Molecules are broken down in the oxygen-free stage of energy metabolism. Biology at the Lyceum

The primary source of energy in living organisms is the Sun. The energy brought by light quanta (photons) is absorbed by the pigment chlorophyll contained in the chloroplasts of green leaves and accumulates as chemical energy in various nutrients.

All cells and organisms can be divided into two main classes depending on what source of energy they use. In the former, called autotrophic (green plants), CO 2 and H 2 O are converted during photosynthesis into elementary organic glucose molecules, from which more complex molecules are then built.

Cells of the second class, called heterotrophic (animal cells), receive energy from various nutrients (carbohydrates, fats and proteins) synthesized by autotrophic organisms. The energy contained in these organic molecules is released mainly as a result of their combination with atmospheric oxygen (ie oxidation) in a process called aerobic respiration. This energy cycle in heterotrophic organisms ends with the release of CO 2 and H 2 O.

Cellular respiration is the oxidation of organic substances, leading to the production of chemical energy (ATP). Most cells use primarily carbohydrates. Polysaccharides are involved in the respiration process only after they are hydrolyzed to monoscharides: starch, glucose (in plants) , glycogen (in animals).

Fats constitute the "first reserve" and are put into action mainly when the supply of carbohydrates is exhausted. However, in skeletal muscle cells, in the presence of glucose and fatty acids, preference is given to fatty acids. Since proteins perform a number of other important functions, they are used only after all the reserves of carbohydrates and fats have been used up, for example, during prolonged starvation.

Stages of energy metabolism: A single process of energy metabolism can be divided into three successive stages:

The first one is preparatory. At this stage, macromolecular organic substances in the cytoplasm are broken down into small molecules under the action of appropriate enzymes: proteins - into amino acids, polysaccharides (starch, glycogen) - into monosaccharides (glucose), fats - into glycerol and fatty acids, nucleic acids - into nucleotides, etc. .d. At this stage, there is no a large number of energy that is dissipated as heat.

Proteins + H 2 O \u003d amino acid + heat (dissipates)

Fats + H 2 O \u003d glycerol + fatty acids + heat

Polysaccharides + H 2 O \u003d glucose + heat

Second phase - anoxic, or incomplete. The substances formed at the preparatory stage - glucose, amino acids, etc. - undergo further enzymatic decomposition without access to oxygen. An example is the enzymatic oxidation of glucose (glycolysis), which is one of the main sources of energy for all living cells. glycolysis- a multi-stage process of glucose breakdown under anaerobic (oxygen-free) conditions to pyruvic acid (PVA), and then to lactic, acetic, butyric acids or ethyl alcohol, occurring in the cytoplasm of the cell. Glucose, under the influence of enzymes, is broken down to two C 3 H 6 O 3 molecules with the release of energy. 60% of this energy is dissipated in the form of heat, 40% in the form of ATP.

The carrier of electrons and protons in these redox reactions is nicotinamide adenine dinucleotide (NAD) and its reduced form NAD *H. The products of glycolysis are pyruvic acid, hydrogen in the form of NADH, and energy in the form of ATP.

At different types fermentation further fate products of glycolysis are different. In animal cells and numerous bacteria, PVC is reduced to lactic acid. The well-known lactic acid fermentation (during the write-off of milk, the formation of sour cream, kefir, etc.) is caused by lactic acid fungi and bacteria.

During alcoholic fermentation, the products of glycolysis are ethyl alcohol and CO 2. For other microorganisms, the fermentation products may be butyl alcohol, acetone, acetic acid, etc.

During oxygen-free splitting, part of the released energy is dissipated in the form of heat, and part is accumulated in ATP molecules.

The third stage of energy metabolism - the stage of oxygen splitting, or aerobic respiration, occurs in mitochondria. Enzymes capable of transferring electrons play an important role in the oxidation process at this stage. The structures that ensure the passage of the third stage are called the electron transport chain. Molecules - energy carriers, which received an energy charge at the second stage of glucose oxidation, enter the electron transport chain. Electrons from molecules - energy carriers, as if in steps, move along the links of the chain from a higher energy level to a lower one. The released energy is used to charge the ATP molecules. The electrons of molecules - energy carriers, which gave energy to the "charging" of ATP, eventually combine with oxygen. As a result, water is formed. In the electron transport chain, oxygen is the final electron receiver. Thus, oxygen is needed by all living beings as the ultimate receiver of electrons. Oxygen provides a potential difference in the electron transport chain and, as it were, attracts electrons from high energy levels of energy carrier molecules to its low energy level. Along the way, energy-rich ATP molecules are synthesized. As a result, 36 ATP is formed at the oxygen stage.

Let us consider in more detail how living organisms release the energy stored in complex organic compounds. Mankind, for example, like a living cell, constantly needs energy. For this, in most cases, fossil fuels (gas, oil, coal) are burned. The chemical energy stored in the fuel is first converted into thermal energy (the energy of superheated steam), then into mechanical energy (the rotation of power plant turbines), and finally into electrical energy, which can be transmitted over power lines over long distances and used for various purposes. The combustion process of organic fuel (for example, methane gas) can be described by a simple equation:

CH 4 + 2O 2 \u003d CO 2 + 2H 2 O + energy (heat)

First, living cells carry out an oxidation reaction in several stages, gradually oxidizing a saturated hydrocarbon to an alcohol, an aldehyde (or ketone), an organic acid, and, finally, carbon dioxide. Conventionally, this can be illustrated by the following sequence of transformations:

CH 4 --> CH 3 OH --> H 2 C=O --> HCOOH --> CO 2

Ultimately, the same amount of energy (heat) will be released in this process as in the simple combustion of methane, but it will be released in portions, in parts.

From this it can be seen that those organic substances are richest in energy, in which carbon is maximally recovered. In cells, these are lipids big amount saturated fatty acids, the complete "burning" of which gives the maximum amount of energy. The breakdown of carbohydrates related to aldehyde alcohols or keto alcohols, or amino acids, will provide about half as much energy, since most of the carbon atoms in the molecules of these compounds are already partially oxidized.

Secondly, not all of the energy released during such reactions is dissipated in the form of heat, since living cells store some of their energy in the form of ATP. For this, the reaction that proceeds with the release of energy is "coupled" with the reaction that proceeds with the absorption of energy - with the formation of ATP from ADP and inorganic phosphate (H 3 PO 4).

Thirdly, to obtain energy completely it is not necessary to oxidize organic substances completely , i.e. to carbon dioxide. Energy will also be released during the oxidation of, for example, alcohol to carboxylic acid, although, of course, its amount will be less than during complete oxidation.

Fourth, living cells can carry out oxidation of organic substances and in the absence of oxygen . Conventionally, this can be illustrated by the following sequence of reactions:

CH 3 - CH 3 --> CH 2 \u003d CH 2 + 2H +,

CH 2 \u003d CH 2 + H 2 O --> CH 3 -CH 2 OH,

CH 3 -CH 2 OH --> CH 3 -HC \u003d O + 2H + ,

CH 3 -HC \u003d O + H 2 O --> CH 3 -HC (OH) 2,

CH 3 -HC(OH) 2 --> CH 3 -COOH + 2H +

So, we see how one of the carbon atoms in the ethane molecule is sequentially oxidized to alcohol, aldehyde and carboxylic acid. Pairs of hydrogen atoms sequentially "torn off" from this carbon atom, which are called recovery equivalents , in cells they attach to the universal acceptors of hydrogen atoms - NAD + or FAD molecules (see the lecture "Organic substances. Lipids. Nucleotides"), restoring them to NADH or FADH 2. These substances can be used in biosynthesis reactions to reduce organic compounds (the reactions shown above proceed in the opposite direction), and in the presence of oxygen, NADH and FADH 2 are oxidized in the mitochondrial respiratory chain, releasing a large amount of energy stored in the form of ATP.

In addition to burning fossil fuels, humanity uses hydroelectric power plants to generate energy: the water accumulated on one side of the dam flows down and rotates turbines, producing electricity. It is interesting to note that living cells learned to use a similar principle long before the emergence of man as a biological species: the oxidation of NADH and FADH 2 in the respiratory chain of mitochondria is accompanied by the transfer of protons through the mitochondrial membrane from the matrix to the intermembrane space and the creation of a significant gradient of their concentration on the membrane; the membrane thus acts as a dam. When protons "flow" into the mitochondria along a concentration gradient through a special channel in the ATP synthetase enzyme molecule, they "rotate" this enzyme (like a water turbine), which leads to the synthesis of ATP (see below).

So, the basis of energy metabolism in cells is made up of sequentially occurring redox reactions. In a chain of such reactions under anaerobic conditions, some organic substances are oxidized (lose hydrogen atoms), while others (mainly NAD+ and FAD) are reduced (add hydrogen atoms). Under aerobic conditions, reduced NADH and FADH 2 are themselves oxidized in mitochondria, donating electrons to oxygen, which is reduced to form water. The energy released during these reactions is partly dissipated in the form of heat, and partly stored in the form of ATP.

The main source of energy for living organisms, including humans, are carbohydrates. Conventionally, the process of their splitting and oxidation, accompanied by the storage of energy in the form of ATP, can be divided into three stages: preparatory, anaerobic (or anoxic) and aerobic (or oxygen) . At the preparatory stage, complex polysaccharides are broken down by digestive enzymes into monomers (glucose). Further transformations of glucose occur in the process of glycolysis.

Anoxic stage of energy metabolism

Sequential reactions of glycolysis are catalyzed by 11 enzymes that are localized in the hyaloplasm. Conventionally, glycolysis can be divided into 2 stages: at the first stage, glucose is converted into glyceraldehyde phosphate with the consumption of ATP, and at the second stage, as a result of redox reactions, ATP and lactic acid are formed. The reduced NADH accumulated as an intermediate product of glycolysis is oxidized during the formation of lactic acid to NAD +, which again returns to glycolysis. In the presence of sufficient oxygen, NADH can be oxidized in the mitochondrial respiratory chain. In this case, glycolysis ends at the stage of formation of not lactic, but pyruvic acid (pyruvate), which enters the Krebs cycle and is completely oxidized to CO 2.

When one glucose molecule breaks down, 2 are spent and 4 ATP molecules are formed, i.e. total the energy yield of glycolysis is 2 ATP molecules . The energy required for this is released as a result of the intramolecular oxidation of the aldehyde group to a carboxyl group. About 30% of the energy released in this case is stored in the form of ATP, which, however, is only 5% of the energy that can be obtained with the complete oxidation of glucose to CO 2 and H 2 O. Thus, glycolysis is energetically less favorable than respiration. Glycolysis also includes other hexoses (galactose, fructose), pentoses, and glycerol. The substrate for glycolysis in animals and fungi can be glycogen (this process is called glycogenolysis), and in plants it can be starch.

By a mechanism similar to glycolysis, the process proceeds fermentation in various microorganisms. Since living organisms apparently first appeared on Earth at a time when its atmosphere was deprived of oxygen, anaerobic fermentation should be considered as the simplest biochemical mechanism for obtaining energy from nutrients. Carbohydrates (hexoses, pentoses), alcohols, organic acids and nitrogenous bases undergo fermentation. Depending on the type of fermentation, its products can be alcohols (ethyl, etc.), organic acids (formic, acetic, lactic, propionic, butyric), acetone, CO 2, and in some cases molecular hydrogen. According to the type of products formed, fermentation is divided into alcohol, lactic, propionic, etc., which formed the basis for the names of a number of groups of bacteria (lactic acid, butyric, propionic, etc.). In the process of alcoholic or lactic acid fermentation, two molecules of pyruvate, ATP and NADH are formed from one molecule of glucose. Since NADH needs to be oxidized and returned to the fermentation cycle, pyruvate is reduced by it to lactic acid (lactate) or ethyl alcohol.

Fermentation plays an important role in the cycle of substances in nature (anaerobic degradation of cellulose and other organic substances), and is also widely used in practice. For many centuries, alcoholic fermentation has been used in winemaking, brewing, baking bread (and in Lately- upon receipt of fuel); lactic acid - for obtaining fermented milk products, when pickling cabbage, pickling cucumbers, ensiling feed for livestock; propionic acid - in cheese making; acetone-butyl - to obtain solvents, etc.

Oxygen stage of energy metabolism

The next step in energy metabolism after glycolysis is cellular respiration , or biological oxidation - the oxygen stage of the oxidation of organic compounds. In the broad sense of the word, respiration is the process of absorbing oxygen (O 2) from the environment and releasing carbon dioxide (CO 2) by living organisms, which is necessary to maintain intracellular oxidative processes that provide energy metabolism. Respiration is divided into external respiration - gas exchange between the body and the environment, and tissue or cellular respiration (biological oxidation) - a set of enzymatic redox reactions, as a result of which complex organic substances are oxidized by oxygen to CO 2 with the release of energy stored by cells in the form ATP.

Cellular respiration in plants, animals and most aerobic microorganisms begins with the removal of CO 2 (decarboxylation) from the pyruvic acid (pyruvate) molecule, which is formed during glycolysis, i.e. Glycolysis is a necessary preparatory stage for cellular respiration during the breakdown of carbohydrates. As a result of this reaction, CO 2 is detached from pyruvate, and the resulting two-carbon residue - the acetic acid radical (acetyl radical) is attached to the molecule of the universal carrier of hydrocarbon radicals - coenzyme A - with the formation of acetyl coenzyme A ( acetyl-CoA ). As a result of this reaction, NAD + is reduced to NADH. Acetyl-CoA and NADH are also formed during the oxidation of fatty acids, which are also substrates of cellular respiration. Further oxidation of acetyl-CoA occurs in the Krebs cycle , and NADH in the mitochondrial respiratory chain . All amino acids can enter the Krebs cycle at various stages. Thus, in this cycle, the oxidation pathways of carbohydrates, fats and proteins converge.

(also called the tricarboxylic acid cycle or the tricarboxylic acid cycle citric acid) is a complex multi-stage redox process, as a result of which the acetic acid residue obtained from acetyl-CoA is completely oxidized to two CO 2 molecules with the formation of three NADH molecules, one FADH 2 molecule and one GTP molecule. All Krebs cycle enzymes, as well as fatty acid oxidation enzymes, are localized in the mitochondrial matrix, and one enzyme, succinate dehydrogenase, is located in the inner mitochondrial membrane.

At the first stage of the Krebs cycle, the acetic acid residue is transferred from acetyl-CoA to the oxaloacetic acid (oxaloacetate) molecule with the formation of citric acid (citrate), which, through the intermediate reaction of the formation of cis-aconitic acid, is converted to isocitric acid (isocitrate). CO 2 and 2 H + atoms are cleaved from isocitric acid, resulting in the formation of a NADH molecule and a-ketoglutaric acid (a-ketoglutarate), which interacts with a coenzyme A molecule. In this case, the second CO 2 molecule is cleaved off and another NADH molecule is formed and energy-rich compound succinyl-CoA, which is cleaved to form free succinic acid (succinate), which is accompanied by the synthesis of GTP from GDP and Fn. Succinic acid is oxidized to fumaric acid (fumarate) with the formation of FADH 2, fumaric acid is converted to malic acid (malate) with the addition of water, and malic acid is oxidized to oxaloacetic acid (oxaloacetate) with the formation of NADH. At this stage, the Krebs cycle closes, i.e. oxaloacetate can recycle and condense with the next acetic acid residue to form citrate.

Thus, the total reaction of the Krebs cycle can be described by the following equation:

Acetyl-CoA + 3NAD + + FAD + GDP + P n + 3H 2 O --> 2CO 2 + 3NADH + 3H + + FADH 2 + GTP + CoA

The energy released during the oxidation of acetyl-CoA is stored in the form of one molecule of GTP (which can be converted into ATP) and four molecules of reducing equivalents (3 molecules of NADH and one FADH 2), which can be used in various biosynthetic processes or oxidized. Their further oxidation occurs in the mitochondrial respiratory chain localized in the inner mitochondrial membrane. The "work" of the respiratory chain of mitochondria is the oxidation of NADH, i.e. in "tearing off" electrons from it, and transferring them to an oxygen molecule. In aerobic bacteria, the respiratory chain is located in special structures of the plasma membrane - mesosomes, and in general terms resembles the respiratory chain of mitochondria.

begins with the oxidation of NADH in the respiratory chain of mitochondria, accompanied by the elimination of two electrons and a proton (H +). The final acceptor of these electrons is O 2, which combines with the H + ions in the matrix to form H 2 O.

Oxidation of NADH is started by the enzyme NADH dehydrogenase, which splits off two electrons and a proton from it, which is released into the matrix.Let us trace the path of electrons split off from the NADH molecule. NADH dehydrogenase is a complex complex consisting of a large number of proteins (about 40) and contains flavin mononucleotide and several iron-sulfur clusters as coenzymes. The electrons torn off from NADH are transferred with the help of these coenzymes to a low molecular weight hydrophobic compound dissolved in the mitochondrial membrane - coenzyme Q (ubiquinone), which transfers them to the chain of electron carriers - cytochromes. Cytochromes are heme-containing proteins (the heme they contain resembles the heme of hemoglobin). Due to the change in the valence of the iron atom that is part of the heme, they are able to reversibly attach and donate an electron (Fe 3+ + e - --> Fe 2+ and then Fe 2+ - e - --> Fe 3+). Coenzyme Q transfers electrons to cytochromes b and c 1, and from them electrons are transferred to cytochrome c. He, in turn, transfers electrons to cytochromes a and a 3 (cytochrome oxidase, copper ions are also involved in the transfer of electrons in this enzyme), which transfer them to the final acceptor - molecular oxygen (O 2).

Electrons “taken away” from NADH are transferred in the respiratory chain from carrier to carrier, losing their reduction potential. Part of the energy released in this case is dissipated in the form of heat, but, in addition, part of the energy is spent on creating a difference in proton concentrations (electrochemical potential) on the inner membrane of mitochondria due to their transfer at several points of the respiratory chain (the so-called interface points) from the matrix to the intermembrane space.

This difference in proton concentrations arises as a result of the fact that the transfer of electrons from NADH to oxygen is accompanied by the "pumping" of protons from the mitochondrial matrix into the intermembrane space.

First, when NADH is oxidized by NADH dehydrogenase,I have at least 4 protons. Secondly, coenzyme Q, receiving electrons from NADH dehydrogenase, captures 2 H + from the matrix; when it is oxidized by cytochromes b and c 1, these protons are ejected into the intermembrane space, and due to the work of the so-called Q-cycle, this amount increases by another 2 H + . Thirdly, 2 protons are ejected from mitochondria during the work of cytochrome oxidase. Thus, NADH oxidation is accompanied by the transfer of at least 10 protons from the matrix through the mitochondrial membrane. When FADH 2 is oxidized, 2 electrons and 2 protons split off from it are transferred immediately to coenzyme Q, therefore, when FADH 2 is oxidized, only 6 protons are transferred through the mitochondrial membrane.

As a result of the work of the respiratory chain of mitochondria, the concentration of H + in the intermembrane space significantly exceeds their concentration in the matrix, which creates a proton concentration gradient directed inside the mitochondria. The mitochondrial membrane is impermeable to them, so it can be compared to a hydroelectric dam that holds water in a reservoir. The energy of this gradient is used by the enzyme ATP synthetase , which transfers H + ions to the matrix and synthesizes ATP from ADP and F n.

For the synthesis of 1 ATP molecule, it is necessary to transfer 3 H + ions inside the mitochondria along the concentration gradient, therefore, due to the oxidation of 1 NADH molecule, 3 ATP molecules can be synthesized, and due to the oxidation of 1 FADH 2 molecule, 2 ATP molecules.

In addition, part of the energy of the proton concentration gradient is spent on the transport of various substances through the inner mitochondrial membrane. The synthesis of ATP in mitochondria by the enzyme ATP synthetase is called oxidative phosphorylation , emphasizing the connection of this process with the oxidation of organic substrates.

Thus, as a result of the complete oxidation of glucose to carbon dioxide and water, a large amount of ATP is formed - 38 molecules. Two of them are synthesized during glycolysis, and the remaining 36 are synthesized during the oxidation of pyruvate. First, in glycolysis, when one molecule of pyruvate is formed, a NADH molecule is reduced, and its oxidation in mitochondria gives 3 ATP molecules. Secondly, during the decarboxylation of pyruvate and the formation of acetyl-CoA, one more NADH molecule (another 3 ATP molecules) is restored. Thirdly, in the Krebs cycle, 3 NADH molecules are formed (and this is 9 ATP molecules), 1 FADH 2 molecule (another 2 ATP molecules) and 1 GTP molecule (exchanges its terminal macroergic phosphate with ADP, which gives another 1 ATP molecule). Thus, the complete oxidation of 1 NADH molecule and 1 pyruvate molecule formed in glycolysis gives 18 ATP molecules, and two - 36 ATP molecules, respectively. Given the 2 ATP molecules formed during glycolysis, the total energy yield of glucose oxidation to carbon dioxide and water during cellular respiration is 38 ATP molecules .

The final equation for this process will look like this:

C 6 H 12 O 6 + 6O 2 + 38ADP + 38F n --> 6CO 2 + 6H 2 O + 38ATP

The efficiency of the complete oxidation of glucose to carbon dioxide and water is very high: from 55 to 70% of the released energy (depending on conditions) is stored in the form of macroergic bonds in ATP molecules; the rest of the energy is dissipated as heat. Thus, ATP is the main product of energy metabolism reactions.

During this reaction, carbon is oxidized by atmospheric oxygen (carbon is reduced to the maximum in methane (CH 4) and oxidized to the maximum in carbon dioxide (CO 2)), which leads to the release of energy in the form of heat. A similar process occurs in living cells of aerobic organisms, but it has a number of significant differences.

energy exchange

energy exchange(dissimilation)- a set of enzymatic reactions in a living organism aimed at breaking down complex organic substances (proteins, nucleic acids, fats, carbohydrates) supplied with food and stored in the body itself (starch, glycogen, etc.) to simple substances with the release of energy.

Conventionally, energy exchange can be divided into several stages.

First stage - preparatory , which includes the splitting of complex substances into simple molecules.

Next stage - anoxic flowing in the cytoplasm of cells without the participation of oxygen.

The most important is oxygen stage . It takes place in the mitochondria and requires the presence of oxygen.

Preparatory stageenergy metabolism It consists in the splitting of large molecules of organic substances into smaller ones.

Their breakdown occurs in various parts of the gastrointestinal tract. Inside the cells, organic substances are broken down with the participation of lysosome enzymes.

Standing out as a result preparatory phase energy is dissipated in the form of heat, and the resulting small molecules are used as building materials.

Anoxic Stageenergy metabolism characterized by the enzymatic breakdown of organic matter under anaerobic conditions.

It goes directly into the cytoplasm of the cell.

Examples of anoxic processes are glycolysis and fermentation.

As a result of the oxygen-free stage of energy metabolism, organisms receive the energy necessary for life; 40% of the energy is spent on the synthesis of ATP, the rest is spent in the form of heat.

Oxygen splitting (oxygen stage)- the stage of energy metabolism, during which the products of the oxygen-free stage are completely oxidized to carbon dioxide and water with the release of energy and its accumulation in ATP molecules.

So, when two lactic acid molecules are oxidized, 36 ATP molecules are formed.Some of the molecules are spent on the oxidation processes themselves, and 21 ATP molecules are transferred to the cytoplasm to ensure the operation of other cellular structures.

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

Oxygen splitting occurs on the inner membrane of mitochondria and in the matrix under the action of numerous cristae enzymes.

ATP molecule (adenosine triphosphoric acid) is a universal carrier and the main accumulator of chemical energy in the cell. It is a nucleotide consisting of adenine, ribose and three phosphoric acid residues.In the body, ATP is synthesized from ADP and inorganic phosphate:

ADP + H 3 PO 4 + energy → ATP + H 2 O.

The small size of the molecules allows them to easily diffuse into various parts of the cell, where it is necessary to provide energy for vital processes.

In the body, ATP is one of the most frequently renewed substances - for example, in humans, the lifespan of one ATP molecule is less than 1 minute. During the day, one ATP molecule goes through an average of 2,000 - 3,000 resynthesis cycles (about 40 kg of ATP per day). Thus, there is practically no ATP reserve in the body, and for normal life it is necessary to constantly synthesize new molecules.

The cell is the functional unit of the body. Various substances continuously enter the cell. It synthesizes new molecules; some of the molecules are destroyed. Some substances are consumed by the cell, others are stored in the reserve, and others are removed from the cell. Substances are constantly moving from one part of the cell to another. In some molecules of the cell, energy is stored, while other molecules are split with the release of the energy necessary for the life of the cell.

Thousands of different enzymatic reactions take place simultaneously in the cell, the totality of which is called metabolism (from Greek metabole - change, transformation) or cellular metabolism. The main role in these reactions belongs to enzymes and ATP, without which they do not proceed. In the process of metabolism, the cell receives energy, which is released when the molecules of fats, carbohydrates and proteins are oxidized. Metabolism provides the cell with building material: new complex molecules are formed in it.

Metabolism includes two groups of interrelated reactions: the synthesis of substances - plastic exchange and breakdown of substances energy metabolism. Let's get acquainted first with the energy exchange.

In the course of energy metabolism, complex molecules of carbohydrates, fats, proteins, with the participation of many enzymes, are oxidized to carbon dioxide and water. The energy released in this process is stored in ATP molecules.

Energy metabolism in aerobes includes three stages:

• preparatory;
• anoxic;
• oxygen.

In the first preparatory stage large molecules break down into "blocks": proteins are broken down into amino acids, polysaccharides - into monosaccharides, fats - into glycerol and fatty acids, nucleic acids - into nucleotides. This process takes place in the lysosomes of the cell. A small amount of the released energy is dissipated in the form of heat.

Second, anoxic stage proceeds in the cytoplasm, where organic substances are broken down to even simpler ones. This stage proceeds without participation of oxygen; a little energy is released at the same time; part of it is dissipated in the form of heat and a small part is spent on the synthesis of two ATP molecules from ADP.

How is ATP produced in cells?

Where does the energy for the synthesis of its molecules come from? It was established 1000 that most of the ATP is synthesized due to the energy of the proton H + and electrons, the source of which are hydrogen atoms. And hydrogen atoms are released during the splitting of molecules of organic substances.

Consider the processes characteristic of the second stage, using the example glycolysis- the process of breaking down glucose without the participation of oxygen. The glucose molecule, which contains 6 carbon atoms, is split into two three-carbon molecules of pyruvic acid - PVA. Cleavage occurs in several stages and includes more than 10 reactions involving a large number of enzymes. This releases energy that is used to synthesize two ATP molecules from ADP.
When a glucose molecule is oxidized, electrons and hydrogen ions are split off from it, which are attached to a special substance NAD +. It passes into the reduced form of NAD H. NAD molecules carry protons and electrons in the cell from one reaction to another, while they themselves do not participate in reactions, are not destroyed, being used repeatedly.

Thus, as a result anoxic stage cleavage of glucose, 2 molecules of PVC, 2 molecules of ATP and 2 molecules of NAD H 2 are formed.

The fate of pyruvic acid (PVA) molecules in the cells of different organisms develops differently. There are microorganisms that live in an oxygen-free environment. They are called anaerobes (from Greek an - negative particle and aer - air). In the cells of anaerobes, only two (in aerobes, three) stages of energy metabolism proceed - preparatory and anoxic, and ATP molecules are synthesized during fermentation. In anaerobes, PVC is converted either into lactic acid, or into ethyl alcohol, or into acetic acid, which still contain a lot of energy.
Lactic acid is formed during the vital activity of lactic acid fermentation bacteria, which occurs during the souring of milk, sauerkraut. Alcoholic fermentation is carried out by yeast fungi, as a result of which ethyl alcohol and carbon dioxide are formed. Fermentation is widely used in human economic activity in the production of dough, beer, wine, sauerkraut, and the production of kefir.