Structure and energy properties of solid crystals. Relationship between spectral lines and electrons (Bohr's discovery)
13.7. Hydrogen atom (hydrogen-like atom) according to Bohr's theory
13.7.2. Emission and absorption of light by an atom
According to Bohr's second postulate(to the frequency rule) light emission occurs during the quantum transition of an atom from a stationary state with a higher energy E m to a stationary state with a lower energy E n ; the energy of the emitted photon is equal to the difference between the energies of the stationary states:
E γ = E m − E n .
The transition of an electron from one energy level to another is always accompanied by the emission or absorption of a photon of the corresponding frequency.
When an electron passes from a stationary state with a higher energy to a stationary state with a lower energy, the atom emits (Fig. 13.17) a photon with a frequency
ν = E 2 − E 1 h ,
where E 1 is the energy of a state with a lower energy (final); E 2 - energy of the state with higher energy (initial); h is Planck's constant, h = 6.626 ⋅ 10 −34 J ⋅ s.
The transition of an electron from a higher energy level to a lower one is accompanied by photon radiation
Rice. 13.17During the transition of an electron from a stationary state with a lower energy to a stationary state with a higher energy, the atom absorbs (Fig. 13.18) a photon with a frequency
ν = E 2 − E 1 h ,
where E 1 is the energy of a state with a lower energy (initial); E 2 - energy of the state with higher energy (final); h is Planck's constant, h = 6.626 ⋅ 10 −34 J ⋅ s.
The transition of an electron from a lower energy level to a higher one is accompanied by photon absorption with an energy equal to the difference between the energies of the indicated energy levels.
For the processes of emission or absorption of a photon during electron transitions between energy levels, it is convenient to use law of conservation of energy:
E 2 \u003d E γ + E 1,
where E 1 is the energy of the lower energy level (with lower energy); E 2 - energy of the upper energy level (with more energy); E γ is the energy of a photon (either emitted or absorbed).
The set of frequencies of emitted or absorbed photons during the transitions of an atom from one stationary state to another is determined by the formula
v = | E n − E m | h ,
where E n , E m are the energies of the electron levels in the atom between which the transition takes place; h is Planck's constant, h = 6.626 ⋅ 10 −34 J ⋅ s.
The set of possible discrete frequencies of photons absorbed or emitted by an atom forms line spectrum atom.
Example 20. The energy of the ground state of an electron in some atom is −28.4 eV. An electron in an atom is in an excited state with an energy of −11.6 eV and, emitting a photon, passes into the ground state. Find the wavelength of the emitted photon.
Solution . During the transition of an electron from a stationary state with a higher energy to a stationary state with a lower energy, the atom emits a photon, the energy of which is determined by the difference in the energies of these states:
E γ = E 2 − E 1 ,
where E 1 is the energy of the ground state, E 1 = −28.4 eV; E 2 is the energy of the excited state, E 2 = −11.6 eV.
The energy of the emitted photon is determined by the formula
E γ = h c λ ,
where h is Planck's constant, h = 6.63 ⋅ 10 −34 J ⋅ s; c is the speed of light in vacuum, c = 3.00 ⋅ 10 8 m/s; λ is the wavelength of the photon.
In explicit form, the law of conservation of energy for the considered transition is written as follows:
h c λ = E 2 − E 1 .
Let us express the wavelength of the emitted photon from here:
λ = h c E 2 − E 1
and calculate:
λ = 6.63 ⋅ 10 − 34 ⋅ 3.00 ⋅ 10 8 (− 11.6 − (− 28.4)) ⋅ 1.6 ⋅ 10 − 19 = 74.0 ⋅ 10 − 9 m = 74.0 nm.
The wavelength of the emitted photon is 74 nm.
Answers to colloquium №2 in physics.
Energy levels of atoms and molecules. Quantum transitions in atoms and molecules. Absorption and emission of energy by atoms and molecules, absorption and emission spectra.
Electronic energy levels in an atom Electrons in the unexcited state fill the lower energy. levels, and the upper levels are free. If an atom receives energy as a result of a collision with other atoms or by absorbing a quantum of light, then it goes into an excited state and any electron of the atom goes from the lower level to one of the upper ones. After a short period of time, it returns to the lower level, emitting a quantum of light of a certain frequency.
Molecules consist of interacting atoms, intramolecular motion is more complicated than intraatomic. In a molecule, in addition to the movement of electrons relative to nuclei, oscillating motion atoms around their equilibrium position and the rotational motion of the molecule as a whole. The electronic, vibrational and rotational motion of a molecule corresponds to three types of energy levels: Eel, Ekol, Heb. total energy molecule is equal to: E \u003d Eel + Eur + Ekol. According to quantum mechanics, the vibrational and rotational energy of molecules, as well as electronic energy, have a quantum character, i.e. change discretely. The distance between vibrational levels is much smaller than between electronic ones. Therefore, each electronic level of a molecule breaks up into a number of vibrational levels (sublevels). In turn, each vibrational level of the molecule corresponds to a number of rotational sublevels, the distance between which is even smaller than between the vibrational levels.
Stationary energy. state: no energy is emitted or absorbed. In quantum transitions, atoms and molecules jumps from one stationary state to another, from one energy level to another. This is due to the energy transitions of electrons.
When moving from higher energy levels to lower ones, an atom or molecule gives off energy, otherwise it absorbs. An atom in its ground state can only absorb energy.
The energy emitted by atoms or molecules forms the emission spectrum, and the absorbed energy forms the absorption spectrum.
At transition of an electron in an atom energy is released or absorbed in the form of a quantum of EM radiation. The atomic emission and absorption spectra are line-like.
Transition in molecules:
As a result of the fact that the electronic levels in a molecule break up into vibrational and rotational sublevels, the number of possible energy transitions increases significantly compared to atoms. Therefore, the molecular spectra become more complicated, they have a continuous character.
∆E >>∆Epol.> ∆Eur.
∆Ee ~ 1-3 EV λ~ 0.5 μV
∆Epol. ~ 10 -2 -10 -1 EP λ~1-100mkV
∆Heb. ~ 10 3 -10 -5 V λ~100-1000mkV
energy absorption of a molecule
2- radiation of the energy of an atom on a molecule
Luminescence, emission and absorption spectra, Stokes rule. The use of luminescence in biophysics and medicine.
Luminescence- radiation, which is an excess over thermal radiation at a given temperature, and having a period t (10 -15) greater than the period of the light wave. Luminescence occurs at any temperature. Luminescent substances glow without heating. It is never balanced.
According to the type of excitation, luminescence is:
1) Photoluminescence - excitation by light;
2) Electroluminescence - excitation electric field;
3) Chemiluminescence (excitation by means of a chemical reaction).
The impact of these sources leads to the excitation of atoms, molecules or ions of the luminescent substance. Radiation occurs as a result of quantum transitions of particles of matter from excited states to the ground (or less excited)
According to the duration of the glow, photoluminescence is divided into:
Fluorescence - short-term afterglow
Phosphorescence - a relatively long afterglow
The luminescence spectrum is continuous.
Basic characteristics of luminescence.
1) quantum yield:
ζ= 100% N(emission)/N(absorption),
where N(radiation) is the number of quanta that gave light,
N(absorb) – number of absorbed quanta.
2) D is the optical density of the sample.
D=lgI 0 /I λ =Esl, where E is the molecular absorption index
3) D= f(λ ) is the absorption spectrum
I λ = f(λ ) is the emission spectrum
Stokes' rule:
The absorption spectrum of a given substance is shifted in relation to the emission spectrum towards shorter wavelengths.
The use of luminescence in biophysics and medicine:
Photoluminescence:
Detection of the initial stage of food spoilage
Sorting of pharmacological preparations
Diagnosis of certain diseases (glow of hair, scales, nails when diagnosing their damage by a fungus or lichen)
Based on photoluminescence, light sources have been created whose spectrum is more similar to daylight than that of incandescent lamps (fluorescent lamps)
Chemiluminescence - application in diagnostics
The use of special fluorescent molecules added to membrane systems from the outside. Such molecules are called fluorescent probes or labels. Changing them makes it possible to detect conformational rearrangements in proteins in membranes.
induced emission. Inverse population of levels. Optical quantum generators (lasers). Properties laser radiation and its use in medicine.
stimulated emission is the forced transition of the excited particle to the lower level. In this case, the number of transitions per second depends on the number of photons entering the substance during the same time, i.e. from the intensity of the light. In addition, forced transitions will be determined by the population of the corresponding energy states. population inversion- such a state of the medium in which the number of particles at one of the upper levels is greater than chm at the lower. The inverse population of the levels is ensured by the fact that electrons can stay at the metastable level 10 -5 times longer than at the excited level.
Vv 1960 the first quantum generator of the visible range of radiation - laser with rubies as a working substance. It creates pulsed radiation with a wavelength of 694.3 nm.
The principle of operation of the laser is similar to luminescence.
Al 2 O 3 + Cr 2 O 3 - ruby
Properties of laser radiation
Laser radiation is always monochromatic
Polarization
The rays do not diverge, strictly parallel
You can get very high intensities.
Laser application:
Identification of DNA and proteins.
Ophthalmology
Treatment of trophic ulcers, malignant tumors
Lasers operating in continuous mode are used for operations on abundantly supplied organs.
X-ray radiation: characteristic and bremsstrahlung; radiation mechanism, spectra of characteristic and bremsstrahlung X-rays.
x-ray radiation is an electromagnetic wave in the range from 80 to 10 -4 nm. Long-wave X-ray radiation is overlapped by short-wave UV radiation, short-wave by long-wave γ-radiation. According to the method of excitation, X-ray radiation is divided into bremsstrahlung and characteristic
brakeRe-radiation.
The radiation that occurs when an electron decelerates in the anode material is called bremsstrahlung X-rays. When electrons slow down, only part of the energy goes to create a photon, the other part is spent on heating the anode. Since the ratio between these parts is random, when a large number of electrons decelerate, a continuous spectrum of X-ray radiation is formed.
Bremsstrahlung spectrum: (1) - at various voltages in x-ray tube)
X-ray flux is measured by the formula:
Ф=КUа-кIZ 2), where U, I - voltage and current in the X-ray tube, Z - serial number of the atomic substance of the anode, k=10 -9 V -1 - - coefficient of proportionality.
Ф=- λ min ˜ ∫Ф λ d λ
If the cathode filament temperature is increased, then the electron emission and the current in the tube will increase. This will increase the number of X-ray photons emitted every second. But its spectral composition does not change.
2) Characteristic x-ray radiation.
Increasing the voltage on the X-ray tube leads to the fact that against the background of a continuous spectrum, a line appears, which corresponds to the characteristic X-ray radiation. Electrons accelerated by a high voltage penetrate deep into the atom and knock electrons out of its inner layers. Electrons from upper levels pass to free places, as a result of which photons of characteristic radiation are emitted.
Characteristic x-ray spectrum atom does not depend on the chemical compound in which this atom is included. Spectrum:
Interaction of X-rays with matter (coherent scattering, photoelectric effect, Compton effect). The law of attenuation of the X-ray intensity. X-ray protection.
Interaction of X-ray radiation with matter
1)
Coherent scattering– scattering of long-wavelength X-rays Condition: hn hn 1 = hn 2, where n is the frequency. 2)
photoelectric effect. hn≥A and. In this case, the X-ray quantum is absorbed, and the electron is detached from the atom. The detached electron acquires kinetic energy. If it is large, then the electron can ionize neighboring atoms by collision. removal of an electron - the process of ionization hn \u003d Eion + m 0 v 2 / 2 3) Compton effect hn>>Ai. In this case, the electron breaks away from the atom and acquires some kinetic energy. The energy of the photon itself decreases. That. as a result of incoherent scattering (Compton effect), secondary scattered radiation is formed and ionization of the atoms of matter occurs. hn \u003d Eion + mv 2 / 2 + hn The law of attenuation of the X-ray intensity: Ix \u003d I 0 e -µx µ=µ p +µ p the contribution of each process term is determined by the photon energy. photoelectric effect Compton effect 6. The device of the X-ray tube and X-ray apparatus. Rigidity and intensity of radiation, their adjustment. Method for obtaining x-rays. The use of x-rays in medicine. X-ray tube device: The tube is a glass flask (with possibly high vacuum), with two electrodes: an anode and a cathode, to which a high voltage is applied. The cathode is the source of electrons. Anode is a metal rod inclined surface in order to direct the emerging x-rays at an angle to the axis of the tube. Under the action of high voltage, the electrons emitted by the hot cathode filament are accelerated to high energy. Kinetic energy acquired by an electron is equal to the work of electric forces and is proportional to the voltage between the cathode and anode. X-ray radiation arises as a result of intense deceleration of fast electrons in the anode material upon collision with its atoms (interaction with an electric field atomic nucleus and electrons). Uа-к≈100∙10 3 V λmin= hc/ eUa-k 1) Short-wavelength X-rays usually have a greater penetrating power than long-wavelength ones and are called hard, and long-wave soft. Soft radiation is more strongly absorbed by matter. Increasing the voltage on the x-ray tube, change the spectral composition of the radiation and increase the rigidity. Getting x-rays. If an inhomogeneous body is placed in the path of X-rays and a fluorescent screen is placed in front of it, then this body, absorbing or attenuating the radiation, forms a shadow on the screen. By the nature of this shadow, one can judge the shape, density, structure, and in many cases the nature of bodies. those. a significant difference in the absorption of x-ray radiation by different tissues allows you to see the image of the internal organs in the shadow projection. µ1<µ2
I 2 < application of x-ray radiation in medicine: X-ray diagnostics: 1) Fluoroscopy (the X-ray tube is located behind the patient. A fluorescent screen is located in front of him. A shadow (positive) image is observed on the screen). 2) radiography (the object is placed on a cassette in which a film with a special photographic emulsion is inserted. The x-ray tube is located above the object. The resulting radiograph gives a negative image, i.e. the opposite in contrast to the picture observed during transmission. In this method, there is a greater image clarity A promising variant of this method is X-ray tomography and computed tomography). 3) Fluorography 4) X-ray therapy - the use of x-rays to destroy malignant tumors. 7. The principle of X-ray tomography. X-ray tomograph. His device. What are the main differences between an x-ray tomogram and an x-ray? 1) Re absorption µ - absorption coefficient, tissue property 2) Re-study Ia \u003d I 0 e -µ x 1 Iv \u003d I 0 e -µ2 x 2 Iv \u003d I 0 e -µ1x1 e -µ2x2 \u003d I 0 e - (µ1x1 + µ2x2) The purpose of the diagnosis. Determine µ3 and ∆µ3 lnI1/I 0 = -(µ1+µ2)∆х lnI2/I 0 = -(µ3+µ4)∆х lnI3/I 0 = -(µ3+µ1)∆х lnI4/I 0 = -(µ4+µ2)∆х I 0 - set I1,I2,I3,I4 - measured (known) ∆x - set (known) find µ1µ2µ3µ4 by solving a system of 4 equations 2048∙2048= 4194304 System of Radon's theorems If I take an infinite number of images of an object, then it is possible to restore it with any accuracy. Technical solution. stage: measurements I1,I2,I3,I4… stage: building an image To each value of µ, the PC assigns its own brightness (color) 1 slice - 1 second Contrast ∆µ/µ=0.1%(by 10%) Layer thickness - 1-2mm Resolution limit 0.2mm With tomography, it is possible to obtain a layered X-ray image of the body with details less than 2 mm. This allows you to distinguish between the gray and white matter of the brain and see very small tumor cells. education 8. Types of ionizing radiation. The action of ionizing radiation on matter. Absorbed, exposure and biological (equivalent) doses, the relationship between them. Units of doses in the SI system and non-systemic units used in medicine. In the process of radiation of the phenomenon of radioactivity, 3 types of rays emitted by radioactive nuclei were discovered. Alpha decay consists in the spontaneous transformation of the nucleus with the emission of α-particles (helium nucleus). A Z X → A-4 Z -2 Y+ 4 2 During α-decay, the daughter nucleus can be formed in an excited state. In this case, the energy of the excited nucleus is most often released in the form of a -quantum. Therefore, alpha decay is accompanied by radiation. beta decay consists in the spontaneous transformation of the nucleus with the emission of electrons (or positrons). A Z X→ A Z +1 Y + 0 -1 β +ν Where ν is the designation of the antineutrino particle. An electron is formed as a result of the intranuclear transformation of a neutron into a proton. Gamma radiation has an electromagnetic nature and is a photon with a wavelength of λ≤10 -10 m Radiation of this type accompanies not only -decay, but also more complex nuclear reactions. Interaction with matter: A charged particle passing through matter loses its energy due to ionization drag. At the same time, its kinetic energy is spent on the excitation and ionization of the atoms of the medium. For a quantitative characteristic of the interaction of a charged particle with a substance, the following quantities are used: linear ionization density, i, number of pairs of ions formed per unit path of the particle: i=dn/dl linear stopping power of a substance (S) is the energy lost by a charged particle per unit path: S=dE/dl the average linear range of a charged ionizing particle (R) is the distance between the beginning and the end of the particle range in a given substance. Characteristic features of the interaction of various types of radiation with matter: Alpha radiation: As the alpha particle advances in the medium, I first increases (with a decrease in speed, the time it spends near the molecule of the medium increases, so the probability of ionization increases), and at the end of the run (x = R) it sharply decreases, which is associated with a decrease in the speed of movement. When the energy of the particle becomes less than the energy required for ionization, the formation of ions stops. Graph of the dependence of the linear ionization density on the path traversed by the alpha particle in the medium: Ionization and excitation are primary processes. Secondary: increase in the speed of molecular thermal motion, characteristic X-ray radiation, radioluminescence, chemical processes. Beta radiation. Causes ionization, excitation, X-ray radiation (when electrons decelerate), characteristic Cherenkov radiation, when the speed of an electron in a medium exceeds the speed of light in this medium. Gamma radiation causes a slight primary ionization, coherent and incoherent scattering, an ionizing photoelectric effect, the formation of electron-positron pairs, photonuclear reactions due to the interaction of a quantum with a nucleus. Absorbed dose (D)
-
a value equal to the ratio of the energy ∆E transferred to the element of the irradiated substance to the mass m of this element: D=∆E /m. In C, the unit of absorbed dose is the gray (Gy). 1 Gy corresponds to the dose of radiation at which the energy of ionizing radiation of 1 J is transferred to the irradiated substance weighing 1 kg. Non-systemic unit 1rad=10 -2 Gy Exposure dose radiation (X) characterizes the ionizing effect of X-ray and γ-radiation in the air surrounding the irradiated body. The SI unit of exposure dose is C/kg. The SI unit of exposure dose is C/kg. 1C/kg corresponds to the exposure dose of photon radiation, at which ions with a charge of 1C of each sign are formed as a result of ionization of 1kg of dry air (n.c.). The unit of exposure dose rate is 1A/kg, and the off-system unit is 1R/s. Since the radiation dose is proportional to the incident ionizing radiation, there should be a proportional relationship between the radiated and exposure doses: D=fX, where f is some conversion factor depending on the irradiated substance and photon energy. Equivalent dose - (N) used to assess the effect of ionizing radiation on biological objects; it has the same dimension as the absorbed radiation dose, but the name is different. In SI: Sievert [Sv], 1Sv=1J/kg Off-system unit: 1ber=10 -2 Sv. There is a relationship between exposure and absorbed doses: H=KD, where K is the quality factor (shows how many times the effectiveness of the biological action of a given type of radiation is greater than that of photon radiation at the same dose of radiation in tissues). 9. dose rate. Relationship between the exposure dose rate and the activity of a radioactive preparation. Dose rate is the value that determines the dose received by the object per unit of time. With a uniform effect of radiation, the dose rate N is numerically equal to the ratio of the dose D to the time t, during which the ionizing radiation acted: N=D/t. The unit of radiation dose rate is gray (Gy), which corresponds to the radiation dose at which ionizing radiation energy of 1J is transferred to an irradiated substance weighing 1kg; radiation dose rate is expressed in Gy/sec. The non-systemic unit of radiation dose is rad (1 rad=10 -2 Gy=100erg/g), its power is rad per second. The exposure dose of radiation (X) characterizes the ionizing effect of X-ray and γ-radiation in the air surrounding the irradiated body. The SI unit of exposure dose is C/kg. 1C/kg corresponds to the exposure dose of photon radiation, at which ions with a charge of 1C of each sign are formed as a result of ionization of 1kg of dry air (n.c.). The unit of exposure dose rate is 1A/kg, and the off-system unit is 1R/s. Since the radiation dose is proportional to the incident ionizing radiation, there should be a proportional relationship between the radiated and exposure doses: D=fX, where f is some conversion factor depending on the irradiated substance and photon energy. For water and human soft tissues, f=1, therefore, the radiation dose in rads is numerically equal to the corresponding exposure dose in roentgens. Relationship between the exposure dose rate and the activity of the radioactive preparation: From the source, γ-photons fly out in all directions. The number of these photons penetrating 1m 2 of the surface of a certain sphere in 1s is proportional to the activity A and inversely proportional to the surface area of the sphere (4πr 2) / The exposure dose rate (X / t) in the volume V depends on the number of photons, since it is they that cause ionization: X /t=k γ A/r 2 Where k γ - which is typical for the given radionuclide. 10. Law of radioactive decay (conclusion). Law chart. artificial radioactivity. Tagged atom method, application in medicine. radioactive decay is a statistical phenomenon. Let dN nuclei decay in a short time interval dt. This number is proportional to the time interval dt, as well as to the total number N of radioactive nuclei: dN=-λNdt, where λ is the decay constant, which is proportional to the decay probability of a radioactive nucleus t, which is different for different radioactive nuclei, decreases with time. We integrate the resulting expression and obtain lnN/N 0 =-λt. N= N 0 e - λt . This is the basic law of radioactive decay: for an equal period of time, the same fraction of the initial number of nuclei decays. Processes of radioactive decay for two substances λ1>λ2. The interpretation of the information given by the spectra is carried out using the theory of the atom. Therefore, we briefly recall some properties of atoms and their radiation. An atom is a system of a nucleus and electrons held together by electrostatic attraction. The energy of an atom (more precisely, its electron shell) can only have certain values, which are called terms, or levels. The lowest energy state is called main, the rest - excited. The scheme of energy levels of the hydrogen atom is shown in fig. 4, where each dash represents a level (the lower one represents the main one). The distances of the levels from the ground state are proportional to the energy of the atom measured from the energy of the ground state. An atom can change its energy by going from one state to another. This is usually due to a change in the motion of the outermost electron. Transitions from higher levels to lower ones can occur spontaneously, without external influence, and the difference in energy is emitted with a "portion" of light radiation, called quantum. The frequency of the emitted light is proportional to the quantum energy. Since strictly defined energy changes correspond to different transitions, the atom emits quanta of only certain frequencies, which give rise to individual lines in the spectrum. Transitions "up", i.e., from the lower levels to the upper ones, cannot spontaneously occur; their implementation requires an external source of energy (for example, quanta of the appropriate frequency). Such a process is called takeover. The number of energy levels of an atom is infinite; they condense to a limit which corresponds to the energy at which the electron completely separates from the atom. The set of lines formed during transitions to a given level from all higher ones is called spectral series. So, hydrogen has a Lyman series corresponding to transitions to the first level, a Balmer series formed during transitions to the second level, etc. Since the first level is very far from the others, transitions to it are accompanied by a large change in energy, and the quanta of the Lyman series lie in the far ultraviolet region. For example, the line L α ;, formed during the transition from the second level to the first, has a wavelength of 1216Å 1, while the violet part of the spectrum corresponds to a wavelength of about 4000 Å. Lines of the Balmer series - H α (6563 Å), H β (4861 Å) and others, lie in the visible part of the spectrum. The rest of the hydrogen series are located in the infrared region. The energy levels of other elements are more complex. To represent them, you need to draw more than one column, as in Fig. 4, but several, with a common upper limit. 1 (Å (angstrom) - a unit of measurement of wavelengths in spectroscopy, equal to 10 -8 cm.) In most atoms, the energy levels are split into several closely spaced sublevels. Therefore, lines often consist of two or three components or more, as a result of which they are called doublets, triplets, etc., or in general - multiplets. Single lines are called singlets. In complex atoms, the level splitting can be very strong. The levels of the hydrogen atom also consist of two sublevels, but their energies are practically the same. Comparing the spectrum and the level scheme of an atom of any element, one can notice that the observed lines do not correspond to all transitions; some transitions don't happen at all. Such transitions are called prohibited. Allowed transitions correspond to strictly defined relationships between energy levels, called selection rules. It should be noted that transitions that violate the selection rules are not absolutely prohibited: under certain conditions, the corresponding lines can still be observed. The "degree" of forbiddenness is best characterized by the time that must elapse, on average, until an atom can spontaneously make a given transition. For an allowed transition, this time is usually 10 -7 - 10 -8 seconds. But in order to make a forbidden transition, the atom must be in the upper state much longer - from 10 -3 sec to several days or more, depending on the degree of prohibition. If there are no allowed "down" transitions from a certain level, then the atom under certain conditions can stay there for a relatively long time. Such levels are called metastable. For most elements and their ions, low levels close to the ground level are metastable. Forbidden lines are usually indicated by the element symbol in square brackets, with a Roman numeral indicating the degree of ionization. For example, [OH] means the forbidden line of ionized oxygen, a - the forbidden line of doubly ionized oxygen. Allowed lines do not have square brackets. In order for an atom to emit a quantum, it must be at one of the excited levels, i.e., it must be given an energy corresponding to the difference between the energies of the level under consideration and the initial one. This energy can be imparted to an atom either when it absorbs a quantum or when it collides with another particle, mainly an electron. In this case, the energy of the electron must be greater than the energy of the corresponding transition. The average particle energy is proportional to the absolute temperature of the gas. However, even if the average energy is insufficient for excitation, there are always electrons in the gas with the required energy, but their number rapidly decreases with increasing energy. Therefore, the number of excitations of a level whose energy is noticeably higher than the average energy will be relatively small 1 . 1 (The excitation frequency depends not only on the number of electrons whose energy is greater than the excitation threshold, but also on the properties of the atom itself, more precisely, on the probability of excitation of a given level of the atom by a fast electron.) The impact of an ion or a neutral atom can also transfer the atom to an excited state, but for this the energy of the ion must be hundreds or thousands of times greater than the energy of the level, because a heavy particle always transfers only a very small part of its energy to a light electron of an atom. Therefore, excitation by heavy particles is usually of no importance under astrophysical conditions, except in cases where the level energy is exceptionally small. If an electron collides with an excited atom, the latter can transfer its energy to it. Then the atom will go to a lower state without emitting a quantum, and the colliding electron will bounce off with increased energy. Such a process is called blow of the second kind, in contrast to the process of excitation of an atom during a collision ( hits of the first kind). Impacts of the second kind are essential in dense gases, where the time interval between collisions of atoms is less than the lifetime of an atom in an excited state. In this case, the atoms collide before they have time to radiate, so that impacts of the second kind significantly attenuate the radiation. For metastable levels, from which there are no allowed "down" transitions and where, therefore, the atom stays much longer, impacts of the second kind can be significant even at low density. That is why we do not observe lines corresponding to forbidden transitions in the laboratory: the excitation usually ends with a second-order impact, and not with radiation. In a very rarefied gas, the interval between collisions can become comparable to the time of existence in a metastable state, and forbidden lines will appear. As will be seen below, it is these conditions that are realized in nebulae. If an electron acquires more energy than the limit of terms, it will break away from the atom. Such a process is called ionization. Ionization of an atom can occur both when a quantum is absorbed and when it collides with a fast electron. A free electron that is not bound to an atom can have any energy. Therefore, in contrast to transitions within an atom, where quanta of only a certain frequency can be absorbed, ionization can be produced by any quantum whose energy is greater than that required for the transition from the initial level to the ionization energy. For example, a hydrogen atom located at the first level can absorb any quantum with a wavelength of less than 912 A, and from the second level - a quantum with a wavelength of less than 3646 A. If the quantum energy is greater than necessary to detach an electron, then the excess energy passes into the kinetic energy of the ejected electron. Impact ionization is possible only if the energy of the impacting electron is greater than that required for detachment. The higher the gas temperature, the greater the number of electrons has the required energy, the more often ionizations occur, and the greater, other things being equal, the proportion of ionized atoms. A heavy particle can ionize an atom only at high energies, much higher than the ionization energy. If the gas has a very high temperature or is in a strong radiation field, then before it has time to catch the electron back, a second, third electron, etc., can be torn off from it. repeatedly ionized atoms. However, the detachment of each subsequent electron requires more and more energy, as the charge of the ion increases. Therefore, multiple ionization requires a very high temperature of the gas or source of ionizing radiation. The reverse process of ionization is called recombination. It represents the capture of an electron by an ion. An electron can be captured by an atom to any level. Therefore, an atom formed as a result of recombination can be either excited or unexcited. During recombination, an energy equal to the difference between the energies of a free and bound electron should be released, usually in the form of a quantum. If an electron with low energy recombines, then the frequency of the quantum is close to the limit of the series of the corresponding level. The greater the energy of the recombining electron, the greater the frequency of the emitted quantum. Since free electrons can have any energy, the spectrum emitted during recombinations has the form of a continuous band, starting immediately after the series limit and stretching, gradually weakening, towards higher frequencies. Such a spectrum is called continuum. The weakening of the brightness is caused by the fact that the fraction of fast electrons always decreases with increasing energy. The higher the gas temperature, the greater the average energy of recombining electrons and the slower the intensity decreases beyond the series limit. We have already said above that an atom is capable of emitting light not only during recombination, but also during transitions from one energy level to another. It turns out that a free electron, moving in the electric field of an ion, is also capable of emitting radiation. A free electron moves in the field of an ion along a hyperbolic orbit, and different orbits correspond to different energies. Just as a quantum is emitted or absorbed during the transition of an electron bound in an atom from one elliptical orbit to another, a quantum can be emitted or absorbed when an electron passes from one hyperbolic orbit to another. Such a process called a free-to-free transition. Since the energy of both orbits can have any value, emission and absorption during free-free transitions occur in a continuous spectrum. The change in energy during free-free transitions is usually a small fraction of the total energy of the electron, so that the energy of the emitted photons is less than the average energy of the electrons. At a not very high temperature, the free-free radiation of the gas is concentrated in the region of long waves - from infrared rays to radio waves. So far we have been talking about the emission and absorption of light by individual atoms. In reality, radiation from a whole layer of gas is always observed. If the layer of gas is transparent at all frequencies, the radiation of individual atoms will simply sum up. It will consist of bright lines against the background of a less bright continuous spectrum formed during recombinations to different levels and during free-to-free transitions. But if you increase the thickness of the gas layer, then part of the radiation will be absorbed by it. First of all, the quanta forming the lines will be absorbed, and therefore the lines will be less contrasted against the background of the continuous spectrum. If the homogeneous layer is practically opaque at all frequencies, then the lines will disappear. The radiation will have a continuous spectrum depending only on the temperature, and the maximum intensity in this spectrum will shift towards short waves as the temperature of the gas layer increases. At the same time, the total amount of energy emitted by a unit surface will increase. The radiation of stars is complicated by the fact that the layer is not homogeneous - its temperature and density increase with depth. Therefore, there are dark lines in the spectrum of stars, and the overall course of intensity in it does not coincide with the spectrum of an opaque layer of gas. Why do gaseous elements absorb or emit radiation with fixed wavelengths, and not radiation with a continuous (continuous) spectrum? The answer to this question was one of Bohr's greatest achievements. He established a correspondence between the lines of the atomic spectrum and the energies of electrons in atoms. He argued that an electron in an atom cannot have arbitrary energy values in the range of continuous change, but must names, only certain fixed energy values. Bohr called these values of energy discrete, or quantum levels. To each such value of energy, Bohr assigned a certain number, which he called the quantum number. An electron can jump from one energy level to another, emitting or absorbing a certain, fixed amount of energy. This portion of energy is called a quantum of energy. An electron at its lowest energy level is considered to be in the ground state. Electrons at higher energy levels are considered to be in excited states. The transition of an electron to a higher energy level is called excitation. What exactly are these energy quanta that are absorbed or emitted during the transitions of electrons from one energy level to another? Bohr was aware of the results of Planck's and Einstein's studies, as well as the fact that, according to Einstein's postulate, light consists of photons. Bohr suggested that an electron can only jump from one energy level to another, higher one, while absorbing a photon of a strictly defined energy. Because the Niels Bohr (1885-1962), Nobel Prize in Physics 1922 Rice. 1.10. Electronic transitions between energy levels. The energy of a photon absorbed or emitted during an electronic transition is equal to the difference between the energies of those levels between which the transition occurs: . (The Greek letter A is used to denote the quantitative change in a property, or the difference between two of its values.) The energy difference is related to the frequency v of a photon by the equation , where A is Planck's constant. Thus, each transition is characterized by a well-defined frequency. This frequency corresponds to a specific line in the atomic spectrum. The energy of a photon is related to its wavelength (or frequency), which means that absorption can only occur if an electron absorbs a photon that has a well-defined frequency. Conversely, when an electron descends from a higher energy level to a lower energy level, it emits a photon that has a certain frequency. This frequency corresponds to a well-defined line in the spectrum. Thus, each line in the atomic spectrum corresponds to the jump of electrons from one specific energy level to another specific energy level (Fig. 1.10). Task number 1.
What parts of a plant cell are visible under a light school microscope?
1. Ribosomes 2. Mitochondria 3. Cell wall 4. Plasma membrane Explanation: large portions of cells are visible under a light microscope. Of the presented, such a large organelle is the cell wall (it is quite thick), and the plasma membrane is difficult to visually (with a small increase) isolate from the cell wall. The correct answer is 3. Task number 2.
A fragment of a DNA molecule, consisting of 12 nucleotides, stores information about
1. 4 amino acids in a protein molecule 2. 16 nucleotides of a tRNA molecule 3. 12 amino acids mRNA molecule 4. 24 nucleotides rRNA molecule Explanation: Three nucleotides of a DNA molecule code for one amino acid (this structure is called a triplet), so 12 nucleotides of DNA code for 4 amino acids in a protein molecule. The correct answer is 1. Task number 3.
The transition of electrons to a higher energy level occurs in the light phase in molecules
1. Chlorophyll 2. Water 3. Glucose 4. Carbon dioxide Explanation: during photosynthesis, it is the chlorophyll molecule that goes into an excited state, since it is from the pigment that the chain of biochemical reactions begins during photosynthesis. The correct answer is 1. Task number 4.
In the course of the individual development of an animal, a multicellular organism develops from a zygote by
1. Meiosis 2. Mitosis 3. Phylogeny 4. Gametogenesis Explanation: after the formation of a zygote, the zygote begins to split up precisely by mitosis (this is the process of division of somatic cells), that is, two diploid cells are formed from one diploid cell: two from one cell, 4 from two, 8 from four, etc. The correct answer is 2. Task number 5.
The peculiarity of asexual reproduction is that a new organism develops from
1. Male gamete 2. Unfertilized egg 3. Zygotes with a double set of chromosomes 4. Somatic cell with a double set of chromosomes Explanation: asexual reproduction is not associated with germ cells, therefore, from the presented answer options, asexual reproduction is possible only when the organism develops from a somatic cell with a double set of chromosomes. With vegetative reproduction, a daughter organism is obtained, identical to the parent. The correct answer is 4. Task number 6.
Determine the crossbreeding pattern if it is known that when a round-fruited tomato, whose genotype is unknown, is crossed with a pear-shaped tomato (recessive trait), 50% of the offspring have pear-shaped fruits. 1. AA x aa 2. Aa x aa 3. Aa x Aa 4. AA x AA Explanation: 50% of the offspring will have pear-shaped fruits only if a heterozygote is crossed with a homozygote for a recessive trait - Aa x aa. The correct answer is 2. Task number 7.
Independent divergence of homologous chromosomes in meiosis is the cause of variability
1. Genomic 2. Combinative 3. Chromosomal 4. Cytoplasmic Explanation: combinative variability - the variability that occurs when the parental genes are recombined. The reasons may be violations in: crossing over in the metaphase of meiosis, divergence of chromosomes in meiosis, fusion of germ cells. The correct answer is 2. Task number 8.
N.I. Vavilov, while studying the characteristics of the inheritance of traits of cultivated plants, substantiated the law 1. Homologous series in hereditary variability 2. Independent inheritance of non-allelic genes 3. Dominance of first generation hybrids 4. Sex-linked inheritance Explanation: N.I. Vavilov formulated the law of homological series, which reads as follows: due to the great similarity of their genotypes (almost identical sets of genes), closely related species have similar potential hereditary variability (similar mutations of identical genes); as the evolutionary-phylogenetic removal of the studied groups (taxa), due to the emerging genotypic differences, the parallelism of hereditary variability becomes less complete. Consequently, the basis of parallelisms in hereditary variability are mutations of homologous genes and genotype regions in representatives of different taxa, that is, truly homologous hereditary variability. However, even within the same species, outwardly similar characters can be caused by mutations in different genes; such phenotypic parallel mutations of various genes can, of course, also occur in different, but rather closely related, species. The correct answer is 1. Task number 9.
The kingdom of bacteria includes
1. Chlamydomonas 2. E. coli 3. Infusoria-shoe 4. Malarial Plasmodium Explanation: chlamydomonas, ciliate slipper and malarial plasmodium are protozoa, and Escherichia coli ( Escherichia coli - bacterium. The correct answer is 2. Task number 10.
Tuber, bulb is
1. Organs of soil respiration 2. Modified shoots 3. Generative organs 4. Rudimentary shoots Explanation: tuber and bulb (as well as, for example, rhizome) are modified shoots. The correct answer is 2. Task number 11.
Ferns, unlike flowering plants, reproduce by
1. Dispute 2. Roots 3. Rhizome 4. Budding Explanation: flowering plants reproduce by seeds, while ferns reproduce by spores. The correct answer is 1. Task number 12.
What class do arthropods have simple eyes and four pairs of walking legs?
1. Insects 2. Cephalopods 3. Shellfish 4. Arachnids Explanation: look at the picture and conclude that the correct answer is 4. Task number 13.
A high metabolic rate allows birds
1. Take care of offspring 2. Lay eggs in nests 3. Eat plant foods 4. Expend a lot of energy during the flight Explanation: a high metabolic rate is one of the adaptations for flying, so we choose to expend a lot of energy during the flight. The correct answer is 4. Task number 14.
If a person suffers from anemia, then in his blood, compared to the norm, the content of
1. Leukocytes 2. Red blood cells 3. Platelets 4. Fibrinogen Explanation: anemia (another name - anemia) is expressed in a reduced content of red blood cells - erythrocytes and / or hemoglobin. Symptoms of the disease: dizziness, increased fatigue, etc. due to insufficient supply of oxygen to cells. The correct answer is 2. Task number 15.
The primary breakdown of complex carbohydrates in the human body occurs in
1. Oral cavity under the action of saliva enzyme 2. Gastric cavities under the action of the enzyme pepsin 3. Liver cells that store glycogen 4. Pancreatic cells that produce hormones Explanation: an enzyme produced in the stomach and oral cavity - amylase begins the breakdown of complex carbohydrates - polymers in the oral cavity. The correct answer is 1. Task number 16.
The function of destroying foreign microorganisms in human blood is performed by
1. Neurons 2. Red blood cells 3. Epithelial cells 4. Lymphocytes Explanation: lymphocytes are immunity cells, that is, they fight foreign microorganisms in human blood. The correct answer is 4. Task number 17.
The impact of twilight light is converted by the human visual analyzer into nerve impulses in
1. The lens of the eye 2. The pupil of the iris 3. Retinal sticks 4. Sclera of the eyeball Explanation: receptors that perceive light signals at dusk and darkness are retinal rods. From them, the signal is transmitted to the central nervous system. The correct answer is 3. Task number 18.
Splinting a broken limb
1. Reduces bleeding 2. Reduces limb edema 3. Prevents the penetration of microorganisms into the fracture site 4. Prevents displacement of broken bones Explanation: a splint is placed on the broken limb to prevent displacement of the bones. The correct answer is 4. Task number 19.
The creative role of natural selection is manifested in
1. Strengthening intraspecific struggle 2. Development by organisms of new habitats 3. The emergence of new mutations 4. The emergence of new species Explanation: natural selection is one of the driving forces of evolution. The goal of evolution is to create the most adapted species in given conditions, therefore the creative role of natural selection is also manifested in the creation of new species. The correct answer is 4. Task number 20.
In modern biological science, a population is considered to be
1. The totality of organisms of one kingdom 2. Individuals forming a food chain 3. Individuals of different species forming a biocenosis 4. Individuals of the same species living in the same territory Explanation: population - a group of individuals of the same species living in the same territory and interbreeding freely. The correct answer is 4. Task number 21.
The phylogenetic series of the horse testifies to (about)
1. Reversibility of the process of evolution 2. Regular return to ancestral forms 3. Similarity of postembryonic development of organisms 4. Historical development of the modern look Explanation: Consider the phylogenetic series of the horse. As can be seen from the picture, the phylogenetic series of a species is a tracking of the stages of its historical development. The correct answer is 4. Task number 22.
Which of the following examples illustrates the competitive relationship between organisms?
1. Squirrel - woodpecker 2. Oak - white mushroom 3. Cow - bull tapeworm 4. Already - a frog Task number 23.
As a result of long-term biotic relations, predator-prey in the natural biocenosis is observed
1. Unregulated increase in the number of predators 2. Regular fluctuation in the abundance of both groups of organisms 3. Accumulation of mutant alleles in the gene pool of victims 4. Manifestation of dominant traits in a population of predators Explanation: with a long-term predator-prey relationship, a constant number of these two species is maintained, but due to changes in environmental conditions, population waves occur and they are gradually extinguished by the constancy of this system (that is, such a system is quite stable). The correct answer is 2. Task number 24.
Thanks to living matter in the biosphere, the circulation of substances
1. Open 2. Involves a lot of chemical elements 3. Increases the diversity of agrocenoses on Earth 4. Provides accumulation of inert gases in the atmosphere Explanation: thanks to living organisms (and especially microorganisms), in nature there are cycles of many elements (and complex substances - carbon dioxide, water), such as: carbon, hydrogen, oxygen, sulfur, and many others. others and even iron. The correct answer is 2. Task number 25.
Are the following statements about the forms of natural selection correct?
A. Stabilizing selection is manifested in conditions of a sudden change in the sex composition of the population.
B. Driving selection contributes to an increase in the number of individuals with an average value of the trait.
1. Only A is true 2. Only B is true 3. Both judgments are correct 4. Both judgments are wrong Explanation: stabilizing selection works only under constant environmental conditions and preserves individuals with an average value of the trait, A - incorrect. Driving selection retains individuals with deviating from the average value of the trait, B - incorrect. The correct answer is 4. Task number 26.
During spermatogenesis
1. Male sex cells are formed 2. Somatic cells are formed 3. Halving the number of chromosomes 4. Four gametes are formed 5. One egg is formed 6. Cells with a diploid set of chromosomes are formed Explanation: spermatogenesis - the process of formation of male germ cells, while four haploid germ cells are formed from the precursor (diploid) cell (that is, the chromosome set is halved). The correct answer is 1, 3, 4. Task number 27.
The systemic circulation in the human body
1. Begins in the left ventricle 2. Originates in the right ventricle 3. Saturated with oxygen in the alveoli of the lungs 4. Supplies organs and tissues with oxygen and nutrients 5. Ends in the right atrium 6. Brings blood to the left side of the heart Explanation: Consider the systemic circulation. As can be seen from the figure, a large circle originates in the left ventricle and carries blood (oxygen, hormones, etc.) to all cells of the body, and carbon dioxide takes it from the cells and carries it back to the heart, namely, to the right atrium. The correct answer is 1, 4, 5. Task number 28.
The processes leading to the formation of new species in nature include
1. Mitotic cell division 2. Jump-like mutation process 3. Modification variability 4. Geographic isolation 5. Asexual reproduction of individuals 6. Natural selection Explanation: various mutations lead to the formation of new species (this is preceded by their accumulation and distribution throughout the population), geographic isolation (it is in the absence of reproduction with individuals of other populations that a new species can appear) and natural selection is a directed process during which the most adapted survive an organism and, subsequently, a species that is maximally adapted to the given environmental conditions is formed. The correct answer is 2, 4, 6. Task number 29.
Establish a correspondence between the characteristics of autotrophic nutrition and its type.
Characteristic Type of autotrophic nutrition A. Oxidation energy is used 1. Photosynthesis inorganic substances 2. Chemosynthesis B. Energy source - sunlight B. Carried out in plant cells D. Oxidation of ammonia occurs D. Oxygen is released into the atmosphere Explanation: photosynthesis is the process of converting the energy of the Sun into the energy of chemical bonds, carried out by phototrophs. Chemosynthesis is the process of converting inorganic substances into organic substances using the energy of oxidation of chemical compounds. Therefore, photosynthesis refers to: the source of energy is sunlight, the process occurs in plant cells and the release of oxygen into the atmosphere. The remaining two provisions refer to chemosynthesis. The correct answer is 21121. Task number 30.
Establish a correspondence between the characteristic and the kingdom of organisms.
Characteristic Kingdom of organisms A. The cell wall contains chitin 1. Mushrooms B. Type of nutrition autotrophic 2. Plants B. Form organic substances from inorganic D. The reserve nutrient is starch. D. In natural systems they are decomposers E. The body is made up of mycelium Explanation: the composition of the cell wall of fungi includes chitin, they are heterotrophs (that is, they consume ready-made organic substances) and decomposers (decompose organic substances to inorganic ones), their body consists of mycelium. Plants are autotrophs (they form organic substances from inorganic substances, starch is the reserve substance. The correct answer is 122211. Task number 31.
Establish a correspondence between the sign of the regulation of functions in the human body and its mechanism.
Sign Mechanism of regulation A. Carried out by the endocrine system 1. Nervous B. Hormones spread 2. Humoral B. Delivered to organs by blood D. The speed of exposure is very high D. Based on bioelectrical phenomena Explanation: nervous regulation - regulation with the help of the nervous system, that is, it is carried out along the processes of nerve cells (with the help of electrical impulses), the speed of such regulation is very high. Humoral (hormonal) regulation - regulation with the help of biologically active substances - hormones that are delivered to the organs by the blood and produced by the glands of the endocrine system. The correct answer is 22211. Task number 32.
Establish a correspondence between the type of organisms and the direction of evolution that is characteristic of it.
Species Direction of evolution A. Gray rat 1. Biological progress B. Snow leopard 2. Biological regression B. Amur tiger G. Creeping wheatgrass D. Przewalski's horse E. common dandelion Explanation: biological progress - an increase in the fitness of individuals, accompanied by an increase in numbers, expansion of the range and intraspecific variability. Biological regression - a decrease in the fitness of individuals, accompanied by a decrease in numbers, a decrease in the range and further extinction of the species. The reduction in numbers is typical for the snow leopard, the Amur tiger and the Przewalski horse. The correct answer is 122121. Task number 33.
Indicate the sequence of processes of geographic speciation.
1. Distribution of a trait in a population 2. The appearance of mutations in new living conditions 3. Spatial isolation of populations 4. Selection of individuals with beneficial modifications 5. Formation of a new species Explanation: speciation begins with isolation, under such conditions, mutations begin to appear, among which only useful ones are selected, then these beneficial mutations spread among all individuals of the population. Geographic speciation ends, which is logical, with the formation of a new species. The correct answer is 32415. Task number 34.
What is the purpose of using yeast mushrooms in baking bread and bakery products? What process takes place? Explanation: when baking bread, yeast is used, since yeast carries out fermentation (oxygen-free respiration), releasing carbon dioxide, which helps to raise the dough (bubbles form in the dough), which subsequently gives the dough splendor. Task number 35.
What division and what phase is shown in the figure? Indicate the set of chromosomes (n), the number of DNA molecules (s) in this period. Justify the answer. Explanation: in the figure, pairs of homologous chromosomes are lined up at the equator of the cell and are ready to diverge (that is, this is the period before anaphase 1), and before anaphase 1, metaphase 1. This means that the diploid set in the cell is 2n (since we see pairs of chromosomes), and the number of molecules DNA - 4c, since each chromosome consists of two chromatids (that is, two DNA molecules). Task number 36.
Find errors in the given text. Indicate the numbers of sentences in which errors were made, correct them.
1. Mitochondria and plastids are two-membrane organelles. 2. Photosensitive pigments are located on the inner membrane of mitochondria and plastids. 3. Unlike plastids, mitochondria contain their own circular nucleic acid molecule. 4. The process of photosynthesis takes place in chloroplasts. 5. The main function of mitochondria is the synthesis of cell lipids. Explanation: 2 - there are no pigments on the mitochondrial membrane (since they do not carry out the process of photosynthesis). 3 - plastids also contain their own nucleic acid. 5 - the main function of mitochondria is the synthesis of ATP (universal energy molecule), and not lipids. Task number 37.
How is the neurohumoral regulation of the separation of gastric juice in the human body? Explain the answer. Explanation: nervous regulation is carried out in two ways: 1. With the help of conditioned reflexes: at the sight of a lemon (or food in general) or the sound of pots in the mouth, saliva begins to be released, that is, the body prepares for eating by releasing enzymes. 2. With irritation of the receptors of the oral cavity and stomach, that is, with the help of unconditioned reflexes. Humoral regulation is carried out only when nutrients enter the blood during absorption. Hormones begin to be released and carried to the cells of the body. Task number 38.
Specify what type criteria are indicated in the text.
All individuals of the agile lizard species have the same karyotype. In males of the quick lizard, the body color is green, while in females it is brown. These animals are most numerous in the tropical and subtropical zones of the globe. Explanation: description of the karyotype - genetic criterion. Description of body color (and differences between females and males) is a morphological criterion. The description of the range of the species is a geographical criterion. Task number 39.
tRNAs with anticodons CCA, GUU, and GAA were successively involved in protein synthesis. Determine the composition of the DNA and mRNA molecules. Explanation: Let's start writing the composition of the molecules in reverse order, that is, first tRNA, then mRNA, and finally DNA. tRNA: CCA GUU GAA mRNA: GGU CAA CUU DNA: CCA GTT GAA Task number 40.
For a crested (A) green (B) female, an analyzing cross was carried out, four phenotypic classes were obtained in the offspring. The resulting crested offspring were crossed among themselves. Is it possible to get offspring without a crest in this crossing? If so, what gender will it be, what phenotype? In canaries, the presence of a crest depends on an autosomal gene, the color of plumage (green or brown) depends on a gene linked to the X chromosome. The heterogametic sex in birds is female. Explanation:
First cross: R: AaHVU x aaHvHv G: AHV, AHV, AU, AU X AHV F1: АаХВХв - crested green male ааХВХв - green male without crest AaHvU - crested brown female We cross a male and a female with a crest: R: AaHVHv x AaHvU G: AHV, AHV, AHV, AHV x AHV, AU, AHV, AU F2: we get 16 genotypes, among which only 4 phenotypes can be distinguished. Phenotypes of individuals without crest: Females: ааХВУ - green female without crest ааХвУ - brown female without crest Males: ааХВХв - green male without crest ааХвХв - brown male without crest.
