Everyone knows that for normal functioning the body needs a regular supply of a number of nutrients, which are needed for healthy metabolism and, accordingly, the balance of the processes of energy production and expenditure. The process of energy production, as is known, occurs in mitochondria, which, thanks to this feature, are called the energy centers of cells. And the sequence chemical reactions, which allows you to obtain energy for the work of every cell of the body, is called the Krebs cycle.
The energy obtained through the Krebs cycle (also the TCA cycle - the tricarboxylic acid cycle) goes to the needs of individual cells, which in turn make up various tissues and, accordingly, organs and systems of our body. Since the body simply cannot exist without energy, mitochondria are constantly working to continuously supply the cells with the energy they need.
Adenosine triphosphate (ATP) - this compound is a universal source of energy necessary for all biochemical processes in our body.
The TCA cycle is the central metabolic pathway, as a result of which the oxidation of metabolites is completed:
During aerobic breakdown, these biomolecules are broken down into smaller molecules that are used to produce energy or synthesize new molecules.
The tricarboxylic acid cycle consists of 8 stages, i.e. reactions:
1. Formation of citric acid:
2. Formation of isocitric acid:
3. Dehydrogenation and direct decarboxylation of isocitric acid.
4. Oxidative decarboxylation of α-ketoglutaric acid
5. Substrate phosphorylation
6. Dehydrogenation of succinic acid with succinate dehydrogenase
7. Formation of malic acid by the enzyme fumarase
8. Formation of oxalacetate
Thus, after the completion of the reactions that make up the Krebs cycle:
NADH and FADH 2 molecules act as electron carriers and are used to form ATP in next stage glucose metabolism - oxidative phosphorylation.
Functions of the Krebs cycle:
A lack of elements necessary for the normal functioning of the Krebs cycle can lead to serious problems in the body associated with a lack of energy.
Thanks to metabolic flexibility, the body is able to use not only glucose as an energy source, but also fats, the breakdown of which also produces molecules that form pyruvic acid (involved in the Krebs cycle). Thus, a properly flowing TCA cycle provides energy and building blocks for the formation of new molecules.
The tricarboxylic acid cycle was first discovered by the English biochemist Krebs. He was the first to postulate the importance of this cycle for the complete combustion of pyruvate, the main source of which is the glycolytic conversion of carbohydrates. It was subsequently shown that the tricarboxylic acid cycle is a “focus” at which almost all metabolic pathways converge.
So, acetyl-CoA formed as a result of oxidative decarboxylation of pyruvate enters the Krebs cycle. This cycle consists of eight consecutive reactions (Fig. 91). The cycle begins with the condensation of acetyl-CoA with oxaloacetate and the formation of citric acid. ( As will be seen below, in the cycle it is not acetyl-CoA itself that undergoes oxidation, but a more complex compound - lemon acid(tricarboxylic acid).)
Then citric acid (a six-carbon compound), through a series of dehydrogenations (removal of hydrogen) and decarboxylation (elimination of CO 2), loses two carbon atoms and again oxaloacetate (a four-carbon compound) appears in the Krebs cycle, i.e., as a result of a complete revolution of the cycle, the acetyl-CoA molecule burns to CO 2 and H 2 O, and the oxaloacetate molecule is regenerated. Below are all eight sequential reactions (stages) of the Krebs cycle.
In the first reaction, catalyzed by the enzyme citrate synthase, acetyl-CoA is condensed with oxaloacetate. As a result, citric acid is formed:
Apparently, in this reaction, citril-CoA bound to the enzyme is formed as an intermediate product. The latter is then spontaneously and irreversibly hydrolyzed to form citrate and HS-CoA.
In the second reaction of the cycle, the resulting citric acid undergoes dehydration to form cis-aconitic acid, which, by adding a water molecule, becomes isocitric acid. These reversible hydration-dehydration reactions are catalyzed by the enzyme aconitate hydratase:
In the third reaction, which appears to be the rate-limiting reaction of the Krebs cycle, isocitric acid is dehydrogenated in the presence of NAD-dependent isocitrate dehydrogenase:
During the isocitrate dehydrogenase reaction, isocitric acid is decarboxylated. NAD-dependent isocitrate dehydrogenase is an allosteric enzyme that requires ADP as a specific activator. In addition, the enzyme requires Mg 2+ or Mn 2+ ions to exhibit its activity.
In the fourth reaction, α-ketoglutaric acid is oxidatively decarboxylated to succinyl-CoA. The mechanism of this reaction is similar to the reaction of oxidative decarboxylation of pyruvate to acetyl-CoA. The α-ketoglutarate dehydrogenase complex is similar in structure to the pyruvate dehydrogenase complex. In both cases, five coenzymes take part in the reaction: TDP, lipoic acid amide, HS-CoA, FAD and NAD. In total, this reaction can be written as follows:
The fifth reaction is catalyzed by the enzyme succinyl-CoA synthetase. During this reaction, succinyl-CoA, with the participation of GDP and inorganic phosphate, is converted into succinic acid(succinate). At the same time, the formation of a high-energy phosphate bond of GTP1 occurs due to the high-energy thioester bond of succinyl-CoA:
In the sixth reaction, succinate is dehydrogenated to fumaric acid. The oxidation of succinate is catalyzed by succinate dehydrogenase, in the molecule of which the coenzyme FAD is covalently bound to the protein:
In the seventh reaction, the resulting fumaric acid is hydrated under the influence of the enzyme fumarate hydratase. The product of this reaction is malic acid (malate). It should be noted that fumarate hydratase is stereospecific - during this reaction L-malic acid is formed:
Finally, in the eighth reaction of the tricarboxylic acid cycle, under the influence of mitochondrial NAD-dependent malate dehydrogenase, L-malate is oxidized to oxaloacetate:
As you can see, in one turn of the cycle, consisting of eight enzymatic reactions, complete oxidation (“combustion”) of one molecule of acetyl-CoA occurs. For continuous operation of the cycle, a constant supply of acetyl-CoA into the system is necessary, and coenzymes (NAD and FAD), which have passed into a reduced state, must be oxidized again and again. This oxidation occurs in the electron transport system (or chain of respiratory enzymes) located in the mitochondria.
The energy released as a result of the oxidation of acetyl-CoA is largely concentrated in the high-energy phosphate bonds of ATP. Of the four pairs of hydrogen atoms, three pairs are transferred through NAD to the electron transport system; in this case, for each pair in the biological oxidation system, three ATP molecules are formed (in the process of conjugate oxidative phosphorylation), and therefore a total of nine ATP molecules. One pair of atoms enters the electron transport system through FAD, resulting in the formation of 2 ATP molecules. During the reactions of the Krebs cycle, 1 molecule of GTP is also synthesized, which is equivalent to 1 molecule of ATP. So, the oxidation of acetyl-CoA in the Krebs cycle produces 12 ATP molecules.
As already noted, 1 molecule of NADH 2 (3 molecules of ATP) is formed during the oxidative decarboxylation of pyruvate into acetyl-CoA. Since the breakdown of one molecule of glucose produces two molecules of pyruvate, when they are oxidized to 2 molecules of acetyl-CoA and the subsequent two turns of the tricarboxylic acid cycle, 30 molecules of ATP are synthesized (hence, the oxidation of one molecule of pyruvate to CO 2 and H 2 O produces 15 molecules ATP).
To this we must add 2 ATP molecules formed during aerobic glycolysis, and 4 ATP molecules synthesized through the oxidation of 2 molecules of extramitochondrial NADH 2, which are formed during the oxidation of 2 molecules of glyceraldehyde-3-phosphate in the dehydrogenase reaction. In total, we find that when 1 glucose molecule is broken down in tissues according to the equation: C 6 H 12 0 6 + 60 2 -> 6CO 2 + 6H 2 O, 36 ATP molecules are synthesized, which contributes to the accumulation of adenosine triphosphate in high-energy phosphate bonds 36 X 34.5 ~ 1240 kJ (or, according to other sources, 36 X 38 ~ 1430 kJ) free energy. In other words, of all the free energy released during aerobic oxidation of glucose (about 2840 kJ), up to 50% of it is accumulated in mitochondria in a form that can be used to perform various physiological functions. There is no doubt that, energetically, the complete breakdown of glucose is a more efficient process than glycolysis. It should be noted that the NADH 2 molecules formed during the conversion of glyceraldehyde-3-phosphate 2 subsequently, upon oxidation, produce not 6 ATP molecules, but only 4. The fact is that the molecules of extramitochondrial NADH 2 themselves are not able to penetrate through the membrane into the mitochondria. However, the electrons they donate can be included in the mitochondrial chain of biological oxidation using the so-called glycerophosphate shuttle mechanism (Fig. 92). As can be seen in the figure, cytoplasmic NADH 2 first reacts with cytoplasmic dihydroxyacetone phosphate to form glycerol 3-phosphate. The reaction is catalyzed by NAD-dependent cytoplasmic glycerol-3-phosphate dehydrogenase.
1)What is a leaf? To which group of plant organs does it belong? 2) What is the role of the leaf in the life of the plant? 3) What changes in the leaves do you notice?known?
4) Name the leaf tissues.
5) What is the role of leaf veins?
I will be very grateful)
phase 3 cycle?
A) 0.1 s
B) 0.3 s
B) 0.5 s
D) 0.7 s
2. At the moment of contraction of the left ventricle of the heart
A) the bicuspid valve opens
B) the bicuspid valve closes
D) the position of the bicuspid and semilunar valves does not change
3. At the moment of contraction of the right ventricle of the heart
A) the tricuspid valve opens
B) semilunar valves close
B) the tricuspid valve closes
D) the position of the tricuspid and semilunar valves does not change
4. What structure of the heart prevents the reverse movement of blood from the left ventricle to the left atrium?
A) pericardial sac
B) bicuspid valve
D) semilunar valves
5. What structure of the heart prevents the movement of blood from the left side of the heart to the right?
A) pericardial sac
B) tricuspid valve
B) septum of the heart muscle
D) semilunar valves
6. It is known that the duration of the cardiac cycle is 0.8 s. How many seconds will the general relaxation phase last if there are 3 phases in one cardiac cycle?
A) 0.4 s
B) 0.5 s
B) 0.6 s
D) 0.7 s
7. Which of the following is a source of automatism in the work of the human heart?
A) nerve center in the thoracic region spinal cord
B) nerve cells located in the pericardial sac
B) special cells of dense fibrous connective tissue
D) special muscle cells of the cardiac muscle conduction system
8. Which part of the heart has the thickest wall?
A) left ventricle
B) right ventricle
B) left atrium
D) right atrium
9. What is the role of the valves located between the atria and ventricles?
A) moisturize the chambers of the heart
B) ensure the movement of blood in the heart
B) contract and push blood into the vessels
D) prevent blood from flowing in the opposite direction
10. Why does a frog’s heart, removed from the body, continue to contract in saline solution for several hours?
A) The leaflet valves work in the heart.
B) The fluid of the pericardial sac moisturizes the heart.
C) Excitation periodically occurs in the fibers of the heart muscle.
D) The cells of the nerve nodes located in the heart muscle contract.
11. The reason for the fatigue of the heart muscle is
A) automatic ability
B) alternating contraction and relaxation
C) structural features of its cells
D) non-simultaneous contraction of the atria and ventricles
12. At what stage of the cardiac cycle does maximum blood pressure occur?
A) relaxation of the ventricles
B) contraction of the ventricles
B) atrial relaxation
D) atrial contraction
13. Heart valves provide
A) regulation blood pressure
B) regulation of heart rate
C) automaticity in the work of the heart
D) blood movement in one direction
