Page 58. Questions and tasks after §
1. What substances are the main sources of energy in cells?
Carbohydrates and fats are used as the main energy material. For example, the complex carbohydrate glycogen and fats are the reserves of "fuel" in the cell. They are consumed by the cells after some periods of starvation of the body. For example, in the morning after sleep, there is an active use of fats, which first break down into glycerol and fatty acids. After eating, the main source of energy in cells is glucose obtained from food.
2. Describe each of the stages of energy metabolism.
Energy exchange takes place in three stages: preparatory anoxic, oxygen. Preparatory stage characterized by the fact that complex organic substances in the body are broken down into monomers. All these processes take place under the action of enzymes. So, proteins obtained with food are broken down into amino acids, carbohydrates - into glucose, fats - into glycerol and fatty acids. The energy released in this case is dissipated in the form of heat in the body, so its amount formed in this case is not large. Using glucose as an example, we can consider the second stage - anoxic - it is called glycolysis (from the Greek "glikis" - sweet, "lysis" - splitting). This is a complex enzymatic process for the breakdown of glucose. This process takes place in the cytoplasm of cells. From one molecule of glucose (1 mol C6H12O6) two molecules of pyruvic acid PVC (2C3H4O3) and two molecules of ATP (2ATP) are formed. Further, if there is not enough oxygen in the cell, pyruvic acid C3H4O3 turns into another organic acid - lactic C3H4O3 (since they are isomers). The next stage - oxygen - is called cellular respiration and takes place in the mitochondria of cells (on the cristae, where respiratory enzymes are located). By its name it is clear that it goes only with the participation of oxygen. At this stage, pyruvic acid is oxidized by molecular oxygen O2 to carbon dioxide and water. The energy released during this oxidation is used very efficiently. For every molecule of glucose, 36 ATP molecules are produced. Thus, during the breakdown of 1 molecule (1 mol) of glucose, 38 ATP are released (in the second stage, 2 molecules and in the third, 36 molecules). This energy is spent on the synthesis of substances needed by the body, and the energy of ATP is converted into different kinds energy - mechanical (movement of flagella), electrical (conduction of a nerve impulse).
3. Why do athletes have faster breathing and muscle pain during intensive training?
With intensive physical work of a person, muscle tissue cells experience oxygen starvation, in this case, with incomplete breakdown of glucose, PVC turns into lactic acid. Its excess accumulates in the muscles, which leads to muscle pain, fatigue, fatigue, shortness of breath - this is a sign of oxygen deficiency.
4. The yield of tomatoes grown in poorly ventilated greenhouses was not high. Explain why.
When growing cultivated plants in greenhouses and greenhouses, it must be remembered that the process of glucose oxidation goes to carbon dioxide and water, and at high temperatures it proceeds more intensively. In addition, only green plant cells carry out photosynthesis, while plant respiration occurs in all cells. In greenhouses, the temperature can reach up to 400C, while the intensity of respiration increases up to 100 times, but the intensity of photosynthesis does not. Therefore, the increase in organic mass is insignificant and the yield of such plants will be low.
5. Explain the meaning of the term "glycolysis", "cellular respiration".
Glycolysis (from the Greek "glikis" - sweet, "lysis" - splitting) is a complex enzymatic process of glucose breakdown, occurring in two stages - anoxic and oxygen. Cellular respiration is the final oxygen stage of glucose breakdown, which takes place in the mitochondria of cells (on the cristae, where respiratory enzymes are located), going in the presence of oxygen.
- Types of nutrition of living organisms
- Photosynthesis
- energy exchange
1. Vitality of all organisms is possible only if they have energy. According to the method of obtaining energy, all cells and organisms are divided into two groups: autotrophs and heterotrophs.
Heterotrophs(Greek heteros - different, different and trophe - food, nutrition) are not able to synthesize organic compounds from inorganic ones themselves, they need to get them from environment. Organic substances serve for them not only as food, but also as a source of energy. Heterotrophs include all animals, fungi, most bacteria, as well as chlorophyll-free land plants and algae.
Heterotrophic organisms are classified according to the way they obtain food. holozoic(animals) that capture solid particles, and osmotrophic(fungi, bacteria) that feed on dissolved substances.
Diverse heterotrophic organisms are capable of jointly decomposing all substances that are synthesized by autotrophs, as well as mineral substances synthesized as a result of human production activities. Heterotrophic organisms, together with autotrophs, constitute a single biological system on Earth, united by trophic relationships.
Autotrophs- organisms that feed (i.e., receive energy) at the expense of not organic compounds these are some bacteria and all green plants. Autotrophs are divided into chemotrophs and phototrophs.
Chemotrophs- organisms that use the energy released during redox reactions. Chemotrophs include nitrifying (nitrogen-fixing) bacteria, sulfur, hydrogen (methane-forming), manganese, iron-forming and carbon monoxide-using bacteria.
Phototrophs- only green plants. Light is their source of energy.
2. Photosynthesis(Greek phos - genus. fall. photos - light and synthesis - connection) - formation with the participation of light energy organic matter cells of green plants, as well as some bacteria, the process of converting light energy into chemical energy. Occurs with the help of pigments (chlorophyll and some others) in the thylakoids of chloroplasts and cell chromatophores. Photosynthesis is based on redox reactions, in which electrons are transferred from a donor-reductant (water, hydrogen, etc.) to an acceptor (Latin acceptor - receiver) - carbon dioxide, acetate with the formation of reduced compounds - carbohydrates and the release of oxygen, if water is oxidized.
Photosynthetic bacteria that use donors other than water do not release oxygen.
Light reactions of photosynthesis(caused by light) flow in the grana of thylakoids of chloroplasts. Visible light quanta (photons) interact with chlorophyll molecules, transferring them to an excited state. An electron in the composition of chlorophyll absorbs a quantum of light of a certain length and, like steps, moves along the chain of electron carriers, losing energy, which serves to phosphorylate ADP into ATP. This is a very efficient process: 30 times more ATP is produced in chloroplasts than in the mitochondria of the same plants. This accumulates the energy necessary for the following - the dark reactions of photosynthesis. Substances act as electron carriers: cytochromes, plastoquinone, ferredoxin, flavoprotein, reductase, etc. Some of the excited electrons are used to reduce NADP + to NADPH. Under the action of sunlight in chloroplasts, water is split - photolysis, in this case, electrons are formed that compensate for their loss by chlorophyll; as a by-product, oxygen is released into the atmosphere of our planet. This is the oxygen that we breathe and which is necessary for all aerobic organisms.
The chloroplasts of higher plants, algae, and cyanobacteria contain two photosystems of different structure and composition. When light quanta are absorbed by pigments (a reaction center - a complex of chlorophyll with a protein that absorbs light with a wavelength of 680 nm - P680) of photosystem II, electrons are transferred from water to an intermediate acceptor and through a chain of carriers to the reaction center of photosystem I. And this photosystem is a reaction center will reveal the foam of the chlorophyll molecule in combination with a special protein-KOM, which absorbs light with a wavelength of 700 nm - P700. In the molecules of chlorophyll F1 there are "holes" - unfilled places of electrons that have passed into PLDPH. These "holes" are filled with electrons formed during the functioning of the FI. That is, photosystem II supplies electrons for photosystem I, which are spent in it for the reduction of NADP + and NADPH. Along the path of movement of photosystem II electrons excited by light to the final acceptor - chlorophyll of photosystem I, ADP is phosphorylated into energy-rich ATP. Thus, the energy of light is stored in ATP molecules and is further consumed for the synthesis of carbohydrates, proteins, nucleic acids and other vital processes of plants, and through them the vital activity of all organisms that feed on plants.
Dark reactions, or carbon fixation reactions, not associated with light, are carried out in the stroma of chloroplasts. The key place in them is occupied by the fixation of carbon dioxide and the conversion of carbon into carbohydrates. These reactions are cyclic in nature, since part of the intermediate carbohydrates undergoes a process of condensation and rearrangement to ribulose diphosphate, the primary acceptor of CO 2 , which ensures the continuous operation of the cycle. This process was first described by the American biochemist Melvin Calvin.
The transformation of the inorganic compound CO 2 into organic compounds - carbohydrates, in the chemical bonds of which solar energy is stored, occurs with the help of a complex enzyme - ribulose-1,5-diphosphate carboxylase. It provides the addition of one CO 2 molecule to five-carbon ribulose-1,5-diphosphate, resulting in the formation of a six-carbon intermediate short-lived compound. This compound, due to hydrolysis, breaks down into two three-carbon molecules of phosphoglyceric acid, which is reduced using ATP and NADPH to three-carbon sugars (triose phosphates). They form the end product of photosynthesis - glucose.
Part of the triose phosphates, having gone through the processes of condensation and rearrangement, turning first into ribulose monophosphate, and then into ribulose diphosphate, is again included in the continuous cycle of creating glucose molecules. Glucose can be enzymatically polymerized into
starch and cellulose - the basic polysaccharide of plants.
A feature of the photosynthesis of some plants (sugarcane, corn, amaranth) is the initial conversion of carbon through four-carbon compounds. Such plants received the index C 4 -plants, and photosynthesis in them carbon metabolism. C 4 -plants attract the attention of researchers due to their high photosynthetic productivity.
Ways to increase the productivity of agricultural plants:
Sufficient mineral nutrition, which can ensure the best course of metabolic processes;
More complete illumination, which can be achieved with the help of certain plant sowing rates, taking into account the light consumption of photophilous and shade-tolerant;
The normal amount of carbon dioxide in the air (with an increase in its content, the process of plant respiration, which is associated with photosynthesis, is disrupted);
Soil moisture content corresponding to the needs of plants in moisture, depending on climatic and agrotechnical conditions.
The importance of photosynthesis in nature.
As a result of photosynthesis on Earth, 150 billion tons of organic matter are formed annually and about 200 billion tons of free oxygen are released. Photosynthesis not only provides and maintains the modern composition of the Earth's atmosphere, necessary for the life of its inhabitants, but also prevents an increase in the concentration of CO 2 in the atmosphere, preventing overheating of our planet (due to the so-called greenhouse effect). Oxygen released during photosynthesis is necessary for the respiration of organisms and to protect them from harmful short-wave ultraviolet radiation.
Chemosynthesis(late Greek chemeta - chemistry and Greek synthesis - connection) - an autotrophic process of creating organic matter by bacteria that do not contain chlorophyll. Chemosynthesis is carried out due to the oxidation of inorganic compounds: hydrogen, hydrogen sulfide, ammonia, iron oxide (II), etc. The assimilation of CO 2 proceeds, as in photosynthesis (Calvin cycle), with the exception of methane-forming, homo-acetate bacteria. The energy obtained from oxidation is stored in bacteria in the form of ATP.
Chemosynthetic bacteria play an extremely important role in biogeochemical cycles. chemical elements in the biosphere. The vital activity of nitrifying bacteria is one of the most important factors in soil fertility. Chemosynthetic bacteria oxidize compounds of iron, manganese, sulfur, etc.
Chemosynthesis was discovered by the Russian microbiologist Sergei Nikolaevich Vinogradsky (1856-1953) in 1887.
3. Energy exchange
Three stages of energy metabolism are carried out with the participation of special enzymes in various parts of cells and organisms.
The first stage is preparatory- proceeds (in animals in the digestive organs) under the action of enzymes that break down molecules with di- and polysaccharides, fats, proteins, nucleic acids into smaller molecules: glucose, glycerol and fatty acids, amino acids, nucleotides. This releases a small amount of energy that is dissipated in the form of heat.
The second stage is anoxic, or incomplete oxidation. It is also called anaerobic respiration (fermentation), or glycolysis. Enzymes of glycolysis are localized in the liquid part of the cytoplasm - hyaloplasm. Glucose undergoes splitting, each molene in which is stepwise split and oxidized with the participation of enzymes to two three-carbon molecules of pyruvic acid CH 3 - CO - COOH, where COOH is a carboxyl group characteristic of organic acids.
Nine enzymes are sequentially involved in this conversion of glucose. In the process of glycolysis, glucose molecules are oxidized, i.e., hydrogen atoms are lost. The hydrogen acceptor (and electron) in these reactions are nicotinamide nindinucleotide (NAD +) molecules, which are similar in string to NADP + and differ only in the absence of a phosphoric acid residue in the ribose molecule. When pyruvic acid is reduced by reduced NAD, the end product of glycolysis, lactic acid, is formed. Phosphoric acid and ATP are involved in the breakdown of glucose.
In summary, this process looks like this:
C 6 H 12 O 6 + 2H 3 P0 4 + 2ADP \u003d 2C 3 H 6 0 3 + 2ATP + 2H 2 0.
In yeast fungi, the glucose molecule, without the participation of oxygen, is converted into ethyl alcohol and carbon dioxide (alcoholic fermentation):
C 6 H 12 O 6 + 2H 3 P0 4 + 2ADP - 2C 2 H b 0H + 2C0 2 + 2ATP + 2H 2 O.
In some microorganisms, the breakdown of glucose without oxygen can result in the formation of acetic acid, acetone, etc. In all cases, the breakdown of one glucose molecule is accompanied by the formation of two ATP molecules, in macroergic bonds of which 40% of energy is stored, the rest is dissipated in the form of heat.
The third stage of energy metabolism(stage of oxygen splitting , or stage of aerobic respiration) is carried out in mitochondria. This stage is associated with the mitochondrial matrix and the inner membrane; enzymes are involved in it, which are an enzymatic ring "conveyor", called the Krebs cycle, named after the scientist who discovered it. This complex and long way of work of many enzymes is also called tricarboxylic acid cycle.
Once in the mitochondria, pyruvic acid (PVA) is oxidized and converted into an energy-rich substance - acetyl coenzyme A, or acetyl-CoA for short. In the Krebs cycle, acetyl-CoA molecules come from different energy sources. In the process of PVC oxidation, electron acceptors NAD + are reduced to NADH, and another type of acceptors is reduced - FAD to FADH 2 (FAD is a flavin adenine dinucleotide). The energy stored in these molecules is used to synthesize ATP, the universal biological energy accumulator. During the stage of aerobic respiration, electrons from NADH and FADH 2 move along a multistage chain of their transfer to the final electron acceptor, molecular oxygen. Several electron carriers are involved in the transfer: coenzyme Q, cytochromes and, most importantly, oxygen. When electrons move from stage to stage of the respiratory conveyor, energy is released, which is spent on ATP synthesis. Inside the mitochondria, H + cations combine with O 2 ~ anions to form water. In the Krebs cycle, CO 2 is formed, and in the electron transport chain - water. At the same time, one molecule of glucose, being completely oxidized with the access of oxygen to CO 2 and H 2 0, contributes to the formation of 38 ATP molecules. From the foregoing, it follows that the oxygen splitting of organic substances, or aerobic respiration, plays the main role in providing the cell with energy. With oxygen deficiency or its complete absence, oxygen-free, anaerobic, splitting of organic substances occurs; the energy of such a process is only enough to create two ATP molecules. Thanks to this, living beings can do without oxygen for a short time.
The living cell has an intrinsically unstable and almost improbable organization; the cell is able to maintain a very specific and beautiful in its complexity orderliness of its fragile structure only due to the continuous consumption of energy.
As soon as the flow of energy stops, the complex structure of the cell disintegrates and it passes into a disordered and devoid of organization state. In addition to providing chemical processes necessary to maintain the integrity of the cell, in various types of cells, due to the conversion of energy, the implementation of a variety of mechanical, electrical, chemical and osmotic processes associated with the vital activity of the organism is ensured.
Having learned in relatively recent times to extract the energy contained in various inanimate sources to perform various work, a person began to comprehend how skillfully and with what high efficiency the cell transforms energy. The transformation of energy in a living cell obeys the same laws of thermodynamics that operate in inanimate nature. According to the first law of thermodynamics, the total energy of a closed system with any physical change always remains constant. According to the second law, energy can exist in two forms: in the form of "free" or useful energy and in the form of useless dissipated energy. The same law states that with any physical change, there is a tendency to dissipate energy, i.e., to reduce the amount of free energy and to increase entropy. Meanwhile, a living cell needs a constant influx of free energy.
The engineer receives the energy he needs mainly from the energy of chemical bonds contained in the fuel. By burning fuel, it converts chemical energy into thermal energy; he can then use the thermal energy to rotate, for example, a steam turbine and in this way obtain electrical energy. Cells also receive free energy by releasing the energy of chemical bonds contained in the "fuel". Energy is stored in these connections by those cells that synthesize the nutrients that serve as such fuel. However, cells use this energy in a very specific way. Since the temperature at which a living cell functions is approximately constant, the cell cannot use thermal energy to do work. In order for heat energy to work, heat must be transferred from a hotter body to a colder one. It is perfectly clear that a cell cannot burn its fuel at the combustion temperature of coal (900°); it also cannot withstand exposure to superheated steam or high voltage current. The cell has to extract and use energy under conditions of a fairly constant and, moreover, low temperature, a dilute iodine environment, and very slight fluctuations in the concentration of hydrogen ions. In order to acquire the ability to receive energy, the cell, over the centuries of evolution of the organic world, has perfected its remarkable molecular mechanisms, which operate unusually effectively in these mild conditions.
The mechanisms of the cell that ensure the extraction of energy are divided into two classes, and based on the differences in these mechanisms, all cells can be divided into two main types. Cells of the first type are called heterotrophic; they include all the cells of the human body and the cells of all higher animals. These cells need a constant supply of ready-made fuel of a very complex nature. chemical composition. Carbohydrates, proteins and fats, i.e., separate components of other cells and tissues, serve as such fuel for them. Heterotrophic cells obtain energy by burning or oxidizing these complex substances (produced by other cells) in a process called respiration, which involves the molecular oxygen (O 2 ) of the atmosphere. Heterotrophic cells use this energy to carry out their biological functions while releasing carbon dioxide into the atmosphere as an end product.
Cells belonging to the second type are called autotrophic. The most typical autotrophic cells are the cells of green plants. In the process of photosynthesis, they bind the energy of sunlight, using it for their needs. In addition, they use solar energy to extract carbon from atmospheric carbon dioxide and use it to build the simplest organic molecule- glucose molecules. From glucose, the cells of green plants and other organisms create more complex molecules that make up their composition. To provide the energy necessary for this, the cells in the process of respiration burn part of the raw materials at their disposal. From this description of the cyclic transformations of energy in the cell, it becomes clear that all living organisms ultimately receive energy from sunlight, and plant cells receive it directly from the sun, and animals - indirectly.
The study of the main questions posed in this article rests on the need for a detailed description of the primary mechanism for extracting energy used by the cell. Most of the steps in the complex cycles of respiration and photosynthesis have already been explored. It has been established in which particular cell organ this or that process occurs. Respiration is carried out by mitochondria, which are present in large numbers in almost all cells; photosynthesis is provided by chloroplasts - cytoplasmic structures contained in the cells of green plants. The molecular mechanisms that are in these cell formations, composing their structure and ensuring the performance of their functions, represent the next important stage in the study of the cell.
The same well-studied molecules - molecules of adenosine triphosphate (ATP) - transfer the free energy received from nutrients or sunlight from the centers of respiration or photosynthesis to all parts of the cell, ensuring the implementation of all processes that occur with energy consumption. ATP was first isolated from muscle tissue by Loman about 30 years ago. The ATP molecule contains three interconnected phosphate groups. In a test tube, the end group can be separated from the ATP molecule by a hydrolysis reaction, which results in adenosine diphosphate (ADP) and inorganic phosphate. During this reaction, the free energy of the ATP molecule is converted into thermal energy, and the entropy increases in accordance with the second law of thermodynamics. In the cell, however, the terminal phosphate group is not simply separated by hydrolysis, but is transferred to a special molecule that serves as an acceptor. At the same time, a significant part of the free energy of the ATP molecule is retained due to the phosphorylation of the acceptor molecule, which now, due to the increased energy, acquires the ability to participate in energy-consuming processes, for example, in the processes of biosynthesis or muscle contraction. After one phosphate group is cleaved off in this coupled reaction, ATP is converted to ADP. In cell thermodynamics, ATP can be considered as an energy-rich, or "charged" form of the energy carrier (adenosine phosphate), and ADP as an energy-poor, or "discharged" form.
The secondary "charging" of the carrier is, of course, carried out by one or the other of the two mechanisms involved in the extraction of energy. In the process of respiration of animal cells, the energy contained in nutrients is released as a result of oxidation and is spent on the construction of ATP from ADP and phosphate. During photosynthesis in plant cells, the energy of sunlight is converted into chemical energy and is spent on "charging" adenosine phosphate, i.e., on the formation of ATP.
Experiments using the radioactive isotope of phosphorus (P 32) have shown that inorganic phosphate is quickly included in the terminal phosphate group of ATP and again leaves it. In the kidney cell, the renewal of the terminal phosphate group is so rapid that its half-life is less than 1 minute; this corresponds to an extremely intensive exchange of energy in the cells of this organ. It should be added that the activity of ATP in a living cell is by no means black magic. Chemists are familiar with many analogous reactions by which chemical energy is transferred in non-living systems. The relatively complex structure of ATP appears to have originated only in the cell - to ensure the most efficient regulation chemical reactions associated with the transfer of energy.
The role of ATP in photosynthesis has only recently been elucidated. This discovery made it possible to largely explain how photosynthetic cells, in the process of carbohydrate synthesis, bind solar energy - the primary source of energy for all living beings.
The energy of sunlight is transmitted in the form of photons, or quanta; light of different colors, or different lengths waves, characterized by different energies. When light falls on some metal surfaces and is absorbed by these surfaces, photons, as a result of collision with metal electrons, transfer their energy to them. This photoelectric effect can be measured due to the resulting electric current. In the cells of green plants, sunlight with certain wavelengths is absorbed green pigment- chlorophyll. The absorbed energy transfers electrons in a complex chlorophyll molecule from the main energy level to a higher one. high level. Such “excited” electrons tend to return to their main stable energy level again, giving away the energy they have absorbed. In a pure preparation of chlorophyll isolated from a cell, the absorbed energy is re-emitted in the form of visible light, just as is the case with other phosphorescent or fluorescent organic and inorganic compounds.
Thus, chlorophyll, being in a test tube, by itself is not able to store or use the energy of light; this energy quickly dissipates, as if a short circuit had occurred. However, in the cell, chlorophyll is sterically bound to other specific molecules; therefore, when, under the influence of light absorption, it comes into an excited state, "hot", or rich in energy, the electrons do not return to their normal (unexcited) energy state; instead, electrons are detached from the chlorophyll molecule and carried by electron carrier molecules, which pass them on to each other in a closed chain of reactions. Making this way outside the chlorophyll molecule, the excited electrons gradually give up their energy and return to their original places in the chlorophyll molecule, which then turns out to be ready to absorb the second photon. Meanwhile, the energy given up by electrons is used to form ATP from ADP and phosphate - in other words, to "charge" the adenosine phosphate system of the photosynthetic cell.
The electron carriers that mediate this process of photosynthetic phosphorylation have not yet been fully established. One of these carriers appears to contain riboflavin (vitamin B2) and vitamin K. Others are tentatively classified as cytochromes (proteins containing iron atoms surrounded by porphyrin groups, which resemble the porphyrin of chlorophyll itself in location and structure). At least two of these electron carriers are capable of binding some of their energy to recover ATP from ADP.
This is the basic scheme for the conversion of light energy into the energy of ATP phosphate bonds, developed by D. Arnon and other scientists.
However, in the process of photosynthesis, in addition to the binding of solar energy, the synthesis of carbohydrates also occurs. It is now believed that some of the "hot" electrons of the excited chlorophyll molecule, together with hydrogen ions originating from water, cause the reduction (i.e., the production of additional electrons or hydrogen atoms) of one of the electron carriers - triphosphopyridine nucleotide (TPN, in reduced form TPN-N).
In a series of dark reactions, so named because they can occur in the absence of light, TPN-N causes the reduction of carbon dioxide to carbohydrate. Most of the energy needed for these reactions comes from ATP. The nature of these dark reactions was investigated mainly by M. Calvin and his co-workers. One of the by-products of the initial photoreduction of ESRD is the hydroxyl ion (OH-). Although we do not yet have complete data, it is assumed that this ion donates its electron to one of the cytochromes in the chain of photosynthetic reactions, the end product of which is molecular oxygen. Electrons move along the chain of carriers, making their energy contribution to the formation of ATP, and, in the end, having spent all their excess energy, they enter the chlorophyll molecule.
As expected from the strictly regular and sequential nature of the process of photosynthesis, the chlorophyll molecules are not randomly arranged in chloroplasts and, of course, are not simply suspended in the liquid filling the chloroplasts. On the contrary, chlorophyll molecules form ordered structures in chloroplasts - grana, between which there is an interlacing of fibers or membranes that separates them. Within each grana, flat chlorophyll molecules lie in piles; each molecule can be considered analogous to a separate plate (electrode) of an element, grains - to elements, and a set of grains (i.e., the entire chloroplast) - to an electric battery.
Chloroplasts also contain all those specialized electron carrier molecules that, together with chlorophyll, are involved in extracting energy from "hot" electrons and using this energy to synthesize carbohydrates. Chloroplasts extracted from the cell can carry out the entire complex process of photosynthesis.
The efficiency of these miniature solar-powered factories is amazing. In the laboratory, under certain special conditions, it can be shown that in the process of photosynthesis up to 75% of the light incident on a chlorophyll molecule is converted into chemical energy; however, this figure cannot be considered quite accurate, and there are still debates on this matter. In the field, due to the unequal illumination of the leaves by the sun, as well as for a number of other reasons, the efficiency of using solar energy is much lower - about a few percent.
Thus, the glucose molecule, which is the end product of photosynthesis, must contain a fairly significant amount of solar energy contained in its molecular configuration. In the process of respiration, heterotrophic cells extract this energy by gradually breaking down the glucose molecule in order to "conserve" the energy contained in it in the newly formed ATP phosphate bonds.
Exist different types heterotrophic cells. Some cells (for example, some marine microorganisms) can live without oxygen; others (like brain cells) absolutely need oxygen; others (for example, muscle cells) are more versatile and are able to function both in the presence of oxygen in the environment and in its absence. In addition, although most cells prefer to use glucose as their main fuel, some of them can exist solely at the expense of amino acids or fatty acids (the main raw material for the synthesis of which is still the same glucose). Nevertheless, the breakdown of a glucose molecule in liver cells can be considered an example of an energy production process typical of most heterotrophs known to us.
The total amount of energy contained in a glucose molecule is very easy to determine. By burning a certain amount (sample) of glucose in the laboratory, it can be shown that when a glucose molecule is oxidized, 6 water molecules and 6 carbon dioxide molecules are formed, and the reaction is accompanied by the release of energy in the form of heat (approximately 690,000 calories per 1 gram molecule, i.e. per 180 grams of glucose). Energy in the form of heat is, of course, useless for a cell that functions at a nearly constant temperature. The gradual oxidation of glucose during respiration occurs, however, in such a way that most of the free energy of the glucose molecule is stored in a form convenient for the cell.
As a result, the cell receives more than 50% of all energy released during oxidation in the form of phosphate bond energy. Such a high efficiency compares favorably with that which is usually achieved in technology, where it is rarely possible to convert more than one third of the thermal energy obtained from the combustion of fuel into mechanical or electrical energy.
The process of glucose oxidation in the cell is divided into two main phases. During the first or preparatory phase, called glycolysis, the six-carbon glucose molecule is broken down into two three-carbon lactic acid molecules. This seemingly simple process consists of not one, but at least 11 steps, with each step being catalyzed by a different enzyme. It might seem that the complexity of this operation contradicts Newton's aphorism "Natura entm simplex esi" ("nature is simple"); however, it should be remembered that the purpose of this reaction is not simply to split the glucose molecule in half, but to isolate the energy contained in this molecule. Each of the intermediates contains phosphate groups, and as a result, two ADP molecules and two phosphate groups are used in the reaction. Ultimately, as a result of the breakdown of glucose, not only two lactic acid molecules are formed, but, in addition, two new ATP molecules.
What does this mean in terms of energy? Thermodynamic equations show that when one gram-molecule of glucose is broken down to form lactic acid, 56,000 calories are released. Since 10,000 calories are bound to each gram-molecule of ATP, the efficiency of the energy capture process at this stage is about 36% - a very impressive figure in terms of what is usually dealt with in technology. However, these 20,000 calories converted into phosphate bond energy represent only a tiny fraction (about 3%) of the total energy contained in a gram-molecule of glucose (690,000 calories). Meanwhile, many cells, for example, anaerobic cells or muscle cells that are in a state of activity (and at this time unable to breathe), exist due to this negligible energy use.
After glucose has been broken down into lactic acid, aerobic cells continue to extract most of their remaining energy through respiration, during which three-carbon lactic acid molecules are broken down into one-carbon carbon dioxide molecules. Lactic acid, or rather its oxidized form, pyruvic acid, undergoes an even more complex series of reactions, each of which is again catalyzed by a specific enzyme system. First, the three-carbon compound decomposes to form the activated form of acetic acid (acetyl coenzyme A) and carbon dioxide. The "two-carbon fragment" (acetyl coenzyme A) then combines with the four-carbon compound, oxaloacetic acid, resulting in citric acid containing six carbon atoms. Citric acid in the course of a series of reactions is again converted into oxaloacetic acid, and the three carbon atoms of pyruvic acid, "served" in this cycle of reactions, ultimately give molecules of carbon dioxide. This “mill”, which “grinds” (oxidizes) not only glucose, but also fat and amino acid molecules, previously broken down to acetic acid, is known as the Krebs cycle or the citric acid cycle.
The cycle was first described by G. Krebs in 1937. This discovery is one of the cornerstones of modern biochemistry, and its author was awarded the Nobel Prize in 1953.
The Krebs cycle traces the oxidation of lactic acid to carbon dioxide; however, this cycle alone cannot explain how the large amounts of energy contained in the lactic acid molecule can be extracted in a form suitable for use in a living cell. This energy extraction process that accompanies the Krebs cycle, in last years intensively studied. The overall picture is more or less clear, but many details remain to be explored. Apparently, during the Krebs cycle, with the participation of enzymes, electrons are detached from intermediate products and transferred along a number of carrier molecules, united under the general name of the respiratory chain. This chain of enzyme molecules represents the final common path of all electrons stripped from nutrient molecules during the process of biological oxidation. In the last link of this chain, the electrons eventually combine with oxygen to form water. Thus, the breakdown of nutrients during respiration is the reverse process of photosynthesis, in which the removal of electrons from water leads to the formation of oxygen. Moreover, the electron carriers in the respiratory chain are chemically very similar to the corresponding carriers involved in the process of photosynthesis. Among them are, for example, riboflavin and cytochrome structures similar to those of the chloroplast. This confirms Newton's aphorism about the simplicity of nature.
As in photosynthesis, the energy of the electrons passing along this chain to oxygen is captured and used to synthesize ATP from ADP and phosphate. In fact, this phosphorylation occurring in the respiratory chain (oxidative phosphorylation) is better studied than the phosphorylation occurring during photosynthesis, which was discovered relatively recently. It has been firmly established, for example, that there are three centers in the respiratory chain in which adenosine phosphate is “charged,” i.e., ATP is formed. Thus, for every pair of electrons cleaved from lactic acid during the Krebs cycle, an average of three ATP molecules is formed.
Based on the total output of ATP, it is currently possible to calculate the thermodynamic efficiency with which the cell extracts the energy made available to it by the oxidation of glucose. Preliminary splitting of glucose into two molecules of lactic acid gives two molecules of ATP. Each lactic acid molecule ultimately transfers six pairs of electrons to the respiratory chain. Since each pair of electrons passing through the chain causes the conversion of three ADP molecules into ATP, 36 ATP molecules are formed in the process of actual respiration. In the formation of each gram molecule of ATP, as we have already indicated, about 10,000 calories are bound and, therefore, 38 gram molecules of ATP bind approximately 380,000 of the 690,000 calories contained in the original gram molecule of glucose. The efficiency of the coupled processes of glycolysis and respiration can thus be considered equal to at least 55%.
The extreme complexity of the process of respiration is another indication that the enzymatic mechanisms involved in it could not function if the constituents were simply mixed in solution. Just as the molecular mechanisms involved in photosynthesis have a specific structural organization and are enclosed in the chloroplast, so the respiratory organs of the cell - mitochondria - represent the same structurally ordered system.
In a cell, depending on its type and the nature of its function, there can be from 50 to 5000 mitochondria (a liver cell contains, for example, about 1000 mitochondria). They are large enough (3-4 microns in length) to be visible with a normal microscope. However, the ultrastructure of mitochondria is discernible only in an electron microscope.
On electron micrographs, it can be seen that the mitochondrion has two membranes, with the inner membrane forming folds extending into the body of the mitochondrion. A recent study of mitochondria isolated from liver cells showed that the molecules of enzymes involved in the Krebs cycle are located in the matrix, or the soluble part of the internal contents of mitochondria, while the enzymes of the respiratory chain in the form of molecular "ensembles" are located in membranes. Membranes consist of alternating layers of protein and lipid (fat) molecules; the membranes in the grana of chloroplasts have the same structure.
Thus, there is a clear similarity in the structure of these two main "power stations" on which the entire life of the cell depends, because one of them "stores" solar energy in the phosphate bonds of ATP, and the other converts the energy contained in nutrients into ATP energy .
Advances in modern chemistry and physics have recently made it possible to clarify the spatial structure of certain large molecules, for example, molecules of a number of proteins and DNA, i.e., molecules containing genetic information.
The next important step in the study of the cell is to find out the location of large enzyme molecules (which themselves are proteins) in the mitochondrial membranes, where they are located together with lipids - an arrangement that ensures the proper orientation of each catalyst molecule and the possibility of its interaction with the subsequent link of the entire working mechanism. The "wiring diagram" of the mitochondria is already clear!
Current information regarding power plants cells show that it leaves far behind not only classical energy, but also the latest, much more brilliant achievements of technology.
Electronics has achieved amazing success in the layout and reduction of the size of the constituent elements of the computing devices. However, all these successes cannot be compared with the absolutely incredible diminutiveness of the most complex energy conversion mechanisms developed in the process of organic evolution and present in every living cell.
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Abundant growth of fat trees,
which are rooted on the barren sand
approved his own, clearly states that
greasy sheets of greasy fat from the air
absorb...
M. V. Lomonosov
How is energy stored in a cell? What is metabolism? What is the essence of the processes of glycolysis, fermentation and cellular respiration? What processes take place in the light and dark phases of photosynthesis? How are the processes of energy and plastic exchange related? What is chemosynthesis?
Lesson-lecture
The ability to convert one type of energy into another (radiant energy into the energy of chemical bonds, chemical energy into mechanical energy, etc.) is one of the fundamental properties of living things. Here we will consider in detail how these processes are realized in living organisms.
ATP - THE MAIN CARRIER OF ENERGY IN THE CELL. For the implementation of any manifestations of the vital activity of cells, energy is needed. Autotrophic organisms receive initial energy from the Sun during photosynthesis reactions, while heterotrophic organisms use organic compounds from food as an energy source. Energy is stored by cells in the chemical bonds of molecules ATP (adenosine triphosphate), which are a nucleotide consisting of three phosphate groups, a sugar residue (ribose) and a nitrogenous base residue (adenine) (Fig. 52).
Rice. 52. ATP molecule
The bond between phosphate residues is called macroergic, since when it breaks, a large amount of energy is released. Normally, a cell extracts energy from ATP by removing only the terminal phosphate group. In this case, ADP (adenosine diphosphate), phosphoric acid is formed and 40 kJ / mol is released:
ATP molecules play the role of the cell's universal energy bargaining chip. They are delivered to the site of an energy-intensive process, whether it is the enzymatic synthesis of organic compounds, the work of proteins - molecular motors or membrane transport proteins, etc. The reverse synthesis of ATP molecules is carried out by attaching a phosphate group to ADP with energy absorption. The storage of energy in the form of ATP by the cell is carried out during the reactions energy metabolism. He is closely associated with plastic exchange during which the cell produces organic compounds necessary for its functioning.
METABOLISM AND ENERGY IN THE CELL (METABOLISM). Metabolism - the totality of all reactions of plastic and energy metabolism, interconnected. In cells, the synthesis of carbohydrates, fats, proteins, nucleic acids is constantly going on. The synthesis of compounds always comes with the expenditure of energy, i.e., with the indispensable participation of ATP. Energy sources for the formation of ATP are enzymatic reactions of oxidation of proteins, fats and carbohydrates entering the cell. This process releases energy, which is stored in ATP. Glucose oxidation plays a special role in cell energy metabolism. Glucose molecules undergo a series of successive transformations.
The first stage, called glycolysis, takes place in the cytoplasm of cells and does not require oxygen. As a result of successive reactions involving enzymes, glucose breaks down into two molecules of pyruvic acid. In this case, two ATP molecules are consumed, and the energy released during oxidation is sufficient to form four ATP molecules. As a result, the energy yield of glycolysis is small and amounts to two ATP molecules:
C 6 H1 2 0 6 → 2C 3 H 4 0 3 + 4H + + 2ATP
Under anaerobic conditions (in the absence of oxygen), further transformations can be associated with various types fermentation.
Everyone knows lactic fermentation(milk souring), which occurs due to the activity of lactic acid fungi and bacteria. It is similar in mechanism to glycolysis, only the final product here is lactic acid. This type of glucose oxidation occurs in oxygen-deficient cells, such as in hard-working muscles. Close in chemistry to lactic and alcoholic fermentation. The difference is that the products of alcoholic fermentation are ethyl alcohol and carbon dioxide.
The next stage, during which pyruvic acid is oxidized to carbon dioxide and water, is called cellular respiration. Respiration-related reactions take place in the mitochondria of plant and animal cells, and only in the presence of oxygen. This is a series of chemical transformations before the formation of the final product - carbon dioxide. At various stages of this process, intermediate products of the oxidation of the initial substance are formed with the elimination of hydrogen atoms. In this case, energy is released, which is "conserved" in the chemical bonds of ATP, and water molecules are formed. It becomes clear that it is precisely in order to bind the split off hydrogen atoms that oxygen is required. This series of chemical transformations is quite complex and occurs with the participation of the inner membranes of mitochondria, enzymes, and carrier proteins.
Cellular respiration has a very high efficiency. There is a synthesis of 30 ATP molecules, two more molecules are formed during glycolysis, and six ATP molecules - as a result of the transformation of glycolysis products on mitochondrial membranes. In total, as a result of the oxidation of one glucose molecule, 38 ATP molecules are formed:
C 6 H 12 O 6 + 6H 2 0 → 6CO 2 + 6H 2 O + 38ATP
In mitochondria, the final stages of oxidation of not only sugars, but also proteins and lipids take place. These substances are used by cells, mainly when the supply of carbohydrates comes to an end. First, fat is consumed, during the oxidation of which much more energy is released than from an equal volume of carbohydrates and proteins. Therefore, fat in animals is the main "strategic reserve" of energy resources. In plants, starch plays the role of an energy reserve. When stored, it takes up significantly more space than an energy-equivalent amount of fat. For plants, this is not a hindrance, since they are motionless and do not carry reserves on themselves, like animals. You can extract energy from carbohydrates much faster than from fats. Proteins perform many important functions in the body, therefore they are involved in energy metabolism only when the resources of sugars and fats are exhausted, for example, during prolonged starvation.
PHOTOSYNTHESIS. Photosynthesis is a process in which energy sun rays is converted into the energy of chemical bonds of organic compounds. In plant cells, photosynthesis-related processes take place in chloroplasts. Inside this organelle there are systems of membranes in which pigments are embedded that capture the radiant energy of the Sun. The main pigment of photosynthesis is chlorophyll, which absorbs mainly blue and violet, as well as red rays of the spectrum. Green light is reflected, so the chlorophyll itself and the plant parts containing it appear green.
There are two phases in photosynthesis - light and dark(Fig. 53). The actual capture and conversion of radiant energy occurs during the light phase. When absorbing light quanta, chlorophyll goes into an excited state and becomes an electron donor. Its electrons are transferred from one protein complex to another along the electron transport chain. The proteins of this chain, like pigments, are concentrated on the inner membrane of chloroplasts. When an electron passes through the carrier chain, it loses energy, which is used to synthesize ATP. Some of the electrons excited by light are used to reduce NDP (nicotinamide adenine dinucleotiphosphate), or NADPH.
Rice. 53. Products of reactions of light and dark phases of photosynthesis
Under the influence of sunlight in chloroplasts, the splitting of water molecules also occurs - photolysis; in this case, electrons arise that compensate for their loss by chlorophyll; Oxygen is formed as a by-product:
Thus, the functional meaning of the light phase lies in the synthesis of ATP and NADP·H by converting light energy into chemical energy.
The dark phase of photosynthesis does not require light. The essence of the processes taking place here is that the ATP and NADP·H molecules obtained in the light phase are used in a series of chemical reactions that “fix” CO2 in the form of carbohydrates. All reactions of the dark phase are carried out inside the chloroplasts, and ADP and NADP released during the "fixation" of carbon dioxide are again used in the reactions of the light phase for the synthesis of ATP and NADP H.
The overall photosynthesis equation is as follows:
RELATIONSHIP AND UNITY OF PROCESSES OF PLASTIC AND ENERGY EXCHANGE. The processes of ATP synthesis occur in the cytoplasm (glycolysis), in mitochondria (cellular respiration) and in chloroplasts (photosynthesis). All reactions taking place during these processes are reactions of energy exchange. The energy stored in the form of ATP is expended in the reactions of plastic exchange for the production of proteins, fats, carbohydrates and nucleic acids necessary for the life of the cell. Note that the dark phase of photosynthesis is a chain of reactions, plastic exchange, and the light phase is energy.
The relationship and unity of the processes of energy and plastic exchange is well illustrated by the following equation:
Reading this equation from left to right, we get the process of oxidation of glucose to carbon dioxide and water during glycolysis and cellular respiration, associated with the synthesis of ATP (energy metabolism). If you read it from right to left, then you get a description of the reactions of the dark phase of photosynthesis, when glucose is synthesized from water and carbon dioxide with the participation of ATP (plastic metabolism).
CHEMOSYNTHESIS. In addition to photoautotrophs, certain bacteria (hydrogen, nitrifying, sulfur bacteria, etc.) are also capable of synthesizing organic substances from inorganic substances. They carry out this synthesis due to the energy released during the oxidation of inorganic substances. They are called chemoautotrophs. These chemosynthetic bacteria play important role in the biosphere. For example, nitrifying bacteria convert ammonium salts inaccessible to plants into salts. nitric acid which are well absorbed by them.
Cellular metabolism is made up of reactions of energy and plastic metabolism. In the course of energy metabolism, the formation of organic compounds with macroergic chemical bonds- ATP. The energy required for this comes from the oxidation of organic compounds during anaerobic (glycolysis, fermentation) and aerobic (cellular respiration) reactions; from the sun's rays, the energy of which is absorbed in the light phase (photosynthesis); from the oxidation of inorganic compounds (chemosynthesis). The energy of ATP is spent on the synthesis of organic compounds necessary for the cell in the course of plastic exchange reactions, which include the reactions of the dark phase of photosynthesis.
- What are the differences between plastic and energy metabolism?
- How is the energy of sunlight converted into the light phase of photosynthesis? What processes take place during the dark phase of photosynthesis?
- Why is photosynthesis called the process of reflection of planetary-cosmic interaction?
The ability to photosynthesis is the main feature of green plants. Plants, like all living organisms, must eat, breathe, remove unnecessary substances, grow, multiply, respond to environmental changes. All this is provided by the work of the corresponding organs of the body. Ordinarily, organs form organ systems that joint work provide the performance of a particular function of a living organism. Thus, a living organism can be represented as a biosystem. Each organ in a living plant performs a specific job. Root absorbs water from the soil with minerals and strengthens the plant in the soil. The stem carries the leaves towards the light. Water moves along the stem, as well as mineral and organic substances. In the chloroplasts of the leaf, in the light, organic substances are formed from inorganic substances, which they feed on. cells all organs plants. Leaves evaporate water.
If the work of any one organ of the body is disturbed, then this can cause a disruption in the work of other organs and the whole organism. If, for example, water stops flowing through the root, then the whole plant may die. If the plant does not produce enough chlorophyll in the leaves, then it will not be able to synthesize a sufficient amount of organic substances for its vital activity.
Thus, the vital activity of the organism is ensured by the interconnected work of all organ systems. Vitality is all the processes that take place in the body.
Through nutrition, the body lives and grows. In the process of nutrition, the necessary substances are absorbed from the environment. They are then absorbed into the body. Plants absorb water and minerals from the soil. The above-ground green organs of plants absorb carbon dioxide from the air. Water and carbon dioxide are used by plants to synthesize organic substances, which are used by the plant to renew body cells, grow and develop.
During respiration, gas exchange takes place. Oxygen is absorbed from the environment, and carbon dioxide and water vapor are released from the body. Oxygen is essential for all living cells to produce energy.
In the process of metabolism, substances that are unnecessary for the body are formed, which are released into the environment.
When a plant reaches a certain size and the age required for its species, if it is in sufficiently favorable environmental conditions, then it begins to reproduce. As a result of reproduction, the number of individuals increases.
Unlike the vast majority of animals, plants grow throughout their lives.
The acquisition of new properties by organisms is called development.
Nutrition, respiration, metabolism, growth and development, as well as reproduction are influenced by the environmental conditions of the plant. If they are not favorable enough, then the plant may grow and develop poorly, its vital processes will be suppressed. Thus, the vital activity of plants depends on the environment.
Question 3_Cell membrane, its functions, composition, structure. Primary and secondary shell.
The cell of any organism is an integral living system. It consists of three inextricably linked parts: membrane, cytoplasm and nucleus. The shell of the cell interacts directly with external environment and interaction with neighboring cells (in multicellular organisms). cell membrane. The cell wall has complex structure. It consists of an outer layer and a plasma membrane located under it. In plants, as well as in bacteria, blue-green algae and fungi, a dense shell, or cell wall, is located on the surface of the cells. In most plants, it consists of fiber. The cell wall plays an extremely important role: it is an outer frame, a protective shell, provides the turgor of plant cells: water, salts, molecules of many organic substances pass through the cell wall.
Cell wall or wall - a rigid shell of the cell, located outside of cytoplasmic membrane and performing structural, protective and transport functions. Found in most bacteria, archaea, fungi and plants. Animals and many protozoa do not have a cell wall.
Functions cell wall:
1. The transport function provides selective regulation of the metabolism between the cell and the external environment, the entry of substances into the cell (due to the semipermeability of the membrane), as well as the regulation of the cell's water balance
1.1. Transmembrane transport (i.e. across the membrane):
- Diffusion
- Passive transport = facilitated diffusion
- Active = selective transport (with the participation of ATP and enzymes).
1.2. Transport in membrane packaging:
- Exocytosis - release of substances from the cell
- Endocytosis (phago- and pinocytosis) - absorption of substances by the cell
2) Receptor function.
3) Support ("skeleton")- maintains the shape of the cell, gives strength. This is mainly a function of the cell wall.
4) Cell isolation(its living contents) from the environment.
5) protective function.
6) contact with neighboring cells. Association of cells into tissues.