The atomic nucleus of an element is made up of. The structure of the atom and the atomic nucleus. What is an atom made of
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 comprises 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 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 envelope directly contacts with the cytoplasm of the cell, has a number of structural features that allow it to be attributed to the proper ER 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 membrane. 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 implementation of the support function is associated: the matrix also contains the primary products of transcription 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.
AT 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: when high level synthesis in the nucleolus is detected big number granules, when the 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 hole 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 indispensable condition for the existence 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.
Nucleus. 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 mitochondrial envelope 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 chemicals (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 intracellular digestion occurs. various substances.
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 matter or may 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 a cylinder wall, 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.
Academician A. F. Ioffe. "Science and Life" No. 1, 1934
The article "The Nucleus of the Atom" by Academician Abram Fedorovich Ioffe opened the first issue of the journal "Science and Life", newly created in 1934.
E. Rutherford.
F. W. Aston.
WAVE NATURE OF MATTER
At the beginning of the 20th century, the atomistic structure of matter ceased to be a hypothesis, and the atom became the same reality as the facts and phenomena common to us are real.
It turned out that the atom is a very complex formation, which undoubtedly includes electric charges, and perhaps only electric charges. Hence, naturally, the question arose about the structure of the atom.
The first model of the atom was modeled after solar system. However, this idea of the structure of the atom soon turned out to be untenable. And it's natural. The idea of the atom as a solar system was a purely mechanical transfer of the picture associated with astronomical scales to the region of the atom, where the scales are only hundred-millionths of a centimeter. Such a sharp quantitative change could not but entail a very significant change quality properties the same phenomena. This difference was primarily reflected in the fact that the atom, unlike the solar system, must be built according to much more stringent rules than those laws that determine the orbits of the planets in the solar system.
There were two difficulties. First, all atoms of a given kind, of a given element, are exactly the same in their physical properties, and, consequently, the orbits of electrons in these atoms must be exactly the same. Meanwhile, the laws of mechanics that govern the motion of celestial bodies give absolutely no grounds for this. Depending on the initial speed The planet's orbit can be, according to these laws, completely arbitrary, the planet can rotate each time with the appropriate speed in any orbit, at any distance from the Sun. If the same arbitrary orbits existed in atoms, then the atoms of the same substance could not be so identical in their properties, for example, give a strictly identical luminescence spectrum. This is one contradiction.
The other was that the motion of an electron around an atomic nucleus, if laws were applied to it, well studied by us on a large scale of laboratory experiments or even astronomical phenomena, would have to be accompanied by a continuous emission of energy. Consequently, the energy of the atom would have to be continuously depleted, and again, the atom could not retain the same and unchanged properties over the course of centuries and millennia, and the whole world and all atoms would have to experience a continuous attenuation, a continuous loss of the energy contained in them. This, too, is in no way incompatible with the basic properties of atoms.
The last difficulty was particularly acute. It seemed to lead the whole of science into an unsolvable dead end.
The outstanding physicist Lorentz ended our conversation on this subject as follows: "I regret that I did not die five years ago, when this contradiction did not yet exist. Then I would have died in the conviction that I had revealed part of the truth in natural phenomena."
At the same time, in the spring of 1924, de Broglie, a young student of Langevin, expressed in his dissertation an idea that, in its further development, led to a new synthesis.
De Broglie's idea, later quite substantially changed, but still largely preserved, was that the motion of an electron rotating around the nucleus in an atom is not just the motion of a certain ball, as was previously imagined, that this motion is accompanied by some wave that travels with the moving electron. An electron is not a ball, but some electrical substance blurred in space, the movement of which is at the same time the propagation of a wave.
This idea, then extended not only to electrons, but also to the motion of any body - and an electron, and an atom, and a whole collection of atoms - asserts that any movement of a body contains two sides, from which we can see especially in individual cases clearly one side, while the other is not noticeable. In one case, we see, as it were, propagating waves and do not notice the movement of particles, in the other case, on the contrary, the moving particles themselves come to the fore, and the wave escapes our observation.
But in fact, both of these sides are always present, and, in particular, in the motion of electrons there is not only the movement of the charges themselves, but also the propagation of the wave.
It cannot be said that there is no movement of electrons along orbits, but there is only pulsation, only waves, that is, something else. No, it would be more correct to say this: we do not at all deny the movement of the electrodes, which we likened to the movement of the planets around the Sun, but this movement itself has the character of a pulsation, and not the character of movement the globe around the sun.
I will not describe here the structure of the atom, the structure of that electron shell that determines all the basic physical properties- cohesion, elasticity, capillarity, chemical properties, etc. All this is the result of the movement of the electron shell, or, as we now say, the pulsation of the atom.
THE PROBLEM OF THE NUCLEAR
The nucleus plays the most important role in the atom. This is the center around which all electrons revolve and whose properties ultimately determine everything else.
The first thing we could learn about the nucleus was its charge. We know that an atom contains a number of negatively charged electrons, but the atom as a whole has no electric charge. This means that there must be corresponding positive charges somewhere. These positive charges are concentrated in the nucleus. The nucleus is a positively charged particle, around which the electron atmosphere pulsates, surrounding the nucleus. The charge of the nucleus determines the number of electrons.
The electrons of iron and copper, glass and wood are exactly the same. It is no harm to an atom to lose a few of its electrons, or even to lose all of its electrons. As long as a positively charged nucleus remains, this nucleus will attract as many electrons from other surrounding bodies as it needs, and the atom will be preserved. An iron atom remains iron as long as its core is intact. If it loses a few electrons, then the positive charge of the nucleus will be greater than the totality of the remaining negative charges, and the entire atom as a whole will acquire an excess positive charge. Then we call it not an atom, but a positive iron ion. In another case, the atom may, on the contrary, attract more negative electrons to itself than it has positive charges - then it will be negatively charged, and we call it a negative ion; it will be the negative ion of the same element. Consequently, the individuality of an element, all its properties exist and are determined by the nucleus, the charge of this nucleus, first of all.
Further, - the mass of an atom in its overwhelming part is determined precisely by the nucleus, and not by electrons, - the mass of electrons is less than one thousandth of the mass of the entire atom; more than 0.999 of the total mass is the mass of the nucleus. This is all the more important because we consider mass to be a measure of the energy reserve that a given substance possesses; mass is the same measure of energy as the erg, kilowatt hour, or calorie.
The complexity of the nucleus was revealed in the phenomenon of radioactivity, discovered shortly after X-rays, on the verge of our century. It is known that radioactive elements continuously emit energy in the form of alpha, beta and gamma rays. But such continuous radiation of energy must have some source. In 1902, Rutherford showed that the only source of this energy should be the atom, in other words, nuclear energy. The other side of radioactivity is that the emission of these rays transfers one element located in one place of the periodic system to another element with other chemical properties. In other words, radioactive processes carry out the transformation of elements. If it is true that the nucleus of an atom determines its individuality, and that as long as the nucleus is intact, so long as the atom remains an atom of a given element, and not of any other, then the transition of one element into another means a change in the very nucleus of the atom.
The rays ejected by radioactive substances provide the first approach that allows one to compose some general idea about what is in the nucleus.
Alpha rays are helium nuclei, and helium is the second element in the periodic table. Therefore, one can think that the composition of the nucleus includes helium nuclei. But measuring the velocities with which alpha rays fly out immediately leads to a very serious difficulty.
GAMOV'S THEORY OF RADIOACTIVITY
The nucleus is positively charged. When approaching it, any charged particle experiences a force of attraction or repulsion. On a large scale in laboratories, the interactions of electric charges are determined by Coulomb's law: two charges interact with each other with a force inversely proportional to the square of the distance between them and directly proportional to the magnitude of one and the other charges. Studying the laws of attraction or repulsion that particles experience when approaching the nucleus, Rutherford found that up to distances very close to the nucleus, on the order of 10 -12 cm, the same Coulomb's law is still valid. If this is the case, then we can easily calculate how much work the nucleus must do by pushing the positive charge away from itself as it exits the nucleus and is thrown out. Alpha particles and charged helium nuclei, flying out of the nucleus, move under the repulsive action of its charge; and now the corresponding calculation shows that, under the action of repulsion alone, the alpha particles should have accumulated a kinetic energy corresponding to at least 10 or 20 million electron volts, i.e., the energy that is obtained when passing through a charge equal to the charge of an electron, potential difference of 20 million volts. But in fact, when they leave the atom, they come out with much less energy, only 1-5 million electron volts. But besides,
it was natural to expect that the nucleus, throwing out an alpha particle, gives it something else in addition. At the moment of ejection, something like an explosion occurs in the nucleus, and this explosion itself imparts some kind of energy; the work of the repulsive forces is added to this, and it turns out that the sum of these energies is less than what one repulsion should give. This contradiction is removed as soon as we abandon the mechanical transfer to this area of views developed in the experience of studying large bodies, where we do not take into account the wave nature of motion. G. A. Gamov was the first to give a correct interpretation of this contradiction and created the wave theory of the nucleus and radioactive processes.
It is known that at sufficiently large distances (more than 10 -12 cm) the nucleus repels a positive charge from itself. On the other hand, there is no doubt that inside the nucleus itself, in which there are many positive charges, for some reason they do not repel each other. The very existence of the nucleus shows that positive charges inside the nucleus mutually attract each other, and outside the nucleus they repel each other.
How can one describe the energy conditions in the nucleus itself and around it? Gamow created the following performance. We will depict on the diagram (Fig. 5) the value of the energy of a positive charge in a given place by the distance from the horizontal line BUT.
As we approach the nucleus, the energy of the charge will increase, because work will be done against the repulsive force. Inside the nucleus, on the contrary, the energy must decrease again, because here there is not mutual repulsion, but mutual attraction. At the boundaries of the nucleus, a sharp decrease in the energy value occurs. Our drawing is depicted on a plane; in fact, one must, of course, imagine it in space with the same distribution of energy and in all other directions. Then we get that around the nucleus there is a spherical layer with high energy, as if some kind of energy barrier that protects the nucleus from the penetration of positive charges, the so-called "Gamow barrier".
If we stand on the point of view of the usual views on the motion of the body and forget about its wave nature, then we must expect that only such a positive charge can penetrate into the nucleus, the energy of which is not less than the height of the barrier. On the contrary, in order to leave the nucleus, the charge must first reach the top of the barrier, after which its kinetic energy will begin to increase as it moves away from the nucleus. If at the top of the barrier the energy was equal to zero, then when it is removed from the atom, it will receive the very 20 million electron volts that are never actually observed. The new understanding of the nucleus, which was introduced by Gamow, is as follows. The motion of a particle must be considered as a wave. Consequently, this motion is affected by the energy not only in the point occupied by the particle, but also in the entire blurred wave of the particle, which covers a fairly large space. Based on the concepts of wave mechanics, we can state that even if the energy at a given point has not reached the limit that corresponds to the top of the barrier, the particle can be on its other side, where it is no longer drawn into the nucleus by the forces of attraction acting there.
Something similar is the following experiment. Imagine that there is a barrel of water behind the wall of the room. From this barrel a pipe is drawn, which passes high above through a hole in the wall and supplies water; water is pouring out at the bottom. This is a well-known device called a siphon. If the barrel on that side is placed higher than the end of the pipe, then water will continuously flow through it at a speed determined by the difference between the water level in the barrel and the end of the pipe. There is nothing surprising here. But if you did not know about the existence of the barrel on the other side of the wall and saw only a pipe through which water flows from a great height, then this fact would seem to you an irreconcilable contradiction. Water flows from a great height and at the same time does not accumulate the energy that corresponds to the height of the pipe. However, the explanation in this case obviously.
We have a similar phenomenon in the nucleus. Charge from its normal position BUT rises to a state of greater energy AT, but does not reach the top of the barrier at all FROM(Fig. 6).
Out of state AT alpha particle, passing through the barrier, starts to repel from the nucleus not from the very top FROM, and from a lower energy height B1. Therefore, when going outside, the energy accumulated by the particle will not depend on the height FROM, but from a smaller height equal to B1(Fig. 7).
This qualitative reasoning can also be given a quantitative form and a law can be given that determines the probability of an alpha particle passing through the barrier as a function of that energy AT, which it has in the nucleus, and, consequently, from the energy that it will receive when it leaves the atom.
With the help of a series of experiments, a very simple law was established that connected the number of alpha particles emitted by radioactive substances with their energy or speed. But the meaning of this law was completely incomprehensible.
Gamow's first success consisted in the fact that this quantitative law of the emission of alpha particles followed from his theory quite precisely and naturally. Now the "Gamow energy barrier" and its wave interpretation are the basis of all our ideas about the nucleus.
The properties of alpha rays are qualitatively and quantitatively well explained by Gamow's theory, but it is known that radioactive substances also emit beta rays - streams of fast electrons. The electron emission model is unable to explain. This is one of the most serious contradictions in the theory of the atomic nucleus, which remained unresolved until very recently, but the solution of which is now, apparently, outlined.
STRUCTURE OF THE NUCLEUS
We now turn to consider what we know about the structure of the nucleus.
More than 100 years ago, Prout expressed the idea that perhaps the elements of the periodic system are not at all separate, unrelated forms of matter, but are only different combinations of the hydrogen atom. If this were so, then one would expect that not only the charges of all nuclei would be integer multiples of the charge of hydrogen, but also the masses of all nuclei would be expressed as integer multiples of the mass of the hydrogen nucleus, i.e. all atomic weights would have to be expressed whole numbers. And indeed, if you look at the table of atomic weights, you can see a large number of integers. For example, carbon is exactly 12, nitrogen is exactly 14, oxygen is exactly 16, fluorine is exactly 19. This, of course, is not an accident. But there are still atomic weights that are far from integers. For example, neon has an atomic weight of 20.2, chlorine has an atomic weight of 35.46. Therefore, Prout's hypothesis remained a partial conjecture and could not become a theory of the structure of the atom. By studying the behavior of charged ions, it is especially easy to study the properties of the nucleus of an atom by acting on them, for example, with an electric and magnetic field.
The method based on this, brought to extremely high accuracy by Aston, made it possible to establish that all elements whose atomic weights were not expressed in whole numbers, in fact, are not a homogeneous substance, but a mixture of two or more - 3, 4, 9 - different types atoms. So, for example, the atomic weight of chlorine, equal to 35.46, is explained by the fact that there are actually several kinds of chlorine atoms. There are chlorine atoms with atomic weights of 35 and 37, and these two kinds of chlorine are mixed together in such a proportion that their average atomic weight is 35.46. It turned out that not only in this one particular case, but in all cases, without exception, where atomic weights are not expressed as integers, we have a mixture of isotopes, i.e., atoms with the same charge, therefore, representing the same element , but with different masses. Each individual kind of atom always has an integer atomic weight.
Thus, Prout's conjecture immediately received significant reinforcement, and the question could be considered solved, if not for one exception, namely, hydrogen itself. The fact is that our system of atomic weights is built not on hydrogen, taken as a unit, but on the atomic weight of oxygen, which is conditionally taken equal to 16. In relation to this weight, atomic weights are expressed in almost exact integers. But hydrogen itself in this system has an atomic weight not one, but somewhat more, namely 1.0078. This number differs from unity quite significantly - by 3/4%, which far exceeds all possible errors in determining the atomic weight.
It turned out that oxygen also has 3 isotopes: in addition to the predominant one, with an atomic weight of 16, another with an atomic weight of 17, and a third with an atomic weight of 18. If we refer all atomic weights to the isotope 16, then the atomic weight of hydrogen will still be slightly greater than unity. Then the second isotope of hydrogen was found - hydrogen with an atomic weight of 2 - deuterium, as the Americans who discovered it, or diplogen, as the British call it. Only about 1/6000 part of this deuterium is mixed in, and therefore the presence of this impurity has very little effect on the atomic weight of hydrogen.
Next to hydrogen, helium has an atomic weight of 4.002. If it were composed of 4 hydrogens, then its atomic weight would obviously have to be 4.031. Therefore, in this case we have some loss in atomic weight, namely: 4.031 - 4.002 = 0.029. Is it possible? As long as we did not consider mass as some measure of matter, of course, this was impossible: this would mean that part of the matter disappeared.
But the theory of relativity established with certainty that mass is not a measure of the amount of matter, but a measure of the energy that this matter possesses. Matter is measured not by mass, but by the number of charges that make up this matter. These charges can have more or less energy. When identical charges approach, the energy increases; when they move away, the energy decreases. But this, of course, does not mean that matter has changed.
When we say that 0.029 atomic weight disappeared during the formation of helium from 4 hydrogens, this means that the energy corresponding to this value disappeared. We know that each gram of matter has an energy equal to 9. 10 20 erg. With the formation of 4 g of helium, an energy equal to 0.029 is lost. 9 . 10 20 ergs. Due to this decrease in energy, 4 hydrogen nuclei will combine into a new nucleus. Excess energy will be released into the surrounding space, and a connection with a slightly lower energy and mass will remain. Thus, if atomic weights are measured not exactly, by integers 4 or 1, but by 4.002 and 1.0078, then it is these thousandths that acquire special significance, because they determine the energy released during the formation of the nucleus.
The more energy released during the formation of the nucleus, i.e., the greater the loss in atomic weight, the stronger the nucleus. In particular, the helium nucleus is very strong, because during its formation, energy is released, corresponding to a loss in atomic weight - 0.029. This is a very big energy. To judge it, it is best to remember this simple ratio: one thousandth of an atomic weight corresponds to about 1 million electron volts. So 0.029 is about 29 million electron volts. In order to destroy a helium nucleus, to decompose it back into 4 hydrogens, colossal energy is needed. The nucleus does not receive such energy, therefore the helium nucleus is extremely stable, and therefore it is precisely from radioactive nuclei that not hydrogen nuclei are released, but entire helium nuclei, alpha particles. These considerations lead us to a new assessment of atomic energy. We already know that almost all the energy of the atom is concentrated in the nucleus, and, moreover, the energy is enormous. 1 g of a substance has, if translated into a more graphic language, as much energy as can be obtained from burning 10 trains of 100 wagons of oil. Therefore, the core is a completely exceptional source of energy. Compare 1 g to 10 trains - this is the ratio of the concentration of energy in the core compared to the energy that we use in our technology.
However, if we think about the facts that we are now considering, we can, on the contrary, come to a completely opposite view of the nucleus. From this point of view, the nucleus is not a source of energy, but its graveyard: the nucleus is the residue after the release of a huge amount of energy, and in it we have the lowest state of energy.
Therefore, if we can talk about the possibility of using the energy of the nucleus, then only in the sense that, perhaps, not all nuclei have reached the lowest possible energy: after all, both hydrogen and helium both exist in nature, and, consequently, not all hydrogen combined into helium, although helium has less energy. If we could fuse the available hydrogen into helium, we would get a certain amount of energy. It's not 10 oil trains, but still it will be about 10 oil wagons. And it's not so bad if it were possible to get as much energy from 1 g of a substance as from burning 10 wagons of oil.
These are the possible reserves of energy in the rearrangement of nuclei. But the possibility is, of course, far from reality.
How can these possibilities be realized? In order to evaluate them, we turn to the consideration of the composition of the atomic nucleus.
We can now say that in all nuclei there are positive hydrogen nuclei, which are called protons, have a unit of atomic weight (more precisely 1.0078) and a unit positive charge. But the nucleus cannot consist of only protons. Take, for example, the heaviest element, 92nd on the periodic table, uranium, with an atomic weight of 238. If we assume that all these 238 units are made up of protons, then uranium would have 238 charges, while it has only 92. Consequently, either not all particles are charged there, or there, in addition to 238 protons, there are 146 negative electrons. Then everything is fine: the atomic weight would be 238, positive charges 238 and negative 146, therefore, the total charge is 92. But we have already established that the assumption of the presence of electrons in the nucleus is incompatible with our ideas: neither in size nor in magnetic properties of electrons in core cannot be placed. There was some contradiction.
DISCOVERY OF THE NEUTRON
This contradiction was destroyed by a new experimental fact, which was discovered about two years ago by Irene Curie and her husband Joliot (Irene Curie is the daughter of Marie Curie, who discovered radium). Irene Curie and Joliot discovered that when beryllium (the fourth element of the periodic system) is bombarded with alpha particles, beryllium emits some strange rays that penetrate through huge thicknesses of matter. It would seem that since they penetrate matter so easily, they should not cause any significant effects there, otherwise their energy would be depleted and they would not penetrate matter. On the other hand, it turns out that these rays, colliding with the nucleus of some atom, throw it away with tremendous force, as if by the impact of a heavy particle. So, on the one hand, one must think that these rays are heavy nuclei, and on the other hand, they are capable of passing through enormous thicknesses without exerting any influence.
The resolution of this contradiction was found in the fact that this particle is not charged. If the particle has no electric charge, then nothing will act on it, and it itself will not act on anything. Only when, during its movement, it bumps somewhere on the core, does it discard it.
Thus, new uncharged particles appeared - neutrons. It turned out that the mass of this particle is approximately the same as the mass of a hydrogen particle - 1.0065 (one thousandth less than a proton, therefore, its energy is approximately 1 million electron volts less). This particle is similar to a proton, but only devoid of a positive charge, it is neutral, it was called a neutron.
As soon as the existence of neutrons became clear, a completely different idea of the structure of the nucleus was proposed. It was first expressed by D. D. Ivanenko, and then developed, especially by Heisenberg, who received Nobel Prize last year. The nucleus can contain protons and neutrons. It could be assumed that the nucleus is composed only of protons and neutrons. Then the entire construction of the periodic system is presented in a completely different, but quite simple way. How, for example, should one imagine uranium? Its atomic weight is 238, that is, there are 238 particles. But some of them are protons, some are neutrons. Each proton has a positive charge, neutrons have no charge at all. If the charge of uranium is 92, then this means that 92 are protons, and everything else is neutrons. This idea has already led to a number of very remarkable successes, it immediately clarified a number of properties of the periodic system, which previously seemed completely mysterious. When there are few protons and neutrons, then, according to modern ideas wave mechanics, one should expect that the number of protons and neutrons in the nucleus is the same. Only the proton has a charge, and the number of protons gives the atomic number. And the atomic weight of an element is the sum of the weights of protons and neutrons, because both of them have a unit of atomic weight. On this basis, we can say that the atomic number is half the atomic weight.
Now there still remains one difficulty, one contradiction. This is a contradiction created by beta particles.
DISCOVERY OF THE POSITRON
We have come to the conclusion that there is nothing in the nucleus but a positively charged proton. But how then are negative electrons ejected from the nucleus, if there are no negative charges there at all? As you can see, we are in a difficult position.
Again, a new experimental fact, a new discovery, leads us out of it. This discovery was made, perhaps for the first time, by D. V. Skobeltsyn, who, having studied cosmic rays for a long time, found that among the charges emitted by cosmic rays, there are also positive light particles. But this discovery was so contrary to everything that had been firmly established that Skobeltsyn did not at first give his observations such an interpretation.
The next who discovered this phenomenon was the American physicist Andersen in Pasadena (California), and after him in England, in Rutherford's laboratory, Blackett. These are positive electrons or, as they are not very well called, positrons. What really are positive electrons can be most easily seen by their behavior in a magnetic field. In a magnetic field, electrons are deflected in one direction, and positrons in the other, and the direction of their deflection determines their sign.
At first, positrons were observed only during the passage of cosmic rays. More recently, the same Irene Curie and Joliot discovered a new remarkable phenomenon. It turned out that there is new type radioactivity, that the nuclei of aluminum, boron, magnesium, which are not radioactive in themselves, when bombarded by alpha rays, become radioactive. For 2 to 14 minutes, they continue to emit particles by themselves, and these particles are no longer alpha and beta rays, but positrons.
The theory of positrons was created much earlier than the positron itself was found. Dirac set himself the task of giving the equations of wave mechanics such a form that they would also satisfy the theory of relativity.
These Dirac equations, however, led to a very strange consequence. The mass enters them symmetrically, i.e., when the sign of the mass is reversed, the equations do not change. This symmetry of the equations with respect to mass allowed Dirac to predict the possibility of the existence of positive electrons.
At that time, no one observed positive electrons, and there was a strong belief that there were no positive electrons (one can judge this by the caution with which both Skobeltsyn and Andersen approached this issue), so Dirac's theory was rejected. Two years later, positive electrons were actually found, and, of course, they remembered Dirac's theory, which predicted their appearance.
"MATERIALIZATION" AND "ANNIHILATION"
This theory is associated with a number of unfounded interpretations that surround it from all sides. Here I would like to analyze the process of materialization, named so on the initiative of Madame Curie - the appearance of a pair of positive and negative electrons during the passage of gamma rays through matter. This experimental fact is interpreted as the transformation of electromagnetic energy into two particles of matter, which did not exist before. This fact, therefore, is interpreted as the creation and disappearance of matter under the influence of those other rays.
But if we take a closer look at what we actually observe, it is easy to see that such an interpretation of the appearance of pairs has no basis. In particular, in the work of Skobeltsyn it is perfectly clear that the appearance of a pair of charges under the influence of gamma rays does not occur at all in empty space, the appearance of pairs is always observed only in atoms. Consequently, here we are dealing not with the materialization of energy, not with the appearance of some new matter, but only with the separation of charges within the matter that already exists in the atom. Where was she? One must think that the process of splitting the positive and negative charges occurs not far from the nucleus, inside the atom, but not inside the nucleus (at a relatively not very large distance of 10 -10 -10 -11 cm, while the radius of the nucleus is 10 -12 -10 -13 cm ).
Exactly the same can be said about the reverse process of "matter annihilation" - the connection of a negative and positive electron with the release of one million electron volts of energy in the form of two quanta of electromagnetic gamma rays. And this process always takes place in the atom, apparently near its nucleus.
Here we come to the possibility of resolving the contradiction we have already noted, which results from the emission of beta rays of negative electrons by a nucleus, which, as we think, does not contain electrons.
Obviously, beta particles do not fly out of the nucleus, but thanks to the nucleus; due to the release of energy inside the nucleus, a process of splitting into positive and negative charges occurs near it, and the negative charge is ejected, and the positive one is drawn into the nucleus and binds with the neutron, forming a positive proton. This is the suggestion that has been made lately.
Here is what we know about the composition of the atomic nucleus.
CONCLUSION
In conclusion, let us say a few words about future prospects.
If in the study of atoms we have reached certain limits beyond which quantitative changes passed into new qualitative properties, then the laws of wave mechanics that we discovered in the atomic shell cease to operate at the boundaries of the atomic nucleus; very vague outlines of a new, even more general theory, in relation to which wave mechanics is only one side of the phenomenon, the other side of which is now beginning to open up - and begins, as always, with contradictions, are beginning to be felt in the core.
Work on the atomic nucleus also has another very interesting side, closely intertwined with the development of technology. The nucleus is very well protected by the Gamow barrier from external influences. If, not limited only to observing the decay of nuclei in radioactive processes, we would like to break through into the nucleus from the outside, rebuild it, then this would require an extremely powerful impact.
The kernel problem urgently requires further technology development, the transition from those voltages that have already been mastered by high-voltage technology, from voltages of several hundred thousand volts, to millions of volts. A new stage is also being created in technology. This work on the creation of new voltage sources, millions of volts, is now being carried out in all countries - both abroad and here, in particular in the Kharkov laboratory, which was the first to start this work, and at the Leningrad Institute of Physics and Technology, and in other places.
The kernel problem is one of the most actual problems of our time in physics; it must be worked on with extreme intensity and perseverance, and in this work it is necessary to have great courage of thought. In my exposition, I have indicated several cases when, in passing to new standards, we became convinced that our logical habits, all our ideas built on limited experience, are not suitable for new phenomena and new standards. It is necessary to overcome this conservatism of common sense inherent in each of us. Common sense is a concentrated experience of the past; it cannot be expected that this experience will fully embrace the future. In the area of the nucleus, more than in any other, one has to constantly keep in mind the possibility of new qualitative properties and not be afraid of them. It seems to me that it is precisely here that the power of the dialectical method, devoid of this conservatism of the method, which also predicted the entire course of development, should be felt. modern physics. Of course, by the dialectical method I mean here not the totality of phrases taken from Engels. Not his words, but their meaning must be transferred to our work; only one dialectical method can take us forward in such a completely new and advanced area as the kernel problem.
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 late XIX centuries, the properties of a discharge tube through which a current is passed have been 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 of the scientist was prompted by an important conclusion of previous studies of the structure of matter: the masses of all chemical elements divided without remainder 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 well known. 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. heavy elements the number of neutrons is much greater.
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.
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Proton-electron theory
By the beginning of $1932$, only three elementary particles were known: electron, proton and neutron. For this reason, it was assumed that the nucleus of an atom consists of protons and electrons (proton-electron hypothesis). It was believed that the composition of the nucleus with number $Z$ in Mendeleev's periodic system of elements and mass number $A$ includes $A$ protons and $Z-A$ neutrons. In accordance with this hypothesis, the electrons that were part of the nucleus acted as a “cementing” agent, with the help of which positively charged protons were retained in the nucleus. Supporters of the proton-electron hypothesis of the composition of the atomic nucleus believed that $\beta ^-$ - radioactivity - is a confirmation of the correctness of the hypothesis. But this hypothesis was not able to explain the results of the experiment and was discarded. One of these difficulties was the impossibility to explain the fact that the spin of the nitrogen nucleus $^(14)_7N$ is equal to the unit $(\hbar)$. According to the proton-electron hypothesis, the $^(14)_7N$ nitrogen nucleus should consist of $14$ protons and $7$ electrons. The spin of protons and electrons is equal to $1/2$. For this reason, the nucleus of the nitrogen atom, which according to this hypothesis consists of $21$ particles, must have spin $1/2,\ 3/2,\ 5/2,\dots 21/2$. This discrepancy between the proton-electron theory is called the "nitrogen catastrophe". It was also incomprehensible that in the presence of electrons in the nucleus, its magnetic moment has a small magnetic moment compared to the magnetic moment of the electron.
In $1932$, J. Chadwick discovered the neutron. After this discovery, D. D. Ivanenko and E. G. Gapon put forward a hypothesis about the proton-neutron structure of the atomic nucleus, which was developed in detail by V. Heisenberg.
Remark 1
The proton-neutron composition of the nucleus is confirmed not only by theoretical conclusions, but also directly by experiments on the splitting of the nucleus into protons and neutrons. It is now generally accepted that the atomic nucleus consists of protons and neutrons, which are also called nucleons(from Latin nucleus kernel, grain).
The structure of the atomic nucleus
Nucleus is the central part of the atom, in which the positive electric charge and the main part of the mass of the atom are concentrated. The dimensions of the nucleus, in comparison with the orbits of electrons, are extremely small: $10^(-15)-10^(-14)\ m$. Nuclei are made up of protons and neutrons, which are almost identical in mass, but only the proton carries an electric charge. The total number of protons is called the atomic number $Z$ of the atom, which is the same as the number of electrons in the neutral atom. Nucleons are held in the nucleus by large forces, by their nature these forces are neither electrical nor gravitational, and in magnitude they are much greater than the forces that bind electrons to the nucleus.
According to the proton-neutron model of the structure of the nucleus:
- the nuclei of all chemical elements consist of nucleons;
- the charge of the nucleus is due only to protons;
- the number of protons in the nucleus is equal to the ordinal number of the element;
- the number of neutrons is equal to the difference between the mass number and the number of protons ($N=A-Z$)
A proton ($^2_1H\ or\ p$) is a positively charged particle: its charge is equal to the charge of an electron $e=1.6\cdot 10^(-19)\ Cl$, and its rest mass is $m_p=1.627\cdot 10^( -27)\kg$. The proton is the nucleus of the nucleon of the hydrogen atom.
To simplify records and calculations, the mass of the nucleus is often determined in atomic mass units (a.m.u.) or in units of energy (by writing down the corresponding energy $E=mc^2$ instead of mass in electron volts). The atomic mass unit is $1/12$ of the mass of the carbon nuclide $^(12)_6C$. In these units we get:
A proton, like an electron, has its own angular momentum - spin, which is equal to $1/2$ (in units of $\hbar $). The latter, in an external magnetic field, can orient only in such a way that its projection and field directions are equal to $+1/2$ or $-1/2$. The proton, like the electron, is subject to Fermi-Dirac quantum statistics, i.e. belongs to fermions.
The proton is characterized by its own magnetic moment, which for a particle with spin $1/2$, charge $e$ and mass $m$ is equal to
For an electron, its own magnetic moment is equal to
To describe the magnetism of nucleons and nuclei, the nuclear magneton is used ($1836$ times smaller than the Bohr magneton):
At first, it was believed that the magnetic moment of the proton is equal to the nuclear magneton, because. its mass is $1836$ times the mass of an electron. But the measurements showed that in fact the intrinsic magnetic moment of the proton is $2.79$ times greater than that of the nuclear magnetron, has a positive sign, i.e. direction coincides with the spin.
Modern physics explains these disagreements by the fact that protons and neutrons are mutually transformed and for some time remain in a state of dissociation into $\pi ^\pm $ - a meson and another nucleon of the corresponding sign:
The rest mass of the $\pi ^\pm $ - meson is $193.63$ MeV, so its own magnetic moment is $6.6$ times greater than the nuclear magneton. Some effective value of the magnetic moment of the proton and $\pi ^+$ -- of the meson environment appears in the measurements.
Neutron ($n$) -- electrically neutral particle; its rest mass
Although the neutron is devoid of charge, it has a magnetic moment $\mu _n=-1.91\mu _Я$. The "$-$" sign shows that behind the direction the magnetic moment is opposite to the spin of the proton. The magnetism of the neutron is determined by the effective value of the magnetic moment of the particles into which it is able to dissociate.
In the free state, the neutron is an unstable particle and randomly decays (half-life $12$ min): emitting a $\beta $ -- particle and an antineutrino, it turns into a proton. The neutron decay scheme is written in the following form:
In contrast to the intranuclear decay of the $\beta $ neutron -- decay belongs to both internal decay and elementary particle physics.
The mutual transformation of the neutron and proton, the equality of spins, the approximation of masses and properties give grounds to assume that we are talking about two varieties of the same nuclear particle - the nucleon. The proton-neutron theory agrees well with experimental data.
As constituents of the nucleus, protons and neutrons are found in numerous fission and fusion reactions.
In arbitrary and piece fission of nuclei, flows of electrons, positrons, mesons, neutrinos and antineutrinos are also observed. The mass $\beta $ of a particle (electron or positron) is $1836$ times less than the mass of a nucleon. Mesons - positive, negative and zero particles - occupy an intermediate place in mass between $\beta $ - particles and nucleons; the lifetime of such particles is very short and amounts to millionths of a second. Neutrinos and antineutrinos are elementary particles whose rest mass is zero. However, electrons, positrons and mesons cannot be constituents of the nucleus. These light particles cannot be localized in a small volume, which is a nucleus with radius $\sim 10^(-15)\ m$.
To prove this, we define the energy of the electrical interaction (for example, an electron with a positron or proton in the nucleus)
and compare it with the self-energy of the electron
Since the energy of the external interaction exceeds the electron's own energy, it cannot exist and retain its own individuality; under the conditions of the nucleus, it will be destroyed. Another situation with nucleons, their own energy is more than $900$ MeV, so they can retain their features in the nucleus.
Light particles are emitted from nuclei in the process of their transition from one state to another.
