Respiration in cells. Cellular respiration and photosynthesis. An excerpt characterizing Cellular Respiration

Cellular respiration is the oxidation of organic substances in the cell, as a result of which ATP molecules are synthesized. The starting raw materials (substrate) are usually carbohydrates, less often fats and even less often proteins. Largest quantity ATP molecules are produced by oxidation with oxygen, and less by oxidation by other substances and electron transfer.

Carbohydrates, or polysaccharides, are broken down into monosaccharides before being used as a substrate for cellular respiration. So in plants, starch, and in animals, glycogen is hydrolyzed to glucose.

Glucose is the main source of energy for almost all cells of living organisms.

The first stage of glucose oxidation is glycolysis. It does not require oxygen and is characteristic of both anaerobic and aerobic respiration.

Biological oxidation

Cellular respiration involves a variety of redox reactions in which hydrogen and electrons move from one compound (or atom) to another. When an atom loses an electron, it oxidizes; when an electron is added - reduction. The oxidized substance is a donor, and the reduced substance is an acceptor of hydrogen and electrons. Redox reactions occurring in living organisms are called biological oxidation, or cellular respiration.

Typically, oxidative reactions release energy. The reason for this lies in physical laws. Electrons in oxidized organic molecules are at a higher energy level than in the reaction products. Electrons, moving from a higher to a lower energy level, release energy. The cell knows how to fix it in the bonds of molecules - the universal “fuel” of living things.

The most common terminal electron acceptor in nature is oxygen, which is reduced. During aerobic respiration, as a result of the complete oxidation of organic substances, carbon dioxide and water.

Biological oxidation occurs in stages, involving many enzymes and electron-transferring compounds. In stepwise oxidation, electrons move along a chain of carriers. At certain stages of the chain, a portion of energy is released sufficient for the synthesis of ATP from ADP and phosphoric acid.

Biological oxidation is very effective compared to various engines. About half of the released energy is ultimately fixed in high-energy bonds of ATP. The other part of the energy is dissipated as heat. Since the oxidation process is stepwise, then thermal energy is released little by little and does not damage cells. At the same time, it serves to maintain a constant body temperature.

Aerobic respiration

Different stages of cellular respiration occur in aerobic eukaryotes

in the mitochondrial matrix -, or the tricarboxylic acid cycle,

on the inner membrane of mitochondria - or the respiratory chain.

At each of these stages, ATP is synthesized from ADP, most of all at the last. Oxygen is used as an oxidizing agent only at the stage of oxidative phosphorylation.

The total reactions of aerobic respiration are as follows.

Glycolysis and the Krebs cycle: C 6 H 12 O 6 + 6H 2 O → 6CO 2 + 12H 2 + 4ATP

Respiratory chain: 12H 2 + 6O 2 → 12H 2 O + 34ATP

Thus, the biological oxidation of one glucose molecule produces 38 ATP molecules. In fact, it is often less.

Anaerobic respiration

During anaerobic respiration in oxidative reactions, the hydrogen acceptor NAD does not ultimately transfer hydrogen to oxygen, which is not present in this case.

Pyruvic acid, formed during glycolysis, can be used as a hydrogen acceptor.

In yeast, pyruvate is fermented to ethanol (alcoholic fermentation). In this case, during the reactions, carbon dioxide is also formed and NAD is used:

CH 3 COCOOH (pyruvate) → CH 3 CHO (acetaldehyde) + CO 2

CH 3 CHO + NAD H 2 → CH 3 CH 2 OH (ethanol) + NAD

Lactic acid fermentation occurs in animal cells experiencing a temporary lack of oxygen, and in a number of bacteria:

CH 3 COCOOH + NAD H 2 → CH 3 CHOHCOOH (lactic acid) + NAD

Both fermentations do not produce ATP. Energy in this case is provided only by glycolysis, and it amounts to only two ATP molecules. Much of the energy from glucose is never recovered. Therefore, anaerobic respiration is considered ineffective.

Basic concepts and key terms: CELLULAR RESPIRATION. ANAEROBIC RESPIRATION. AEROBIC RESPIRATION.

Remember! What is breathing?

Introductory exercise

Determine the sequence of digestive processes in the human body after oral cavity got a piece of chocolate banana cake: d) cavity digestion in duodenum proteins, fats and carbohydrates; e) slow grinding of food and its moistening; m) breakdown of carbohydrates present in the cake by salivary amylases;

f) sticking of food into food lumps and their movement through the esophagus into the stomach; i) final wall digestion of complex molecules and absorption of small molecules into the blood and lymph; p) breakdown of biscuit proteins and milk fats in the stomach; i) transport of amino acids, fatty acids and glucose into cells using blood and lymph. What word did you receive?

What is the biological significance of cellular respiration?

The main nutrients for cells are amino acids, fatty acid and glucose. Respiration is the process by which these substances are broken down and released chemical energy. There are two main types of cellular respiration: anaerobic and aerobic.

AEROBIC RESPIRATION is a set of processes of biological oxidation of nutrients and production of energy with the participation of oxygen. The breakdown of organic substances occurs with the formation of final oxidation products H 2 O and CO 2. Aerobic respiration is characteristic of the vast majority of eukaryotic cells. Glycolysis begins in the cytoplasm and continues in the mitochondria.

In aerobic oxidation, oxygen serves as an acceptor (receiver) of electrons and hydrogen protons to form water. Aerobic respiration is the most advanced way to obtain energy. Its energy effect is approximately 20 times greater than that of anaerobic respiration.

Respiration processes are similar in many ways in the cells of organisms from different kingdoms of living nature. Signs of similarity are the formation of such universal substances as pyruvic acid and ATP, the use of oxygen as an acceptor of electrons and hydrogen, splitting into the final products H 2 O and CO 2, etc.

So, CELL RESPIRATION is a set of processes of biological oxidation of nutrients with the release of chemical energy, which is accumulated in ATP.

What processes are the basis of anaerobic respiration of cells?

Most cells primarily use glucose to release energy during respiration. Interestingly, there are cells (for example, brain cells, skeletal muscles, mature red blood cells), which receive energy only from the molecules of this monosaccharide.

Why is glucose the main source of energy for cells? Polar glucose molecules interact very well with water, therefore they move easily and quickly in the cell; their transport into the cell is carried out by facilitated diffusion, which does not require energy expenditure. In addition, glucose can be converted by cells into reserve carbohydrates: in a plant cell - into starch, in animal and fungal cells - into glycogen.

The oldest and most universal process of oxygen-free breakdown of glucose is glycolysis (from the Greek sweet and splitting), which occurs in the cytoplasm of cells. Glycolysis is a set of enzymatic reactions that ensure the oxygen-free breakdown of glucose molecules with the formation of lactic acid and ATP. Glycolysis is a process common to anaerobic and aerobic respiration. The energy effect of glycolysis is about 200 kJ (120 kJ for heat, 80 kJ for ATP):

The energy of glycolysis is only 5 - 7% of the potential energy of glucose. Despite low efficiency, glycolysis is of great biological importance. This process provides the body with energy in conditions of oxygen deficiency. Even in vertebrates and humans, glycolysis serves effective way obtaining energy during short periods of intense stress.

Another mechanism for the anaerobic transformation of glucose is fermentation. Fermentation is the process of decomposition of organic substances (mainly carbohydrates) under oxygen-free conditions. Louis Pasteur called fermentation processes “life without oxygen.” Fermentation is characteristic of yeast cells, lactic acid bacteria, mucor fungi, etc. In addition to alcoholic and lactic acid fermentation, butyric, acetic, propionic acid, methane, etc. fermentations also occur in organisms.

So, the main processes of anaerobic respiration in cells are glycolysis and fermentation.

What are the main stages of aerobic respiration of cells?

The life processes of cells are very complex. But their understanding is very important, since it is at the cellular level that all vital functions of organisms are determined. To illustrate this statement, consider aerobic respiration of cells.

The oxygen stage of respiration occurs in mitochondria with the participation of oxygen, and at the same time the main part of the energy (more than 90%) is released with the formation of H 2 O and CO 2. The energy effect of such splitting is large (for example, for glucose - about 2,600 kJ):

At this stage of catabolism, scientists distinguish three stages: oxidative decarboxylation, the Krebs cycle (or tricarboxylic acid cycle) and oxidative phosphorylation (Fig. 48).

First stage. Oxidative decarboxylation is the conversion of pyruvic acid (a product of the oxygen-free breakdown of small biomolecules) into acetyl coenzyme A (acetyl-CoA).

Second stage. The Krebs cycle (tricarboxylic acid cycle) is a sequence of enzymatic reactions in the mitochondrial matrix, as a result of which acetyl-CoA is oxidized to CO 2 with the release of energy and the formation of hydrogen atoms.

Third stage. Oxidative phosphorylation is the biosynthesis of ATP from ADP and inorganic orthophosphate due to the energy released and accumulated with the participation of enzymes of the respiratory chain. This process occurs on the cristae of mitochondria.

So, thanks to the reactions of the oxygen stage, a total of 36 moles of ATP are synthesized. The total energetic result of the complete breakdown of glucose is 2800 kJ of energy (200 kJ + 2600 kJ), of which 38 ATP molecules accumulate

55% is lost, and 45% is dissipated in the form of heat. Complete equation The breakdown of glucose has the form:

So, the main role in providing cells with energy is played by the complete oxygen breakdown of glucose.

ACTIVITY

Assignment to develop practical skills

In the process of glucose catabolism in human muscles, 4 moles of glucose were broken down, of which only half underwent complete oxygen breakdown. Determine: a) how much lactic acid (in moles) accumulated in the muscles; b) how much energy was released; c) how much ATP (in moles) was formed?

1. How much lactic acid (in moles) has accumulated in human muscles?

2. What amount of energy was released during the incomplete breakdown of 2 moles of glucose and the complete breakdown of 2 moles of glucose?

3. How much ATP (in moles) was formed?

ATTITUDE Biology + Health

The breakdown of nutrients in the body occurs in three stages. Use the table to compare these stages. Prove the need for knowledge about cell respiration for healthy image life.

STAGES OF NUTRIENT BREAKDOWN USING THE EXAMPLE OF CARBOHYDRATES

	Self-control tasks
	1. What is cellular respiration? 2. Name the main types of cellular respiration. 3. What is anaerobic respiration? 4. Name the main mechanisms of anaerobic respiration. 5. What is aerobic respiration? 6. Name the main processes of aerobic respiration.
	7. What is the biological significance of cellular respiration? 8. What processes are the basis of anaerobic respiration of cells?
	10. Why is oxygen decomposition of organic compounds more energetically efficient than oxygen-free decomposition?

This is textbook material

Energy metabolism- this is a stage-by-stage decomposition of complex organic compounds, occurring with the release of energy, which is stored in the high-energy bonds of ATP molecules and is then used in the process of cell life, including for biosynthesis, i.e. plastic exchange.

In aerobic organisms there are:

1. Preparatory- splitting of biopolymers into monomers.
2. Oxygen-free- glycolysis - the breakdown of glucose into pyruvic acid.
3. Oxygen- splitting of pyruvic acid into carbon dioxide and water.

Preparatory stage

At the preparatory stage of energy metabolism, the breakdown of organic compounds supplied with food into simpler ones, usually monomers, occurs. This is how carbohydrates are broken down into sugars, including glucose; proteins - to amino acids; fats - to glycerol and fatty acids.

Although energy is released, it is not stored in ATP and therefore cannot be used later. Energy is dissipated as heat.

The breakdown of polymers in multicellular complex animals occurs in digestive tract under the influence of enzymes secreted here by the glands. The resulting monomers are then absorbed into the blood mainly through the intestines. The blood carries nutrients throughout the cells.

However, not all substances decompose into monomers in the digestive system. The breakdown of many occurs directly in cells, in their lysosomes. In single-celled organisms, absorbed substances enter the digestive vacuoles, where they are digested.

The resulting monomers can be used for both energy and plastic exchange. In the first case, they are broken down, in the second, the components of the cells themselves are synthesized from them.

Oxygen-free stage of energy metabolism

The oxygen-free stage occurs in the cytoplasm of cells and, in the case of aerobic organisms, includes only glycolysis - enzymatic multi-stage oxidation of glucose and its breakdown to pyruvic acid, which is also called pyruvate.

The glucose molecule contains six carbon atoms. During glycolysis, it is broken down into two molecules of pyruvate, which contains three carbon atoms. In this case, part of the hydrogen atoms is split off, which are transferred to the coenzyme NAD, which, in turn, will then participate in the oxygen stage.

Part of the energy released during glycolysis is stored in ATP molecules. For one molecule of glucose, only two molecules of ATP are synthesized.

The energy remaining in pyruvate, stored in NAD, in aerobes will be further extracted in the next stage of energy metabolism.

Under anaerobic conditions, when the oxygen stage of cellular respiration is absent, pyruvate is “neutralized” into lactic acid or undergoes fermentation. In this case, energy is not stored. Thus, here the useful energy output is provided only by ineffective glycolysis.

Oxygen stage

The oxygen stage occurs in mitochondria. It has two substages: the Krebs cycle and oxidative phosphorylation. The oxygen entering the cells is used only in the second. The Krebs cycle produces and releases carbon dioxide.

Krebs cycle occurs in the mitochondrial matrix and is carried out by many enzymes. It is not the pyruvic acid molecule itself (or fatty acid, amino acid) that enters it, but the acetyl group separated from it with the help of coenzyme A, which includes two carbon atoms of the former pyruvate. During the multi-stage Krebs cycle, the acetyl group is split into two CO 2 molecules and hydrogen atoms. Hydrogen combines with NAD and FAD. The synthesis of the GDP molecule also occurs, leading to the synthesis of ATP.

For one molecule of glucose, from which two pyruvates are formed, there are two Krebs cycles. Thus, two ATP molecules are formed. If the energy exchange ended here, then the total breakdown of a glucose molecule would produce 4 ATP molecules (two from glycolysis).

Oxidative phosphorylation occurs on cristae - outgrowths of the inner membrane of mitochondria. It is provided by a conveyor belt of enzymes and coenzymes, forming the so-called respiratory chain, ending with the enzyme ATP synthetase.

The respiratory chain transmits hydrogen and electrons received from the coenzymes NAD and FAD. The transfer is carried out in such a way that hydrogen protons accumulate on the outside of the inner mitochondrial membrane, and the last enzymes in the chain transfer only electrons.

Ultimately, the electrons are transferred to oxygen molecules located on the inside of the membrane, causing them to become negatively charged. A critical level of electrical potential gradient arises, leading to the movement of protons through the channels of ATP synthetase. The energy of movement of hydrogen protons is used to synthesize ATP molecules, and the protons themselves combine with oxygen anions to form water molecules.

The energy output of the functioning of the respiratory chain, expressed in ATP molecules, is large and in total ranges from 32 to 34 ATP molecules per initial glucose molecule.

Aerobic respiration - it is an oxidative process that consumes oxygen. During respiration, the substrate is completely broken down into energy-poor inorganic substances with a high energy yield. The most important substrates for respiration are carbohydrates. In addition, fats and proteins can be consumed during respiration.

Aerobic respiration includes two main stages:

1. oxygen-free, during which the substrate is gradually broken down with the release of hydrogen atoms and binding to coenzymes (transporters such as NAD and FAD);

2. oxygen, during which further abstraction of hydrogen atoms occurs from derivatives of the respiratory substrate and gradual oxidation of hydrogen atoms as a result of the transfer of their electrons to oxygen.

At the first stage, high-molecular organic substances (polysaccharides, lipids, proteins, nucleic acids, etc.) are broken down into simpler compounds (glucose, higher carboxylic acids, glycerol, amino acids, nucleotides, etc.) under the action of enzymes. This process occurs in the cytoplasm of cells and is accompanied by the release of a small amount of energy, which is dissipated in the form of heat. Next, enzymatic breakdown of simple organic compounds occurs.

An example of such a process is glycolysis - the multi-stage oxygen-free breakdown of glucose. In glycolysis reactions, a six-carbon molecule of glucose (C 6) is split into two three-carbon molecules of pyruvic acid (C 3). In this case, two ATP molecules are formed and hydrogen atoms are released. The latter attach to the NAD + transporter (nicotinamide adenine dinucleotide), which transforms into its reduced form NAD ∙ H + H +. NAD is a coenzyme similar in structure to NADP. Both of them are derivatives of nicotinic acid, one of the B vitamins. The molecules of both coenzymes are electropositive (they lack one electron) and can act as a carrier of both electrons and hydrogen atoms. When a pair of hydrogen atoms is accepted, one of the atoms dissociates into a proton and an electron:

H → H + + e — ,

and the second joins NAD or NADP entirely:

NAD + + H + [H + + e - ] → NAD ∙ H + H + .

The free proton is later used to reverse oxidize the coenzyme.

In total, the glycolysis reaction has the form

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

The product of glycolysis - pyruvic acid (C 3 H 4 O 3) - contains a significant part of the energy, and its further release is carried out in the mitochondria. Here, complete oxidation of pyruvic acid to CO 2 and H 2 O occurs. This process can be divided into three main stages:

1) oxidative decarboxylation of pyruvic acid, 2) tricarboxylic acid cycle (Krebs cycle); 3) the final stage of oxidation is the electron transport chain.

In the first stage, pyruvic acid reacts with a substance called coenzyme A (abbreviated as CoA), resulting in the formation of adethyl coenzyme A with a high-energy bond. In this case, the CO 2 molecule (the first) and hydrogen atoms are split off from the pyruvic acid molecule, which are stored in the form of NAD ∙ H + H +.

The second stage is the Krebs cycle (named after the English scientist Hans Krebs who discovered it).

Acetyl-CoA formed in the previous stage enters the Krebs cycle. Acetyl-CoA reacts with oxaloacetic acid (a four-carbon compound), resulting in the formation of a six-carbon lemon acid. This reaction requires energy; it is supplied by the high-energy acetyl-CoA bond. Further, the transformation proceeds through the formation of a number of organic acids, as a result of which the acetyl groups entering the cycle during the hydrolysis of acetyl-CoA are dehydrogenated, releasing four pairs of hydrogen atoms and decarboxylated to form two CO 2 molecules. During decarboxylation, oxygen is split off from water molecules to oxidize carbon atoms to CO2. At the end of the cycle, oxaloacetic acid is regenerated in its original form. Now it is able to react with a new acetyl-CoA molecule, and the cycle repeats. During the cycle, three water molecules are used, two CO 2 molecules and four pairs of hydrogen atoms are released, which restore the corresponding coenzymes (FAD - flavin-denine dinucleotide and NAD). The overall reaction of the cycle can be expressed by the following equation:

acetyl-CoA + ZN 2 O + ZNAD + + FAD + ADP + H 3 PO 4 → CoA + 2CO 2 + ZNAD ∙ H + H + + FAD ∙ H 2 + ATP.

Thus, as a result of the decomposition of one molecule of pyruvic acid in the aerobic phase (decarboxylation of PVA and the Krebs cycle), 3CO 2, 4NAD ∙ H + H +, FAD ∙ H 2 are released.

The total reaction of glycolysis, oxidative decarboxylation and the Krebs cycle can be written as follows:

C 6 H 12 O 6 + 6 H 2 O + 10 NAD + 2 FAD → 6 CO 2 + 4 ATP + 10 NAD ∙ H + H + + 2 FAD ∙ H 2.

The third stage is the electron transport chain.

Pairs of hydrogen atoms, split off from intermediate products in dehydrogenation reactions during glycolysis and in the Krebs cycle, are ultimately oxidized by molecular oxygen to H 2 O with the simultaneous phosphorylation of ADP into ATP. This happens when hydrogen, separated from NAD ∙ H 2 and FAD ∙ H 2, is transferred along a chain of carriers built into the inner membrane of mitochondria. Pairs of hydrogen atoms 2H can be considered as 2H + + 2e - . It is in this form that they are transmitted along the chain of carriers. The path of transfer of hydrogen and electrons from one carrier molecule to another is a redox process. In this case, the molecule that donates an electron or a hydrogen atom is oxidized, and the molecule that receives an electron or a hydrogen atom is reduced. The driving force for the transport of hydrogen atoms in the respiratory target is the potential difference.

With the help of carriers, hydrogen ions H + are transferred from the inner side of the membrane to its outer side, in other words, from the mitochondrial matrix to the intermembrane space.

When a pair of electrons is transferred from NAD to oxygen, they cross the membrane three times, and this process is accompanied by the release of six protons to the outer side of the membrane. At the final stage, electrons are transferred to the inner side of the membrane and accepted by oxygen.

½O 2 + 2e - → O 2- .

As a result of this transfer of H + ions to the outer side of the mitochondrial membrane, an increased concentration of them is created in the perimitochondrial space, i.e., an electrochemical gradient of protons (ΔμH +) arises.

The proton gradient is like a reservoir of free energy. This energy is used by the flow of protons back across the membrane to synthesize ATP. In some cases, direct use of proton gradient energy (ΔμH +) can be observed. It can provide osmotic work and transport of substances through the membrane against their concentration gradient, be used for mechanical work, etc. Thus, the cell has two forms of energy - ATP and ΔμH +. The first form is chemical. ATP dissolves in water and is easily used in the aqueous phase. The second (ΔμH +) - electrochemical - is inextricably linked with membranes. These two forms of energy can change into each other. During the formation of ATP, energy ΔμH + is used; during the breakdown of ATP, energy can be accumulated in the form of ΔμH +.

When the proton gradient reaches a certain value, hydrogen ions from the H + reservoir move through special channels in the membrane, and their energy reserve is used for the synthesis of ATP. In the matrix they combine with charged O 2- particles, and water is formed: 2H + + O 2- → H 2 O.

The process of ATP formation as a result of the transfer of H + ions across the mitochondrial membrane is called oxidative phosphorylation. It is carried out with the participation of the enzyme ATP synthetase. ATP synthetase molecules are located in the form of spherical granules on the inner side of the inner mitochondrial membrane.

As a result of the cleavage of two molecules of pyruvic acid and the transfer of hydrogen ions through the membrane through special channels, a total of 36 ATP molecules are synthesized (2 molecules in the Krebs cycle and 34 molecules as a result of the transfer of H + ions through the membrane).

It should be noted that enzyme systems are oriented in mitochondria opposite to what is the case in chloroplasts: in chloroplasts the H + reservoir is located on the inner side of the inner membrane, and in mitochondria - on its outer side; during photosynthesis, electrons move mainly from water to hydrogen atom carriers, while in respiration, hydrogen carriers that transfer electrons to the electron transport chain are located on the inside of the membrane, and the electrons are ultimately included in the resulting water molecules.

The oxygen stage thus provides 18 times more energy than is stored as a result of glycolysis. The overall equation for aerobic respiration can be expressed as follows:

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

It is quite obvious that aerobic respiration will cease in the absence of oxygen, since it is oxygen that serves as the final acceptor of hydrogen. If cells do not receive enough oxygen, all hydrogen carriers will soon be completely saturated and will not be able to transmit it further. As a result, the main source of energy for the formation of ATP will be blocked.

Photosynthesis and respiration are two processes that underlie life. They both take place in a cell. The first - in plants and some bacterial ones, the second - in animals, and in plants, and in fungi, and in bacteria.

We can say that cellular respiration and photosynthesis are processes that are opposite to each other. This is partly correct, since with the first, oxygen is absorbed and released, and with the second, vice versa. However, it is incorrect to even compare these two processes, since they occur in different organelles using different substances. The purposes for which they are needed are also different: photosynthesis is necessary to obtain nutrients, and cellular respiration is necessary to produce energy.

Photosynthesis: where and how does it happen?

This is a chemical reaction aimed at producing organic substances from inorganic ones. A prerequisite for photosynthesis to occur is the presence of sunlight, since its energy acts as a catalyst.

Photosynthesis characteristic of plants can be expressed by the following equation:

• 6CO 2 + 6H 2 O = C 6 H 12 O 6 + 6O 2.

That is, from six molecules of carbon dioxide and the same number of water molecules in the presence of sunlight, a plant can obtain one molecule of glucose and six oxygen.

This is the simplest example of photosynthesis. In addition to glucose, plants can synthesize other, more complex carbohydrates, as well as organic substances from other classes.

Here is an example of amino acid production from inorganic compounds:

• 6CO 2 + 4H 2 O + 2SO 4 2- + 2NO 3 - + 6H + = 2C 3 H 7 O 2 NS + 13O 2.

Aerobic cellular respiration is characteristic of all other organisms, including animals and plants. It occurs with the participation of oxygen.

In representatives of fauna, cellular respiration occurs in special organelles. They are called mitochondria. In plants, cellular respiration also occurs in mitochondria.

Stages

Cellular respiration occurs in three stages:

1. Preparatory stage.
2. Glycolysis (anaerobic process, does not require oxygen).
3. Oxidation (aerobic stage).

Preparatory stage

The first stage is that complex substances in the digestive system are broken down into simpler ones. Thus, amino acids are obtained from proteins, fatty acids and glycerol are obtained from lipids, and complex carbohydrates- glucose. These compounds are transported into the cell and then directly into the mitochondria.

Glycolysis

It lies in the fact that under the action of enzymes, glucose is broken down into pyruvic acid and hydrogen atoms. In this case, this process is formed. This process can be expressed by the following equation:

• C 6 H 12 O 6 = 2C 3 H 3 O 3 + 4H + 2ATP.

Thus, in the process of glycolysis, the body can obtain two ATP molecules from one glucose molecule.

Oxidation

At this stage, formed during glycolysis under the action of enzymes reacts with oxygen, resulting in the formation of carbon dioxide and hydrogen atoms. These atoms are then transported to the cristae where they are oxidized to form water and 36 ATP molecules.

So, in the process of cellular respiration, a total of 38 ATP molecules are formed: 2 in the second stage and 36 in the third. Adenosine triphosphoric acid is the main source of energy that mitochondria supply the cell with.

Mitochondria structure

Organelles in which respiration occurs are found in animals, plants, and plants. They are spherical in shape and about 1 micron in size.

Mitochondria, like chloroplasts, have two membranes separated by an intermembrane space. What is inside the membranes of this organelle is called the matrix. It contains ribosomes, mitochondrial DNA (mtDNA) and mtRNA. Glycolysis and the first stage of oxidation take place in the matrix.

Folds similar to ridges form from the inner membrane. They are called cristae. Here the second stage of the third stage of cellular respiration takes place. During it, the most ATP molecules are formed.

Origin of double-membrane organelles

Scientists have proven that the structures that ensure photosynthesis and respiration appeared in the cell through symbiogenesis. That is, they were once separate organisms. This explains the fact that both mitochondria and chloroplasts have their own ribosomes, DNA and RNA.