Atomic nucleus. School encyclopedia Component of the atomic nucleus
Chromatin
1) heterochromatin;
2) euchromatin.
Heterochromatin
Structural
Optional
Euchromatin
a) histone proteins;
b) nonhistone proteins.
Yo Histone proteins (histones
Yo Non-histone proteins
nucleolus
ЁSize - 1-5 microns.
The form is spherical.
Granular component
Fibrillar
nuclear envelope
1. External nuclear membrane (m. nuclearis externa),
inner nuclear membrane
Features:
Karyoplasm
cell reproduction
nuclear apparatus
The nucleus is present in all eukaryotic cells, with the exception of mature erythrocytes and plant sieve tubes. Cells usually have a single nucleus, but sometimes multinucleated cells are found.
The nucleus is spherical or oval.
Some cells have segmented nuclei. The size of the nuclei is from 3 to 10 microns in diameter. The nucleus is essential for the life of the cell. It regulates cell activity. The nucleus stores hereditary information contained in DNA. This information, thanks to the nucleus, is transmitted to daughter cells during cell division. The nucleus determines the specificity of proteins synthesized in the cell. The nucleus contains many proteins necessary for its functions. RNA is synthesized in the nucleus.
cell nucleus consists of membrane, nuclear sap, one or more nucleoli and chromatin.
Functional role nuclear envelope is the isolation of genetic material (chromosome) eukaryotic cell from the cytoplasm with its inherent numerous metabolic reactions, as well as the regulation of bilateral interactions of the nucleus and cytoplasm. The nuclear envelope consists of two membranes - outer and inner, between which is located perinuclear (perinuclear) space. The latter can communicate with the tubules of the cytoplasmic reticulum. outer membrane The nuclear membrane directly contacts with the cytoplasm of the cell, has a number of structural features that allow it to be attributed to the proper EPR membrane system. It contains a large number of ribosomes, as well as on the membranes of ergastoplasm. The inner membrane of the nuclear envelope does not have ribosomes on its surface, but is structurally associated with nuclear lamina- fibrous peripheral layer of the nuclear protein matrix.
The nuclear envelope contains nuclear pores with a diameter of 80-90 nm, which are formed due to numerous zones of fusion of two nuclear membranes and are, as it were, rounded, through perforations of the entire nuclear membrane. Pores play an important role in the transport of substances into and out of the cytoplasm. Nuclear pore complex (NPC) with a diameter of about 120 nm has a certain structure (consists of more than 1000 proteins - nucleoporins, whose mass is 30 times greater than the ribosome), which indicates a complex mechanism for the regulation of nuclear-cytoplasmic movements of substances and structures. In the process of nuclear-cytoplasmic transport, nuclear pores function as a kind of molecular sieve, passively passing particles of a certain size along a concentration gradient (ions, carbohydrates, nucleotides, ATP, hormones, proteins up to 60 kDa). Pores are not permanent formations. The number of pores increases during the period of greatest nuclear activity. The number of pores depends on the functional state of the cell. The higher the synthetic activity in the cell, the greater their number. It has been calculated that in lower vertebrates in erythroblasts, where hemoglobin is intensively formed and accumulated, there are about 30 pores per 1 μm2 of the nuclear envelope. In mature erythrocytes of these animals that retain nuclei, up to five pores remain per 1 μg of the membrane, i.e. 6 times less.
In the region of the feather complex, the so-called dense plate - a protein layer that underlies the entire length of the inner membrane of the nuclear envelope. This structure primarily performs a supporting function, since in its presence the shape of the nucleus is preserved even if both membranes of the nuclear envelope are destroyed. It is also assumed that the regular connection with the substance of the dense plate contributes to the ordered arrangement of chromosomes in the interphase nucleus.
Nuclear sap (karyoplasm or matrix)- the internal contents of the nucleus, is a solution of proteins, nucleotides, ions, more viscous than hyaloplasm. It also contains fibrillar proteins. The karyoplasm contains nucleoli and chromatin. Nuclear juice forms the internal environment of the nucleus, and therefore it plays an important role in ensuring the normal functioning of the genetic material. The composition of nuclear juice contains filamentous, or fibrillar, proteins, with which the performance of the support function is associated: the matrix also contains the primary products of transcription of genetic information - heteronuclear RNA (hnRNA), which are processed here, turning into mRNA.
nucleolus- an obligatory component of the nucleus, are found in interphase nuclei and are small bodies, spherical in shape. The nucleoli are denser than the nucleus. In the nucleoli, the synthesis of rRNA, other types of RNA and the formation of subunits takes place. ribosome. The emergence of nucleoli is associated with certain zones of chromosomes called nucleolar organizers. The number of nucleoli is determined by the number of nucleolar organizers. They contain rRNA genes. rRNA genes occupy certain areas (depending on the type of animal) of one or more chromosomes (in humans, 13-15 and 21-22 pairs) - nucleolar organizers, in which the nucleoli are formed. Such regions in metaphase chromosomes look like constrictions and are called secondary constrictions. Using an electron microscope, filamentous and granular components are revealed in the nucleolus. The filamentous (fibrillar) component is represented by complexes of protein and giant RNA precursor molecules, from which smaller molecules of mature rRNA are then formed. In the process of maturation, fibrils are transformed into ribonucleoprotein grains (granules), which represent the granular component.
Chromatin structures in the form of lumps, scattered in the nucleoplasm are an interphase form of existence chromosomes cells.
Ribosome - it is a rounded ribonucleoprotein particle with a diameter of 20-30 nm. Ribosomes are non-membrane cell organelles. Ribosomes combine amino acid residues into polypeptide chains (protein synthesis). Ribosomes are very small and numerous.
It consists of small and large subunits, the combination of which occurs in the presence of messenger (messenger) RNA (mRNA). The small subunit includes protein molecules and one molecule of ribosomal RNA (rRNA), while the second one contains proteins and three rRNA molecules. Protein and rRNA by mass in equal amounts participate in the formation of ribosomes. rRNA is synthesized in the nucleolus.
One mRNA molecule usually combines several ribosomes like a string of beads. Such a structure is called polysome. Polysomes are freely located in the ground substance of the cytoplasm or attached to the membranes of the rough cytoplasmic reticulum. In both cases, they serve as a site for active protein synthesis. Comparison of the ratio of the number of free and membrane-attached polysomes in embryonic undifferentiated and tumor cells, on the one hand, and in specialized cells of an adult organism, on the other hand, led to the conclusion that proteins are formed on hyaloplasmic polysomes for their own needs (for "home" use) of this cell, while on the polysomes of the granular network proteins are synthesized that are removed from the cell and used for the needs of the body (for example, digestive enzymes, breast milk proteins). Ribosomes can be freely located in the cytoplasm or be associated with the endoplasmic reticulum, being part of the rough ER. Proteins formed on ribosomes connected to the ER membrane usually enter the ER tanks. Proteins synthesized on free ribosomes remain in the hyaloplasm. For example, hemoglobin is synthesized on free ribosomes in erythrocytes. Ribosomes are also present in mitochondria, plastids, and prokaryotic cells.
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The structure of the nucleus and its chemical composition
The nucleus consists of chromatin, nucleolus, karyoplasm (nucleoplasm), and nuclear envelope.
In a cell that divides, in most cases there is one nucleus, but there are cells that have two nuclei (20% of liver cells are binuclear), as well as multinuclear (bone tissue osteoclasts).
ЁSizes - range from 3-4 to 40 microns.
Each type of cell is characterized by a constant ratio of the volume of the nucleus to the volume of the cytoplasm. This ratio is called the Hertwing index. Depending on the value of this index, cells are divided into two groups:
1. nuclear - the Hertwing index is of greater importance;
2. cytoplasmic - the Hertwing index has insignificant values.
Yoform - can be spherical, rod-shaped, bean-shaped, annular, segmented.
Yolocalization - the nucleus is always localized in a certain place in the cell. For example, in the cylindrical cells of the stomach, it is in a basal position.
The nucleus in a cell can be in two states:
a) mitotic (during division);
b) interphase (between divisions).
In a living cell, the interphase nucleus looks like an optically empty one; only the nucleolus is found. The structures of the nucleus in the form of threads, grains can be observed only when damaging factors act on the cell, when it goes into a state of paranecrosis (a borderline state between life and death). From this state, the cell can return to normal life or die. After cell death, morphologically, the following changes are distinguished in the nucleus:
1) karyopyknosis - compaction of the nucleus;
2) karyorrhexis - decomposition of the nucleus;
3) karyolysis - dissolution of the nucleus.
Functions: 1) storage and transmission of genetic information,
2) protein biosynthesis, 3) formation of ribosome subunits.
Chromatin
Chromatin (from the Greek chroma - color paint) is the main structure of the interphase nucleus, which stains very well with basic dyes and determines the chromatin pattern of the nucleus for each cell type.
Due to the ability to stain well with various dyes, and especially with the main ones, this component of the nucleus was called "chromatin" (Flemming 1880).
Chromatin is a structural analogue of chromosomes and in the interphase nucleus is the carrier DNA of the body.
Morphologically, two types of chromatin are distinguished:
1) heterochromatin;
2) euchromatin.
Heterochromatin(heterochromatinum) corresponds to parts of chromosomes partially condensed in the interphase and is functionally inactive. This chromatin stains very well and it is this chromatin that can be seen on histological preparations.
Heterochromatin, in turn, is divided into:
1) structural; 2) optional.
Structural heterochromatin is the segments of chromosomes that are constantly in a condensed state.
Optional heterochromatin is heterochromatin capable of decondensing and turning into euchromatin.
Euchromatin- these are regions of chromosomes decondensed in interphase. This is a working, functionally active chromatin. This chromatin is not stained and is not detected on histological preparations.
During mitosis, all euchromatin is maximally condensed and becomes part of the chromosomes. During this period, the chromosomes do not perform any synthetic functions. In this regard, cell chromosomes can be in two structural and functional states:
1) active (working), sometimes they are partially or completely decondensed and with their participation in the nucleus, the processes of transcription and reduplication occur;
2) inactive (non-working, metabolic dormancy), when they are maximally condensed, they perform the function of distribution and transfer of genetic material to daughter cells.
Sometimes, in some cases, the whole chromosome during the interphase can remain in a condensed state, while it looks like smooth heterochromatin. For example, one of the X-chromosomes of the somatic cells of the female body is subject to heterochromatization at the initial stages of embryogenesis (during cleavage) and does not function. This chromatin is called sex chromatin or Barr bodies.
In different cells, sex chromatin has a different appearance:
a) in neutrophilic leukocytes - a type of drumstick;
b) in the epithelial cells of the mucosa - the appearance of a hemispherical lump.
Sex chromatin determination is used to establish genetic sex, as well as to determine the number of X chromosomes in an individual's karyotype (it is equal to the number of sex chromatin bodies + 1).
Electron microscopic studies have shown that preparations of isolated interphase chromatin contain elementary chromosomal fibrils 20–25 nm thick, which consist of fibrils 10 nm thick.
Chemically, chromatin fibrils are complex complexes of deoxyribonucleoproteins, which include:
b) special chromosomal proteins;
The quantitative ratio of DNA, protein and RNA is 1:1.3:0.2. The share of DNA in the chromatin preparation is 30-40%. The length of individual linear DNA molecules varies within indirect limits and can reach hundreds of micrometers and even centimeters. The total length of DNA molecules in all chromosomes of one human cell is about 170 cm, which corresponds to 6x10-12g.
Chromatin proteins make up 60-70% of its dry mass and are represented by two groups:
a) histone proteins;
b) nonhistone proteins.
Yo Histone proteins (histones) - alkaline proteins containing basic amino acids (mainly lysine, arginine) are unevenly arranged in blocks along the length of the DNA molecule. One block contains 8 histone molecules that form the nucleosome. The size of the nucleosome is about 10 nm. The nucleosome is formed by compaction and supercoiling of DNA, which leads to a shortening of the length of the chromosome fibril by about 5 times.
Yo Non-histone proteins make up 20% of the number of histones and in the interphase nuclei form a structural network inside the nucleus, which is called the nuclear protein matrix. This matrix represents the framework that determines the morphology and metabolism of the nucleus.
The perichromatin fibrils are 3-5 nm thick, the granules are 45 nm in diameter, and the interchromatin granules are 21-25 nm in diameter.
nucleolus
The nucleolus (nucleolus) is the densest structure of the nucleus, which is clearly visible in a living unstained cell and is a derivative of the chromosome, one of its loci with the highest concentration and active synthesis of RNA in the interphase, but is not an independent structure or organelle.
ЁSize - 1-5 microns.
The form is spherical.
The nucleolus has a heterogeneous structure. In a light microscope, its fine-fibrous organization is visible.
Electron microscopy reveals two main components:
a) granular; b) fibrillar.
Granular component represented by granules with a diameter of 15-20 nm, these are maturing subunits of ribosomes. Sometimes the granular component forms filamentous structures - nucleolonemes, about 0.2 µm thick. The granular component is localized along the periphery.
Fibrillar the component is ribonucleoprotein strands of ribosome precursors, which are concentrated in the central part of the nucleolus.
The ultrastructure of the nucleoli depends on the activity of RNA synthesis: at a high level of synthesis, a large number of granules are detected in the nucleolus, when synthesis is stopped, the number of granules decreases and the nucleoli turn into dense fibrillar strands of a basophilic nature.
nuclear envelope
The nuclear envelope (nuclolemma) consists of:
Physics of the atomic nucleus. Core composition.
The outer nuclear membrane (m. nuclearis externa),
2. The inner membrane (m. nuclearis interna), which are separated by the perinuclear space or the cistern nuclear envelope (cisterna nucleolemmae), 20-60 nm wide.
Each membrane has a thickness of 7-8nm. In general, the nuclear membrane resembles a hollow two-layer bag that separates the contents of the nucleus from the cytoplasm.
Outer membrane of the nuclear envelope, which is in direct contact with the cytoplasm of the cell, has a number of structural features that allow it to be attributed to the proper membrane system of the endoplasmic reticulum. These features include: the presence of numerous polyribosomes on it from the side of the hyaloplasm, and the outer nuclear membrane itself can directly pass into the membranes of the granular endoplasmic reticulum. The surface of the outer nuclear membrane in most animal and plant cells is not smooth and forms outgrowths of various sizes towards the cytoplasm in the form of vesicles or long tubular formations.
inner nuclear membrane associated with the chromosomal material of the nucleus. From the side of the karyoplasm, the so-called fibrillar layer, consisting of fibrils, is adjacent to the inner nuclear membrane, but it is not characteristic of all cells.
The nuclear envelope is not continuous. The most characteristic structures of the nuclear envelope are nuclear pores. Nuclear pores are formed by the fusion of two nuclear membranes. In this case, rounded through holes (perforations, annulus pori) are formed, which have a diameter of about 80-90 nm. These holes in the nuclear membrane are filled with complex globular and fibrillar structures. The combination of membrane perforations and these structures is called the pore complex (complexus pori). The pore complex consists of three rows of granules, eight in each row, the diameter of the granules is 25 nm; fibrillar processes extend from these granules. Granules are located on the border of the opening in the nuclear envelope: one row lies on the side of the nucleus, the second - on the side of the cytoplasm, the third in the central part of the pore. Fibrils extending from peripheral granules can converge in the center and create, as it were, a partition, a diaphragm across the pore (diaphragma pori). The pore sizes of this cell are usually stable. The number of nuclear pores depends on the metabolic activity of the cells: the more intense the synthetic processes in the cell, the more pores per unit surface of the cell nucleus.
Features:
1. Barrier - separates the contents of the nucleus from the cytoplasm, limits the free transport of macromolecules between the nucleus and the cytoplasm.
2. Creation of intranuclear order - fixation of chromosomal material in the three-dimensional lumen of the nucleus.
Karyoplasm
Karyoplasm is the liquid part of the nucleus, in which nuclear structures are located, it is an analogue of hyaloplasm in the cytoplasmic part of the cell.
cell reproduction
One of the most important biological phenomena, which reflects general patterns and is an essential condition for the existence of biological systems for a sufficiently long period of time, is the reproduction (reproduction) of their cellular composition. Reproduction of cells, according to cell theory, is carried out by dividing the original. This position is one of the main ones in the cell theory.
The nucleus (nucleus) of the cell
CORE FUNCTIONS
Chromatin -
Chromosomes
which include:
- histone proteins
– small amounts of RNA;
nuclear matrix
Consists of 3 components:
laying the nuclear envelope.
What is a nucleus - is it in biology: properties and functions
Intranuclear network (skeleton).
3. "Residual" nucleolus.
It consists of:
- outer nuclear membrane;
Nucleoplasm (karyoplasm)- the liquid component of the nucleus, in which chromatin and nucleoli are located. Contains water and a number
nucleolus
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The nucleus (nucleus) of the cell- system of genetic determination and regulation of protein synthesis.
CORE FUNCTIONS
● storage and maintenance of hereditary information
● implementation of hereditary information
The nucleus consists of chromatin, nucleolus, karyoplasm (nucleoplasm) and a nuclear envelope that separates it from the cytoplasm.
Chromatin - these are zones of dense matter in the nucleus, which
Rosho perceives different dyes, especially basic ones.
In non-dividing cells, chromatin is found in the form of clumps and granules, which is an interphase form of the existence of chromosomes.
Chromosomes- chromatin fibrils, which are complex complexes of deoxyribonucleoproteins (DNP), in the composition
which include:
- histone proteins
- non-histone proteins - make up 20%, these are enzymes, perform structural and regulatory functions;
– small amounts of RNA;
- small amounts of lipids, polysaccharides, metal ions.
nuclear matrix– is a framework intranuclear system
mine, the unifying backbone for chromatin, nucleolus, nuclear envelope. This structural network is the basis that determines the morphology and metabolism of the nucleus.
Consists of 3 components:
1. Lamina (A, B, C) - peripheral fibrillar layer, sub-
laying the nuclear envelope.
2. Intranuclear network (skeleton).
3. "Residual" nucleolus.
Nuclear envelope (karyolemma) is a membrane that separates the contents of the nucleus from the cytoplasm of the cell.
It consists of:
- outer nuclear membrane;
- the inner nuclear membrane, between which is the perinuclear space;
- the double-membrane nuclear envelope has a pore complex.
Nucleoplasm (karyoplasm)- the liquid component of the nucleus, in which chromatin and nucleoli are located.
Core. Kernel Components
Contains water and a number
substances dissolved and suspended in it: RNA, glycoproteins,
ions, enzymes, metabolites.
nucleolus- the densest structure of the nucleus, formed by specialized areas - loops of chromosomes, which are called nucleolar organizers.
There are 3 components of the nucleolus:
1. The fibrillar component is the primary rRNA transcripts.
2. The granular component is an accumulation of pre-
ribosome subunits.
3. Amorphous component - areas of the nucleolar organizer,
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The nucleus is the main regulatory component of the cell. Its structure and functions.
The nucleus is an essential part of eukaryotic cells. This is the main regulatory component of the cell. It is responsible for the storage and transmission of hereditary information, controls all metabolic processes in the cell. . Not an organoid, but a component of a cell.
The core consists of:
1) the nuclear envelope (nuclear membrane), through the pores of which the exchange between the cell nucleus and the cytoplasm takes place.
2) nuclear juice, or karyoplasm, is a semi-liquid, weakly stained plasma mass that fills all the nuclei of the cell and contains the remaining components of the nucleus;
3) chromosomes that are visible in the non-dividing nucleus only with the help of special microscopy methods. The set of chromosomes in a cell is called aryotype. Chromatin on stained cell preparations is a network of thin strands (fibrils), small granules or clumps.
4) one or more spherical bodies - nucleoli, which are a specialized part of the cell nucleus and are associated with the synthesis of ribonucleic acid and proteins.
two kernel states:
1. interphase nucleus - has nuclei. sheath - karyolemma.
2. nucleus during cell divisions. only chromatin is present in a different state.
The nucleolus includes two zones:
1. inner-fibrillar-protein molecules and pre-RNA
2. outer - granular - form subunits of ribosomes.
The nuclear envelope consists of two membranes separated by a perinuclear space. Both of them are permeated with numerous pores, thanks to which the exchange of substances between the nucleus and the cytoplasm is possible.
The main components of the nucleus are chromosomes, formed from a DNA molecule and various proteins. In a light microscope, they are clearly distinguishable only during the period of cell division (mitosis, meiosis). In a non-dividing cell, the chromosomes look like long thin threads distributed throughout the entire volume of the nucleus.
The main functions of the cell nucleus are as follows:
- data storage;
- transfer of information to the cytoplasm using transcription, i.e., the synthesis of information-carrying i-RNA;
- transfer of information to daughter cells during replication - division of cells and nuclei.
- regulates biochemical, physiological and morphological processes in the cell.
takes place in the nucleus replication- duplication of DNA molecules, as well as transcription- synthesis of RNA molecules on a DNA template. In the nucleus, the synthesized RNA molecules undergo some modifications (for example, during splicing insignificant, meaningless regions are excluded from messenger RNA molecules), after which they enter the cytoplasm . Ribosome assembly also occurs in the nucleus, in special formations called nucleoli. The compartment for the nucleus - the karyotheque - is formed by expanding and merging with each other the tanks of the endoplasmic reticulum in such a way that the nucleus has double walls due to the narrow compartments of the nuclear membrane surrounding it. The cavity of the nuclear envelope is called lumen or perinuclear space. The inner surface of the nuclear envelope is underlain by the nuclear lamina- a rigid protein structure formed by lamins proteins, to which strands of chromosomal DNA are attached. In some places, the inner and outer membranes of the nuclear envelope merge and form the so-called nuclear pores through which material exchange occurs between the nucleus and the cytoplasm.
12. Two-membrane organelles (mitochondria, plastids). Their structure and functions.
Mitochondria - these are rounded or rod-shaped structures, often branching, 0.5 µm thick and usually up to 5-10 µm long.
The shell of mitochondria consists of two membranes that differ in chemical composition, a set of enzymes, and functions. Inner membrane forms invaginations of leaf-like (cristae) or tubular (tubules) shape. The space bounded by the inner membrane is matrix
organelles. Using an electron microscope, grains with a diameter of 20-40 nm are detected in it. They accumulate calcium and magnesium ions, as well as polysaccharides, such as glycogen.
The matrix contains its own organelle protein biosynthesis apparatus. It is represented by 2-6 copies of a circular and histone-free (as in prokaryotes) DNA molecule, ribosomes, a set of transport RNA (tRNA), enzymes for DNA replication, transcription and translation of hereditary information. Main function mitochondria consists in the enzymatic extraction of energy from certain chemical substances (by their oxidation) and the accumulation of energy in a biologically usable form (by the synthesis of adenosine triphosphate -ATP molecules). In general, this process is called oxidative phosphorylation.
Among the side functions of mitochondria, one can name participation in the synthesis of steroid hormones and some amino acids (glutamine).
plastids - these are semi-autonomous (they can exist relatively autonomously from the nuclear DNA of the cell) two-membrane organelles characteristic of photosynthetic eukaryotic organisms. There are three main types of plastids: chloroplasts, chromoplasts and leukoplasts.The totality of plastids in a cell is calledplastidoma . Each of these types, under certain conditions, can pass one into another. Like mitochondria, plastids contain their own DNA molecules. Therefore, they are also able to reproduce independently of cell division. Plastids are found only in plant cells.
Chloroplasts. The length of chloroplasts ranges from 5 to 10 microns, the diameter is from 2 to 4 microns. Chloroplasts are bounded by two membranes. The outer membrane is smooth, the inner one has a complex folded structure. The smallest fold is called t ilakoid. A group of thylakoids stacked like a stack of coins is called a g wound. The granules are connected to each other by flattened channels - lamellae. The thylakoid membranes contain photosynthetic pigments and enzymes that provide ATP synthesis. The main photosynthetic pigment is chlorophyll, which determines the green color of chloroplasts.
The inner space of chloroplasts is filled stroma. The stroma contains circular naked DNA, ribosomes, enzymes of the Calvin cycle, and starch grains. Inside each thylakoid there is a proton reservoir, there is an accumulation of H +. Chloroplasts, like mitochondria, are capable of autonomous reproduction by dividing in two. The chloroplasts of lower plants are called chromatophores.
Leucoplasts. The outer membrane is smooth, the inner one forms small thylakoids. The stroma contains circular "naked" DNA, ribosomes, enzymes for the synthesis and hydrolysis of reserve nutrients. There are no pigments. Especially many leukoplasts have cells of the underground organs of the plant (roots, tubers, rhizomes, etc.). .). Amyloplasts- synthesize and store starch , elaioplast- oils , proteinoplasts- proteins. Different substances can accumulate in the same leukoplast.
Chromoplasts. The outer membrane is smooth, the inner or also smooth, or forms single thylakoids. The stroma contains circular DNA and pigments. - carotenoids, giving chromoplasts a yellow, red, or orange color. The form of accumulation of pigments is different: in the form of crystals, dissolved in lipid drops, etc. Chromoplasts are considered the final stage in the development of plastids.
Plastids can mutually transform into each other: leukoplasts - chloroplasts - chromoplasts.
Single-membrane organelles (ER, Golgi apparatus, lysosomes). Their structure and functions.
tubular and vacuolar system formed by communicating or separate tubular or flattened (cistern) cavities, limited by membranes and spreading throughout the cytoplasm of the cell. In this system, there are rough and smooth cytoplasmic reticulum. A feature of the structure of the rough network is the attachment of polysomes to its membranes. Because of this, it performs the function of synthesizing a certain category of proteins that are mainly removed from the cell, for example, secreted by gland cells. In the area of the rough network, the formation of proteins and lipids of cytoplasmic membranes, as well as their assembly. Densely packed into a layered structure, cisterns of a rough network are the sites of the most active protein synthesis and are called ergastoplasm.
The membranes of the smooth cytoplasmic reticulum are devoid of polysomes. Functionally, this network is associated with the metabolism of carbohydrates, fats and other non-protein substances, such as steroid hormones (in the gonads, adrenal cortex). Through the tubules and cisterns, substances move, in particular, the material secreted by the glandular cell, from the site of synthesis to the packing area into granules. In areas of liver cells rich in smooth network structures, harmful toxic substances and some drugs (barbiturates) are destroyed and rendered harmless. In the vesicles and tubules of the smooth network of striated muscles, calcium ions are stored (deposited), which play an important role in the contraction process.
Golgi complex-is a stack of flat membrane sacs called cisterns. The tanks are completely isolated from each other and are not interconnected. Numerous tubules and vesicles branch off from the cisterns along the edges. Vacuoles (vesicles) with synthesized substances are laced from the EPS from time to time, which move to the Golgi complex and connect with it. Substances synthesized in the EPS become more complex and accumulate in the Golgi complex. Functions of the Golgi complex :1- In the tanks of the Golgi complex, there is a further chemical transformation and complication of substances that have entered it from the EPS. For example, substances are formed that are necessary to renew the cell membrane (glycoproteins, glycolipids), polysaccharides.
2- In the Golgi complex there is an accumulation of substances and their temporary "storage"
3- Formed substances are “packed” into vesicles (in vacuoles) and in this form move through the cell.
4- In the Golgi complex, lysosomes are formed (spherical organelles with degrading enzymes).
Lysosomes- small spherical organelles, the walls of which are formed by a single membrane; contain lytic(cleaving) enzymes. At first, the lysosomes, laced from the Golgi complex, contain inactive enzymes. Under certain conditions, their enzymes are activated. When a lysosome fuses with a phagocytic or pinocytic vacuole, a digestive vacuole is formed, in which various substances are digested intracellularly.
Functions of lysosomes :1- Carry out the splitting of substances absorbed as a result of phagocytosis and pinocytosis. Biopolymers are broken down into monomers that enter the cell and are used for its needs.
The nucleus and its structural components
For example, they can be used to synthesize new organic substances, or they can be further broken down for energy.
2- Destroy old, damaged, excess organelles. Splitting of organelles can also occur during starvation of the cell.
Vacuoles- spherical single-membrane organelles, which are reservoirs of water and substances dissolved in it. Vacuoles include: phagocytic and pinocytic vacuoles, digestive vacuoles, vesicles, laced from the EPS and the Golgi complex. Animal cell vacuoles are small and numerous, but their volume does not exceed 5% of the total cell volume. Their main function - transport of substances through the cell, the implementation of the relationship between organelles.
In a plant cell, vacuoles account for up to 90% of the volume.
In a mature plant cell, there is only one vacuole, it occupies a central position. The vacuole membrane of a plant cell is the tonoplast, its contents are cell sap. Functions of vacuoles in a plant cell: maintaining the cell membrane in tension, accumulation of various substances, including waste products of the cell. Vacuoles supply water for photosynthesis. May include:
- reserve substances that can be used by the cell itself (organic acids, amino acids, sugars, proteins). - substances that are excreted from the metabolism of the cell and accumulate in the vacuole (phenols, tannins, alkaloids, etc.) - phytohormones, phytoncides,
- pigments (coloring substances) that give the cell sap a purple, red, blue, violet color, and sometimes yellow or cream. It is the pigments of cell sap that color flower petals, fruits, root crops.
14. Non-membrane organelles (microtubules, cell center, ribosomes). Their structure and functions.Ribosome - a non-membrane organelle of the cell that performs protein synthesis. Consists of two subunits - small and large. The ribosome consists of 3-4 rRNA molecules that form its framework, and several dozen molecules of various proteins. Ribosomes are synthesized in the nucleolus. In a cell, ribosomes can be located on the surface of the granular ER or in the hyaloplasm of the cell in the form of polysomes. Polysome - it is a complex of i-RNA and several ribosomes that read information from it. Function ribosome- protein biosynthesis. If ribosomes are located on the ER, then the proteins synthesized by them are used for the needs of the whole organism, hyaloplasmic ribosomes synthesize proteins for the needs of the cell itself. The ribosomes of prokaryotic cells are smaller than those of eukaryotes. The same small ribosomes are found in mitochondria and plastids.
microtubules - hollow cylindrical structures of the cell, consisting of the irreducible protein tubulin. Microtubules are incapable of contraction. The walls of the microtubule are formed by 13 strands of the protein tubulin. Microtubules are located in the thickness of the hyaloplasm of cells.
Cilia and flagella - organelles of movement. Main function - movement of cells or movement along the cells of the fluid or particles surrounding them. In a multicellular organism, cilia are characteristic of the epithelium of the respiratory tract, fallopian tubes, and flagella are characteristic of spermatozoa. Cilia and flagella differ only in size - the flagella are longer. They are based on microtubules arranged in a 9(2) + 2 system. This means that 9 double microtubules (doublets) form the wall of a cylinder, in the center of which there are 2 single microtubules. The cilia and flagella are supported by the basal bodies. The basal body has a cylindrical shape, formed by 9 triplets (triplets) of microtubules; there are no microtubules in the center of the basal body.
Cl e exact center — mitotic center, a permanent structure in almost all animal and some plant cells, determines the poles of a dividing cell (see Mitosis) . The cell center usually consists of two centrioles - dense granules 0.2-0.8 in size micron, located at right angles to each other. During the formation of the mitotic apparatus, centrioles diverge towards the poles of the cell, determining the orientation of the spindle of cell division. Therefore, it is more correct to K. c. call mitotic center, reflecting by this its functional significance, especially since only in some cells K. c. located in its center. In the course of development of the organism, they change as the position of K. c. in cells, so is the shape of it. When a cell divides, each of the daughter cells receives a pair of centrioles. The process of their duplication occurs more often at the end of the previous cell division. Emergence of a number of pathological forms of cell division is connected with abnormal division To. c.
Long before the emergence of reliable data on the internal structure of all things, Greek thinkers imagined matter in the form of the smallest fiery particles that were in constant motion. Probably, this vision of the world order of things was derived from purely logical conclusions. Despite some naivety and absolute lack of evidence for this statement, it turned out to be true. Although scientists were able to confirm a bold guess only twenty-three centuries later.
The structure of atoms
At the end of the 19th century, the properties of a discharge tube through which a current was passed were investigated. Observations have shown that two streams of particles are emitted:
The negative particles of the cathode rays were called electrons. Subsequently, particles with the same charge-to-mass ratio were found in many processes. Electrons seemed to be universal constituents of various atoms, quite easily separated by the bombardment of ions and atoms.
Particles carrying a positive charge were represented by fragments of atoms after they lost one or more electrons. In fact, the positive rays were groups of atoms devoid of negative particles, and therefore having a positive charge.
Thompson model
On the basis of experiments, it was found that positive and negative particles represented the essence of the atom, were its constituents. The English scientist J. Thomson proposed his theory. In his opinion, the structure of the atom and the atomic nucleus was a kind of mass in which negative charges were squeezed into a positively charged ball, like raisins in a cupcake. Charge compensation made the cake electrically neutral.
Rutherford model
The young American scientist Rutherford, analyzing the tracks left after alpha particles, came to the conclusion that the Thompson model is imperfect. Some alpha particles were deflected by small angles - 5-10 o . In rare cases, alpha particles were deflected at large angles of 60-80 o , and in exceptional cases, the angles were very large - 120-150 o . Thompson's model of the atom could not explain such a difference.
Rutherford proposes a new model that explains the structure of the atom and the atomic nucleus. The physics of processes states that an atom must be 99% empty, with a tiny nucleus and electrons revolving around it, which move in orbits.
He explains the deviations during impacts by the fact that the particles of the atom have their own electric charges. Under the influence of bombarding charged particles, atomic elements behave like ordinary charged bodies in the macrocosm: particles with the same charges repel each other, and with opposite charges they attract.
State of atoms
At the beginning of the last century, when the first particle accelerators were launched, all theories explaining the structure of the atomic nucleus and the atom itself were waiting for experimental verification. By that time, the interactions of alpha and beta rays with atoms had already been thoroughly studied. Until 1917, it was believed that atoms were either stable or radioactive. Stable atoms cannot be split, the decay of radioactive nuclei cannot be controlled. But Rutherford managed to refute this opinion.
First proton
In 1911, E. Rutherford put forward the idea that all nuclei consist of the same elements, the basis for which is the hydrogen atom. This idea was prompted by an important conclusion of previous studies of the structure of matter: the masses of all chemical elements are divided without a trace by the mass of hydrogen. The new assumption opened up unprecedented possibilities, allowing us to see the structure of the atomic nucleus in a new way. Nuclear reactions had to confirm or disprove the new hypothesis.
Experiments were carried out in 1919 with nitrogen atoms. By bombarding them with alpha particles, Rutherford achieved an amazing result.
The N atom absorbed the alpha particle, then turned into an oxygen atom O 17 and emitted a hydrogen nucleus. This was the first artificial transformation of an atom of one element into another. Such an experience gave hope that the structure of the atomic nucleus, the physics of existing processes make it possible to carry out other nuclear transformations.
The scientist used in his experiments the method of scintillation - flashes. From the frequency of flashes, he drew conclusions about the composition and structure of the atomic nucleus, about the characteristics of the particles born, about their atomic mass and serial number. The unknown particle was named by Rutherford the proton. It had all the characteristics of a hydrogen atom stripped of its single electron - a single positive charge and a corresponding mass. Thus it was proved that the proton and the nucleus of hydrogen are the same particles.
In 1930, when the first large accelerators were built and launched, Rutherford's model of the atom was tested and proved: each hydrogen atom consists of a lone electron, the position of which cannot be determined, and a loose atom with a lone positive proton inside. Since protons, electrons, and alpha particles can fly out of an atom when bombarded, scientists thought that they were the constituents of any atom's nucleus. But such a model of the atom of the nucleus seemed unstable - the electrons were too large to fit in the nucleus, in addition, there were serious difficulties associated with the violation of the law of momentum and conservation of energy. These two laws, like strict accountants, said that the momentum and mass during the bombardment disappear in an unknown direction. Since these laws were generally accepted, it was necessary to find explanations for such a leak.
Neutrons
Scientists around the world set up experiments aimed at discovering new constituents of the nuclei of atoms. In the 1930s, German physicists Becker and Bothe bombarded beryllium atoms with alpha particles. In this case, an unknown radiation was registered, which it was decided to call G-rays. Detailed studies revealed some features of the new beams: they could propagate strictly in a straight line, did not interact with electric and magnetic fields, and had a high penetrating power. Later, the particles that form this type of radiation were found in the interaction of alpha particles with other elements - boron, chromium and others.
Chadwick's hypothesis
Then James Chadwick, a colleague and student of Rutherford, gave a short report in Nature magazine, which later became public knowledge. Chadwick drew attention to the fact that the contradictions in the conservation laws are easily resolved if we assume that the new radiation is a stream of neutral particles, each of which has a mass approximately equal to the mass of a proton. Considering this assumption, physicists significantly supplemented the hypothesis explaining the structure of the atomic nucleus. Briefly, the essence of the additions was reduced to a new particle and its role in the structure of the atom.
Properties of the neutron
The discovered particle was given the name "neutron". The newly discovered particles did not form electromagnetic fields around themselves and easily passed through matter without losing energy. In rare collisions with light nuclei of atoms, the neutron is able to knock out the nucleus from the atom, losing a significant part of its energy. The structure of the atomic nucleus assumed the presence of a different number of neutrons in each substance. Atoms with the same nuclear charge but different numbers of neutrons are called isotopes.
Neutrons have served as an excellent replacement for alpha particles. Currently, they are used to study the structure of the atomic nucleus. Briefly, their significance for science cannot be described, but it was thanks to the bombardment of atomic nuclei by neutrons that physicists were able to obtain isotopes of almost all known elements.
The composition of the nucleus of an atom
At present, the structure of the atomic nucleus is a collection of protons and neutrons held together by nuclear forces. For example, a helium nucleus is a lump of two neutrons and two protons. Light elements have an almost equal number of protons and neutrons, while heavy elements have a much larger number of neutrons.
This picture of the structure of the nucleus is confirmed by experiments at modern large accelerators with fast protons. The electric forces of repulsion of protons are balanced by vigorous forces that act only in the nucleus itself. Although the nature of nuclear forces is not yet fully understood, their existence is practically proven and fully explains the structure of the atomic nucleus.
Relationship between mass and energy
In 1932, a cloud chamber captured an amazing photograph proving the existence of positive charged particles, with the mass of an electron.
Prior to this, positive electrons were theoretically predicted by P. Dirac. A real positive electron was also discovered in cosmic radiation. The new particle was called the positron. When colliding with its twin - an electron, annihilation occurs - the mutual annihilation of two particles. This releases a certain amount of energy.
Thus, the theory developed for the macrocosm was fully suitable for describing the behavior of the smallest elements of matter.
A feature of radioactive contamination, in contrast to contamination by other pollutants, is that it is not the radionuclide (pollutant) itself that has a harmful effect on humans and environmental objects, but the radiation, the source of which it is.
However, there are cases when a radionuclide is a toxic element. For example, after the accident at the Chernobyl nuclear power plant, plutonium 239, 242 Pu was released into the environment with particles of nuclear fuel. In addition to the fact that plutonium is an alpha emitter and poses a significant danger when it enters the body, plutonium itself is a toxic element.
For this reason, two groups of quantitative indicators are used: 1) to assess the content of radionuclides and 2) to assess the impact of radiation on an object.
Activity- a quantitative measure of the content of radionuclides in the analyzed object. Activity is determined by the number of radioactive decays of atoms per unit time. The SI unit of activity is the Becquerel (Bq) equal to one disintegration per second (1Bq = 1 decay/s). Sometimes an off-system activity measurement unit is used - Curie (Ci); 1Ci = 3.7 × 1010 Bq.
Radiation dose is a quantitative measure of the impact of radiation on an object.
Due to the fact that the impact of radiation on an object can be assessed at different levels: physical, chemical, biological; at the level of individual molecules, cells, tissues or organisms, etc., several types of doses are used: absorbed, effective equivalent, exposure.
To assess the change in the dose of radiation over time, the indicator "dose rate" is used. Dose rate is the ratio of dose to time. For example, the dose rate of external exposure from natural sources of radiation in Russia is 4-20 μR/h.
The main standard for humans - the main dose limit (1 mSv / year) - is introduced in units of the effective equivalent dose. There are standards in units of activity, levels of land pollution, VDU, GWP, SanPiN, etc.
The structure of the atomic nucleus.
An atom is the smallest particle of a chemical element that retains all of its properties. In its structure, an atom is a complex system consisting of a positively charged nucleus of a very small size (10 -13 cm) located in the center of the atom and negatively charged electrons rotating around the nucleus in various orbits. The negative charge of the electrons is equal to the positive charge of the nucleus, while in general it turns out to be electrically neutral.
Atomic nuclei are made up of nucleons - nuclear protons ( Z- number of protons) and nuclear neutrons (N is the number of neutrons). "Nuclear" protons and neutrons differ from particles in a free state. For example, a free neutron, unlike a bound one in a nucleus, is unstable and turns into a proton and an electron.
The number of nucleons Am (mass number) is the sum of the numbers of protons and neutrons: Am = Z + N.
Proton - elementary particle of any atom, it has a positive charge equal to the charge of an electron. The number of electrons in the shell of an atom is determined by the number of protons in the nucleus.
Neutron - another kind of nuclear particles of all elements. It is absent only in the nucleus of light hydrogen, which consists of one proton. It has no charge and is electrically neutral. In the atomic nucleus, neutrons are stable, while in the free state they are unstable. The number of neutrons in the nuclei of atoms of the same element can fluctuate, so the number of neutrons in the nucleus does not characterize the element.
Nucleons (protons + neutrons) are held inside the atomic nucleus by nuclear forces of attraction. Nuclear forces are 100 times stronger than electromagnetic forces and therefore keeps like-charged protons inside the nucleus. Nuclear forces manifest themselves only at very small distances (10 -13 cm), they constitute the potential binding energy of the nucleus, which is partially released during some transformations and passes into kinetic energy.
For atoms differing in the composition of the nucleus, the name "nuclides" is used, and for radioactive atoms - "radionuclides".
Nuclides call atoms or nuclei with a given number of nucleons and a given charge of the nucleus (nuclide designation A X).
Nuclides having the same number of nucleons (Am = const) are called isobars. For example, the nuclides 96 Sr, 96 Y, 96 Zr belong to a series of isobars with the number of nucleons Am = 96.
Nuclides that have the same number of protons (Z= const) are called isotopes. They differ only in the number of neutrons, therefore they belong to the same element: 234 U , 235 U, 236 U , 238 U .
isotopes- nuclides with the same number of neutrons (N = Am -Z = const). Nuclides: 36 S, 37 Cl, 38 Ar, 39 K, 40 Ca belong to the isotope series with 20 neutrons.
Isotopes are usually denoted as Z X M, where X is the symbol of a chemical element; M is the mass number equal to the sum of the number of protons and neutrons in the nucleus; Z is the atomic number or charge of the nucleus, equal to the number of protons in the nucleus. Since each chemical element has its own constant atomic number, it is usually omitted and limited to writing only the mass number, for example: 3 H, 14 C, 137 Cs, 90 Sr, etc.
Atoms of the nucleus that have the same mass numbers, but different charges and, consequently, different properties are called "isobars", for example, one of the phosphorus isotopes has a mass number of 32 - 15 Р 32, one of the sulfur isotopes has the same mass number - 16 S 32 .
Nuclides can be stable (if their nuclei are stable and do not decay) or unstable (if their nuclei are unstable and undergo changes that eventually increase the stability of the nucleus). Unstable atomic nuclei that can spontaneously decay are called radionuclides. The phenomenon of spontaneous decay of the nucleus of an atom, accompanied by the emission of particles and (or) electromagnetic radiation, is called radioactivity.
As a result of radioactive decay, both a stable and a radioactive isotope can be formed, in turn, spontaneously decaying. Such chains of radioactive elements connected by a series of nuclear transformations are called radioactive families.
Currently, IUPAC (International Union of Pure and Applied Chemistry) has officially named 109 chemical elements. Of these, only 81 have stable isotopes, the heaviest of which is bismuth. (Z= 83). For the remaining 28 elements, only radioactive isotopes are known, with uranium (u~ 92) is the heaviest element found in nature. The largest of the natural nuclides has 238 nucleons. In total, the existence of about 1700 nuclides of these 109 elements has now been proven, with the number of isotopes known for individual elements ranging from 3 (for hydrogen) to 29 (for platinum).
atomic nucleus is the central part of the atom, made up of protons and neutrons (collectively called nucleons).
The nucleus was discovered by E. Rutherford in 1911 while studying the passage α -particles through matter. It turned out that almost the entire mass of an atom (99.95%) is concentrated in the nucleus. The size of the atomic nucleus is of the order of 10 -1 3 -10 - 12 cm, which is 10,000 times smaller than the size of the electron shell.
The planetary model of the atom proposed by E. Rutherford and his experimental observation of hydrogen nuclei knocked out α -particles from the nuclei of other elements (1919-1920), led the scientist to the idea of proton. The term proton was introduced in the early 20s of the XX century.
Proton (from Greek. protons- first, symbol p) is a stable elementary particle, the nucleus of a hydrogen atom.
Proton- a positively charged particle, the charge of which is equal in absolute value to the charge of an electron e\u003d 1.6 10 -1 9 Cl. The mass of a proton is 1836 times the mass of an electron. Rest mass of a proton m p= 1.6726231 10 -27 kg = 1.007276470 amu
The second particle in the nucleus is neutron.
Neutron (from lat. neuter- neither one nor the other, a symbol n) is an elementary particle that has no charge, i.e., neutral.
The mass of the neutron is 1839 times the mass of the electron. The mass of a neutron is almost equal to (slightly larger than) that of a proton: the rest mass of a free neutron m n= 1.6749286 10 -27 kg = 1.0008664902 amu and exceeds the proton mass by 2.5 electron masses. Neutron, along with the proton under the common name nucleon is part of the atomic nucleus.
The neutron was discovered in 1932 by D. Chadwig, a student of E. Rutherford, during the bombardment of beryllium α -particles. The resulting radiation with high penetrating power (it overcame an obstacle made of a lead plate 10–20 cm thick) intensified its effect when passing through the paraffin plate (see figure). The estimation of the energy of these particles from the tracks in the cloud chamber made by the Joliot-Curies and additional observations made it possible to exclude the initial assumption that this γ -quanta. The great penetrating power of new particles, called neutrons, was explained by their electrical neutrality. After all, charged particles actively interact with matter and quickly lose their energy. The existence of neutrons was predicted by E. Rutherford 10 years before the experiments of D. Chadwig. On hit α -particles in the nuclei of beryllium, the following reaction occurs:
Here is the symbol of the neutron; its charge is zero, and the relative atomic mass is approximately equal to one. A neutron is an unstable particle: a free neutron in a time of ~ 15 min. decays into a proton, an electron and a neutrino - a particle devoid of rest mass.
After the discovery of the neutron by J. Chadwick in 1932, D. Ivanenko and W. Heisenberg independently proposed proton-neutron (nucleon) model of the nucleus. According to this model, the nucleus consists of protons and neutrons. Number of protons Z coincides with the serial number of the element in the table of D. I. Mendeleev.
Core charge Q determined by the number of protons Z, which are part of the nucleus, and is a multiple of the absolute value of the electron charge e:
Q = + Ze.
Number Z called nuclear charge number or atomic number.
Mass number of the nucleus A called the total number of nucleons, i.e., protons and neutrons contained in it. The number of neutrons in a nucleus is denoted by the letter N. So the mass number is:
A = Z + N.
The nucleons (proton and neutron) are assigned a mass number equal to one, and the electron is assigned a zero value.
The idea of the composition of the nucleus was also facilitated by the discovery isotopes.
Isotopes (from the Greek. isos equal, the same and topoa- place) - these are varieties of atoms of the same chemical element, the atomic nuclei of which have the same number of protons ( Z) and a different number of neutrons ( N).
The nuclei of such atoms are also called isotopes. Isotopes are nuclides one element. Nuclide (from lat. nucleus- nucleus) - any atomic nucleus (respectively, an atom) with given numbers Z and N. The general designation of nuclides is ……. where X- symbol of a chemical element, A=Z+N- mass number.
Isotopes occupy the same place in the Periodic Table of the Elements, hence their name. As a rule, isotopes differ significantly in their nuclear properties (for example, in their ability to enter into nuclear reactions). The chemical (and almost equally physical) properties of isotopes are the same. This is explained by the fact that the chemical properties of an element are determined by the charge of the nucleus, since it is this charge that affects the structure of the electron shell of the atom.
The exception is isotopes of light elements. Isotopes of hydrogen 1 H — protium, 2 H— deuterium, 3 H — tritium they differ so much in mass that their physical and chemical properties are different. Deuterium is stable (i.e., not radioactive) and is included as a small impurity (1: 4500) in ordinary hydrogen. Deuterium combines with oxygen to form heavy water. It boils at normal atmospheric pressure at 101.2°C and freezes at +3.8°C. Tritium β is radioactive with a half-life of about 12 years.
All chemical elements have isotopes. Some elements have only unstable (radioactive) isotopes. For all elements, radioactive isotopes have been artificially obtained.
Isotopes of uranium. The element uranium has two isotopes - with mass numbers 235 and 238. The isotope is only 1/140 of the more common.
By studying the composition of matter, scientists came to the conclusion that all matter consists of molecules and atoms. For a long time, the atom (translated from Greek as "indivisible") was considered the smallest structural unit of matter. However, further studies have shown that the atom has a complex structure and, in turn, includes smaller particles.
What is an atom made of?
In 1911, the scientist Rutherford suggested that the atom has a central part that has a positive charge. Thus, for the first time, the concept of the atomic nucleus appeared.
According to Rutherford's scheme, called the planetary model, an atom consists of a nucleus and elementary particles with a negative charge - electrons moving around the nucleus, just as the planets orbit around the Sun.
In 1932, another scientist, Chadwick, discovered the neutron, a particle that has no electric charge.
According to modern concepts, the nucleus corresponds to the planetary model proposed by Rutherford. The nucleus carries most of the atomic mass. It also has a positive charge. The atomic nucleus contains protons - positively charged particles and neutrons - particles that do not carry a charge. Protons and neutrons are called nucleons. Negatively charged particles - electrons - orbit around the nucleus.
The number of protons in the nucleus is equal to those moving in orbit. Therefore, the atom itself is a particle that does not carry a charge. If an atom captures foreign electrons or loses its own, then it becomes positive or negative and is called an ion.
Electrons, protons and neutrons are collectively referred to as subatomic particles.
The charge of the atomic nucleus
The nucleus has a charge number Z. It is determined by the number of protons that make up the atomic nucleus. Finding out this amount is simple: just refer to the periodic system of Mendeleev. The atomic number of the element to which an atom belongs is equal to the number of protons in the nucleus. Thus, if the chemical element oxygen corresponds to the serial number 8, then the number of protons will also be equal to eight. Since the number of protons and electrons in an atom is the same, there will also be eight electrons.
The number of neutrons is called the isotopic number and is denoted by the letter N. Their number may vary in an atom of the same chemical element.
The sum of protons and electrons in the nucleus is called the mass number of the atom and is denoted by the letter A. Thus, the formula for calculating the mass number looks like this: A \u003d Z + N.
isotopes
In the case when elements have an equal number of protons and electrons, but a different number of neutrons, they are called isotopes of a chemical element. There can be one or more isotopes. They are placed in the same cell of the periodic system.
Isotopes are of great importance in chemistry and physics. For example, an isotope of hydrogen - deuterium - in combination with oxygen gives a completely new substance, which is called heavy water. It has a different boiling and freezing point than usual. And the combination of deuterium with another isotope of hydrogen - tritium leads to a thermonuclear fusion reaction and can be used to generate a huge amount of energy.
Mass of the nucleus and subatomic particles
The size and mass of atoms are negligible in the minds of man. The size of the nuclei is approximately 10 -12 cm. The mass of the atomic nucleus is measured in physics in the so-called atomic mass units - a.m.u.
For one a.m.u. take one twelfth of the mass of a carbon atom. Using the usual units of measurement (kilograms and grams), the mass can be expressed as follows: 1 a.m.u. \u003d 1.660540 10 -24 g. Expressed in this way, it is called the absolute atomic mass.
Despite the fact that the atomic nucleus is the most massive component of the atom, its dimensions relative to the electron cloud surrounding it are extremely small.
nuclear forces
Atomic nuclei are extremely stable. This means that protons and neutrons are held in the nucleus by some forces. These cannot be electromagnetic forces, since protons are like-charged particles, and it is known that particles with the same charge repel each other. The gravitational forces are too weak to hold the nucleons together. Consequently, the particles are held in the nucleus by a different interaction - nuclear forces.
Nuclear interaction is considered the strongest of all existing in nature. Therefore, this type of interaction between the elements of the atomic nucleus is called strong. It is present in many elementary particles, as well as electromagnetic forces.
Features of nuclear forces
- Short action. Nuclear forces, in contrast to electromagnetic forces, manifest themselves only at very small distances comparable to the size of the nucleus.
- Charge independence. This feature is manifested in the fact that nuclear forces act equally on protons and neutrons.
- Saturation. The nucleons of the nucleus interact only with a certain number of other nucleons.
Core binding energy
Something else is closely connected with the concept of strong interaction - the binding energy of nuclei. Nuclear binding energy is the amount of energy required to split an atomic nucleus into its constituent nucleons. It is equal to the energy required to form a nucleus from individual particles.
To calculate the binding energy of a nucleus, it is necessary to know the mass of subatomic particles. Calculations show that the mass of a nucleus is always less than the sum of its constituent nucleons. The mass defect is the difference between the mass of the nucleus and the sum of its protons and electrons. Using the relationship between mass and energy (E \u003d mc 2), you can calculate the energy generated during the formation of the nucleus.
The strength of the binding energy of the nucleus can be judged by the following example: the formation of several grams of helium produces the same amount of energy as the combustion of several tons of coal.
Nuclear reactions
The nuclei of atoms can interact with the nuclei of other atoms. Such interactions are called nuclear reactions. Reactions are of two types.
- Fission reactions. They occur when heavier nuclei break down into lighter ones as a result of the interaction.
- Synthesis reactions. The process is the reverse of fission: the nuclei collide, thereby forming heavier elements.
All nuclear reactions are accompanied by the release of energy, which is subsequently used in industry, in the military, in energy, and so on.
Having become acquainted with the composition of the atomic nucleus, we can draw the following conclusions.
- An atom consists of a nucleus containing protons and neutrons, and electrons around it.
- The mass number of an atom is equal to the sum of the nucleons of its nucleus.
- Nucleons are held together by the strong force.
- The enormous forces that give the atomic nucleus stability are called the binding energies of the nucleus.
