptiness.

In 1950, with the advent electronic theory Daniel and Dawson were already confirmed by practical observations - microphotographs of the cell membrane clearly showed layers of lipid and protein heads and also an empty space between them.

In 1960, the American biologist J. Robertson developed a theory about the three-layer structure of cell membranes, which for a long time was considered the only true one, but with further development science, doubts about its infallibility began to appear. So, for example, from the point of view of cells, it would be difficult and laborious to transport the necessary useful substances through the entire “sandwich”

And only in 1972, the American biologists S. Singer and G. Nicholson were able to explain the inconsistencies of Robertson's theory with the help of a new fluid-mosaic model of the cell membrane. In particular, they found that the cell membrane is not homogeneous in composition, moreover, it is asymmetric and filled with liquid. In addition, cells are in constant motion. And the notorious proteins that make up the cell membrane have different structures and functions.

Properties and functions of the cell membrane

Now let's look at what functions the cell membrane performs:

The barrier function of the cell membrane - the membrane, as a real border guard, stands guard over the boundaries of the cell, delaying, not letting through harmful or simply inappropriate molecules

The transport function of the cell membrane - the membrane is not only a border guard at the gates of the cell, but also a kind of customs checkpoint, exchange constantly passes through it useful substances with other cells and the environment.

Matrix function - it is the cell membrane that determines the location relative to each other, regulates the interaction between them.

Mechanical function - is responsible for the restriction of one cell from another and in parallel for the correct connection of cells with each other, for their formation into a homogeneous tissue.

The protective function of the cell membrane is the basis for building a protective shield of the cell. In nature, an example of this function would be hardwood, dense peel, protective shell, all thanks to protective function membranes.

The enzymatic function is another important function performed by some cell proteins. For example, due to this function, the synthesis of digestive enzymes occurs in the intestinal epithelium.

Also, in addition to all this, cell metabolism is carried out through the cell membrane, which can take place in three different reactions:

• Phagocytosis is a cellular exchange in which phagocytic cells embedded in the membrane capture and digest various nutrients.
• Pinocytosis - is the process of capture by the cell membrane, fluid molecules in contact with it. To do this, special tendrils are formed on the surface of the membrane, which seem to surround a drop of liquid, forming a bubble, which is subsequently “swallowed” by the membrane.
• Exocytosis - is the reverse process, when the cell releases secretory functional fluid through the membrane to the surface.

The structure of the cell membrane

There are three classes of lipids in the cell membrane:

• phospholipids (they are a combination of fats and phosphorus),
• glycolipids (combination of fats and carbohydrates),
• cholesterol.

Phospholipids and glycolipids, in turn, consist of a hydrophilic head, into which two long hydrophobic tails extend. Cholesterol, on the other hand, occupies the space between these tails, preventing them from bending, all this in some cases makes the membrane of certain cells very rigid. In addition to all this, cholesterol molecules regulate the structure of the cell membrane.

But be that as it may, but most important part The structure of the cell membrane is a protein, or rather different proteins that play various important roles. Despite the diversity of proteins contained in the membrane, there is something that unites them - annular lipids are located around all membrane proteins. Annular lipids are special structured fats that serve as a kind of protective shell for proteins, without which they simply would not work.

The structure of the cell membrane has three layers: the basis of the cell membrane is a homogeneous liquid lipid layer. Proteins cover it on both sides like a mosaic. It is proteins that, in addition to the functions described above, also play the role of peculiar channels through which substances pass through the membrane that are unable to penetrate the liquid layer of the membrane. These include, for example, potassium and sodium ions; for their penetration through the membrane, nature provides special ion channels of cell membranes. In other words, proteins provide the permeability of cell membranes.

If we look at the cell membrane through a microscope, we will see a layer of lipids formed by small spherical molecules on which proteins float like on the sea. Now you know what substances are part of the cell membrane.

Cell membrane, video

And finally, an educational video about the cell membrane.

The study of the structure of organisms, as well as plants, animals and humans, is the branch of biology called cytology. Scientists have found that the contents of the cell, which is inside it, is quite complex. It is surrounded by the so-called surface apparatus, which includes the outer cell membrane, supra-membrane structures: glycocalyx and microfilaments, pelicule and microtubules that form its submembrane complex.

In this article, we will study the structure and functions of the outer cell membrane, which is part of the surface apparatus various kinds cells.

What are the functions of the outer cell membrane?

As described earlier, the outer membrane is part of the surface apparatus of each cell, which successfully separates its internal contents and protects cell organelles from adverse environmental conditions. Another function is to ensure the exchange of substances between the cell contents and the tissue fluid, therefore, the outer cell membrane transports molecules and ions entering the cytoplasm, and also helps to remove toxins and excess toxic substances from the cell.

The structure of the cell membrane

Membranes, or plasmalemmas, of different types of cells are very different from each other. Mainly, chemical structure, as well as the relative content of lipids, glycoproteins, proteins in them and, accordingly, the nature of the receptors located in them. External which is determined primarily by the individual composition of glycoproteins, takes part in the recognition of environmental stimuli and in the reactions of the cell itself to their actions. Some types of viruses can interact with proteins and glycolipids of cell membranes, as a result of which they penetrate into the cell. Herpes and influenza viruses can use to build their protective shell.

And viruses and bacteria, the so-called bacteriophages, attach to the cell membrane and dissolve it at the point of contact with the help of a special enzyme. Then a molecule of viral DNA passes into the hole formed.

Features of the structure of the plasma membrane of eukaryotes

Recall that the outer cell membrane performs the function of transport, that is, the transfer of substances into and out of it into the external environment. To carry out such a process, a special structure is required. Indeed, the plasmalemma is a constant, universal system of the surface apparatus for all. This is a thin (2-10 Nm), but fairly dense multilayer film that covers the entire cell. Its structure was studied in 1972 by such scientists as D. Singer and G. Nicholson, they also created a fluid-mosaic model of the cell membrane.

The main chemical compounds that form it are ordered molecules of proteins and certain phospholipids, which are interspersed in a liquid lipid environment and resemble a mosaic. Thus, the cell membrane consists of two layers of lipids, the non-polar hydrophobic "tails" of which are located inside the membrane, and the polar hydrophilic heads face the cytoplasm of the cell and the intercellular fluid.

The lipid layer is penetrated by large protein molecules that form hydrophilic pores. It is through them that aqueous solutions glucose and mineral salts. Some protein molecules are located both on the outer and on inner surface plasmalemma. Thus, on the outer cell membrane in the cells of all organisms with nuclei, there are carbohydrate molecules bound by covalent bonds with glycolipids and glycoproteins. The content of carbohydrates in cell membranes ranges from 2 to 10%.

The structure of the plasmalemma of prokaryotic organisms

The outer cell membrane in prokaryotes performs similar functions to the plasma membranes of cells of nuclear organisms, namely: the perception and transmission of information coming from the external environment, the transport of ions and solutions into and out of the cell, and the protection of the cytoplasm from foreign reagents from the outside. It can form mesosomes - structures that arise when the plasmalemma protrudes into the cell. They may contain enzymes involved in the metabolic reactions of prokaryotes, for example, in DNA replication, protein synthesis.

Mesosomes also contain redox enzymes, while photosynthetics contain bacteriochlorophyll (in bacteria) and phycobilin (in cyanobacteria).

The role of outer membranes in intercellular contacts

Continuing to answer the question of what functions the outer cell membrane performs, let us dwell on its role in plant cells. In plant cells, pores are formed in the walls of the outer cell membrane, passing into the cellulose layer. Through them, the exit of the cytoplasm of the cell to the outside is possible; such thin channels are called plasmodesmata.

Thanks to them, the connection between neighboring plant cells is very strong. In human and animal cells, the sites of contact between adjacent cell membranes are called desmosomes. They are characteristic of endothelial and epithelial cells, and are also found in cardiomyocytes.

Auxiliary formations of the plasmalemma

To understand how plant cells differ from animals, it helps to study the structural features of their plasma membranes, which depend on what functions the outer cell membrane performs. Above it in animal cells is a layer of glycocalyx. It is formed by polysaccharide molecules associated with proteins and lipids of the outer cell membrane. Thanks to the glycocalyx, adhesion (sticking) occurs between cells, leading to the formation of tissues, therefore it takes part in the signaling function of the plasmalemma - the recognition of environmental stimuli.

How is the passive transport of certain substances across cell membranes

As mentioned earlier, the outer cell membrane is involved in the process of transporting substances between the cell and external environment. There are two types of transport through the plasmalemma: passive (diffusion) and active transport. The first includes diffusion, facilitated diffusion and osmosis. The movement of substances along the concentration gradient depends primarily on the mass and size of the molecules passing through the cell membrane. For example, small non-polar molecules easily dissolve in the middle lipid layer of the plasmalemma, move through it and end up in the cytoplasm.

Large molecules of organic substances penetrate into the cytoplasm with the help of special carrier proteins. They are species-specific and, when combined with a particle or ion, passively transport them across the membrane along a concentration gradient (passive transport) without expending energy. This process underlies such property of the plasmalemma as selective permeability. In the process, the energy of ATP molecules is not used, and the cell saves it for other metabolic reactions.

Active transport of chemical compounds across the plasmalemma

Since the outer cell membrane ensures the transfer of molecules and ions from the external environment into the cell and back, it becomes possible to remove the products of dissimilation, which are toxins, to the outside, that is, to the intercellular fluid. occurs against a concentration gradient and requires the use of energy in the form of ATP molecules. It also involves carrier proteins called ATPases, which are also enzymes.

An example of such transport is the sodium-potassium pump (sodium ions pass from the cytoplasm to the external environment, and potassium ions are pumped into the cytoplasm). The epithelial cells of the intestine and kidneys are capable of it. Varieties of this method of transfer are the processes of pinocytosis and phagocytosis. Thus, having studied what functions the outer cell membrane performs, it can be established that heterotrophic protists, as well as cells of higher animal organisms, for example, leukocytes, are capable of pino- and phagocytosis.

Bioelectric processes in cell membranes

It has been established that there is a potential difference between the outer surface of the plasmalemma (it is positively charged) and the parietal layer of the cytoplasm, which is negatively charged. It was called the resting potential, and it is inherent in all living cells. And the nervous tissue has not only a resting potential, but is also capable of conducting weak biocurrents, which is called the process of excitation. The outer membranes of nerve cells-neurons, receiving irritation from receptors, begin to change charges: sodium ions massively enter the cell and the surface of the plasmalemma becomes electronegative. And the parietal layer of the cytoplasm, due to an excess of cations, receives positive charge. This explains why the outer cell membrane of the neuron is recharged, which causes the conduction of nerve impulses that underlie the excitation process.

1 - polar head of the phospholipid molecule

2 - fatty acid tail of the phospholipid molecule

3 - integral protein

4 - peripheral protein

5 - semi-integral protein

6 - glycoprotein

7 - glycolipid

The outer cell membrane is inherent in all cells (animals and plants), has a thickness of about 7.5 (up to 10) nm and consists of lipid and protein molecules.

At present, the fluid-mosaic model of the construction of the cell membrane is widespread. According to this model, lipid molecules are arranged in two layers, with their water-repellent ends (hydrophobic - fat-soluble) facing each other, and water-soluble (hydrophilic) - to the periphery. Protein molecules are embedded in the lipid layer. Some of them are located on the outer or inner surface of the lipid part, others are partially immersed or penetrate the membrane through and through.

Membrane functions :

Protective, border, barrier;

Transport;

Receptor - is carried out at the expense of proteins - receptors, which have a selective ability for certain substances (hormones, antigens, etc.), enter into chemical interactions with them, conduct signals inside the cell;

Participate in the formation of intercellular contacts;

They provide the movement of some cells (amoeboid movement).

Animal cells have a thin layer of glycocalyx on top of the outer cell membrane. It is a complex of carbohydrates with lipids and carbohydrates with proteins. The glycocalyx is involved in intercellular interactions. The cytoplasmic membranes of most cell organelles have exactly the same structure.

In plant cells outside of the cytoplasmic membrane. the cell wall is made up of cellulose.

Transport of substances across the cytoplasmic membrane .

There are two main mechanisms for the entry of substances into the cell or out of the cell to the outside:

1. Passive transport.

2. Active transport.

Passive transport of substances occurs without the expenditure of energy. An example of such transport is diffusion and osmosis, in which the movement of molecules or ions is carried out from a region of high concentration to a region of lower concentration, for example, water molecules.

Active transport - in this type of transport, molecules or ions penetrate the membrane against a concentration gradient, which requires energy. An example of active transport is the sodium-potassium pump, which actively pumps sodium out of the cell and absorbs potassium ions from the external environment, transferring them into the cell. The pump is a special membrane protein that sets it in motion with ATP.

Active transport maintains a constant cell volume and membrane potential.

Substances can be transported by endocytosis and exocytosis.

Endocytosis - the penetration of substances into the cell, exocytosis - out of the cell.

During endocytosis, the plasma membrane forms an invagination or outgrowths, which then envelop the substance and, lacing off, turn into vesicles.

There are two types of endocytosis:

1) phagocytosis - the absorption of solid particles (phagocyte cells),

2) pinocytosis - the absorption of liquid material. Pinocytosis is characteristic of amoeboid protozoa.

By exocytosis, various substances are removed from the cells: undigested food residues are removed from the digestive vacuoles, their liquid secret is removed from the secretory cells.

Cytoplasm -(cytoplasm + nucleus form protoplasm). The cytoplasm consists of a watery ground substance (cytoplasmic matrix, hyaloplasm, cytosol) and various organelles and inclusions in it.

Inclusions– cell waste products. There are 3 groups of inclusions - trophic, secretory (gland cells) and special (pigment) values.

Organelles - These are permanent structures of the cytoplasm that perform certain functions in the cell.

Isolate organelles general meaning and special. Special ones are found in most cells, but are present in significant numbers only in cells that perform a specific function. These include microvilli of intestinal epithelial cells, cilia of the epithelium of the trachea and bronchi, flagella, myofibrils (providing muscle contraction, etc.).

Organelles of general importance include EPS, the Golgi complex, mitochondria, ribosomes, lysosomes, centrioles of the cell center, peroxisomes, microtubules, microfilaments. Plant cells contain plastids and vacuoles. Organelles of general importance can be divided into organelles having a membrane and non-membrane structure.

Organelles having a membrane structure are two-membrane and one-membrane. Two-membrane cells include mitochondria and plastids. To single-membrane - endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, vacuoles.

Membraneless organelles: ribosomes, cell center, microtubules, microfilaments.

Mitochondria – These are round or oval organelles. They consist of two membranes: internal and external. The inner membrane has outgrowths - cristae, which divide the mitochondria into compartments. The compartments are filled with a substance - a matrix. The matrix contains DNA, mRNA, tRNA, ribosomes, calcium and magnesium salts. This is where protein biosynthesis takes place. The main function of mitochondria is the synthesis of energy and its accumulation in ATP molecules. New mitochondria are formed in the cell as a result of the division of old ones.

plastids – organelles found predominantly in plant cells. They are of three types: chloroplasts containing a green pigment; chromoplasts (pigments of red, yellow, orange color); leucoplasts (colorless).

Chloroplasts, thanks to the green pigment chlorophyll, are able to synthesize organic matter from inorganic, using the energy of the sun.

Chromoplasts give bright colors to flowers and fruits.

Leucoplasts are able to accumulate reserve nutrients: starch, lipids, proteins, etc.

Endoplasmic reticulum ( EPS ) is a complex system of vacuoles and channels that are limited by membranes. There are smooth (agranular) and rough (granular) EPS. Smooth has no ribosomes on its membrane. It contains the synthesis of lipids, lipoproteins, the accumulation and removal of toxic substances from the cell. Granular EPS has ribosomes on membranes in which proteins are synthesized. Then the proteins enter the Golgi complex, and from there out.

Golgi complex (Golgi apparatus) is a stack of flattened membrane sacs - cisterns and a system of bubbles associated with them. The stack of cisterns is called a dictyosome.

Functions of the Golgi complex : protein modification, polysaccharide synthesis, substance transport, cell membrane formation, lysosome formation.

Lysosomes – are membrane-bound vesicles containing enzymes. They carry out intracellular cleavage of substances and are divided into primary and secondary. Primary lysosomes contain enzymes in an inactive form. After entering the organelles various substances enzymes are activated and the process of digestion begins - these are secondary lysosomes.

Peroxisomes have the appearance of bubbles bounded by a single membrane. They contain enzymes that break down hydrogen peroxide, which is toxic to cells.

Vacuoles – These are plant cell organelles that contain cell sap. Cell sap may contain spare nutrients, pigments, and waste products. Vacuoles are involved in the creation of turgor pressure, in the regulation of water-salt metabolism.

Ribosomes – organelles made up of large and small subunits. They can be located either on the ER or located freely in the cell, forming polysomes. They are composed of rRNA and protein and are produced in the nucleolus. Protein synthesis takes place in ribosomes.

Cell Center – found in the cells of animals, fungi, lower plants and absent in higher plants. It consists of two centrioles and a radiant sphere. The centriole has the form of a hollow cylinder, the wall of which consists of 9 triplets of microtubules. When dividing, cells form threads of the mitotic spindle, which ensure the divergence of chromatids in the anaphase of mitosis and homologous chromosomes during meiosis.

microtubules – tubular formations of various lengths. They are part of the centrioles, mitotic spindle, flagella, cilia, perform a supporting function, promote the movement of intracellular structures.

Microfilaments – filamentous thin formations located throughout the cytoplasm, but there are especially many of them under the cell membrane. Together with microtubules, they form the cytoskeleton of the cell, determine the flow of the cytoplasm, intracellular movements of vesicles, chloroplasts, and other organelles.

cell evolution

There are two stages in cell evolution:

1.Chemical.

2. Biological.

Chemical stage began about 4.5 billion years ago. Under the influence ultraviolet radiation, radiation, lightning discharges (sources of energy), first simple chemical compounds - monomers, and then more complex ones - polymers and their complexes (carbohydrates, lipids, proteins, nucleic acids) were formed.

The biological stage of cell formation begins with the appearance of probionts - separate complex systems capable of self-reproduction, self-regulation and natural selection. Probionts appeared 3-3.8 billion years ago. The first prokaryotic cells, bacteria, originated from probionts. Eukaryotic cells evolved from prokaryotes (1-1.4 billion years ago) in two ways:

1) By symbiosis of several prokaryotic cells - this is a symbiotic hypothesis;

2) By invagination of the cell membrane. The essence of the invagination hypothesis is that the prokaryotic cell contained several genomes attached to the cell membrane. Then invagination took place - invagination, detachment of the cell membrane, and these genomes turned into mitochondria, chloroplasts, and the nucleus.

Cell differentiation and specialization .

Differentiation is the formation of different types of cells and tissues during development multicellular organism. One of the hypotheses relates differentiation to gene expression during individual development. Expression is the process of turning certain genes into work, which creates conditions for directed synthesis of substances. Therefore, there is a development and specialization of tissues in one direction or another.

Similar information.

delimitative ( barrier) - separate the cellular contents from the external environment;

Regulate the exchange between the cell and the environment;

Divide cells into compartments, or compartments, designed for certain specialized metabolic pathways ( dividing);

It is the site of some chemical reactions (light reactions of photosynthesis in chloroplasts, oxidative phosphorylation during respiration in mitochondria);

Provide communication between cells in the tissues of multicellular organisms;

Transport- carries out transmembrane transport.

Receptor- are the site of localization of receptor sites that recognize external stimuli.

Transport of substances through the membrane is one of the leading functions of the membrane, which ensures the exchange of substances between the cell and the external environment. Depending on the energy costs for the transfer of substances, there are:

passive transport, or facilitated diffusion;

active (selective) transport with the participation of ATP and enzymes.

transport in membrane packaging. There are endocytosis (into the cell) and exocytosis (out of the cell) - mechanisms that transport large particles and macromolecules through the membrane. During endocytosis, the plasma membrane forms an invagination, its edges merge, and a vesicle is laced into the cytoplasm. The vesicle is delimited from the cytoplasm by a single membrane, which is part of the outer cytoplasmic membrane. Distinguish between phagocytosis and pinocytosis. Phagocytosis is the absorption of large particles, rather solid. For example, phagocytosis of lymphocytes, protozoa, etc. Pinocytosis is the process of capturing and absorbing liquid droplets with substances dissolved in it.

Exocytosis is the process of removing various substances from the cell. During exocytosis, the membrane of the vesicle or vacuole merges with the outer cytoplasmic membrane. The contents of the vesicle are removed from the cell surface, and the membrane is included in the outer cytoplasmic membrane.

At the core passive transport of uncharged molecules is the difference between the concentrations of hydrogen and charges, i.e. electrochemical gradient. Substances will move from an area with a higher gradient to an area with a lower one. The transport speed depends on the gradient difference.

Simple diffusion is the transport of substances directly through the lipid bilayer. Characteristic of gases, non-polar or small uncharged polar molecules, soluble in fats. Water quickly penetrates through the bilayer, because. its molecule is small and electrically neutral. The diffusion of water across membranes is called osmosis.

Diffusion through membrane channels is the transport of charged molecules and ions (Na, K, Ca, Cl) that penetrate the membrane due to the presence in it of special channel-forming proteins that form water pores.

Facilitated diffusion is the transport of substances with the help of special transport proteins. Each protein is responsible for a strictly defined molecule or group of related molecules, interacts with it and moves through the membrane. For example, sugars, amino acids, nucleotides and other polar molecules.

active transport carried out by proteins - carriers (ATPase) against an electrochemical gradient, with the expenditure of energy. Its source is ATP molecules. For example, the sodium-potassium pump.

The concentration of potassium inside the cell is much higher than outside it, and sodium - vice versa. Therefore, potassium and sodium cations passively diffuse along the concentration gradient through the water pores of the membrane. This is due to the fact that the permeability of the membrane for potassium ions is higher than for sodium ions. Accordingly, potassium diffuses faster out of the cell than sodium into the cell. However, for the normal functioning of the cell, a certain ratio of 3 potassium and 2 sodium ions is necessary. Therefore, there is a sodium-potassium pump in the membrane, which actively pumps sodium out of the cell, and potassium into the cell. This pump is a transmembrane membrane protein capable of conformational rearrangements. Therefore, it can attach to itself both potassium ions and sodium ions (antiport). The process is energy intensive:

Sodium ions and an ATP molecule enter the pump protein from the inside of the membrane, and potassium ions from the outside.

Sodium ions combine with protein molecule, and the protein acquires ATPase activity, i.e. the ability to cause ATP hydrolysis, which is accompanied by the release of energy that drives the pump.

The phosphate released during ATP hydrolysis is attached to the protein, i.e. phosphorylates a protein.

Phosphorylation causes a conformational change in the protein, it is unable to retain sodium ions. They are released and go outside the cell.

The new conformation of the protein promotes the addition of potassium ions to it.

The addition of potassium ions causes dephosphorylation of the protein. He again changes his conformation.

The change in protein conformation leads to the release of potassium ions inside the cell.

The protein is again ready to attach sodium ions to itself.

In one cycle of operation, the pump pumps 3 sodium ions out of the cell and pumps 2 potassium ions.

Cytoplasm- an obligatory component of the cell, enclosed between the surface apparatus of the cell and the nucleus. It is a complex heterogeneous structural complex, consisting of:

hyaloplasm

organelles (permanent components of the cytoplasm)

inclusions - temporary components of the cytoplasm.

cytoplasmic matrix(hyaloplasm) is the inner contents of the cell - a colorless, thick and transparent colloidal solution. The components of the cytoplasmic matrix carry out the processes of biosynthesis in the cell, contain the enzymes necessary for the formation of energy, mainly due to anaerobic glycolysis.

Basic properties of the cytoplasmic matrix.

Determines the colloidal properties of the cell. Together with the intracellular membranes of the vacuolar system, it can be considered as a highly heterogeneous or multiphase colloidal system.

Provides a change in the viscosity of the cytoplasm, the transition from a gel (thicker) to a sol (more liquid), which occurs under the influence of external and internal factors.

Provides cyclosis, amoeboid movement, cell division and movement of pigment in chromatophores.

Determines the polarity of the location of intracellular components.

Provides mechanical properties of cells - elasticity, ability to merge, rigidity.

Organelles- permanent cellular structures that ensure the performance of specific functions by the cell. Depending on the features of the structure, there are:

membranous organelles - have a membrane structure. They can be single-membrane (ER, Golgi apparatus, lysosomes, vacuoles of plant cells). Double membrane (mitochondria, plastids, nucleus).

Non-membrane organelles - do not have a membrane structure (chromosomes, ribosomes, cell center, cytoskeleton).

General purpose organelles - characteristic of all cells: nucleus, mitochondria, cell center, Golgi apparatus, ribosomes, ER, lysosomes. If organelles are characteristic of certain types of cells, they are called special organelles (for example, myofibrils that contract a muscle fiber).

Endoplasmic reticulum- a single continuous structure, the membrane of which forms many invaginations and folds that look like tubules, microvacuoles and large cisterns. EPS membranes, on the one hand, are associated with the cellular cytoplasmic membrane, and on the other hand, with the outer shell of the nuclear membrane.

There are two types of EPS - rough and smooth.

In rough or granular ER, cisterns and tubules are associated with ribosomes. is the outer side of the membrane. There is no connection with ribosomes in a smooth or agranular EPS. This is the inside of the membrane.

Outside, the cell is covered with a plasma membrane (or outer cell membrane) about 6-10 nm thick.

The cell membrane is a dense film of proteins and lipids (mainly phospholipids). Lipid molecules are arranged in an orderly manner - perpendicular to the surface, in two layers, so that their parts that interact intensively with water (hydrophilic) are directed outward, and the parts that are inert to water (hydrophobic) are directed inward.

Protein molecules are located in a non-continuous layer on the surface of the lipid framework on both sides. Some of them are immersed in the lipid layer, and some pass through it, forming areas permeable to water. These proteins perform various functions - some of them are enzymes, others are transport proteins involved in the transfer of certain substances from the environment to the cytoplasm and vice versa.

Basic Functions of the Cell Membrane

One of the main properties of biological membranes is selective permeability (semipermeability)- some substances pass through them with difficulty, others easily and even towards a higher concentration. So, for most cells, the concentration of Na ions inside is much lower than in environment. For K ions, the reverse ratio is characteristic: their concentration inside the cell is higher than outside. Therefore, Na ions always tend to enter the cell, and K ions - to go outside. The equalization of the concentrations of these ions is prevented by the presence in the membrane of a special system that plays the role of a pump that pumps Na ions out of the cell and simultaneously pumps K ions inside.

The desire of Na ions to move from outside to inside is used to transport sugars and amino acids into the cell. With the active removal of Na ions from the cell, conditions are created for the entry of glucose and amino acids into it.

In many cells, absorption of substances also occurs by phagocytosis and pinocytosis. At phagocytosis the flexible outer membrane forms a small depression where the captured particle enters. This recess increases, and, surrounded by a portion of the outer membrane, the particle is immersed in the cytoplasm of the cell. The phenomenon of phagocytosis is characteristic of amoeba and some other protozoa, as well as leukocytes (phagocytes). Similarly, the cells absorb liquids containing the substances necessary for the cell. This phenomenon has been called pinocytosis.

The outer membranes of different cells differ significantly in both chemical composition their proteins and lipids, and by their relative content. It is these features that determine the diversity in the physiological activity of the membranes of various cells and their role in the life of cells and tissues.

FROM outer membrane the endoplasmic reticulum of the cell is connected. With the help of outer membranes, different types intercellular contacts, i.e. communication between individual cells.

Many types of cells are characterized by the presence on their surface a large number protrusions, folds, microvilli. They contribute both to a significant increase in the surface area of ​​cells and improve metabolism, as well as to stronger bonds of individual cells with each other.

On the outside of the cell membrane, plant cells have thick membranes that are clearly visible in an optical microscope, consisting of cellulose (cellulose). They create a strong support for plant tissues (wood).

Some cells of animal origin also have a number of external structures that are located on top of the cell membrane and have a protective character. An example is the chitin of the integumentary cells of insects.

Functions of the cell membrane (briefly)