Use of different starting substrates

The initial substrates for respiration can be various substances that are converted during specific metabolic processes into Acetyl-CoA with the release of a number of by-products. The reduction of NAD (NADP) and the formation of ATP can already occur at this stage, but most of them are formed in the tricarboxylic acid cycle during the processing of Acetyl-CoA.

Glycolysis

Glycolysis - the path of enzymatic breakdown of glucose - is a process common to almost all living organisms. In aerobes it precedes cellular respiration itself; in anaerobes it ends with fermentation. Glycolysis itself is a completely anaerobic process and does not require the presence of oxygen to occur.

Its first stage proceeds with the energy consumption of 2 molecules of ATP and includes the splitting of a glucose molecule into 2 molecules of glyceraldehyde-3-phosphate. At the second stage, NAD-dependent oxidation of glyceraldehyde-3-phosphate occurs, accompanied by substrate phosphorylation, that is, the addition of a phosphoric acid residue to the molecule and the formation of a high-energy bond in it, after which the residue is transferred to ADP with the formation of ATP.

Thus, the equation of glycolysis is as follows:

Glucose + 2NAD + + 4ADP + 2ATP + 2P n = 2PVK + 2NAD∙H + 2 ADP + 4ATP + 2H 2 O + 4H + .

Reducing ATP and ADP from the left and right sides of the reaction equation, we get:

Glucose + 2NAD + + 2ADP + 2P n = 2NAD∙H + 2PVK + 2ATP + 2H 2 O + 4H + .

Oxidative decarboxylation of pyruvate

Pyruvic acid (pyruvate) formed during glycolysis, under the action of the pyruvate dehydrogenase complex (a complex structure of 3 different enzymes and more than 60 subunits), breaks down into carbon dioxide and acetaldehyde, which, together with Coenzyme A, forms Acetyl-CoA. The reaction is accompanied by the restoration of NAD to NADH.

In eukaryotes, the process takes place in the mitochondrial matrix.

β-oxidation of fatty acids

Main article: β-oxidation

Finally, in the fourth stage, the resulting β-keto acid is cleaved by β-ketothiolase in the presence of coenzyme A into acetyl-CoA and new acyl-CoA, in which the carbon chain is 2 atoms shorter. The β-oxidation cycle is repeated until all the fatty acid is converted to acetyl-CoA.

Tricarboxylic acid cycle

Total reaction equation:

Acetyl-CoA + 3NAD + + FAD + GDP + Pn + 2H 2 O + CoA-SH = 2CoA-SH + 3NADH + 3H + + FADH 2 + GTP + 2CO 2

In eukaryotes, the enzymes of the cycle are in a free state in the mitochondrial matrix, only succinate dehydrogenase is built into the inner mitochondrial membrane.

The bulk of ATP molecules are produced by oxidative phosphorylation at the last stage of cellular respiration: in the electron transport chain. Here, the oxidation of NADH and FADN 2, reduced in the processes of glycolysis, β-oxidation, the Krebs cycle, etc., occurs. The energy released during these reactions is due to the chain of electron carriers localized in the inner membrane of mitochondria (in prokaryotes - in cytoplasmic membrane), is transformed into a transmembrane proton potential. The enzyme ATP synthase uses this gradient to synthesize ATP, converting its energy into the energy of chemical bonds. It is calculated that a molecule of NAD∙H can produce 2.5 molecules of ATP during this process, FADH 2 - 1.5 molecules.

The final electron acceptor in the aerobic respiratory chain is oxygen.

Anaerobic respiration

General equation of respiration, ATP balance

Stage	Coenzyme yield	ATP output (GTP)	Method for obtaining ATP
First phase of glycolysis		−2	Phosphorylation of glucose and fructose 6-phosphate using 2 ATP from the cytoplasm.
Second phase of glycolysis		4	Substrate phosphorylation
	2 NADH	3 (5)	Oxidative phosphorylation. Only 2 ATP is formed from NADH in the electron transport chain, since the coenzyme is formed in the cytoplasm and must be transported to the mitochondria. When the malate-aspartate shuttle is used for transport into mitochondria, 3 moles of ATP are formed from NADH. When using a glycerophosphate shuttle, 2 moles of ATP are formed.
Decarboxylation of pyruvate	2 NADH	5	Oxidative phosphorylation
Krebs cycle		2	Substrate phosphorylation
	6 NADH	15	Oxidative phosphorylation
	2 FADN 2	3	Oxidative phosphorylation
General output		30 (32) ATP	With the complete oxidation of glucose to carbon dioxide and the oxidation of all resulting coenzymes.

Notes

see also

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BREATH Modern encyclopedia

BREATH- a set of processes that ensure the entry of oxygen into the body and the removal of carbon dioxide (external respiration), as well as the use of oxygen by cells and tissues for the oxidation of organic substances with the release of energy necessary for... ... Big Encyclopedic Dictionary

Breath- BREATHING, a set of processes that ensure the entry of oxygen into the body and the removal of carbon dioxide (external respiration), as well as the use of oxygen by cells and tissues for the oxidation of organic substances with the release of energy,... ... Illustrated Encyclopedic Dictionary

BREATH- BREATHING, I, cf. 1. The process of absorption of oxygen and release of carbon dioxide by living organisms. Respiratory system. Cellular d. (special). 2. Inhalation and release of air by the lungs. Smooth d. Restrain d. D. spring (translated). Second wind rush... ... Ozhegov's Explanatory Dictionary

breath- BREATHING, BREATHING, I; Wed 1. The intake and release of air by the lungs or (in some animals) other relevant organs as a process of absorption of oxygen and release of carbon dioxide by living organisms. Respiratory system. Noisy, heavy... encyclopedic Dictionary

Breath- in a commonly used sense, denotes a series of movements of the chest continuously alternating during life in the form of inhalation and exhalation and determining, on the one hand, an influx of fresh air into the lungs, and on the other, the removal of already spoiled air from them... ... Encyclopedic Dictionary F.A. Brockhaus and I.A. Ephron

Breath- I Respiration (respiration) is a set of processes that ensure the supply of oxygen to the body from atmospheric air, its use in the biological oxidation of organic substances and the removal of carbon dioxide from the body. As a result... ... Medical encyclopedia

Flow of energy in a cell

The flow of energy in a cell is based on the processes of nutrition of organisms and cellular respiration.

1. Food– the process of acquiring matter and energy by living organisms.

2. Cellular respiration- the process by which living organisms release energy from organic substances rich in it when they are enzymatically broken down (dissimilated) into simpler ones. Cellular respiration can be aerobic or anaerobic.

3. Aerobic respiration– energy is obtained with the participation of oxygen in the process of breakdown of organic substances. It is also called the oxygen (aerobic) stage of energy metabolism.

Anaerobic respiration– obtaining energy from food without using free atmospheric oxygen. In general, the flow of energy in a cell can be represented as follows (Fig. 5.3.)

FOOD

SUGAR, FATTY ACIDS, AMINO ACIDS

CELLULAR RESPIRATION

ATP

CO 2, H 2 O, NH 3

CHEMICAL, MECHANICAL, ELECTRICAL, OSMOTIC WORK

ADP + H 3 PO 4

Fig.5.3. Flow of energy in a cell

Chemical work: biosynthesis in the cell of proteins, nucleic acids, fats, polysaccharides.

Mechanical work: contraction of muscle fibers, beating of cilia, divergence of chromosomes during mitosis.

Electrical work– maintaining a potential difference across the cell membrane.

Osmotic work– maintenance of substance gradients in the cell and its environment.

The process of aerobic respiration takes place in three stages: 1) preparatory; 2) oxygen-free; 3) oxygen.

First stage – preparatory or digestive stage, which includes the enzymatic breakdown of polymers into monomers: proteins into amino acids, fats into glycerol and fatty acids, glycogen and starch into glucose, nucleic acids into nucleotides. It occurs in the gastrointestinal tract with the participation of digestive enzymes and in the cytoplasm of cells with the participation of lysosome enzymes.

At this stage, a small amount of energy is released, dissipated in the form of heat, and the resulting monomers undergo further breakdown in cells or are used as building material.

Second phase – anaerobic (oxygen-free). It occurs in the cytoplasm of cells without the participation of oxygen. The monomers formed in the first stage undergo further cleavage. An example of such a process is glycolysis – oxygen-free incomplete breakdown of glucose.

In glycolysis reactions, one molecule of glucose (C 6 H 12 O 6) produces two molecules of pyruvic acid (C 3 H 4 O 3 - PVK). In this case, 4 H+ atoms are split off from each glucose molecule and 2 ATP molecules are formed. Hydrogen atoms attach to NAD + (nicotinamide adenine dinucleotide; the function of NAD and similar carriers is to accept Hydrogen in the first reaction (reduce), and give it away (oxidize) in the other.

The overall equation for glycolysis looks like this:

C 6 H 12 O 6 + 2ADP + 2H 3 PO 4 + 2NAD + → 2C 3 H 4 O 3 + 2ATP + 2H 2 O + 2NAD H 2

During glycolysis, 200 kJ/mol of energy is released, of which 80 kJ or 40% goes to ATP synthesis, and 120 kJ (60%) is dissipated as heat.

a) in animal cells 2 molecules of lactic acid are formed, which are subsequently converted into glycogen and deposited in the liver;

b) alcoholic fermentation occurs in plant cells with the release of CO 2. The final product is ethanol.

Anaerobic respiration, compared to oxygen respiration, is an evolutionarily earlier, but less efficient form of obtaining energy from nutrients.

Third stage – aerobic(oxygen, tissue respiration) occurs in mitochondria and requires the presence of oxygen.

Organic compounds formed at the previous oxygen-free stage are oxidized by eliminating hydrogen to CO 2 and H 2 O. The separated Hydrogen atoms are transferred to Oxygen with the help of carriers, interact with it and form water. This process is accompanied by the release of a significant amount of energy, part of which (55%) goes to the formation of water. In the oxygen stage, reactions of the Krebs cycle and oxidative phosphorylation reactions can be distinguished.

Krebs cycle(tricarboxylic acid cycle) occurs in the mitochondrial matrix. It was discovered by the English biochemist H. Krebs in 1937.

The Krebs cycle begins by the reaction of pyruvic acid with acetic acid. In this case, citric acid is formed, which, after a series of successive transformations, again becomes acetic acid and the cycle repeats.

During the reactions of the Krebs cycle, 4 pairs of Hydrogen atoms, two CO 2 molecules, and one ATP molecule are formed from one PVC molecule. Carbon dioxide is removed from the cell, and Hydrogen atoms join the carrier molecules - NAD and FAD (flavin adenine dinucleotide), resulting in the formation of NADH 2 and FADH 2.

The transfer of energy from NADH 2 and FADH 2, which were formed in the Krebs cycle and at the previous anaerobic stage, to ATP takes place on the inner membrane of mitochondria in the respiratory chain.

Respiratory chain or electron transport chain (electron transport chain) found in the inner membrane of mitochondria. It is based on electron carriers, which are part of enzyme complexes that catalyze redox reactions.

Hydrogen pairs are split off from NADH 2 and FADH 2, in the form of protons and electrons (2H + +2e), and enter electron transport chain. In the respiratory chain they enter into a series of biochemical reactions, the final result of which is the synthesis of ATP (Fig. 5.4.)

Rice. 5.4 Electron transport chain

Electrons and protons are captured by molecules of respiratory chain carriers and transported: electrons to the inner side of the membrane, and protons to the outer. Electrons combine with Oxygen. Oxygen atoms become negatively charged:

O 2 + e - = O 2 -

Protons (H +) accumulate on the outside of the membrane, and anions (O 2-) accumulate on the inside. As a result, the potential difference increases.

In some places of the membrane, molecules of the enzyme for the synthesis of ATP (ATP synthetase), which has an ion (proton) channel, are embedded. When the potential difference across the membrane reaches 200 mV, protons (H +) are pushed through the channel by the force of the electric field and pass to the inner side of the membrane where they interact with O 2 -, forming H 2 O

½ O 2 + 2H + = H 2 O

Oxygen entering the mitochondria is necessary for the attachment of electrons (e -), and then protons (H+). In the absence of O2, processes associated with the transport of protons and electrons stop. In these cases, many cells synthesize ATP by breaking down nutrients through the fermentation process.

Summary equation of the oxygen stage

2C 3 H 4 O 3 + 36 H 3 PO 4 + 6 O 2 + 36 ADP = 6 CO 2 + 42 H 2 O + 36 ATP + 2600 kJ

1440 (40·36) accumulated in ATP

1160 kJ released as heat

Summary equation of oxygen respiration, including oxygen-free and oxygen stages :

C 6 H 12 O 6 + 38ADP + 38H 3 PO 4 + 6O 2 = 38ATP + 6CO 2 + 44H 2 O

The end products of energy metabolism (CO 2 , H 2 O, NH 3), as well as excess energy, are released from the cell through the cell membrane, the structure and functions of which deserve special attention.

metabolism

Metabolism is a set of reactions of biosynthesis and breakdown of substances in a cell. A certain sequence of enzymatic transformations of a substance in a cell is called a metabolic pathway, and the resulting intermediate products are metabolites.

Two aspects of metabolism interconnected in space and time are plastic and energy metabolism.

The set of reactions of biological synthesis, when complex organic substances similar to the contents of the cell are formed from simple substances entering the cell from the outside, is called anabolism (plastic metabolism). Assimilation occurs. These reactions take place using the energy generated as a result of the breakdown reactions of organic substances supplied with food. The most intensive plastic exchange occurs during the growth of the organism. The most important processes of anabolism are photosynthesis and protein synthesis.

Catabolism (energy metabolism) – enzymatic breakdown (hydrolysis, oxidation) of complex organic compounds into simpler ones. Dissimilation occurs. These reactions release energy.

Stages of energy metabolism. Cellular respiration.

The process opposite to biosynthesis is dissimilation, or catabolism, a set of cleavage reactions. When high-molecular compounds are broken down, the energy required for biosynthesis reactions is released. Therefore, dissimilation is also called energy metabolism of the cell. Heterotrophic organisms obtain the energy necessary for life from food. The chemical energy of nutrients is contained in various covalent bonds between atoms in the molecule of organic compounds. Part of the energy released from nutrients is dissipated in the form of heat, and part is accumulated, i.e. accumulates in energy-rich high-energy phosphate bonds of ATP. It is ATP that provides energy for all types of cellular functions: biosynthesis, mechanical work, active transport of substances across membranes, etc. ATP synthesis occurs in mitochondria. Cellular respiration is the enzymatic decomposition of organic substances (glucose) in the cell to carbon dioxide and water in the presence of free oxygen, associated with the storage of energy released.

Energy metabolism is divided into a series of stages, each of which is carried out with the participation of special enzymes in certain areas of cells.

The first stage is preparatory. In humans and animals, during the digestion process, large food molecules, including oligo-, polysaccharides, lipids, proteins, nucleic acids, break down into smaller molecules - glucose, glycerol, fatty acids, amino acids, nucleotides. At this stage, a small amount of energy is released and dissipated as heat. These molecules are absorbed in the intestines into the blood and delivered to various organs and tissues, where they can serve as building material for the synthesis of new substances needed by the body, and to provide the body with energy.

The second stage is oxygen-free, or incomplete, anaerobic respiration (glycolysis or fermentation). The substances formed at this stage undergo further breakdown with the participation of enzymes.

Glycolysis is one of the central pathways of glucose catabolism, when the breakdown of carbohydrate with the formation of ATP occurs under oxygen-free conditions. In aerobic organisms (plants, animals) this is one of the stages of cellular respiration; in microorganisms, fermentation is the main way of obtaining energy. Glycolysis enzymes are localized in the cytoplasm. The process occurs in two stages in the absence of oxygen.

1). Preparatory stage - activation of glucose molecules occurs as a result of the addition of phosphate groups, which occurs with the consumption of ATP, with the formation of two 3-carbon molecules of glyceraldehyde phosphate.

2), redox stage - enzymatic reactions of substrate phosphorylation occur, when energy is extracted in the form of ATP immediately at the moment of oxidation of the substrate. Thus, the glucose molecule undergoes further stepwise cleavage and oxidation to two 3-carbon molecules of pyruvic acid. In summary, the glycolysis process looks like this:

C 6 H 12 O 6 + 2 H 3 PO 4 + 2 ADP → 2 C 3 H 6 O 3 + 2 ATP + 2 H 2 O

During the oxidation of glucose, protons are removed and electrons are stored in the form of NADH. In muscle, anaerobic respiration breaks down a glucose molecule into two PLA molecules, which are then reduced to lactic acid using reduced NADH. In yeast fungi, a glucose molecule without the participation of oxygen is converted into ethyl alcohol and carbon dioxide (alcoholic fermentation):

C 6 H 12 O 6 + 2 H 3 PO 4 + 2 ADP → 2 C 3 H 5 OH + 2 CO 2 + 2 ATP + 2 H 2 O

In other microorganisms, the breakdown of glucose - glycolysis - can result in the formation of acetone, acetic acid, etc.

In all cases, the breakdown of one glucose molecule is accompanied by the formation of 4 ATP molecules. In this case, in the reactions of glucose breakdown, 2 ATP molecules are consumed. Thus, during the oxygen-free breakdown of glucose, 2 ATP molecules are formed. In general, the energy efficiency of glycolysis is low, because 40% of the energy is stored as a chemical bond in the ATP molecule, and the rest of the energy is dissipated as heat.

The third stage is the stage of oxygen splitting, or aerobic respiration. Aerobic respiration occurs in the mitochondria of the cell when oxygen is available. The process of cellular respiration also consists of 3 stages.

Oxidative decarboxylation of PVC, formed at the previous stage from glucose and entering the mitochondrial matrix. With the participation of a complex enzyme complex, a carbon dioxide molecule is split off and an acetyl-coenzyme A compound is formed, as well as NADH.

Tricarboxylic acid cycle (Krebs cycle). This stage includes a large number of enzymatic reactions. Inside the mitochondrial matrix, acetyl coenzyme A (which can be formed from various substances) is broken down, releasing another molecule of carbon dioxide, as well as the formation of ATP, NADH and FADH. Carbon dioxide enters the blood and is removed from the body through the respiratory system. The energy stored in the NADH and FADH molecules is used to synthesize ATP in the next stage of cellular respiration.

Oxidative phosphorylation is a multistage transfer of electrons from reduced forms of NADH and FADH along the electron transport chain built into the inner membrane of mitochondria to the final acceptor oxygen, coupled with the synthesis of ATP. The electron transport chain includes a number of components: ubiquinone (coenzyme Q), cytochromes b, c, a, which act as electron carriers. As a result of the functioning of the electron transport chain, hydrogen atoms from NADH and FADH are separated into protons and electrons. Electrons are gradually transferred to oxygen, so water is formed, and protons are pumped into the intermembrane space of mitochondria, using the energy of the electron flow. Then the protons return to the mitochondrial matrix, passing through special channels as part of the ATP synthetase enzyme built into the membrane. This produces ATP from ADP and phosphate. In the electron transport chain there are 3 sites of conjugation of oxidation and phosphorylation, i.e. sites of ATP formation. The mechanism for the formation of energy and the form of ATP in mitochondria is explained by the chemiosmotic theory of P. Mitchell. Oxygen respiration is accompanied by the release of a large amount of energy and its accumulation in ATP molecules. The overall equation for aerobic respiration looks like this?

C 6 H 12 O 6 + 6 O 2 + 38 H 3 PO 4 +38 ADP → 6 CO 2 + 6 H 2 O + 38 ATP

Thus, with the complete oxidation of one glucose molecule to the final products - carbon dioxide and water, with the access of oxygen, 38 ATP molecules are formed. Consequently, aerobic respiration plays the main role in providing the cell with energy.

Similarities between photosynthesis and aerobic respiration:

A mechanism for the exchange of carbon dioxide and oxygen is required.

Special organelles (chloroplasts, mitochondria) are required.

An electron transport chain embedded in the membranes is required.

Energy conversion occurs (ATP synthesis as a result of phosphorylation).

Cyclic reactions occur (Calvin cycle, Krebs cycle).

Differences between photosynthesis and aerobic respiration:

Photosynthesis	Aerobic respiration
An anabolic process as a result of which carbohydrate molecules are synthesized from simple inorganic compounds.	The process of dissimilation, as a result of which carbohydrate molecules are broken down into simple inorganic compounds.
ATP energy is accumulated and stored in carbohydrates.	Energy is stored in the form of ATP.
Oxygen is released.	Oxygen is consumed.
Carbon dioxide and water are consumed.	Carbon dioxide and water are released.
There is an increase in organic mass.	There is a decrease in organic mass.
In eukaryotes, the process occurs in chloroplasts.	In eukaryotes, the process occurs in mitochondria.
Occurs only in cells containing chlorophyll in the light.	Occurs continuously in all cells throughout life.

Having worked through these topics, you should be able to:

1. Describe the concepts below and explain the relationships between them:
   • polymer, monomer;
   • carbohydrate, monosaccharide, disaccharide, polysaccharide;
   • lipid, fatty acid, glycerol;
   • amino acid, peptide bond, protein;
   • catalyst, enzyme, active site;
   • nucleic acid, nucleotide.
2. List 5-6 reasons that make water such an important component of living systems.
3. Name the four main classes of organic compounds found in living organisms; describe the role of each of them.
4. Explain why enzyme-controlled reactions depend on temperature, pH, and the presence of coenzymes.
5. Explain the role of ATP in the energy economy of the cell.
6. Name the starting materials, main steps and end products of light-induced reactions and carbon fixation reactions.
7. Give a brief description of the general scheme of cellular respiration, from which it would be clear what place the reactions of glycolysis, the H. Krebs cycle (citric acid cycle) and the electron transport chain occupy.
8. Compare respiration and fermentation.
9. Describe the structure of the DNA molecule and explain why the number of adenine residues is equal to the number of thymine residues, and the number of guanine residues is equal to the number of cytosine residues.
10. Make a brief diagram of RNA synthesis from DNA (transcription) in prokaryotes.
11. Describe the properties of the genetic code and explain why it should be a triplet code.
12. Based on the given DNA chain and codon table, determine the complementary sequence of the messenger RNA, indicate the codons of the transfer RNA and the amino acid sequence that is formed as a result of translation.
13. List the stages of protein synthesis at the ribosome level.

Algorithm for solving problems.

Type 1. Self-copying of DNA.

One of the DNA chains has the following nucleotide sequence:
AGTACCGATACCGATTTACCG...
What nucleotide sequence does the second chain of the same molecule have?

To write the nucleotide sequence of the second strand of a DNA molecule, when the sequence of the first strand is known, it is enough to replace thymine with adenine, adenine with thymine, guanine with cytosine, and cytosine with guanine. Having made this replacement, we get the sequence:
TATTGGGCTATGAGCTAAAATG...

Type 2. Protein coding.

The chain of amino acids of the ribonuclease protein has the following beginning: lysine-glutamine-threonine-alanine-alanine-alanine-lysine...
What nucleotide sequence does the gene corresponding to this protein begin with?

To do this, use the genetic code table. For each amino acid, we find its code designation in the form of the corresponding triple of nucleotides and write it down. By arranging these triplets one after another in the same order as the corresponding amino acids, we obtain the formula for the structure of a section of messenger RNA. As a rule, there are several such triplets, the choice is made according to your decision (but only one of the triplets is taken). Accordingly, there may be several solutions.
ААААААААЦУГЦГГЦУГЦГАAG

What sequence of amino acids does a protein begin with if it is encoded by the following sequence of nucleotides:
ACCTTCCATGGCCGGT...

Using the principle of complementarity, we find the structure of a section of messenger RNA formed on a given segment of a DNA molecule:
UGCGGGGUACCGGCCCA...

Then we turn to the table of the genetic code and for each triple of nucleotides, starting from the first, we find and write out the corresponding amino acid:
Cysteine-glycine-tyrosine-arginine-proline-...

Ivanova T.V., Kalinova G.S., Myagkova A.N. "General Biology". Moscow, "Enlightenment", 2000

• Topic 4. "Chemical composition of the cell." §2-§7 pp. 7-21
• Topic 5. "Photosynthesis." §16-17 pp. 44-48
• Topic 6. "Cellular respiration." §12-13 pp. 34-38
• Topic 7. "Genetic information." §14-15 pp. 39-44

CELLULAR RESPIRATION

The main processes that provide the cell with energy are photosynthesis, chemosynthesis, respiration, fermentation and glycolysis as a stage of respiration.

With blood, oxygen penetrates into the cell, or rather into special cellular structures - mitochondria. They are found in all cells except bacterial cells, blue-green algae and mature blood cells (red blood cells). In mitochondria, oxygen enters into a multi-stage reaction with various nutrients - proteins, carbohydrates, fats, etc. This process is called cellular respiration. As a result, chemical energy is released, which the cell stores in a special substance - adenosine triphosphoric acid, or ATP. This is a universal store of energy that the body spends on growth, movement, and maintaining its vital functions.

Respiration is an oxidative breakdown of organic nutrients with the participation of oxygen, accompanied by the formation of chemically active metabolites and the release of energy that is used by cells for vital processes.

The general breathing equation is as follows:

Where Q=2878 kJ/mol.

But breathing, unlike combustion, is a multi-stage process. There are two main stages in it: glycolysis and the oxygen stage.

Glycolysis

ATP, precious for the body, is formed not only in mitochondria, but also in the cytoplasm of the cell as a result of glycolysis (from the Greek “glykis” - “sweet” and “lysis” - “decay”). Glycolysis is not a membrane-dependent process. It occurs in the cytoplasm. However, glycolytic enzymes are associated with cytoskeletal structures.

Glycolysis is a very complex process. This is a process of breakdown of glucose under the action of various enzymes, which does not require the participation of oxygen. For the breakdown and partial oxidation of a glucose molecule, the coordinated occurrence of eleven sequential reactions is necessary. In glycolysis, one molecule of glucose makes it possible to synthesize two molecules of ATP. The products of glucose breakdown can then enter into a fermentation reaction, turning into ethyl alcohol or lactic acid. Alcoholic fermentation is characteristic of yeast, and lactic acid fermentation is characteristic of animal cells and some bacteria. Many are aerobic, i.e. Living exclusively in an oxygen-free environment, organisms have enough energy generated as a result of glycolysis and fermentation. But aerobic organisms need to supplement this small reserve, and quite significantly.

Oxygen stage of respiration

The products of glucose breakdown enter the mitochondria. There, a molecule of carbon dioxide is first split off from them, which is removed from the body upon exit. “Afterburning” occurs in the so-called Krebs cycle (Appendix No. 1) (named after the English biochemist who described it) - a sequential chain of reactions. Each of the enzymes participating in it enters into compounds, and after several transformations is again released in its original form. The biochemical cycle is not at all aimless walking in circles. It is more like a ferry that scurries between two shores, but in the end people and cars move in the right direction. As a result of the reactions occurring in the Krebs cycle, additional ATP molecules are synthesized, additional carbon dioxide molecules and hydrogen atoms are split off.

Fats are also involved in this chain, but their breakdown takes time, so if energy is needed urgently, the body uses carbohydrates rather than fats. But fats are a very rich source of energy. Proteins can also be oxidized for energy needs, but only in extreme cases, for example, during prolonged fasting. Proteins are an emergency supply for the cell.

The most efficient process of ATP synthesis occurs with the participation of oxygen in the multi-stage respiratory chain. Oxygen is capable of oxidizing many organic compounds and at the same time releasing a lot of energy at once. But such an explosion would be disastrous for the body. The role of the respiratory chain and everything aerobic, i.e. associated with oxygen, breathing consists precisely in providing the body with energy continuously and in small portions - to the extent that the body needs it. An analogy can be drawn with gasoline: spilled on the ground and set on fire, it will instantly flare up without any benefit. And in a car, burning little by little, gasoline will do useful work for several hours. But this requires such a complex device as an engine.

The respiratory chain, in combination with the Krebs cycle and glycolysis, makes it possible to increase the “yield” of ATP molecules from each glucose molecule to 38. But during glycolysis, this ratio was only 2:1. Thus, the efficiency of aerobic respiration is much greater.

How does the respiratory chain work?

The mechanism of ATP synthesis during glycolysis is relatively simple and can be easily reproduced in vitro. However, it has never been possible to simulate respiratory ATP synthesis in the laboratory. In 1961, the English biochemist Peter Mitchell suggested that enzymes - neighbors in the respiratory chain - observe not only a strict sequence, but also a clear order in the space of the cell. The respiratory chain, without changing its order, is fixed in the inner shell (membrane) of the mitochondria and “stitches” it several times as if with stitches. Attempts to reproduce the respiratory synthesis of ATP failed because the role of the membrane was underestimated by researchers. But the reaction also involves enzymes concentrated in mushroom-shaped growths on the inner side of the membrane. If these growths are removed, then ATP will not be synthesized.

Breathing is harmful.

Molecular oxygen is a powerful oxidizing agent. But as a potent medicine, it can also have side effects. For example, the direct interaction of oxygen with lipids causes the formation of toxic peroxides and disrupts the structure of cells. Reactive oxygen compounds can also damage proteins and nucleic acids.

Why doesn’t poisoning with these poisons occur? Because they have an antidote. Life arose in the absence of oxygen, and the first creatures on Earth were anaerobic. Then photosynthesis appeared, and oxygen as its byproduct began to accumulate in the atmosphere. In those days, this gas was dangerous for all living things. Some anaerobes died, others found oxygen-free corners, for example, settling in lumps of soil; still others began to adapt and change. It was then that mechanisms appeared that protected the living cell from random oxidation. These are a variety of substances: enzymes, including the destroyer of harmful hydrogen peroxide - catalysis, as well as many other non-protein compounds.

Breathing in general first appeared as a way to remove oxygen from the atmosphere surrounding the body and only then became a source of energy. Anaerobes that adapted to the new environment became aerobes, gaining enormous advantages. But the hidden danger of oxygen still remains for them. The power of antioxidant “antidotes” is not unlimited. That is why in pure oxygen, and even under pressure, all living things die quite quickly. If the cell is damaged by any external factor, then the protective mechanisms usually fail first, and then oxygen begins to harm even at normal atmospheric concentrations