Respiration of microbes. Aerobic and anaerobic. Incomplete oxidation. Aerobic oxidation of carbohydrates In the process of aerobic oxidation of glucose,

Under aerobic conditions, glucose is oxidized to CO 2 and H 2 O. The overall equation is:

C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O + 2880 kJ/mol.

This process includes several stages:

    Aerobic glycolysis . In it, 1 glucose is oxidized to 2 PVC, with the formation of 2 ATP (first 2 ATP are consumed, then 4 are formed) and 2 NADH 2;

    Conversion of 2 PVK into 2 acetyl-CoA with the release of 2 CO 2 and the formation of 2 NADH 2;

    CTK. It oxidizes 2 acetyl-CoA with the release of 4 CO 2, the formation of 2 GTP (yielding 2 ATP), 6 NADH 2 and 2 FADH 2;

    Oxidative phosphorylation chain. In it, 10 (8) NADH 2, 2 (4) FADH 2 are oxidized with the participation of 6 O 2, while 6 H 2 O is released and 34 (32) ATP is synthesized.

As a result of aerobic oxidation of glucose, 38 (36) ATP is formed, of which: 4 ATP in reactions of substrate phosphorylation, 34 (32) ATP in reactions of oxidative phosphorylation. The efficiency of aerobic oxidation will be 65%.

Anaerobic oxidation of glucose

Glucose catabolism without O2 occurs in anaerobic glycolysis and PFS (PFP).

    During anaerobic glycolysis 1 glucose is oxidized to 2 molecules of lactic acid with the formation of 2 ATP (first 2 ATP are consumed, then 4 are formed). Under anaerobic conditions, glycolysis is the only source of energy. The overall equation is: C 6 H 12 O 6 + 2H 3 PO 4 + 2ADP → 2C 3 H 6 O 3 + 2ATP + 2H 2 O.

    During PFP Pentoses and NADPH 2 are formed from glucose. During PFS Only NADPH 2 is formed from glucose.

GLYCOLYSIS

Glycolysis is the main pathway for the catabolism of glucose (as well as fructose and galactose). All its reactions take place in the cytosol.

Aerobic glycolysis is the process of oxidation of glucose to PVC, occurring in the presence of O 2.

Anaerobic glycolysis is the process of oxidation of glucose to lactate, occurring in the absence of O 2.

Anaerobic glycolysis differs from aerobic glycolysis only in the presence of the last 11 reactions; the first 10 reactions are common to them.

Stages of glycolysis

In any glycolysis, 2 stages can be distinguished:

    Stage 1 is preparatory, it consumes 2 ATP. Glucose is phosphorylated and broken down into 2 phosphotrioses;

    Stage 2 is associated with ATP synthesis. At this stage, phosphotrioses are converted to PVC. The energy of this stage is used for the synthesis of 4 ATP and the reduction of 2NADH 2, which under aerobic conditions is used for the synthesis of 6 ATP, and under anaerobic conditions they reduce PVA to lactate.

Energy balance of glycolysis

Thus, the energy balance of aerobic glycolysis is:

8ATP = -2ATP + 4ATP + 6ATP (from 2NADH 2)

Energy balance of anaerobic glycolysis:

2ATP = -2ATP + 4ATP

General reactions of aerobic and anaerobic glycolysis

1. Hexokinase (hexokinase II, ATP: hexose-6-phosphotransferase) in muscles phosphorylates mainly glucose, less fructose and galactose. Km<0,1 ммоль/л. Ингибитор глюкозо-6-ф, АТФ. Активатор адреналин. Индуктор инсулин.

Glucokinase (hexokinase IV, ATP: glucose-6-phosphotransferase) phosphorylates glucose. Km - 10 mmol/l, active in the liver and kidneys. Glucose-6-ph is not inhibited. Insulin inducer. Hexokinases carry out phosphorylation of hexoses.

2. Phosphohexose isomerase (glucose-6ph-fructose-6ph-isomerase) carries out aldo-ketoisomerization of open forms of hexoses.

3. Phosphofructokinase 1 (ATP: fructose-6ph-1-phosphotransferase) carries out phosphorylation of fructose-6ph. The reaction is irreversible and the slowest of all glycolysis reactions, determining the rate of all glycolysis. Activated by: AMP, fructose-2,6-df (a powerful activator, formed with the participation of phosphofructokinase 2 from fructose-6ph), fructose-6-ph, Fn. Inhibited by: glucagon, ATP, NADH 2, citrate, fatty acids, ketone bodies. Insulin response inducer.

4. Aldolaza A (fructose-1,6-ph: DAP-lyase). Aldolases act on open forms of hexoses, have 4 subunits, and form several isoforms. Most tissues contain Aldolase A. The liver and kidneys contain Aldolase B.

5. Phosphotriose isomerase (DAP-PHA isomerase).

6. 3-PHA dehydrogenase (3-PHA: NAD+ oxidoreductase (phosphorylating)) consists of 4 subunits. Catalyzes the formation of a high-energy bond in 1,3-PGA and the reduction of NADH 2, which are used under aerobic conditions for the synthesis of 8 (6) ATP molecules.

7. Phosphoglycerate kinase (ATP: 3PGA-1-phosphotransferase). Carries out substrate phosphorylation of ADP to form ATP.

In the following reactions, the low-energy phosphoester is converted to high-energy phosphate.

8. Phosphoglycerate mutase (3-PGA-2-PGA isomerase) transfers the phosphate residue to PGA from position 3 to position 2.

9. Enolase (2-PHA: hydro-lyase) splits off a water molecule from 2-PHA and forms a high-energy bond with phosphorus. Inhibited by F - ions.

10. Pyruvate kinase (ATP: PVK-2 phosphotransferase) carries out substrate phosphorylation of ADP to form ATP. Activated by fructose-1,6-df, glucose. Inhibited by ATP, NADH 2, glucagon, adrenaline, alanine, fatty acids, Acetyl-CoA. Inducer: insulin, fructose.

The resulting enol form of PVK is then non-enzymatically converted to a more thermodynamically stable keto form. This reaction is the last for aerobic glycolysis.

Further catabolism of 2 PVK and the use of 2 NADH 2 depends on the availability of O 2 .

Anaerobic oxidation of carbohydrates occurs in cells, organs and tissues without the participation of oxygen. If the process begins with the conversion of glucose and ends with the formation of lactic acid, then it is called anaerobic glycolysis, if it begins with the conversion of glycogen - glycogenolysis.

Glycolysis

At the 1st stage of glycolysis (Fig. 7), phosphorylation of glucose occurs under the action of the enzyme hexokinase in the presence of ATP and Mg 2+ ions with the formation glucose-6-phosphate(G-6-F), which at the 2nd stage turns into fructose 6-phosphate(F-6-F). This reaction is catalyzed glucose phosphate isomerase . During the 3rd stage, fructose-6-phosphate is phosphorylated, resulting in the formation fructose 1,6-bisphosphate(F-1,6-F). Catalysis of this reaction is ensured in the presence of Mg 2+ ions and ATPphosphofructokinase .

At the 4th stage F-1,6-F under the influence aldolases splits into 2 phosphotrioses - glyceraldehyde-3-phosphate(GA-3-F) and dihydroxyacetone phosphate(DAF).

DAF under the influence triosephosphate isomerase turns into GA-3-F. Thus, from one glucose molecule we get 2 molecules of GA-3-P.

GA-3-F in the presence glyceraldehyde phosphate dehydrogenase , coenzyme NAD + and phosphoric acid are oxidized to form 1,3-diphosphoglycerate(1,3-DPG) and NADH(H+). The energy released in this reaction is accumulated in the high-energy bond of 1,3-DPG, which is further converted into 3-phosphoglycerate Under the influence phosphoglycerate kinase . This process is associated with phosphorylation of ADP (formation ATP at the level substrate phosphorylation).

3-FY with the participation phosphoglycerate mutase turns into 2-phosphoglycerate(2-PG), which undergoes dehydration in the presence enolases and Mg 2+ ions. As a result, phosphoenolpyruvate(PEP), containing a high-energy bond. PEP under the action of pyruvate kinase in the presence of Mg 2+ ions is converted into the enol form pyruvate. This process is associated with ATP synthesis at the level of substrate phosphorylation. The enol form of pyruvate tautomerizes to the ketone form.

At the final stage of glycolysis, pyruvate is reduced to L-lactate in the presence of NADH(H+) and enzyme lactate dehydrogenase . The coenzyme NAD + formed in this reaction is subsequently used in the oxidation reaction of GA-3-P.

Thus, the final products of glycolysis are 2 molecules of L-lactate and 2 molecules of ATP per 1 molecule of glucose.

Regulation glycolysis occurs at the level phosphofructokinase reactions. The enzyme is inhibited by high levels of ATP and citrate. Allosteric activators of phosphofructokinase are AMP, ADP, P-6-P.

Glycogenolysis

Glycogen is a branched polysaccharide consisting of α-D-glucose residues linked to each other in the linear regions of the molecule by α-1,4-glycosidic bonds, and at branch points by α-1,6-glycosidic bonds.

Under the influence glycogen phosphorylase cleavage of one glucose residue occurs in linear sections with its transfer to a phosphoric acid molecule, resulting in the formation glucose-1-phosphate(G-1-F). Glycogen phosphorylase works until there are 4 glucose residues left before the nearest branch point, then the enzyme starts working oligosaccharide transferase , transferring a fragment of 3 glucose residues to the adjacent branch, thus leaving one glucose residue at the branch point. It is cleaved hydrolytically with the help of -1,6-glycosidase in the form of a free glucose molecule, and conditions are again created for glycogen phosphorylase to work in a linear chain.

Product of glycogen phosphorylase reaction G-1-F further under the influence phosphoglucomutase turns into G-6-F, which is involved in glycolytic path (Fig. 8).

Rice. 8. Scheme of glycogen breakdown in muscles and liver.

The end products of glycogenolysis are 2 molecules of L-lactate and 3 molecules of ATP per 1 molecule of glucose.

Most organisms in the biosphere are in aerobic conditions. In the presence of oxygen in the body, complete “burning” of carbohydrates and other “cellular fuel” molecules occurs to the final products - C0 2 and H 2 0.

Rice. 19.1. Scheme of complete oxidation of glucose to six CO2 molecules and the energy efficiency of this process (ATP balance); ATP formation pathways:

SF - substrate phosphorylation; OP - oxidative phosphorylation

The overall process of complete oxidation of glucose under aerobic conditions is described by the stoichiometric equation

In this complex, multi-stage process of glucose oxidation, three stages can be distinguished (Fig. 19.1).

  • At the first stage, aerobic glycolysis reactions occur, during which glucose is split into two pyruvate molecules. This stage constitutes the initial phase of carbohydrate decomposition; it is called “preparatory”.
  • At the second stage, a chain of reactions of oxidative decarboxylation of pyruvate occurs, resulting in the formation of one of the central metabolites of the cell, acetyl-S-CoA, and the oxidation of one carbon atom of pyruvate to CO 2 . Since two molecules of pyruvate are formed per one molecule of glucose, at this stage the oxidation of two carbon atoms of glucose to CO 2 already occurs.
  • The third stage is an extremely important set of reactions of complete oxidation of the acetyl residue, which is called the tricarboxylic acid cycle (TCA cycle).

The process of aerobic oxidation of carbohydrates is accompanied by the release of a large amount of energy (2880 kJ/mol glucose). If we add up the total ATP yield in this process, it will be 38 molecules (see Fig. 19.1). As noted earlier (Chapter 15), the synthesis of one high-energy ATP bond

31 kJ is needed, and 1178 kJ is consumed for the synthesis of 38 ATP molecules, i.e., more than 40% of the free energy of complete oxidation of glucose is stored in ATP molecules. This indicates the high efficiency of oxidative processes occurring under aerobic conditions compared to anaerobic ones. In the process of aerobic oxidation, metabolically available energy is accumulated in reduced NADH and FADH 2 molecules, which are then oxidized in an oxygen-dependent process oxidative phosphorylation, the result of which is the formation of 34 ATP molecules, and only 4 ATP molecules are formed by substrate phosphorylation: 2ATP in glycolysis (stage I) and 2ATP in the TCA cycle (2 turns, stage III).

It should be noted that if the first stage of aerobic oxidation of carbohydrates - glycolysis - is a specific process of glucose catabolism, then the next two - oxidative decarboxylation of pyruvate and the TCA cycle belong to the general pathways of catabolism (GCP). After the formation of pyruvate (C 3 fragment) and acetyl-CoA (C 2 fragment), formed during the breakdown of not only glucose, but also lipids and amino acids, the oxidation pathways of these substances to the final products occur in the same way according to the mechanism of OPC reactions.

At the first stage, glucose is split into 2 trioses:

Thus, at the first stage of glycolysis, 2 molecules of ATP are spent on activating glucose and 2 molecules of 3-phosphoglyceraldehyde are formed.

In the second stage, 2 molecules of 3-phosphoglyceraldehyde are oxidized to two molecules of lactic acid.

The significance of the lactate dehydrogenase reaction (LDH) is to oxidize NADH 2 to NAD under oxygen-free conditions and allow the dehydrogenase reaction of 3-phosphoglyceraldehyde to occur.

Summary equation of glycolysis:

glucose + 2ADP + 2H 3 PO 4 → 2 lactate + 2ATP + 2H 2 O

Glycolysis occurs in the cytosol. Its regulation is carried out by key enzymes - phosphofructokinase, pyruvate kinase. These enzymes are activated by ADP and NAD and inhibited by ATP and NADH 2 .

The energy efficiency of anaerobic glycolysis comes down to the difference between the number of ATP molecules consumed and the number of ATP molecules produced. 2 ATP molecules are consumed per glucose molecule in the hexokinase reaction and the phosphofructokinase reaction. 2 molecules of ATP are formed per molecule of triose (1/2 glucose) in the glycerokinase reaction and pyruvate kinase reaction. For a molecule of glucose (2 trioses), 4 molecules of ATP are formed, respectively. Total balance: 4 ATP – 2 ATP = 2 ATP. 2 ATP molecules accumulate ≈ 20 kcal, which is about 3% of the energy of complete oxidation of glucose (686 kcal).

Despite the relatively low energy efficiency of anaerobic glycolysis, it has an important biological significance in that it the only one a method of generating energy in oxygen-free conditions. In conditions of oxygen deficiency, it ensures intense muscle work during the initial period of physical activity.

In fetal tissue Anaerobic glycolysis is very active under conditions of oxygen deficiency. It remains active during newborns, gradually giving way to aerobic oxidation.

Further conversion of lactic acid

  • With an intensive supply of oxygen under aerobic conditions, lactic acid is converted into PVA and, through acetyl CoA, is included in the Krebs cycle, providing energy.
  • Lactic acid is transported from muscles to the liver, where it is used for glucose synthesis - the R. Cori cycle.

Measles cycle

  • At high concentrations of lactic acid in tissues, it can be released through the kidneys and sweat glands to prevent acidosis.

Aerobic glucose oxidation



Aerobic oxidation of glucose includes 3 stages:

Stage 1 occurs in the cytosol and involves the formation of pyruvic acid:

Glucose → 2 PVK + 2 ATP + 2 NADH 2;

Stage 2 occurs in mitochondria:

2 PVC → 2 acetyl - CoA + 2 NADH 2;

Stage 3 occurs inside the mitochondria:

2 acetyl-CoA → 2 TCA cycle.

Due to the fact that 2 molecules of NADH 2 are formed in the cytosol at the first stage, and they can only be oxidized in the mitochondrial respiratory chain, hydrogen transfer from NADH 2 of the cytosol to the intramitochondrial electron transport chain is necessary. Mitochondria are impermeable to NADH 2 , so special shuttle mechanisms exist for the transfer of hydrogen from the cytosol to mitochondria. Their essence is reflected in the diagram, where X is the oxidized form of the hydrogen carrier, and XH 2 is its reduced form:

Depending on which substances are involved in the transfer of hydrogen across the mitochondrial membrane, several shuttle mechanisms are distinguished.

Glycerophosphate shuttle mechanism in which the loss of two ATP molecules occurs, because instead of two molecules of NADH 2 (potentially 6 molecules of ATP), 2 molecules of FADH 2 are formed (actually 4 molecules of ATP).

Malate shuttle mechanism works to remove hydrogen from the mitochondrial matrix:

Energy efficiency of aerobic oxidation.

  1. glucose → 2 PVK + 2 ATP + 2 NADH 2 (→8 ATP).
  2. 2 PVK → 2 acetyl CoA + 2 NADH 2 (→ 6 ATP).
  3. 2 acetyl CoA → 2 TCA cycle (12*2 = 24 ATP).

In total, 38 ATP molecules can be formed, from which it is necessary to subtract 2 ATP molecules lost in the glycerophosphate shuttle mechanism. Thus, it is formed 36 ATP.

36 ATP (about 360 kcal) is from 686 kcal. 50-60% is the energy efficiency of aerobic glucose oxidation, which is twenty times higher than the efficiency of anaerobic glucose oxidation. Therefore, when oxygen enters the tissues, the anaerobic pathway is blocked, and this phenomenon is called Pasteur effect. In newborns the aerobic pathway begins to activate in the first 2-3 months of life.



6.5. 2. Biosynthesis of glucose (gluconeogenesis)

Gluconeogenesis is a pathway for the synthesis of glucose in the body from non-carbohydrate substances, which is capable of maintaining glucose levels for a long time in the absence of carbohydrates in the diet. The starting materials for it are lactic acid, PVC, amino acids, glycerin. Gluconeogenesis occurs most actively in the liver and kidneys. This process is intracellularly localized partly in the cytosol, partly in the mitochondria. In general, gluconeogenesis is the reverse process of glycolysis.

Glycolysis has three irreversible stages catalyzed by enzymes:

· pyruvate kinase;

· phosphofructokinase;

· hexokinase.

Therefore in gluconeogenesis Instead of these enzymes, there are specific enzymes that bypass these irreversible stages:

  • pyruvate carboxylase and carboxykinase (“bypass” pyruvate kinase);
  • fructose-6-phosphatase (“bypasses” phosphofructokinase);
  • glucose-6-phosphatase (“bypasses” hexokinase).

Glucose-6-phosphate, under the action of glucose-6-phosphatase, is converted into glucose, which exits the hepatocytes into the blood.

The key enzymes for gluconeogenesis are pyruvate carboxylase And fructose 1,6-biphosphatase. The activator for them is ATP (the synthesis of one glucose molecule requires 6 ATP molecules).

Thus, a high concentration of ATP in cells activates gluconeogenesis, which requires energy, and at the same time inhibits glycolysis (at the stage of phosphofructokinase), leading to the formation of ATP. This situation is illustrated by the graph below.

Vitamin H

Vitamin H (biotin, antiseborrheic vitamin), which by its chemical nature is a sulfur-containing heterocycle with valeric acid residues, participates in gluconeogenesis. It is widely distributed in animal and plant products (liver, yolk). The daily requirement for it is 0.2 mg. Vitamin deficiency manifests itself as dermatitis, nail damage, an increase or decrease in the formation of sebum (seborrhea). Biological role of vitamin H:

  • participates in carboxylation reactions;
  • participates in transcarboxylation reactions;
  • participates in the exchange of purine bases and some amino acids.

Gluconeogenesis is active in recent months intrauterine development. After the birth of a child, the activity of the process increases, starting from the third month of life.

BELARUSIAN STATE UNIVERSITY OF INFORMATICS AND RADIO ELECTRONICS

Department of ETT

« Aerobic oxidation of carbohydrates. Biological oxidation and reduction"

MINSK, 2008


Aerobic oxidation of carbohydrates- the main way of energy production for the body. Indirect - dichotomous and direct - apotomic.

The direct pathway of glucose breakdown is pentose cycle– leads to the formation of pentoses and the accumulation of NADPH 2. The pentose cycle is characterized by the sequential elimination of each of its 6 carbon atoms from glucose molecules with the formation of 1 molecule of carbon dioxide and water during one cycle. The breakdown of the entire glucose molecule occurs over 6 repeating cycles.

The importance of the pentose phosphate cycle of carbohydrate oxidation in metabolism is great:

1. It supplies reduced NADP, necessary for the biosynthesis of fatty acids, cholesterol, etc. Due to the pentose cycle, 50% of the body's need for NADPH 2 is covered.

2. Supply of pentose phosphates for the synthesis of nucleic acids and many coenzymes.

The reactions of the pentose cycle take place in the cytoplasm of the cell.

In a number of pathological conditions, the proportion of the pentose pathway of glucose oxidation increases.

Indirect path– breakdown of glucose to carbon dioxide and water with the formation of 36 molecules of ATP.

1. Breakdown of glucose or glycogen to pyruvic acid

2. Conversion of pyruvic acid to acetyl-CoA

Oxidation of acetyl-CoA in the Krebs cycle to carbon dioxide and water

C 6 H 12 O 6 + 6 O 2 ® 6 CO 2 + 6 H 2 O + 686 kcal

In the case of aerobic conversion, pyruvic acid undergoes oxidative decarboxylation to form acetyl-CoA, which is then oxidized to carbon dioxide and water.

The oxidation of pyruvate to acetyl-CoA is catalyzed by the pyruvate dehydrogenase system and occurs in several stages. Total reaction:

Pyruvate + NADH + NS-CoA ® acetyl-CoA + NADH 2 + CO 2 reaction is almost irreversible

Complete oxidation of acetyl-CoA occurs in the tricarboxylic acid cycle or Krebs cycle. This process takes place in mitochondria.

The cycle consists of 8 consecutive reactions:

In this cycle, a molecule containing 2 carbon atoms (acetic acid in the form of acetyl-CoA) reacts with a molecule of oxaloacetic acid, resulting in the formation of a compound with 6 carbon atoms - citric acid. During the process of dehydrogenation, decarboxylation and preparatory reaction, citric acid is converted back into oxaloacetic acid, which easily combines with another acetyl-CoA molecule.

1) acetyl-CoA + oxaloacetate (SCHUK) ®citric acid

citrate synthase

2) citric acid® isocitric acid

aconitate hydratase

3) isocitric acid + NAD®a-ketoglutaric acid + NADH 2 + CO 2

isocitrate dehydrogenase

4) a-ketoglutaric acid + NS-CoA + NAD®succinylSCoA + NADH 2 + CO 2

5) succinyl-CoA+GDP+Fn®succinic acid+GTP+HS-CoA

succinyl CoA synthetase

6) succinic acid+FAD®fumaric acid+FADN 2

succinate dehydrogenase

7) fumaric acid + H 2 O® L malic acid

fumarate hydratase

8) malate + NAD®oxaloacetate + NADH 2

malate dehydrogenase

In total, when a glucose molecule is broken down in tissues, 36 ATP molecules are synthesized. Undoubtedly, this is an energetically more efficient process than glycolysis.

The Krebs cycle is the common final pathway by which the metabolism of carbohydrates, fatty acids and amino acids is completed. All these substances are included in the Krebs cycle at one stage or another. Next, biological oxidation or tissue respiration occurs, the main feature of which is that it proceeds gradually through numerous enzymatic stages. This process occurs in mitochondria, cellular organelles in which a large number of enzymes are concentrated. The process involves pyridine-dependent dehydrogenases, flavin-dependent dehydrogenases, cytochromes, coenzyme Q - ubiquinone, proteins containing non-heme iron.

The rate of respiration is controlled by the ATP/ADP ratio. The lower this ratio, the more intense respiration occurs, ensuring the production of ATP.

Also, the citric acid cycle is the main source of carbon dioxide in the cell for carboxylation reactions, which begin the synthesis of fatty acids and gluconeogenesis. The same carbon dioxide supplies carbon for urea and some units of the purine and pyrimidine rings.

The relationship between the processes of carbohydrate and nitrogen metabolism is also achieved through intermediate products of the citric acid cycle.

There are several pathways through which citric acid cycle intermediates are incorporated into the process of lipogenesis. The breakdown of citrate leads to the formation of acetyl-CoA, which plays the role of a precursor in the biosynthesis of fatty acids.

Isocitrate and malate provide the formation of NADP, which is consumed in the subsequent reductive stages of fat synthesis.

The role of the key factor determining the conversion of NADH is played by the state of adenine nucleotides. High ADP and low ATP indicate low energy reserves. In this case, NADH is involved in the reactions of the respiratory chain, enhancing the processes of oxidative phosphorylation associated with energy storage. The opposite phenomenon is observed at low ADP content and high ATP content. By limiting the electron transport system, they promote the use of NADH in other reducing reactions such as glutamate synthesis and gluconeogenesis.

Biological oxidation and reduction.

Cellular respiration is the totality of enzymatic processes occurring in each cell, as a result of which molecules of carbohydrates, fatty acids and amino acids are ultimately broken down into carbon dioxide and water, and the released biologically useful energy is stored by the cell and then used. Many enzymes that catalyze these reactions are located in the walls and cristae of mitochondria.

It is known that for all manifestations of life - growth, movement, irritability, self-reproduction - the cell must expend energy. All living cells obtain biologically useful energy through enzymatic reactions in which electrons are transferred from one energy level to another. For most organisms, the final electron acceptor is oxygen, which reacts with electrons and hydrogen ions to form a water molecule. The transfer of electrons to oxygen occurs with the participation of an enzyme system located in mitochondria - the electron transfer system. ATP serves as the “energy currency” of the cell and is used in all metabolic reactions that require energy. Energy-rich molecules do not move freely from one cell to another, but are formed in that place. where they should be used. For example, high-energy ATP bonds, which serve as a source of energy for reactions associated with muscle contraction, are formed in the muscle cells themselves.

The process in which atoms or molecules lose electrons (e -) is called oxidation, and the reverse process - the addition (attachment) of electrons to an atom or molecule - is called reduction.

A simple example of oxidation and reduction is the reversible reaction - Fe 2+ ®Fe 3+ + e -

Reaction going to the right - oxidation, removal of an electron

To the left - reduction (addition of an electron)

All oxidation reactions (in which an electron is removed) must be accompanied by reduction - a reaction in which electrons are captured by some other molecule, because they do not exist in a free state.

The transfer of electrons through the electron transport system occurs through a series of sequential oxidation-reduction reactions, which together are called biological oxidation. If the energy of the electron flow accumulates in the form of high-energy phosphate bonds (~P), then the process is called oxidative phosphorylation. Specific compounds that form an electron transport system and that are alternately oxidized and reduced are called cytochromes. Each of the cytochromes is a protein molecule to which is attached a chemical group called heme; at the center of the heme is an iron atom, which is alternately oxidized and reduced, giving or accepting one electron.

All biological oxidation reactions occur with the participation of enzymes, and each enzyme is strictly specific and catalyzes either the oxidation or the reduction of very specific chemical compounds.

Another component of the electron transfer system, ubiquinone or coenzyme Q, is capable of acquiring or donating electrons.

Mitochondria are contained in the cytoplasm of the cell and are microscopic rod-shaped or other shaped formations, the number of which in one cell amounts to hundreds or thousands.

What are mitochondria, what is their structure? The internal space of mitochondria is surrounded by two continuous membranes, with the outer membrane being smooth and the inner one forming numerous folds or cristae. The intramitochondrial space, bounded by the inner membrane, is filled with the so-called matrix, which consists of approximately 50% protein and has a very fine structure. Mitochondria contain a large number of enzymes. The outer membrane of mitochondria does not contain any of the components of the respiratory catalyst chain. Based on the enzyme composition of the outer membrane, it is still difficult to answer the question of what its purpose is. Perhaps it plays the role of a partition separating the internal, working part of the mitochondria from the rest of the cell. Enzymes of the respiratory chain are associated with the inner membrane. The matrix contains a number of Krebs cycle enzymes.

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