Abstract: Cell structure and functions. Cellular structure of the body

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Plan

1. Cell, its structure and functions

2. Water in the life of the cell

3. Metabolism and energy in the cell

4. Cell nutrition. Photosynthesis and chemosynthesis

5. Genetic code. Protein synthesis in the cell

6. Regulation of transcription and translation in the cell and body

Bibliography

1. Cell, its structure and functions

Cells are located in the intercellular substance, which provides their mechanical strength, nutrition and respiration. The main parts of any cell are the cytoplasm and the nucleus.

The cell is covered with a membrane consisting of several layers of molecules, ensuring selective permeability of substances. The cytoplasm contains the smallest structures - organelles. Cell organelles include: endoplasmic reticulum, ribosomes, mitochondria, lysosomes, Golgi complex, cell center.

The cell consists of: surface apparatus, cytoplasm, nucleus.

The structure of an animal cell

Outer or plasma membrane- delimits the contents of the cell from the environment (other cells, intercellular substance), consists of lipid and protein molecules, ensures communication between cells, transport of substances into the cell (pinocytosis, phagocytosis) and out of the cell.

Cytoplasm- the internal semi-liquid environment of the cell, which provides communication between the nucleus and organelles located in it. The main life processes take place in the cytoplasm.

Cell organelles:

1) endoplasmic reticulum (ER)- a system of branching tubules, participates in the synthesis of proteins, lipids and carbohydrates, in the transport of substances, in the cell;

2) ribosomes- bodies containing rRNA are located on the ER and in the cytoplasm and participate in protein synthesis. EPS and ribosomes are a single apparatus for protein synthesis and transport;

3) mitochondria- “power stations” of the cell, delimited from the cytoplasm by two membranes. The inner one forms cristae (folds), increasing its surface. Enzymes on the cristae accelerate the oxidation of organic substances and the synthesis of energy-rich ATP molecules;

4) Golgi complex- a group of cavities delimited by a membrane from the cytoplasm, filled with proteins, fats and carbohydrates, which are either used in vital processes or removed from the cell. The membranes of the complex carry out the synthesis of fats and carbohydrates;

5) lysosomes- bodies filled with enzymes accelerate the breakdown of proteins into amino acids, lipids into glycerol and fatty acids, polysaccharides into monosaccharides. In lysosomes, dead parts of the cell, whole cells, are destroyed.

Cellular inclusions- accumulations of reserve nutrients: proteins, fats and carbohydrates.

Core- the most important part of the cell.

It is covered with a two-membrane shell with pores, through which some substances penetrate into the nucleus, while others enter the cytoplasm.

Chromosomes are the main structures of the nucleus, carriers of hereditary information about the characteristics of the organism. It is transmitted during the division of the mother cell to daughter cells, and with germ cells to daughter organisms.

The nucleus is the site of DNA, mRNA, and rRNA synthesis.

Chemical composition of the cell

The cell is the elementary unit of life on Earth. It has all the characteristics of a living organism: it grows, reproduces, exchanges substances and energy with the environment, and reacts to external stimuli. The beginning of biological evolution is associated with the appearance of cellular life forms on Earth. Unicellular organisms are cells that exist separately from each other. The body of all multicellular organisms - animals and plants - is built from a greater or lesser number of cells, which are a kind of blocks that make up a complex organism. Regardless of whether a cell is an integral living system - a separate organism or constitutes only a part of it, it is endowed with a set of characteristics and properties common to all cells.

About 60 elements of Mendeleev's periodic table, which are also found in inanimate nature, were found in cells. This is one of the proofs of the commonality of living and inanimate nature. In living organisms, the most abundant are hydrogen, oxygen, carbon and nitrogen, which make up about 98% of the mass of cells. This is due to the peculiar chemical properties of hydrogen, oxygen, carbon and nitrogen, as a result of which they turned out to be most suitable for the formation of molecules that perform biological functions. These four elements are capable of forming very strong covalent bonds by pairing electrons belonging to two atoms. Covalently bonded carbon atoms can form the frameworks of countless different organic molecules. Since carbon atoms easily form covalent bonds with oxygen, hydrogen, nitrogen, and sulfur, organic molecules achieve exceptional complexity and structural diversity.

In addition to the four main elements, the cell contains noticeable amounts (10th and 100ths of a percent) of iron, potassium, sodium, calcium, magnesium, chlorine, phosphorus and sulfur. All other elements (zinc, copper, iodine, fluorine, cobalt, manganese, etc.) are found in the cell in very small quantities and are therefore called trace elements.

Chemical elements are part of inorganic and organic compounds. Inorganic compounds include water, mineral salts, carbon dioxide, acids and bases. Organic compounds are proteins, nucleic acids, carbohydrates, fats (lipids) and lipoids. In addition to oxygen, hydrogen, carbon and nitrogen, they may contain other elements. Some proteins contain sulfur. Phosphorus is a component of nucleic acids. The hemoglobin molecule includes iron, magnesium is involved in the construction of the chlorophyll molecule. Microelements, despite their extremely low content in living organisms, play an important role in life processes. Iodine is part of the thyroid hormone - thyroxine, cobalt is part of vitamin B 12, the hormone of the islet part of the pancreas - insulin - contains zinc.

Organic cell matter

Squirrels.

Among the organic substances of the cell, proteins are in first place both in quantity (10 - 12% of the total mass of the cell) and in importance. Proteins are high-molecular polymers (with a molecular weight from 6000 to 1 million and above), the monomers of which are amino acids. Living organisms use 20 amino acids, although there are many more. Any amino acid contains an amino group (-NH2), which has basic properties, and a carboxyl group (-COOH), which has acidic properties. Two amino acids are combined into one molecule by establishing an HN-CO bond, releasing a water molecule. The bond between the amino group of one amino acid and the carboxyl group of another is called a peptide bond.

Proteins are polypeptides containing tens and hundreds of amino acids. Molecules of various proteins differ from each other in molecular weight, number, composition of amino acids and the sequence of their location in the polypeptide chain. It is therefore clear that proteins are extremely diverse; their number in all types of living organisms is estimated at 1010 - 1012.

A chain of amino acid units connected covalently by peptide bonds in a specific sequence is called the primary structure of the protein.

In cells, proteins look like spirally twisted fibers or balls (globules). This is explained by the fact that in natural protein the polypeptide chain is laid out in a strictly defined way, depending on the chemical structure of its constituent amino acids.

First, the polypeptide chain folds into a spiral. Attraction occurs between atoms of neighboring turns and hydrogen bonds are formed, in particular, between NH and CO groups located on adjacent turns. A chain of amino acids, twisted in the form of a spiral, forms the secondary structure of the protein. As a result of further folding of the helix, a configuration specific to each protein arises, called the tertiary structure. The tertiary structure is due to the action of cohesive forces between hydrophobic radicals present in some amino acids and covalent bonds between the SH groups of the amino acid cysteine (S-S bonds). The number of amino acids with hydrophobic radicals and cysteine, as well as the order of their arrangement in the polypeptide chain, are specific to each protein. Consequently, the features of the tertiary structure of a protein are determined by its primary structure. The protein exhibits biological activity only in the form of a tertiary structure. Therefore, replacing even one amino acid in a polypeptide chain can lead to a change in the configuration of the protein and to a decrease or loss of its biological activity.

In some cases, protein molecules combine with each other and can only perform their function in the form of complexes. Thus, hemoglobin is a complex of four molecules and only in this form is it capable of attaching and transporting oxygen. Such aggregates represent the quaternary structure of the protein. Based on their composition, proteins are divided into two main classes - simple and complex. Simple proteins consist only of amino acids, nucleic acids (nucleotides), lipids (lipoproteins), Me (metalloproteins), P (phosphoproteins).

The functions of proteins in a cell are extremely diverse..

One of the most important is the construction function: proteins are involved in the formation of all cell membranes and cell organelles, as well as intracellular structures. The enzymatic (catalytic) role of proteins is extremely important. Enzymes accelerate chemical reactions occurring in the cell by 10 and 100 million times. Motor function is provided by special contractile proteins. These proteins are involved in all types of movements that cells and organisms are capable of: the flickering of cilia and the beating of flagella in protozoa, muscle contraction in animals, the movement of leaves in plants, etc.

The transport function of proteins is to attach chemical elements (for example, hemoglobin adds O) or biologically active substances (hormones) and transport them to the tissues and organs of the body. The protective function is expressed in the form of the production of special proteins, called antibodies, in response to the penetration of foreign proteins or cells into the body. Antibodies bind and neutralize foreign substances. Proteins play an important role as sources of energy. With complete splitting 1g. 17.6 kJ (~4.2 kcal) of proteins are released. cell membrane chromosome

Carbohydrates.

Carbohydrates, or saccharides, are organic substances with the general formula (CH2O)n. Most carbohydrates have twice the number of H atoms as the number of O atoms, as in water molecules. That's why these substances were called carbohydrates. In a living cell, carbohydrates are found in quantities not exceeding 1-2, sometimes 5% (in the liver, in the muscles). Plant cells are the richest in carbohydrates, where their content in some cases reaches 90% of the dry matter mass (seeds, potato tubers, etc.).

Carbohydrates are simple and complex.

Simple carbohydrates are called monosaccharides. Depending on the number of carbohydrate atoms in the molecule, monosaccharides are called trioses, tetroses, pentoses or hexoses. Of the six carbon monosaccharides - hexoses - the most important are glucose, fructose and galactose. Glucose is contained in the blood (0.1-0.12%). The pentoses ribose and deoxyribose are found in nucleic acids and ATP. If two monosaccharides are combined in one molecule, the compound is called a disaccharide. Table sugar, obtained from cane or sugar beets, consists of one molecule of glucose and one molecule of fructose, milk sugar - of glucose and galactose.

Complex carbohydrates formed from many monosaccharides are called polysaccharides. The monomer of polysaccharides such as starch, glycogen, cellulose is glucose. Carbohydrates perform two main functions: construction and energy. Cellulose forms the walls of plant cells. The complex polysaccharide chitin serves as the main structural component of the exoskeleton of arthropods. Chitin also performs a construction function in fungi.

Carbohydrates play the role of the main source of energy in the cell. During the oxidation of 1 g of carbohydrates, 17.6 kJ (~4.2 kcal) is released. Starch in plants and glycogen in animals are deposited in cells and serve as an energy reserve.

Nucleic acids.

The importance of nucleic acids in a cell is very great. The peculiarities of their chemical structure provide the possibility of storing, transferring and inheriting to daughter cells information about the structure of protein molecules that are synthesized in each tissue at a certain stage of individual development.

Since most of the properties and characteristics of cells are determined by proteins, it is clear that the stability of nucleic acids is the most important condition for the normal functioning of cells and entire organisms. Any changes in the structure of cells or the activity of physiological processes in them, thus affecting vital activity. The study of the structure of nucleic acids is extremely important for understanding the inheritance of traits in organisms and the patterns of functioning of both individual cells and cellular systems - tissues and organs.

There are 2 types of nucleic acids - DNA and RNA.

DNA is a polymer consisting of two nucleotide helices arranged to form a double helix. Monomers of DNA molecules are nucleotides consisting of a nitrogenous base (adenine, thymine, guanine or cytosine), a carbohydrate (deoxyribose) and a phosphoric acid residue. The nitrogenous bases in the DNA molecule are connected to each other by an unequal number of H-bonds and are arranged in pairs: adenine (A) is always against thymine (T), guanine (G) against cytosine (C). Schematically, the arrangement of nucleotides in a DNA molecule can be depicted as follows:

Fig. 1. Location of nucleotides in a DNA molecule

From Fig.1. it is clear that the nucleotides are connected to each other not randomly, but selectively. The ability for selective interaction of adenine with thymine and guanine with cytosine is called complementarity. The complementary interaction of certain nucleotides is explained by the peculiarities of the spatial arrangement of atoms in their molecules, which allow them to come closer and form H-bonds.

In a polynucleotide chain, neighboring nucleotides are linked to each other through a sugar (deoxyribose) and a phosphoric acid residue. RNA, like DNA, is a polymer whose monomers are nucleotides.

The nitrogenous bases of three nucleotides are the same as those that make up DNA (A, G, C); the fourth - uracil (U) - is present in the RNA molecule instead of thymine. RNA nucleotides differ from DNA nucleotides in the structure of the carbohydrate they contain (ribose instead of deoxyribose).

In a chain of RNA, nucleotides are joined by forming covalent bonds between the ribose of one nucleotide and the phosphoric acid residue of another. The structure differs between two-stranded RNA. Double-stranded RNAs are the custodians of genetic information in a number of viruses, i.e. They perform the functions of chromosomes. Single-stranded RNA transfers information about the structure of proteins from the chromosome to the place of their synthesis and participates in protein synthesis.

There are several types of single-stranded RNA. Their names are determined by their function or location in the cell. Most of the RNA in the cytoplasm (up to 80-90%) is ribosomal RNA (rRNA), contained in ribosomes. rRNA molecules are relatively small and consist of an average of 10 nucleotides.

Another type of RNA (mRNA) that carries information about the sequence of amino acids in proteins that must be synthesized to ribosomes. The size of these RNAs depends on the length of the DNA region from which they were synthesized.

Transfer RNAs perform several functions. They deliver amino acids to the site of protein synthesis, “recognize” (by the principle of complementarity) the triplet and RNA corresponding to the transferred amino acid, and carry out the precise orientation of the amino acid on the ribosome.

Fats and lipoids.

Fats are compounds of high-molecular fatty acids and trihydric alcohol glycerol. Fats do not dissolve in water - they are hydrophobic.

There are always other complex hydrophobic fat-like substances called lipoids in the cell. One of the main functions of fats is energy. During the breakdown of 1 g of fats into CO 2 and H 2O, a large amount of energy is released - 38.9 kJ (~ 9.3 kcal).

The main function of fats in the animal (and partly plant) world is storage.

Fats and lipids also perform a construction function: they are part of cell membranes. Due to poor thermal conductivity, fat is capable of a protective function. In some animals (seals, whales) it is deposited in the subcutaneous adipose tissue, forming a layer up to 1 m thick. The formation of some lipoids precedes the synthesis of a number of hormones. Consequently, these substances also have the function of regulating metabolic processes.

2. Water in the life of the cell

Chemical substances that make up the cell: inorganic (water, mineral salts)

Ensuring cell elasticity.

The consequences of cell loss of water are wilting of leaves and drying out of fruits.

Accelerating chemical reactions by dissolving substances in water.

Ensuring the movement of substances: the entry of most substances into the cell and their removal from the cell in the form of solutions.

Ensuring the dissolution of many chemicals (a number of salts, sugars).

Participation in a number of chemical reactions.

Participation in the process of thermoregulation due to the ability to slowly heat up and slowly cool down.

Water. H 2ABOUT - the most common compound in living organisms. Its content in different cells varies within fairly wide limits.

The extremely important role of water in supporting life processes is due to its physicochemical properties.

The polarity of molecules and the ability to form hydrogen bonds make water a good solvent for a huge number of substances. Most chemical reactions occurring in a cell can only occur in an aqueous solution.

Water is also involved in many chemical transformations.

The total number of hydrogen bonds between water molecules varies depending on t °. At t ° When ice melts, approximately 15% of hydrogen bonds are destroyed, at t° 40°C - half. Upon transition to the gaseous state, all hydrogen bonds are destroyed. This explains the high specific heat capacity of water. When the temperature of the external environment changes, water absorbs or releases heat due to the rupture or new formation of hydrogen bonds.

In this way, fluctuations in temperature inside the cell turn out to be smaller than in the environment. The high heat of evaporation underlies the efficient mechanism of heat transfer in plants and animals.

Water as a solvent takes part in the phenomena of osmosis, which plays an important role in the life of the body’s cells. Osmosis is the penetration of solvent molecules through a semi-permeable membrane into a solution of a substance.

Semi-permeable membranes are those that allow solvent molecules to pass through, but do not allow solute molecules (or ions) to pass through. Therefore, osmosis is the one-way diffusion of water molecules in the direction of the solution.

Mineral salts.

Most of the inorganic substances in cells are in the form of salts in a dissociated or solid state.

The concentration of cations and anions in the cell and in its environment is not the same. The osmotic pressure in the cell and its buffering properties largely depend on the concentration of salts.

Buffering is the ability of a cell to maintain the slightly alkaline reaction of its contents at a constant level. The content of mineral salts in the cell in the form of cations (K+, Na+, Ca2+, Mg2+) and anions (--HPO|~, - H 2PC>4, -SG, -NSS*z). The balance of the content of cations and anions in the cell, ensuring the constancy of the internal environment of the body. Examples: in the cell the environment is slightly alkaline, inside the cell there is a high concentration of K+ ions, and in the environment surrounding the cell there is a high concentration of Na+ ions. Participation of mineral salts in metabolism.

3 . ABOUTmetabolism and energy in the cell

Energy metabolism in the cell

Adenosine triphosphate (abbr. ATP, English Asia-Pacific) - nucleotide, plays an extremely important role in the exchange of energy and substances in organisms; First of all, the compound is known as a universal source of energy for all biochemical processes occurring in living systems.

ATP provides energy for all cell functions: mechanical work, biosynthesis, division, etc. On average, the ATP content in a cell is about 0.05% of its mass, but in those cells where ATP costs are high (for example, in liver cells, transverse -striated muscles), its content can reach up to 0.5%. ATP synthesis in cells occurs mainly in mitochondria. As you remember (see 1.7), it is necessary to spend 40 kJ to synthesize 1 mole of ATP from ADP.

Energy metabolism in the cell is divided into three stages.

The first stage is preparatory.

During this process, large food polymer molecules break down into smaller fragments. Polysaccharides break down into di- and monosaccharides, proteins into amino acids, fats into glycerol and fatty acids. During these transformations, little energy is released, it is dissipated as heat, and ATP is not formed.

The second stage is incomplete, oxygen-free, breakdown of substances.

At this stage, the substances formed during the preparatory stage are decomposed by enzymes in the absence of oxygen.

Let's look at this stage using the example of glycolysis - the enzymatic breakdown of glucose. Glycolysis occurs in animal cells and in some microorganisms. In total, this process can be represented as the following equation:

C 6H 12O 6 + 2H 3P 04 + 2ADP > 2C 3H 603 + 2ATP + 2H 2O

Thus, during glycolysis, two molecules of three-carbon pyruvic acid (C 3H 4O 3) are formed from one molecule of glucose, which in many cells, for example, muscle cells, is converted into lactic acid (C 3H 6O 3), and the energy released in this case enough to convert two molecules of ADP into two molecules of ATP.

Despite its apparent simplicity, glycolysis is a multi-stage process, numbering more than ten stages, catalyzed by different enzymes. Only 40% of the released energy is stored by the cell in the form of ATP, and the remaining 60% is dissipated in the form of heat. Due to the many stages of glycolysis, the released small portions of heat do not have time to heat the cell to a dangerous level.

Glycolysis occurs in the cytoplasm of cells.

In most plant cells and some fungi, the second stage of energy metabolism is represented by alcoholic fermentation:

C 6H 12O 6 + 2H 3PO 4 + 2ADP>2C 2H 5OH + 2C 02 + 2ATP + 2H2O

The initial products of alcoholic fermentation are the same as those of glycolysis, but the result is the formation of ethyl alcohol, carbon dioxide, water and two ATP molecules. There are microorganisms that decompose glucose into acetone, acetic acid and other substances, but in any case, the “energy gain” of the cell is two ATP molecules.

The third stage of energy metabolism is complete oxygen breakdown, or cellular respiration.

In this case, the substances formed in the second stage are destroyed to the final products - CO 2 and H 2O. This stage can be imagined as follows:

2C 3H 6O 3 + 6O 2 + 36H 3PO 4 + 36 ADP > 6CO 2 + 42 H 2O + 36ATP.

Thus, the oxidation of two molecules of three-carbon acid, formed during the enzymatic breakdown of glucose to CO 2 and H 2O, leads to the release of a large amount of energy, sufficient for the formation of 36 ATP molecules.

Cellular respiration occurs at the cristae of mitochondria. The efficiency of this process is higher than that of glycolysis and is approximately 55%. As a result of the complete breakdown of one glucose molecule, 38 ATP molecules are formed.

To obtain energy in cells, in addition to glucose, other substances can be used: lipids, proteins. However, the leading role in energy metabolism in most organisms belongs to sugars.

4 . Pfoodcells. Photosynthesis and chemosynthesis

Cell nutrition occurs as a result of a number of complex chemical reactions, during which substances that enter the cell from the external environment (carbon dioxide, mineral salts, water) enter the body of the cell itself in the form of proteins, sugars, fats, oils, nitrogen and phosphorus compounds.

All living organisms living on Earth can be divided into two groups depending on how they obtain the organic substances they need.

First group - autotrophs, which translated from Greek means “self-feeding”. They are able to independently create all the organic substances they need to build cells and vital processes from inorganic ones - water, carbon dioxide and others. They receive energy for such complex transformations either from sunlight and are called phototrophs, or from the energy of chemical transformations of mineral compounds, in which case they are called chemotrophs. But both phototrophic and chemotrophic organisms do not require organic substances from outside. Autotrophs include all green plants and many bacteria.

A fundamentally different way of obtaining the necessary organic compounds from heterotrophs. Heterotrophs cannot independently synthesize such substances from inorganic compounds and require constant absorption of ready-made organic substances from the outside. Then they “rearrange” the molecules obtained from outside to suit their needs.

Heterotrophic organisms are directly dependent on the products of photosynthesis produced by green plants. For example, by eating cabbage or potatoes, we receive substances synthesized in plant cells using the energy of sunlight. If we eat the meat of domestic animals, then we must remember that these animals eat plant foods: grass, grain, etc. Thus, their meat is built from molecules obtained from plant foods.

Heterotrophs include fungi, animals and many bacteria. Some cells of a green plant are also heterotrophic: cambium and root cells. The fact is that the cells of these parts of the plant are not capable of photosynthesis and are nourished by organic substances synthesized by the green parts of the plant.

Cell nutrition: lysosomes and intracellular digestion

Lysosomes, the number of which reaches several hundred in one cell, form a typical space.

Lysosomes come in various shapes and sizes; Their internal structure is particularly diverse. This diversity is reflected in morphological terminology. There are many terms for the particles we now know as lysosomes. Among them: dense bodies, residual bodies, cytosomes, cytosegresomas and many others.

From a chemical point of view, digesting food means subjecting it to hydrolysis, i.e. using water to break down the various bonds through which the building blocks of natural macromolecules are connected. For example, peptide bonds that connect amino acids in proteins, glycolytic bonds that connect sugars in polysaccharides, and ester bonds between acids and alcohols. For the most part, these bonds are very stable, breaking only under severe temperature and pH conditions (acidic or alkaline).

Living organisms are unable to create or withstand such conditions, yet they digest food without difficulty. And they do this with the help of special catalysts - hydrolytic enzymes, or hydrolases, which are secreted in the digestive system. Hydrolases are specific catalysts. Each of them breaks only a strictly defined type of chemical bond. Since food typically consists of many components with diverse chemical bonds, digestion requires the simultaneous coordinated or sequential participation of various enzymes. Indeed, digestive juices secreted into the gastrointestinal tract contain a large number of different hydrolases, which allows the human body to absorb many complex foods of plant and animal origin. However, this ability is limited and the human body is unable to digest cellulose.

These basic provisions essentially apply to lysosomes. In each lysosome we find a whole collection of different hydrolases - more than 50 species have been identified - which together are capable of completely or almost completely digesting many of the main natural substances, including proteins, polysaccharides, nucleic acids, combinations and derivatives thereof. However, like the human gastrointestinal tract, lysosomes are characterized by some limitations in their digestive capacity.

In the intestine, the end products of digestion (digested) are "cleaned" as a result of intestinal absorption: they are removed by mucosal cells, usually with the help of active pumps, and enter the bloodstream. Something similar happens in lysosomes.

Various small molecules formed during digestion are transported through the lysosomal membrane into the cytoplasm, where they are used by the cell's metabolic systems.

But sometimes digestion does not occur or is incomplete and does not reach the stage at which its products can be purified. In most simple organisms and lower invertebrates, such situations do not cause any special consequences, because their cells have the ability to get rid of the contents of their old lysosomes, simply throwing them into the environment.

In higher animals, many cells are unable to empty their lysosomes in this way. They are in a state of chronic constipation. It is this serious deficiency that underlies numerous pathological conditions associated with lysosome overload. Dyspepsia, hyperacidity, constipation and other digestive disorders.

Afthotrophic nutrition

Life on Earth depends on autotrophic organisms. Almost all organic substances needed by living cells are produced through the process of photosynthesis.

Photosynthesis(from the Greek photos - light and synthesis - connection, combination) - the transformation by green plants and photosynthetic microorganisms of inorganic substances (water and carbon dioxide) into organic ones due to solar energy, which is converted into the energy of chemical bonds in the molecules of organic substances.

Phases of photosynthesis.

During the process of photosynthesis, energy-poor water and carbon dioxide are converted into energy-intensive organic matter - glucose. In this case, solar energy is accumulated in the chemical bonds of this substance. In addition, during the process of photosynthesis, oxygen is released into the atmosphere, which is used by organisms for respiration.

It has now been established that photosynthesis occurs in two phases - light and dark.

During the light phase, due to solar energy, chlorophyll molecules are excited and ATP is synthesized.

Simultaneously with this reaction, water (H 20) decomposes under the influence of light, releasing free oxygen (02). This process was called photolysis (from the Greek photos - light and lysis - dissolution). The resulting hydrogen ions bind to a special substance - the hydrogen ion transporter (NADP) and are used in the next phase.

The presence of light is not necessary for tempo phase reactions to occur. The source of energy here is ATP molecules synthesized in the light phase. In the tempo phase, carbon dioxide is absorbed from the air, its reduction with hydrogen ions and the formation of glucose due to the use of ATP energy.

The influence of environmental conditions on photosynthesis.

Photosynthesis uses only 1% of the solar energy falling on the leaf. Photosynthesis depends on a number of environmental conditions. Firstly, this process occurs most intensively under the influence of red rays of the solar spectrum (Fig. 58). The intensity of photosynthesis is determined by the amount of oxygen released, which displaces water from the cylinder. The rate of photosynthesis also depends on the degree of illumination of the plant. An increase in daylight hours leads to an increase in the productivity of photosynthesis, i.e., the amount of organic substances produced by the plant.

The meaning of photosynthesis.

Photosynthesis products are used:

· organisms as nutrients, a source of energy and oxygen for vital processes;

· in human food production;

· as a building material for housing construction, in the production of furniture, etc.

Humanity owes its existence to photosynthesis.

All fuel reserves on Earth are products formed as a result of photosynthesis. Using coal and wood, we obtain energy that was stored in organic matter during photosynthesis. At the same time, oxygen is released into the atmosphere.

Scientists estimate that without photosynthesis, the entire supply of oxygen would be used up in 3,000 years.

Chemosynthesis.

In addition to photosynthesis, there is another known method for obtaining energy and synthesizing organic substances from inorganic ones. Some bacteria are capable of extracting energy by oxidizing various inorganic substances. They do not need light to create organic substances. The process of synthesis of organic substances from inorganic ones, which takes place thanks to the energy of oxidation of inorganic substances, is called chemosynthesis (from the Latin chemistry - chemistry and the Greek synthesis - connection, combination).

Chemosynthetic bacteria were discovered by Russian scientist S.N. Vinogradsky. Depending on the oxidation of which substance releases energy, chemosynthesizing iron bacteria, sulfur bacteria and azotobacteria are distinguished.

5 . Ggeneticcue code. Protein synthesis in the cell

Genetic code- a unified system for recording hereditary information in nucleic acid molecules in the form of a nucleotide sequence. The genetic code is based on the use of an alphabet consisting of only four letters-nucleotides, distinguished by nitrogenous bases: A, T, G, C.

The main properties of the genetic code are as follows:

1. The genetic code is triplet. A triplet (codon) is a sequence of three nucleotides encoding one amino acid. Since proteins contain 20 amino acids, it is obvious that each of them cannot be encoded by one nucleotide (since there are only four types of nucleotides in DNA, in this case 16 amino acids remain unencoded). Two nucleotides are also not enough to encode amino acids, since in this case only 16 amino acids can be encoded. This means that the smallest number of nucleotides encoding one amino acid is three. (In this case, the number of possible nucleotide triplets is 43 = 64).

2. Redundancy (degeneracy) of the code is a consequence of its triplet nature and means that one amino acid can be encoded by several triplets (since there are 20 amino acids and 64 triplets). The exceptions are methionine and tryptophan, which are encoded by only one triplet. In addition, some triplets perform specific functions.

So, in the mRNA molecule, three of them UAA, UAG, UGA are stop codons, i.e. stop signals that stop the synthesis of the polypeptide chain. The triplet corresponding to methionine (AUG), located at the beginning of the DNA chain, does not code for an amino acid, but performs the function of initiating (exciting) reading.

3. Along with redundancy, the code is characterized by the property of unambiguity, which means that each codon corresponds to only one specific amino acid.

4. The code is collinear, i.e. the sequence of nucleotides in a gene exactly matches the sequence of amino acids in a protein.

5. The genetic code is non-overlapping and compact, that is, it does not contain “punctuation marks.” This means that the reading process does not allow for the possibility of overlapping columns (triplets), and, starting at a certain codon, reading proceeds continuously, triplet after triplet, until the stop signals (termination codons). For example, in mRNA the following sequence of nitrogenous bases AUGGGUGTSUAUAUGUG will be read only by such triplets: AUG, GUG, TSUU, AAU, GUG, and not AUG, UGG, GGU, GUG, etc. or AUG, GGU, UGC, CUU, etc. etc. or in some other way (for example, codon AUG, punctuation mark G, codon UGC, punctuation mark U, etc.).

6. The genetic code is universal, that is, the nuclear genes of all organisms encode information about proteins in the same way, regardless of the level of organization and systematic position of these organisms.

Protein synthesis in the cell

Protein biosynthesis occurs in every living cell. It is most active in young growing cells, where proteins are synthesized to build their organelles, as well as in secretory cells, where enzyme proteins and hormone proteins are synthesized.

The main role in determining the structure of proteins belongs to DNA. A piece of DNA containing information about the structure of one protein is called a gene. A DNA molecule contains several hundred genes. The DNA molecule contains a code for the sequence of amino acids in a protein in the form of specifically combined nucleotides. The DNA code was almost completely deciphered. Its essence is as follows. Each amino acid corresponds to a section of a DNA chain consisting of three adjacent nucleotides.

For example, the section T--T--T corresponds to the amino acid lysine, the section A--C--A - cystine, C--A--A - valine, etc. There are 20 different amino acids, the number of possible combinations of 4 nucleotides 3 each equals 64. Consequently, there are more than enough triplets to encode all the amino acids.

Protein synthesis is a complex multi-stage process, representing a chain of synthetic reactions proceeding according to the principle of matrix synthesis.

Since DNA is located in the cell nucleus, and protein synthesis occurs in the cytoplasm, there is an intermediary that transfers information from DNA to ribosomes. This messenger is mRNA. :

In protein biosynthesis, the following stages are determined, occurring in different parts of the cell:

1. The first stage - the synthesis of mRNA occurs in the nucleus, during which the information contained in the DNA gene is transcribed into mRNA. This process is called transcription (from the Latin “transcript” - rewriting).

2. At the second stage, amino acids are combined with tRNA molecules, which sequentially consist of three nucleotides - anticodons, with the help of which their triplet codon is determined.

3. The third stage is the process of direct synthesis of polypeptide bonds, called translation. It occurs in ribosomes.

4. At the fourth stage, the formation of the secondary and tertiary structure of the protein occurs, that is, the formation of the final structure of the protein.

Thus, in the process of protein biosynthesis, new protein molecules are formed in accordance with the exact information contained in the DNA. This process ensures the renewal of proteins, metabolic processes, cell growth and development, that is, all the life processes of the cell.

Chromosomes (from the Greek “chroma” - color, “soma” - body) - very important structures of the cell nucleus. They play a major role in the process of cell division, ensuring the transmission of hereditary information from one generation to another. They are thin strands of DNA linked to proteins. The threads are called chromatids consisting of DNA, basic proteins (histones) and acidic proteins.

In a non-dividing cell, chromosomes fill the entire volume of the nucleus and are not visible under a microscope. Before division begins, DNA spiralization occurs and each chromosome becomes visible under a microscope.

During spiralization, chromosomes are shortened tens of thousands of times. In this state, the chromosomes look like two identical strands (chromatids) lying next to each other, connected by a common section - the centromere.

Each organism is characterized by a constant number and structure of chromosomes. In somatic cells, chromosomes are always paired, that is, in the nucleus there are two identical chromosomes that make up one pair. Such chromosomes are called homologous, and paired sets of chromosomes in somatic cells are called diploid.

Thus, the diploid set of chromosomes in humans consists of 46 chromosomes, forming 23 pairs. Each pair consists of two identical (homologous) chromosomes.

The structural features of chromosomes make it possible to distinguish them into 7 groups, which are designated by the Latin letters A, B, C, D, E, F, G. All pairs of chromosomes have serial numbers.

Men and women have 22 pairs of identical chromosomes. They are called autosomes. A man and a woman differ in one pair of chromosomes, which are called sex chromosomes. They are designated by letters - large X (group C) and small Y (group C). In the female body there are 22 pairs of autosomes and one pair (XX) of sex chromosomes. Men have 22 pairs of autosomes and one pair (XY) of sex chromosomes.

Unlike somatic cells, germ cells contain half the set of chromosomes, that is, they contain one chromosome from each pair! This set is called haploid. The haploid set of chromosomes arises during cell maturation.

6 . Rregulation of transcription and translation in the cell andbody

Operon and repressor.

It is known that the set of chromosomes, i.e. the set of DNA molecules, is the same in all cells of one organism.

Consequently, each cell of the body is capable of synthesizing any amount of each protein characteristic of a given organism. Fortunately, this never happens, since the cells of a particular tissue must have a certain set of proteins necessary to perform their function in a multicellular organism, and in no case synthesize “extraneous” proteins that are characteristic of the cells of other tissues.

For example, in root cells it is necessary to synthesize plant hormones, and in leaf cells - enzymes to ensure photosynthesis. Why aren’t all the proteins, information about which is contained in its chromosomes, synthesized in one cell at once?

Such mechanisms are better studied in prokaryotic cells. Despite the fact that prokaryotes are single-celled organisms, their transcription and translation are also regulated, since at one point in time a cell may need a certain protein, and at another moment the same protein may become harmful to it.

The genetic unit of the mechanism for regulating protein synthesis should be considered an operon, which includes one or more structural genes, i.e., genes that carry information about the structure of mRNA, which, in turn, carries information about the structure of the protein. Before these genes, at the beginning of the operon, there is a promoter - a “landing site” for the enzyme RNA polymerase. Between the promoter and the structural genes in the operon there is a section of DNA called the operator. If a special protein, a repressor, is associated with the operator, then RNA polymerase cannot begin synthesis of mRNA.

The mechanism of regulation of protein synthesis in eukaryotes.

The regulation of gene function in eukaryotes, especially if we are talking about a multicellular organism, is much more complex. Firstly, the proteins necessary to provide any function can be encoded in the genes of different chromosomes (recall that in prokaryotes, the DNA in the cell is represented by a single molecule). Secondly, in eukaryotes the genes themselves are more complex than in prokaryotes; they have “silent” regions from which mRNA is not read, but which are capable of regulating the functioning of neighboring DNA sections. Thirdly, in a multicellular organism it is necessary to precisely regulate and coordinate the work of genes in cells of different tissues.

This coordination is carried out at the level of the whole organism and mainly with the help of hormones. They are produced both in the cells of the endocrine glands and in the cells of many other tissues, such as the nervous one. These hormones bind to special receptors located either on the cell membrane or inside the cell. As a result of the interaction of the receptor with the hormone in the cell, certain genes are activated or, conversely, repressed, and protein synthesis in a given cell changes its character. For example, the adrenal hormone adrenaline activates the breakdown of glycogen into glucose in muscle cells, which leads to an improvement in the energy supply of these cells. Another hormone, insulin, secreted by the pancreas, on the contrary, promotes the formation of glycogen from glucose and its storage in liver cells.

It should also be taken into account that 99.9% of DNA in all people is the same and only the remaining 0.1% determines the unique individuality of each person: appearance, character traits, metabolism, susceptibility to certain diseases, individual reaction to medications and much more. .

One could assume that some of the “non-functioning” genes in certain cells are lost and destroyed. However, a number of experiments have proven that this is not so. From a tadpole intestinal cell, under certain conditions, it is possible to grow a whole frog, which is only possible if all the genetic information is preserved in the nucleus of this cell, although some of it was not expressed in the form of proteins while the cell was part of the intestinal wall. Consequently, in each cell of a multicellular organism, only part of the genetic information contained in its DNA is used. This means that there must be mechanisms that “turn on” or “turn off” the work of a particular gene in different cells.

The total length of the DNA molecules contained in the 46 human chromosomes is almost 2 meters. If the letters of the alphabet were encoded with a genetic triplet code, then the DNA of one human cell would be enough to encrypt 1000 thick volumes of text!

All organisms on Earth are made up of cells. There are unicellular and multicellular organisms.

Organisms without nuclear cells are called prokaryotes, and those with nuclei in their cells are called eukaryotes. On the outside, each cell is covered with a biological membrane. Inside the cell there is a cytoplasm in which the nucleus (in eukaryotes) and other organelles are located. The nucleus is filled with karyoplasm, in which chromatin and nucleoli are located. Chromatin is DNA bound to proteins that forms chromosomes during cell division.

The chromosome set of a cell is called a karyotype.

In the cytoplasm of eukaryotic cells there is a cytoskeleton - a complex system that performs support, motor and transport functions. The most important organelles of the cell: nucleus, endoplasmic reticulum, Golgi complex, ribosomes, mitochondria, lysosomes, plastids. Some cells have organelles of movement: flagella, cilia.

There are significant structural differences between prokaryotic and eukaryotic cells.

Viruses are non-cellular life forms.

For the normal functioning of a cell and an entire multicellular organism, a constant internal environment, called homeostasis, is necessary.

Homeostasis is maintained by metabolic reactions, which are divided into assimilation (anabolism) and dissimilation (catabolism). All metabolic reactions occur with the participation of biological catalysts - enzymes. Each enzyme is specific, that is, it participates in the regulation of strictly defined life processes. Therefore, many enzymes “work” in each cell.

All energy costs of any cell are provided by the universal energy substance - ATP. ATP is formed from the energy released during the oxidation of organic substances. This process is multi-stage, and the most efficient oxygen breakdown occurs in the mitochondria.

According to the method of obtaining organic substances necessary for life, all cells are divided into autotrophs and heterotrophs. Autotrophs are divided into photosynthetics and chemosynthetics, and all of them are capable of independently synthesizing the organic substances they need. Heterotrophs obtain most organic compounds from outside.

Photosynthesis is the most important process that underlies the emergence and existence of the vast majority of organisms on Earth. As a result of photosynthesis, complex organic compounds are synthesized using solar radiation energy. With the exception of chemosynthetics, all organisms on Earth depend directly or indirectly on photosynthetics.

The most important process that occurs in all cells (with the exception of cells that have lost DNA during development) is protein synthesis. Information about the sequence of amino acids that make up the primary structure of a protein is contained in the sequence of triplet combinations of DNA nucleotides. A gene is a section of DNA that encodes information about the structure of one protein. Transcription is the process of synthesis of mRNA that encodes the amino acid sequence of a protein. mRNA leaves the nucleus (in eukaryotes) into the cytoplasm, where the formation of the protein amino acid chain occurs in ribosomes. This process is called translation. Each cell contains many genes, but the cell uses only a strictly defined part of the genetic information, which is ensured by the presence in the genes of special mechanisms that turn on or off the synthesis of a particular protein in the cell.

Bibliography

1. Darevsky, I.S.; Orlov, N.L. Rare and endangered animals. Amphibians and reptiles; M.: Higher School, 1988. - 463 p.

2. Linnaeus, Karl Philosophy of Botany; M.: Nauka, 1989. - 456 p.

3. Oparin, A.I. Matter. Life. Intelligence; M.: Nauka, 1977. - 208 p.

5. Attenborough, David Living Planet; M.: Mir, 1988. - 328 p.

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What are somatic cells?

This is the name given to all cells of living organisms, except the reproductive cells. All of them have a double set of chromosomes, in contrast to the same germ cells, which have a single set. All living organisms in the world, with the exception of viruses, are formed from them. Their division is based on a process called mitosis.

What is mitosis and what is its role in nature?

During this process, two identical daughter cells are formed from one cell, with exactly the same set of chromosomes as the mother’s. This is the only way of reproduction of all single-celled eukaryotes; this process also underlies the regeneration of tissues of plants, animals and fungi. Mitosis plays a crucial role not only in asexual reproduction, but also in sexual reproduction, ensuring the division of embryonic cells. Cells of plants, fungi and animals divide in exactly the same way during the growth of the body.

What is meiosis?

This is the second way in which somatic cells divide. However, it is somewhat specific. During the process of meiosis, one cell with a double set of chromosomes produces several daughter cells with a single set. It is in this way that sex cells, that is, gametes, are produced.

Phases of mitosis

Somatic cell division occurs in several stages, each of which has its own distinctive features. The whole process lasts about three hours. There are four stages, not counting interphase: prophase, anaphase, metaphase and telophase. First things first.

Interphase

This is the period of time between cell divisions during which it prepares for mitosis. In this phase, the cell develops and exhibits its usual signs of vital activity. This period is not directly included in the process of mitosis.

Prophase

This is the longest phase of mitosis. Along its length, the cell nucleus increases, chromosomes form in a spiral. During this period, all chromosomes are two chromatids, which are connected by centromeres - a kind of constrictions. These structures look like the letter X. Then the nuclear envelope and nucleolus are destroyed, and the chromosomes move into the cytoplasm. The centrioles of the cell are located at its poles and form filaments of the spindle between themselves, which are then, at the end of the phase, attached to the centromeres.

Metaphase

This is the next step in the process by which somatic cells divide. During this phase, chromosomes align along the equator of the cell. In this way, the metaphase plate is formed. At this time, the chromosomes are very small in size, as they are tightly twisted into spirals. However, they are clearly visible through a microscope due to their clear location. Therefore, the study of cell chromosomes is usually carried out at this stage of mitosis.

Anaphase

This is the shortest stage of cell division through mitosis. During this period, the spindle threads formed by the centrioles begin to pull the centromeres of the chromosome in opposite directions, resulting in its division into two separate chromatids. Now at each pole of the cell there are identical sets of chromatids.

Telophase

This is the last stage of mitosis. During its course, processes are observed that are opposite to those that occurred in the three previous phases. Namely: the chromosome spirals unwind, nuclear membranes and nucleoli are formed again. Also at this stage, the division itself occurs directly: the cytoplasm is divided, and each daughter cell receives its own set of organelles. In plants, a cellulose wall is also formed around the membrane of two newly formed structures.

Meiosis

Another process by which somatic cells divide. It involves the formation of gametes, that is, sex cells with a single set of chromosomes. Somatic cells divide twice in succession during this process. Thus, meiosis I and meiosis II are distinguished. Each of them consists of phases with the same names as mitosis. Let us take a closer look at the processes that occur in the cell during the various stages of meiosis.

Meiosis I

During this process, the cell divides in such a way that two daughter cells are formed with a halved set of chromosomes:

Prophase. At this stage, an interesting process occurs - crossing over. It lies in the fact that chromatids intertwine and exchange individual sections of DNA. As a result, a recombination of the genetic information of the cell occurs, which ensures the diversity of organisms of the same species. Then the chromatids are separated, and the same thing happens as in prophase of mitosis: the nuclear membrane and nucleolus disappear and the spindle is formed.
Metaphase. At this time, chromosomes line up along the equator of the cell, with homologous chromosomes arranged in pairs.
Anaphase. At this stage, chromosomes move to different poles of the cell. That is, each pair of homologous structures is divided, one of the chromosomes is located on one side, the other on the other.
Telophase. Here, nuclear membranes and nucleoli are re-formed, the cytoplasm and organelles are separated, and two daughter cells with a single set of chromosomes are formed.

Meiosis II

Immediately after the first meiosis, the second begins. Prophase very short. Following her comes anaphase, during which the chromosomes occupy a position along the equator, the spindle threads are attached to them. In anaphase, the individual halves of the chromosomes move towards the poles. IN telophase four cells with a single set of genetic information are formed. Together, meiosis I and meiosis II are called gametogenesis.

Cell diversity

Somatic cells of vertebrates and other organisms are divided into groups, depending on their purpose, the role and functions of the tissues that consist of them. In this regard, they have slightly different structures.

Types of tissues and characteristics of their cells

Among animal tissues, the following types are distinguished: integumentary, connective, nervous, muscle, blood, lymph. All of them consist of somatic cells, but slightly different in structure:

Video tutorial 1: Cell division. Mitosis

Video tutorial 2: Meiosis. Phases of meiosis

Lecture: A cell is the genetic unit of a living thing. Chromosomes, their structure (shape and size) and functions

Cell - genetic unit of living things

The basic unit of life is the individual cell. It is at the cellular level that processes occur that distinguish living matter from nonliving matter. In each cell, hereditary information about the chemical structure of proteins that must be synthesized in it is stored and intensively used, and therefore it is called the genetic unit of the living. Even anucleated red blood cells in the initial stages of their existence have mitochondria and a nucleus. Only in a mature state do they not have structures for protein synthesis.

To date, science does not know any cells that do not contain DNA or RNA as a carrier of genomic information. In the absence of genetic material, the cell is not capable of protein synthesis, and therefore life.

DNA is not only found in nuclei; its molecules are contained in chloroplasts and mitochondria; these organelles can multiply inside the cell.

DNA in a cell is found in the form of chromosomes - complex protein-nucleic acid complexes. Eukaryotic chromosomes are localized in the nucleus. Each of them is a complex structure of:

The only long DNA molecule, 2 meters of which is packed into a compact structure measuring (in humans) up to 8 microns;

Special histone proteins, whose role is to pack chromatin (the substance of the chromosome) into the familiar rod-shaped shape;

Chromosomes, their structure (shape and size) and functions

This dense packing of genetic material is produced by the cell before dividing. It is at this moment that the densely packed formed chromosomes can be examined under a microscope. When DNA is folded into compact chromosomes called heterochromatin, messenger RNA cannot be synthesized. During the period of cell mass gain and interphase development, the chromosomes are in a less packed state, which is called interchromatin, in which mRNA is synthesized and DNA replication occurs.

The main elements of chromosome structure are:

Centromere. This is a part of a chromosome with a special nucleotide sequence. It connects two chromatids and participates in conjugation. It is to this that the protein filaments of the cell division spindle tubes are attached.

Telomeres. These are the terminal sections of chromosomes that are not capable of connecting with other chromosomes; they play a protective role. They consist of repeating sections of specialized DNA that form complexes with proteins.

DNA replication initiation points.

Prokaryotic chromosomes are very different from eukaryotic ones, being DNA-containing structures located in the cytoplasm. Geometrically, they are a ring molecule.

The chromosome set of a cell has its own name - karyotype. Each type of living organism has its own characteristic composition, number and shape of chromosomes.

Somatic cells contain a diploid (double) chromosome set, half of which is received from each parent.

Chromosomes responsible for encoding the same functional proteins are called homologous. The ploidy of cells can be different - as a rule, gametes in animals are haploid. In plants, polyploidy is currently a fairly common phenomenon, used in the creation of new varieties as a result of hybridization. Violation of the amount of ploidy in warm-blooded animals and humans causes serious congenital diseases such as Down syndrome (the presence of three copies of chromosome 21). Most often, chromosomal abnormalities lead to the inability of the organism.

In humans, the complete chromosome set consists of 23 pairs. The largest known number of chromosomes, 1600, was found in the simplest planktonic organisms, radiolarians. Australian black bulldog ants have the smallest chromosome set - only 1.

Life cycle of a cell. Phases of mitosis and meiosis

Interphase, in other words, the period of time between two divisions is defined by science as the life cycle of a cell.

During interphase, vital chemical processes occur in the cell, it grows, develops, and accumulates reserve substances. Preparation for reproduction involves doubling the contents - organelles, vacuoles with nutritional contents, and the volume of the cytoplasm. It is thanks to division, as a way to quickly increase the number of cells, that long life, reproduction, an increase in the size of the body, its survival from wounds and tissue regeneration are possible. The following stages are distinguished in the cell cycle:

Interphase. Time between divisions. First, the cell grows, then the number of organelles, the volume of reserve substance increases, and proteins are synthesized. In the last part of interphase, the chromosomes are ready for subsequent division - they consist of a pair of sister chromatids.

Mitosis. This is the name of one of the methods of nuclear division, characteristic of bodily (somatic) cells, during which 2 cells are obtained from one, with an identical set of genetic material.

Gametogenesis is characterized by meiosis. Prokaryotic cells have retained the ancient method of reproduction - direct division.

Mitosis consists of 5 main phases:

Prophase. Its beginning is considered to be the moment when the chromosomes become so densely packed that they are visible under a microscope. Also, at this time, the nucleoli are destroyed and a spindle is formed. Microtubules are activated, the duration of their existence decreases to 15 seconds, but the rate of formation also increases significantly. The centrioles diverge to opposite sides of the cell, forming a huge number of constantly synthesized and disintegrated protein microtubules, which extend from them to the centromeres of the chromosomes. This is how the fission spindle is formed. Membrane structures such as the ER and Golgi apparatus break up into separate vesicles and tubes, randomly located in the cytoplasm. Ribosomes are separated from the ER membranes.

Metaphase. A metaphase plate is formed, consisting of chromosomes balanced in the middle of the cell by the efforts of opposite centriole microtubules, each pulling them in their own direction. At the same time, the synthesis and disintegration of microtubules continues, a kind of “bulkhead” of them. This phase is the longest.

Anaphase. The forces of microtubules tear off chromosome connections in the centromere region and forcefully stretch them towards the poles of the cell. In this case, chromosomes sometimes take a V-shape due to the resistance of the cytoplasm. A ring of protein fibers appears in the area of the metaphase plate.
Telophase. Its beginning is considered to be the moment when the chromosomes reach the division poles. The process of restoration of the internal membrane structures of the cell begins - the ER, Golgi apparatus, and nucleus. The chromosomes are unpacked. Nucleoli assemble and ribosome synthesis begins. The fission spindle disintegrates.
Cytokinesis. The last phase in which the protein ring that appears in the central region of the cell begins to shrink, pushing the cytoplasm towards the poles. The cell divides into two and a protein ring of the cell membrane is formed in place.

Regulators of the mitosis process are specific protein complexes. The result of mitotic division is a pair of cells with identical genetic information. In heterotrophic cells, mitosis occurs faster than in plant cells. In heterotrophs, this process can take from 30 minutes, in plants – 2-3 hours.

To generate cells with half the normal number of chromosomes, the body uses another division mechanism - meiosis.

It is associated with the need to produce germ cells; in multicellular organisms, it avoids the constant doubling of the number of chromosomes in the next generation and makes it possible to obtain new combinations of allelic genes. It differs in the number of phases, being longer. The resulting decrease in the number of chromosomes leads to the formation of 4 haploid cells. Meiosis consists of two divisions following each other without interruption.

The following phases of meiosis are defined:

Prophase I. Homologous chromosomes move closer to each other and unite longitudinally. This combination is called conjugation. Then crossing over occurs - double chromosomes cross their arms and exchange sections.

Metaphase I. Chromosomes separate and occupy positions at the equator of the cell spindle, taking on a V-shape due to the tension of the microtubules.

Anaphase I. Homologous chromosomes are stretched by microtubules towards the cell poles. But unlike mitotic division, they separate as whole chromatids rather than as separate ones.

The result of the first meiotic division is the formation of two cells with half the number of intact chromosomes. Between divisions of meiosis, interphase is practically absent, chromosome doubling does not occur, they are already bichromatid.

Immediately following the first, the repeated meiotic division is completely analogous to mitosis - in it, the chromosomes are divided into separate chromatids, distributed equally between new cells.

oogonia go through the stage of mitotic reproduction at the embryonic stage of development, so that the female body is already born with a constant number of them;

spermatogonia are capable of reproduction at any time during the reproductive period of the male body. A much larger number of them are generated than female gametes.

Gametogenesis of animal organisms occurs in the gonads - gonads.

The process of transformation of spermatogonia into spermatozoa occurs in several stages:

Mitotic division transforms spermatogonia into first-order spermatocytes.

As a result of a single meiosis, they turn into second-order spermatocytes.

The second meiotic division produces 4 haploid spermatids.

The period of formation begins. In the cell, the nucleus becomes compacted, the amount of cytoplasm decreases, and a flagellum forms. Also, proteins are stored and the number of mitochondria increases.

The formation of eggs in an adult female body occurs as follows:

From the 1st order oocyte, of which there is a certain number in the body, as a result of meiosis with a halving of the number of chromosomes, 2nd order oocytes are formed.

As a result of the second meiotic division, a mature egg and three small reduction bodies are formed.

This unbalanced distribution of nutrients between the 4 cells is intended to provide a large resource of nutrients for the new living organism.

Ovules in ferns and mosses are formed in archegonia. In more highly organized plants - in special ovules located in the ovary.

Cell membrane. The cell (Fig. 1.1) as a living system needs to maintain certain internal conditions: the concentration of various substances, temperature inside the cell, etc. Some of these parameters are maintained at a constant level, since their change will lead to the death of the cell, others are of less importance for the preservation her life activity.

Rice. 1.1.

Cell membrane must ensure separation of the cell contents from the environment to maintain the required concentration of substances inside the cell, at the same time it must be permeable for constant exchange of substances between the cell and the environment (Fig. 1.2). Membranes also limit the internal structures of the cell - organoids (organelles) - from the cytoplasm. However, these are not just dividing barriers. Cell membranes themselves are the most important organ of the cell, providing not only its structure, but also many functions. In addition to separating cells from each other and separating them from the external environment, membranes unite cells into tissues, regulate the exchange between the cell and the external environment, are themselves the site of many biochemical reactions, and serve as transmitters of information between cells.

Rice. 1.2.

According to modern data, plasma membranes are lipoprotein structures (lipoproteins are compounds of protein and fat molecules). Lipids (fats) spontaneously form a double layer, and membrane proteins “float” in it, like islands in the ocean. Membranes contain several thousand different proteins: structural, transporters, enzymes, etc. In addition, there are pores between protein molecules through which some substances can pass. Special glycosyl groups are connected to the surface of the membrane, which are involved in the process of cell recognition during tissue formation.

Different types of membranes differ in their thickness (usually it ranges from 5 to 10 nm). The consistency of the membrane resembles olive oil. The most important property of the cell membrane is semi-permeability, those. the ability to pass only certain substances. The passage of various substances through the plasma membrane is necessary for the delivery of nutrients and oxygen into the cell, the removal of toxic waste, and the creation of differences in the concentration of individual microelements to maintain nervous and muscle activity. Mechanisms of transport of substances across the membrane.

diffusion - gases, fat-soluble molecules penetrate directly through the plasma membrane, including facilitated diffusion, when a water-soluble substance passes through the membrane through a special channel;
osmosis - diffusion of water through semi-permeable membranes towards a lower ion concentration;
active transport - the transfer of molecules from an area of lower concentration to an area of higher concentration using special transport proteins;
endocytosis - transfer of molecules using vesicles (vacuoles) formed by retraction of the membrane; distinguish between phagocytosis (absorption of solid particles) and pinocytosis (absorption of liquids) (Fig. 1.3);

Rice. 1.3.

Exocytosis is the reverse process of endocytosis; through it, solid particles and liquid secretions can be removed from the cells (Fig. 1.4).

Rice. 1.4.

Diffusion and osmosis do not require additional energy; active transport, endocytosis and exocytosis need to be provided with energy, which the cell receives when the nutrients it has absorbed are thawed.

Regulation of the passage of various substances through the plasma membrane is one of its most important functions. Depending on external conditions, the structure of the membrane may change: it may become more liquid, active and permeable. The regulator of membrane permeability is the fat-like substance cholesterol.

The external structure of the cell is supported by a denser structure - cell membrane. The cell membrane can have a very different structure (be elastic, have a rigid frame, bristles, antennae, etc.) and perform quite complex functions.

Core found in all cells of the human body, with the exception of red blood cells. As a rule, a cell contains only one nucleus, but there are exceptions - for example, striated muscle cells contain many nuclei. The core has a spherical shape, its dimensions range from 10 to 20 μm (Fig. 1.5).

The nucleus is separated from the cytoplasm nuclear membrane, consisting of two membranes - outer and inner, similar to the cell membrane, and a narrow gap between them containing a semi-liquid medium; through the pores of the nuclear membrane, intensive exchange of substances occurs between the nucleus and the cytoplasm. On the outer membrane of the shell there are many ribosomes - organelles that synthesize protein.

Rice. 1.5.

Under the nuclear envelope is karyoplasm (nuclear juice), which receives substances from the cytoplasm. Karyoplasm contains chromosomes (oblong structures containing DNA, in which information about the structure of proteins specific to a given cell is “recorded” - hereditary, or genetic information) and nucleoli (rounded structures inside the nucleus in which ribosomes are formed).

The set of chromosomes contained in the nucleus is called chromosome set. The number of chromosomes in somatic cells is even - diploid (in humans there are 44 autosomes and 2 sex chromosomes that determine sex), the sex cells involved in fertilization carry half the set (in humans there are 22 autosomes and 1 sex chromosome) (Fig. 1.6).

The most important function of the nucleus is the transfer of genetic information to daughter cells: when a cell divides, the nucleus is divided in two, and the DNA in it is copied (DNA replication) - this allows each daughter cell to have complete information received from the original (mother) cell (see. Cell reproduction).

Cytoplasm(cytosol) is a gelatinous substance containing about 90% water, in which all organelles are located, true and colloidal solutions of nutrients and insoluble wastes of metabolic processes are contained, biochemical processes take place: glycolysis, synthesis of fatty acids, nucleic acids and other substances. Organelles in the cytoplasm move, the cytoplasm itself also undergoes periodic active movement - the oz cycle.

Cellular structures(organoids , or organelles) are the “internal organs” of the cell (Table 1.1). They ensure the vital processes of the cell, the production of certain substances by the cell (secrets, hormones, enzymes); the general activity of the body’s tissues and the ability to perform functions specific to a given tissue depend on their vital activity. Cell structures, like the cell itself, go through their life cycles: they are born (created through reproduction), actively function, age and are destroyed. Most cells of the body are able to recover at the subcellular level due to the reproduction and renewal of the organelles included in its structure.

Rice. 1.6.

Table 1.1

Cellular organelles, their structure and functions

Organoids	Structure
Cytoplasm	Enclosed in an outer membrane, it includes various organelles. It is represented by a colloidal solution of salts and organic substances, permeated with a cytoskeleton (a system of protein threads)	Unites all cellular structures into a single system, provides an environment for biochemical reactions, exchange of substances and energy in the cell
Outer cell membrane	Two layers of monomolecular protein, between which there is a bimolecular layer of lipids; in the lipid layer there are holes - pores	Limits the cell, separates it from the environment, has selective permeability, actively regulates the metabolism and energy with the external environment, is responsible for the connection of cells in tissue, provides pinocytosis and phagocytosis; regulates the water balance of the cell and removes “waste” from it - waste products
Endoplasmic reticulum (ER)	A system of tubes, tubules, cisterns, vesicles formed by ultramicroscopic membranes, combined into a single whole with the outer membrane of the nuclear envelope and the outer cell membrane. Granular ES carries ribosomes, smooth ES does not have ribosomes	Transport of substances within a cell and between neighboring cells; division of the cell into sectors in which various processes can take place. Granular ES is involved in protein synthesis. Protein and fat synthesis and ATP transport occur in ES channels.
Ribosomes	Small spherical organelles composed of RNA and protein	Carry out protein synthesis
Golgi apparatus	Microscopic single-membrane organelles consisting of a stack of flat cisterns, along the edges of which tubes branch off, separating small bubbles	The products of the cell's metabolic processes accumulate in the bubbles. Packed in vesicles, they enter the cytoplasm and are either used or excreted as waste.
Lysosomes	Single-membrane organelles, the number of which depends on the vital activity of the cell. Lysosomes contain enzymes formed in ribosomes	Digestion of nutrients. Protective function. Autolysis (self-dissolution of organelles and the cell itself under conditions of food or oxygen starvation)

Cell Reproduction

All cells are formed through division. The cell life cycle includes two stages: interphase and mitosis. During interphase the cell mass increases (the cell “grows”). Some cells (for example, cells of nervous tissue) remain in this stage without moving to the next; in others (cells of most tissues capable of growth and regeneration), with an increase in mass during interphase, chromosomal DNA doubles, and the cell enters the stage of mitosis ( Fig. 1.7).

Mitosis divided into prophase (the nuclear membrane is destroyed, chromosomes are separated and connected to special microtubules, which will direct their movement to the poles of the dividing cell - centrioles); metaphase (chromosomes line up along the equator of the dividing cell and finally uncouple); anaphase (chromosomes move to the poles of the cell); telophase (the cell divides in two in the equatorial plane, the spindle filaments are destroyed, and nuclear membranes form around the chromosomes). Mitosis is called asexual division, or cloning: each daughter cell receives an identical set of chromosomes and can again continue growth and development - enter the interphase stage. This process usually takes about an hour.

Another type of reproduction - sexual - is called meiosis. This type of cell division allows, as a result of two successive divisions, which in their mechanism are close to the processes of mitosis, to form gametes – sex cells with half the set of chromosomes (one chromosome from each pair). When two parent gametes merge into zygote (fertilization) hereditary information received from two parents is combined and forms the basis for the development of the future organism. The random nature of the processes of chromosome divergence during cell division and the joining of chromosomes of male and female gametes leads to the emergence of new combinations of genes and provides variability in various characteristics of a biological species. Subsequently, the zygote divides by mitosis and develops into an independent organism, bearing the characteristics of both parents in manifested or unmanifested form.