What is the importance of air and oxygen for human life, plants and all living organisms? How long can a healthy person, a human brain, live without air or oxygen? What is the record for holding a person's breath underwater? Why do we breathe? Oxygen
Oxygen- one of the most common elements not only in nature, but also in the composition of the human body.
The special properties of oxygen as a chemical element have made it, during the evolution of living beings, a necessary partner in the fundamental processes of life. The electronic configuration of the oxygen molecule is such that it has unpaired electrons, which are highly reactive. Possessing therefore high oxidizing properties, the oxygen molecule is used in biological systems as a kind of trap for electrons, the energy of which is extinguished when they are associated with oxygen in a water molecule.
There is no doubt that oxygen is “at home” for biological processes as an electron acceptor. The solubility of oxygen in both the aqueous and lipid phases is also very useful for an organism whose cells (especially biological membranes) are built from physically and chemically diverse materials. This allows it to diffuse relatively easily to any structural formations of cells and participate in oxidative reactions. True, oxygen is several times more soluble in fats than in an aqueous environment, and this is taken into account when using oxygen as a therapeutic agent.
Each cell of our body requires uninterrupted supply of oxygen, where it is used in various metabolic reactions. In order to deliver and sort it into cells, you need a fairly powerful transport apparatus.
Under normal conditions, the cells of the body need to supply about 200-250 ml of oxygen every minute. It is easy to calculate that the need for it per day is considerable (about 300 liters). With hard work, this need increases tenfold.
The diffusion of oxygen from the pulmonary alveoli into the blood occurs due to the alveolar-capillary difference (gradient) of oxygen tension, which when breathing normal air is: 104 (pO 2 in the alveoli) - 45 (pO 2 in the pulmonary capillaries) = 59 mm Hg. Art.
Alveolar air (with an average lung capacity of 6 liters) contains no more than 850 ml of oxygen, and this alveolar reserve can supply the body with oxygen for only 4 minutes, given that the body's average oxygen requirement in normal conditions is approximately 200 ml per minute.
It has been calculated that if molecular oxygen were simply dissolved in blood plasma (and it dissolves poorly in it - 0.3 ml in 100 ml of blood), then in order to ensure the cells’ normal need for it, it is necessary to increase the speed of vascular blood flow to 180 l in a minute. In fact, blood moves at a speed of only 5 liters per minute. Oxygen delivery to tissues is carried out by a wonderful substance - hemoglobin.
Hemoglobin contains 96% protein (globin) and 4% non-protein component (heme). Hemoglobin, like an octopus, captures oxygen with its four tentacles. The role of “tentacles” that specifically grasp oxygen molecules in the arterial blood of the lungs is played by heme, or rather the divalent iron atom located in its center. Iron is “attached” inside the porphyrin ring using four bonds. This complex of iron with porphyrin is called protoheme or simply heme. The other two iron bonds are directed perpendicular to the plane of the porphyrin ring. One of them goes to the protein subunit (globin), and the other is free, it directly catches molecular oxygen.
The polypeptide chains of hemoglobin are arranged in space in such a way that their configuration approaches a spherical one. Each of the four globules has a “pocket” in which heme is placed. Each heme is capable of capturing one oxygen molecule. A hemoglobin molecule can bind a maximum of four oxygen molecules.
How does hemoglobin “work”?
Observations of the respiratory cycle of the “molecular lung” (as the famous English scientist M. Perutz called hemoglobin) reveal the amazing features of this pigment protein. It turns out that all four gems work in concert, rather than independently. Each of the gems is, as it were, informed about whether its partner has added oxygen or not. In deoxyhemoglobin, all the “tentacles” (iron atoms) protrude from the plane of the porphyrin ring and are ready to bind an oxygen molecule. Having caught an oxygen molecule, iron is drawn inside the porphyrin ring. The first oxygen molecule is the most difficult to attach, and each subsequent one gets better and easier. In other words, hemoglobin acts according to the proverb “appetite comes with eating.” The addition of oxygen even changes the properties of hemoglobin: it becomes a stronger acid. This fact is of great importance in the transfer of oxygen and carbon dioxide.
Having become saturated with oxygen in the lungs, hemoglobin in the red blood cells carries it through the bloodstream to the cells and tissues of the body. However, before saturating hemoglobin, oxygen must dissolve in the blood plasma and pass through the red blood cell membrane. In practice, especially when using oxygen therapy, it is important for a doctor to take into account the potential capabilities of erythrocyte hemoglobin to retain and deliver oxygen.
One gram of hemoglobin under normal conditions can bind 1.34 ml of oxygen. Reasoning further, we can calculate that with an average hemoglobin content in the blood of 14-16 ml%, 100 ml of blood binds 18-21 ml of oxygen. If we take into account the blood volume, which averages about 4.5 liters in men and 4 liters in women, then the maximum binding activity of erythrocyte hemoglobin is about 750-900 ml of oxygen. Of course, this is only possible if all the hemoglobin is saturated with oxygen.
When breathing atmospheric air, hemoglobin is incompletely saturated - 95-97%. You can saturate it by using pure oxygen for breathing. It is enough to increase its content in the inhaled air to 35% (instead of the usual 24%). In this case, the oxygen capacity will be maximum (equal to 21 ml O 2 per 100 ml of blood). Oxygen will no longer be able to bind due to the lack of free hemoglobin.
A small amount of oxygen remains dissolved in the blood (0.3 ml per 100 ml of blood) and is transferred in this form to the tissues. Under natural conditions, the needs of tissues are satisfied by oxygen bound to hemoglobin, because oxygen dissolved in plasma is an insignificant amount - only 0.3 ml in 100 ml of blood. This leads to the conclusion: if the body needs oxygen, then it cannot live without hemoglobin.
During its life (it is approximately 120 days), the red blood cell does a tremendous job, transferring about a billion oxygen molecules from the lungs to the tissues. However, hemoglobin has an interesting feature: it does not always absorb oxygen with the same greed, nor does it give it to surrounding cells with the same willingness. This behavior of hemoglobin is determined by its spatial structure and can be regulated by both internal and external factors.
The process of saturation of hemoglobin with oxygen in the lungs (or dissociation of hemoglobin in cells) is described by an S-shaped curve. Thanks to this dependence, a normal supply of oxygen to cells is possible even with small differences in the blood (from 98 to 40 mm Hg).
The position of the S-shaped curve is not constant, and its change indicates important changes in the biological properties of hemoglobin. If the curve shifts to the left and its bend decreases, then this indicates an increase in the affinity of hemoglobin for oxygen and a decrease in the reverse process - the dissociation of oxyhemoglobin. On the contrary, a shift of this curve to the right (and an increase in the bend) indicates the exact opposite picture - a decrease in the affinity of hemoglobin for oxygen and a better release of it to tissues. It is clear that shifting the curve to the left is advisable to capture oxygen in the lungs, and to the right to release it to the tissues.
The dissociation curve of oxyhemoglobin changes depending on the pH of the environment and temperature. The lower the pH (shift to the acidic side) and the higher the temperature, the worse oxygen is captured by hemoglobin, but the better it is given to tissues during the dissociation of oxyhemoglobin. Hence the conclusion: in a hot atmosphere, oxygen saturation of the blood occurs ineffectively, but with an increase in body temperature, the unloading of oxyhemoglobin from oxygen is very active.
Red blood cells also have their own regulatory devices. It is 2,3-diphosphoglyceric acid, formed during the breakdown of glucose. The “mood” of hemoglobin in relation to oxygen also depends on this substance. When 2,3-diphosphoglyceric acid accumulates in red blood cells, it reduces the affinity of hemoglobin for oxygen and promotes its release to tissues. If there is not enough of it, the picture is the opposite.
Interesting events also occur in capillaries. At the arterial end of the capillary, oxygen diffusion occurs perpendicular to the movement of blood (from the blood into the cell). The movement occurs in the direction of the difference in partial pressures of oxygen, i.e., into the cells.
Cells give preference to physically dissolved oxygen, and it is used first. At the same time, oxyhemoglobin is unloaded from its burden. The more intensely an organ works, the more oxygen it requires. When oxygen is released, the hemoglobin tentacles are released. Due to the absorption of oxygen by tissues, the content of oxyhemoglobin in venous blood drops from 97 to 65-75%.
The unloading of oxyhemoglobin simultaneously promotes the transport of carbon dioxide. The latter, formed in tissues as the final product of combustion of carbon-containing substances, enters the blood and can cause a significant decrease in the pH of the environment (acidification), which is incompatible with life. In fact, the pH of arterial and venous blood can fluctuate within an extremely narrow range (no more than 0.1), and for this it is necessary to neutralize carbon dioxide and remove it from the tissues to the lungs.
It is interesting that the accumulation of carbon dioxide in the capillaries and a slight decrease in the pH of the environment just contribute to the release of oxygen by oxyhemoglobin (the dissociation curve shifts to the right, and the S-shaped bend increases). Hemoglobin, which plays the role of the blood buffer system itself, neutralizes carbon dioxide. In this case, bicarbonates are formed. Some of the carbon dioxide is bound by hemoglobin itself (resulting in the formation of carbhemoglobin). It is estimated that hemoglobin is directly or indirectly involved in the transport of up to 90% of carbon dioxide from tissues to the lungs. In the lungs, reverse processes occur, because oxygenation of hemoglobin leads to an increase in its acidic properties and the release of hydrogen ions into the environment. The latter, combining with bicarbonates, form carbonic acid, which is broken down by the enzyme carbonic anhydrase into carbon dioxide and water. Carbon dioxide is released by the lungs, and oxyhemoglobin, binding cations (in exchange for split-off hydrogen ions), moves to the capillaries of peripheral tissues. Such a close connection between the acts of supplying tissues with oxygen and removing carbon dioxide from tissues to the lungs reminds us that when using oxygen for medicinal purposes, one should not forget about another function of hemoglobin - to free the body from excess carbon dioxide.
The arterial-venous difference or oxygen pressure difference along the capillary (from the arterial to the venous end) gives an idea of the oxygen demand of tissues. The length of the capillary travel of oxyhemoglobin varies in different organs (and their oxygen needs are not the same). Therefore, for example, oxygen tension in the brain drops less than in the myocardium.
Here, however, it is necessary to make a reservation and recall that the myocardium and other muscle tissues are in special conditions. Muscle cells have an active system for capturing oxygen from the flowing blood. This function is performed by myoglobin, which has the same structure and works on the same principle as hemoglobin. Only myoglobin has one protein chain (and not four, like hemoglobin) and, accordingly, one heme. Myoglobin is like a quarter of hemoglobin and captures only one molecule of oxygen.
The unique structure of myoglobin, which is limited only to the tertiary level of organization of its protein molecule, is associated with interaction with oxygen. Myoglobin binds oxygen five times faster than hemoglobin (has a high affinity for oxygen). The myoglobin saturation (or oxymyoglobin dissociation) curve with oxygen has the shape of a hyperbola rather than an S-shape. This makes great biological sense, since myoglobin, located deep in muscle tissue (where the partial pressure of oxygen is low), greedily grabs oxygen even under conditions of low tension. A kind of oxygen reserve is created, which is spent, if necessary, on the formation of energy in the mitochondria. For example, in the heart muscle, where there is a lot of myoglobin, during diastole a reserve of oxygen is formed in the cells in the form of oxymyoglobin, which during systole satisfies the needs of muscle tissue.
Apparently, the constant mechanical work of the muscular organs required additional devices for catching and reserving oxygen. Nature created it in the form of myoglobin. It is possible that non-muscle cells also have some as yet unknown mechanism for capturing oxygen from the blood.
In general, the usefulness of the work of red blood cell hemoglobin is determined by how much it was able to carry to the cell and transfer oxygen molecules to it and remove the carbon dioxide that accumulates in the tissue capillaries. Unfortunately, this worker sometimes does not work at full capacity and through no fault of his own: the release of oxygen from oxyhemoglobin in the capillary depends on the ability of biochemical reactions in cells to consume oxygen. If little oxygen is consumed, then it seems to “stagnate” and, due to its low solubility in a liquid medium, no longer comes from the arterial bed. Doctors observe a decrease in the arteriovenous oxygen difference. It turns out that hemoglobin uselessly carries some of the oxygen, and besides, it carries less carbon dioxide. The situation is not pleasant.
Knowledge of the operating patterns of the oxygen transport system in natural conditions allows the doctor to draw a number of useful conclusions for the correct use of oxygen therapy. It goes without saying that it is necessary to use, together with oxygen, agents that stimulate zytropoiesis, increase blood flow in the affected body and help the use of oxygen in the tissues of the body.
At the same time, it is necessary to clearly know for what purposes oxygen is spent in cells, ensuring their normal existence?
On its way to its place of participation in metabolic reactions inside cells, oxygen overcomes many structural formations. The most important of them are biological membranes.
Every cell has a plasma (or outer) membrane and a bizarre variety of other membrane structures that bound subcellular particles (organelles). Membranes are not just partitions, but formations that perform special functions (transport, breakdown and synthesis of substances, energy production, etc.), which are determined by their organization and the composition of the biomolecules included in them. Despite the variability in membrane shapes and sizes, they consist predominantly of proteins and lipids. Other substances also found in membranes (for example, carbohydrates) are connected through chemical bonds to either lipids or proteins.
We will not dwell on the details of the organization of protein-lipid molecules in membranes. It is important to note that all models of the structure of biomembranes (“sandwich”, “mosaic”, etc.) assume the presence in the membranes of a bimolecular lipid film held together by protein molecules.
The lipid layer of the membrane is a liquid film that is in constant motion. Oxygen, due to its good solubility in fats, passes through the double lipid layer of membranes and enters the cells. Some of the oxygen is transferred to the internal environment of cells through carriers such as myoglobin. Oxygen is believed to be in a soluble state in the cell. Probably, it dissolves more in lipid formations, and less in hydrophilic ones. Let us remember that the structure of oxygen perfectly meets the criteria of an oxidizing agent used as an electron trap. It is known that the main concentration of oxidative reactions occurs in special organelles, mitochondria. The figurative comparisons that biochemists gave to mitochondria speak about the purpose of these small (0.5 to 2 microns in size) particles. They are called both “energy stations” and “power stations” of the cell, thereby emphasizing their leading role in the formation of energy-rich compounds.
It’s probably worth making a small digression here. As you know, one of the fundamental characteristics of living things is the efficient extraction of energy. The human body uses external sources of energy - nutrients (carbohydrates, lipids and proteins), which are crushed into smaller pieces (monomers) with the help of hydrolytic enzymes of the gastrointestinal tract. The latter are absorbed and delivered to the cells. Only those substances that contain hydrogen, which has a large supply of free energy, have energy value. The main task of the cell, or rather the enzymes contained in it, is to process substrates in such a way as to remove hydrogen from them.
Almost all enzyme systems that perform a similar role are localized in mitochondria. Here, the glucose fragment (pyruvic acid), fatty acids and carbon skeletons of amino acids are oxidized. After final processing, the remaining hydrogen is “stripped off” from these substances.
Hydrogen, which is separated from combustible substances with the help of special enzymes (dehydrogenases), is not in free form, but in connection with special carriers - coenzymes. They are derivatives of nicotinamide (vitamin PP) - NAD (nicotinamide adenine dinucleotide), NADP (nicotinamide adenine dinucleotide phosphate) and derivatives of riboflavin (vitamin B 2) - FMN (flavin mononucleotide) and FAD (flavin adenine dinucleotide).
Hydrogen does not burn immediately, but gradually, in portions. Otherwise, the cell could not use its energy, because when hydrogen interacts with oxygen, an explosion would occur, which is easily demonstrated in laboratory experiments. In order for hydrogen to release the energy contained in it in parts, there is a chain of electron and proton carriers in the inner membrane of mitochondria, otherwise called the respiratory chain. At a certain section of this chain, the paths of electrons and protons diverge; electrons jump through cytochromes (which, like hemoglobin, consist of protein and heme), and protons escape into the environment. At the end point of the respiratory chain, where cytochrome oxidase is located, electrons “slip” onto oxygen. In this case, the energy of the electrons is completely extinguished, and oxygen, binding protons, is reduced to a water molecule. Water no longer has energy value for the body.
The energy given off by electrons jumping along the respiratory chain is converted into the energy of chemical bonds of adenosine triphosphate - ATP, which serves as the main energy accumulator in living organisms. Since two acts are combined here: oxidation and the formation of energy-rich phosphate bonds (present in ATP), the process of energy formation in the respiratory chain is called oxidative phosphorylation.
How does the combination of the movement of electrons along the respiratory chain and the capture of energy during this movement occur? It's not entirely clear yet. Meanwhile, the action of biological energy converters would make it possible to solve many issues related to the salvation of body cells affected by a pathological process, which, as a rule, experience energy starvation. According to experts, revealing the secrets of the mechanism of energy formation in living beings will lead to the creation of more technically promising energy generators.
These are perspectives. For now, it is known that the capture of electron energy occurs in three sections of the respiratory chain and, therefore, the combustion of two hydrogen atoms produces three ATP molecules. The efficiency of such an energy transformer is close to 50%. Considering that the share of energy supplied to the cell during the oxidation of hydrogen in the respiratory chain is at least 70-90%, the colorful comparisons that were awarded to mitochondria become clear.
ATP energy is used in a variety of processes: for the assembly of complex structures (for example, proteins, fats, carbohydrates, nucleic acids) from building proteins, mechanical activity (muscle contraction), electrical work (the emergence and propagation of nerve impulses), transport and accumulation of substances inside cells, etc. In short, life without energy is impossible, and as soon as there is a sharp shortage of it, living beings die.
Let us return to the question of the place of oxygen in energy generation. At first glance, the direct participation of oxygen in this vital process seems disguised. It would probably be appropriate to compare the combustion of hydrogen (and the resulting formation of energy) with a production line, although the respiratory chain is a line not for assembling, but for “disassembling” matter.
At the origin of the respiratory chain is hydrogen. From it, the flow of electrons rushes to the final destination - oxygen. In the absence of oxygen or its shortage, the production line either stops or does not work at full capacity, because there is no one to unload it, or the efficiency of unloading is limited. No flow of electrons - no energy. According to the apt definition of the outstanding biochemist A. Szent-Gyorgyi, life is controlled by the flow of electrons, the movement of which is set by an external source of energy - the Sun. It is tempting to continue this thought and add that since life is controlled by the flow of electrons, then oxygen maintains the continuity of this flow
Is it possible to replace oxygen with another electron acceptor, unload the respiratory chain and restore energy production? In principle it is possible. This is easily demonstrated in laboratory experiments. For the body, selecting an electron acceptor such as oxygen so that it is easily transported, penetrates all cells and participates in redox reactions is still an incomprehensible task.
So, oxygen, while maintaining the continuity of the flow of electrons in the respiratory chain, under normal conditions contributes to the constant formation of energy from substances entering the mitochondria.
Of course, the situation presented above is somewhat simplified, and we did this in order to more clearly show the role of oxygen in the regulation of energy processes. The effectiveness of such regulation is determined by the operation of the apparatus for transforming the energy of moving electrons (electric current) into the chemical energy of ATP bonds. If nutrients are present even in the presence of oxygen. burn in the mitochondria “in vain”, the thermal energy released in this case is useless for the body, and energy starvation may occur with all the ensuing consequences. However, such extreme cases of impaired phosphorylation during electron transfer in tissue mitochondria are hardly possible and have not been encountered in practice.
Much more frequent are cases of dysregulation of energy production associated with insufficient oxygen supply to the cells. Does this mean immediate death? It turns out not. Evolution decided wisely, leaving a certain reserve of energy strength for human tissues. It is provided by an oxygen-free (anaerobic) pathway for the formation of energy from carbohydrates. Its efficiency, however, is relatively low, since the oxidation of the same nutrients in the presence of oxygen provides 15-18 times more energy than without it. However, in critical situations, body tissues remain viable precisely due to anaerobic energy production (through glycolysis and glycogenolysis).
This is a small digression that talks about the potential for the formation of energy and the existence of an organism without oxygen, further evidence that oxygen is the most important regulator of life processes and that existence is impossible without it.
However, no less important is the participation of oxygen not only in energy, but also in plastic processes. This side of oxygen was pointed out back in 1897 by our outstanding compatriot A. N. Bach and the German scientist K. Engler, who developed the position “on the slow oxidation of substances with activated oxygen.” For a long time, these provisions remained in oblivion due to too much interest of researchers in the problem of the participation of oxygen in energy reactions. Only in the 60s of our century the question of the role of oxygen in the oxidation of many natural and foreign compounds was again raised. As it turned out, this process has nothing to do with the generation of energy.
The main organ that uses oxygen to introduce it into the molecule of the oxidized substance is the liver. In liver cells, many foreign compounds are neutralized in this way. And if the liver is rightly called a laboratory for the neutralization of drugs and poisons, then oxygen in this process is given a very honorable (if not dominant) place.
Briefly about the localization and design of the oxygen consumption apparatus for plastic purposes. In the membranes of the endoplasmic reticulum, which penetrates the cytoplasm of liver cells, there is a short electron transport chain. It differs from a long (with a large number of carriers) respiratory chain. The source of electrons and protons in this chain is reduced NADP, which is formed in the cytoplasm, for example, during the oxidation of glucose in the pentose phosphate cycle (hence glucose can be called a full partner in the detoxification of substances). Electrons and protons are transferred to a special protein containing flavin (FAD) and from it to the final link - a special cytochrome called cytochrome P-450. Like hemoglobin and mitochondrial cytochromes, it is a heme-containing protein. Its function is dual: it binds the oxidized substance and participates in the activation of oxygen. The end result of such a complex function of cytochrome P-450 is that one oxygen atom enters the molecule of the oxidized substance, and the second enters the water molecule. The differences between the final acts of oxygen consumption during the formation of energy in mitochondria and during the oxidation of substances in the endoplasmic reticulum are obvious. In the first case, oxygen is used to form water, and in the second - to form both water and an oxidized substrate. The proportion of oxygen consumed in the body for plastic purposes can be 10-30% (depending on the conditions for the favorable occurrence of these reactions).
Raising the question (even purely theoretically) about the possibility of replacing oxygen with other elements is pointless. Considering that this path of oxygen utilization is also necessary for the exchange of the most important natural compounds - cholesterol, bile acids, steroid hormones - it is easy to understand how far the functions of oxygen extend. It turns out that it regulates the formation of a number of important endogenous compounds and the detoxification of foreign substances (or, as they are now called, xenobiotics).
It should, however, be noted that the enzymatic system of the endoplasmic reticulum, which uses oxygen to oxidize xenobiotics, has some costs, which are as follows. Sometimes, when oxygen is introduced into a substance, a more toxic compound is formed than the original one. In such cases, oxygen acts as an accomplice in poisoning the body with harmless compounds. Such costs take a serious turn, for example, when carcinogens are formed from procarcinogens with the participation of oxygen. In particular, the well-known component of tobacco smoke, benzopyrene, which was considered a carcinogen, actually acquires these properties when oxidized in the body to form oxybenzpyrene.
The above facts force us to pay close attention to those enzymatic processes in which oxygen is used as a building material. In some cases, it is necessary to develop preventive measures against this method of oxygen consumption. This task is very difficult, but it is necessary to look for approaches to it in order to use various techniques to direct the regulating potencies of oxygen in the direction necessary for the body.
The latter is especially important in the case of the use of oxygen in such an “uncontrolled” process as peroxide (or free radical) oxidation of unsaturated fatty acids. Unsaturated fatty acids are part of various lipids in biological membranes. The architecture of membranes, their permeability and the functions of the enzymatic proteins included in the membranes are largely determined by the ratio of various lipids. Lipid peroxidation occurs either with the help of enzymes or without them. The second option is no different from free radical oxidation of lipids in conventional chemical systems and requires the presence of ascorbic acid. The participation of oxygen in lipid peroxidation is, of course, not the best way to utilize its valuable biological qualities. The free radical nature of this process, which can be initiated by divalent iron (the center of radical formation), allows it to quickly lead to the disintegration of the lipid backbone of membranes and, consequently, to cell death.
Such a catastrophe does not occur in natural conditions, however. Cells contain natural antioxidants (vitamin E, selenium, some hormones) that break the chain of lipid peroxidation, preventing the formation of free radicals. Nevertheless, the use of oxygen in lipid peroxidation, according to some researchers, also has positive aspects. Under biological conditions, lipid peroxidation is necessary for membrane self-renewal, since lipid peroxides are more water-soluble compounds and are more easily released from the membrane. They are replaced by new, hydrophobic lipid molecules. Only the excessiveness of this process leads to the collapse of membranes and pathological changes in the body.
It's time to take stock. So, oxygen is the most important regulator of vital processes, used by the cells of the body as a necessary component for the formation of energy in the respiratory chain of mitochondria. The oxygen requirements of these processes are met unequally and depend on many conditions (on the power of the enzymatic system, the abundance in the substrate and the availability of oxygen itself), but still the lion's share of oxygen is spent on energy processes. Hence, the “living wage” and the functions of individual tissues and organs during an acute lack of oxygen are determined by endogenous oxygen reserves and the power of the oxygen-free pathway of energy production.
However, it is no less important to supply oxygen to other plastic processes, although a smaller part of it is consumed for this. In addition to a number of necessary natural syntheses (cholesterol, bile acids, prostaglandins, steroid hormones, biologically active products of amino acid metabolism), the presence of oxygen is especially necessary for the neutralization of drugs and poisons. In case of poisoning by foreign substances, one can perhaps assume that oxygen is of greater vital importance for plastic than for energy purposes. In case of intoxication, this side of the action finds practical application. And only in one case does the doctor have to think about how to put a barrier to oxygen consumption in the cells. We are talking about inhibition of the use of oxygen in lipid peroxidation.
As we can see, knowledge of the characteristics of the delivery and routes of oxygen consumption in the body is the key to unraveling the disorders that arise during various types of hypoxic conditions, and to the correct tactics for the therapeutic use of oxygen in the clinic.
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1. In unicellular organisms, the cell performs all the functions characteristic of any living organism. Name these functions. 2. In a multicellular organism, life The world of living organisms is diverse. However, representatives of various kingdoms of the organic world have common properties. Select the signscharacteristic: A - for plants; B - animals; B - all living
organisms:
1 - have a cellular structure;
2 - feed on ready-made organic substances;
3 - create organic substances during photosynthesis;
4 - when breathing, they absorb oxygen and release carbon dioxide;
5 - consist of inorganic and organic substances;
6 - cells contain plastids and vacuoles with cell sap;
7 - capable of metabolism and energy;
8 - the majority are practically motionless;
9 - capable of active movement;
10 - adapted to environmental conditions:
11 - the end product of metabolism is urea;
12 - the plasma membrane is covered with a cellulose cell wall;
13 - characteristically limited growth;
14 - cells contain a cell center and small vacuoles without cell sap.
education). Coal _______________ (what living organisms took part in its formation) =)))
The cause of death of living organisms can be: other living organisms, diseases, lack of food, unfavorable living conditions. Is it possible to attributeYou probably know that breathing is necessary so that the oxygen necessary for life enters the body with the inhaled air, and when exhaling, the body releases carbon dioxide.
All living things breathe - including animals,
both birds and plants.
Why do living organisms need oxygen so much that life is impossible without it? And where does carbon dioxide come from in cells, from which the body needs to constantly get rid of?
The fact is that each cell of a living organism represents a small but very active biochemical production. Do you know that no production is possible without energy. All processes that occur in cells and tissues occur with the consumption of large amounts of energy.
Where does it come from?
With the food we eat - carbohydrates, fats and proteins. In cells these substances oxidize. Most often, a chain of transformations of complex substances leads to the formation of a universal source of energy - glucose. As a result of the oxidation of glucose, energy is released. Oxygen is precisely what is needed for oxidation. The energy that is released as a result of these reactions is stored by the cell in the form of special high-energy molecules - they, like batteries or accumulators, release energy as needed. And the end product of nutrient oxidation is water and carbon dioxide, which are removed from the body: from the cells it enters the blood, which carries carbon dioxide to the lungs, and there it is expelled out during exhalation. In one hour, a person releases from 5 to 18 liters of carbon dioxide and up to 50 grams of water through the lungs.
By the way...
High-energy molecules that are the “fuel” for biochemical processes are called ATP - adenosine triphosphoric acid. In humans, the lifespan of one ATP molecule is less than 1 minute. The human body synthesizes about 40 kg of ATP per day, but all of it is almost immediately spent, and practically no ATP reserve is created in the body. For normal life, it is necessary to constantly synthesize new ATP molecules. That is why, without oxygen, a living organism can live for a maximum of a few minutes.
Are there living organisms that do not need oxygen?
Each of us is familiar with the processes of anaerobic respiration! Thus, the fermentation of dough or kvass is an example of an anaerobic process carried out by yeast: they oxidize glucose to ethanol (alcohol); the process of souring milk is the result of the work of lactic acid bacteria, which carry out lactic acid fermentation - convert milk sugar lactose into lactic acid.
Why do you need oxygen breathing if you have oxygen-free breathing?
Then, aerobic oxidation is many times more effective than anaerobic oxidation. Compare: during the anaerobic breakdown of one glucose molecule, only 2 ATP molecules are formed, and as a result of the aerobic breakdown of a glucose molecule, 38 ATP molecules are formed! For complex organisms with high speed and intensity of metabolic processes, anaerobic respiration is simply not enough to maintain life - for example, an electronic toy that requires 3-4 batteries to operate simply will not turn on if only one battery is inserted into it.
Is oxygen-free respiration possible in the cells of the human body?
Certainly! The first stage of the breakdown of the glucose molecule, called glycolysis, takes place without the presence of oxygen. Glycolysis is a process common to almost all living organisms. During glycolysis, pyruvic acid (pyruvate) is formed. It is she who sets off on the path of further transformations leading to the synthesis of ATP during both oxygen and oxygen-free respiration.
Thus, ATP reserves in muscles are very small - they are only enough for 1-2 seconds of muscle work. If a muscle needs short-term but active activity, anaerobic respiration is the first to be mobilized in it - it is activated faster and provides energy for about 90 seconds of active muscle work. If the muscle works actively for more than two minutes, then aerobic respiration kicks in: with it, ATP production occurs slowly, but it provides enough energy to maintain physical activity for a long time (up to several hours).
Everything about everything. Volume 5 Likum Arkady
Why do we need oxygen?
Why do we need oxygen?
Animals can survive without food for several weeks, without water for several days. But without oxygen they die within minutes. Oxygen is a chemical element, and one of the most common on earth. It is found all around us, making up about one-fifth of the air (and almost the rest is nitrogen). Oxygen combines with almost all other elements. In living organisms it combines with hydrogen, carbon and other substances, making up approximately two-thirds of the total weight in the human body.
At normal temperatures, oxygen reacts with other elements very slowly, forming new substances called oxides. This process is called an oxidation reaction. Oxidation occurs constantly in living organisms. Food is the fuel of living cells.
When food is oxidized, energy is released that the body uses to move and for its own growth. The slow oxidation that occurs in living beings is often called internal respiration. A person inhales oxygen through the lungs. From the lungs it enters the circulatory system and is carried throughout the body. By breathing air, we supply the cells of our body with oxygen for their internal respiration. Thus, we need oxygen to obtain energy, thanks to which the body can function.
People with breathing problems are often placed in oxygen chambers, where the patient breathes air that is forty to sixty percent oxygen, and he does not have to expend much energy to obtain the amount of oxygen he needs. Although oxygen is constantly taken from the air by living beings for breathing, its reserves, however, never run out. Plants release it during their nutrition, thereby replenishing our oxygen supplies.
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Excess oxygen
Lack of oxygenium
Causes:
- Decrease in partial pressure of O2 in inhaled air;
Why do we breathe?
You probably know that breathing is necessary so that the oxygen necessary for life enters the body with the inhaled air, and when exhaling, the body releases carbon dioxide.
All living things breathe - animals, birds, and plants.
Why do living organisms need oxygen so much that life is impossible without it? And where does carbon dioxide come from in cells, from which the body needs to constantly get rid of?
The fact is that each cell of a living organism represents a small but very active biochemical production. Do you know that no production is possible without energy. All processes that occur in cells and tissues occur with the consumption of large amounts of energy.
Where does it come from?
With the food we eat - carbohydrates, fats and proteins. In cells, these substances are oxidized. Most often, a chain of transformations of complex substances leads to the formation of a universal source of energy - glucose. As a result of the oxidation of glucose, energy is released. Oxygen is precisely what is needed for oxidation. The energy that is released as a result of these reactions is stored by the cell in the form of special high-energy molecules - they, like batteries or accumulators, release energy as needed. And the end product of nutrient oxidation is water and carbon dioxide, which are removed from the body: from the cells it enters the blood, which carries carbon dioxide to the lungs, and there it is expelled out during exhalation. In one hour, a person releases from 5 to 18 liters of carbon dioxide and up to 50 grams of water through the lungs.
By the way.
High-energy molecules that are the “fuel” for biochemical processes are called ATP - adenosine triphosphoric acid. In humans, the lifespan of one ATP molecule is less than 1 minute. The human body synthesizes about 40 kg of ATP per day, but all of it is almost immediately spent, and practically no ATP reserve is created in the body. For normal life, it is necessary to constantly synthesize new ATP molecules. That is why, without oxygen, a living organism can live for a maximum of a few minutes.
Are there living organisms that do not need oxygen?
Each of us is familiar with the processes of anaerobic respiration! Thus, the fermentation of dough or kvass is an example of an anaerobic process carried out by yeast: they oxidize glucose to ethanol (alcohol); the process of souring milk is the result of the work of lactic acid bacteria, which carry out lactic acid fermentation - convert milk sugar lactose into lactic acid.
Why do you need oxygen breathing if oxygen-free breathing is available?
Then, aerobic oxidation is many times more effective than anaerobic oxidation. Compare: during the anaerobic breakdown of one glucose molecule, only 2 ATP molecules are formed, and as a result of the aerobic breakdown of a glucose molecule, 38 ATP molecules are formed! For complex organisms with high speed and intensity of metabolic processes, anaerobic respiration is simply not enough to maintain life - for example, an electronic toy that requires 3-4 batteries to operate simply will not turn on if only one battery is inserted into it.
Is oxygen-free respiration possible in the cells of the human body?
Certainly! The first stage of the breakdown of the glucose molecule, called glycolysis, takes place without the presence of oxygen. Glycolysis is a process common to almost all living organisms. During glycolysis, pyruvic acid (pyruvate) is formed. It is she who sets off on the path of further transformations leading to the synthesis of ATP during both oxygen and oxygen-free respiration.
Thus, ATP reserves in muscles are very small - they are only enough for 1-2 seconds of muscle work. If a muscle needs short-term but active activity, anaerobic respiration is the first to be mobilized in it - it is activated faster and provides energy for about 90 seconds of active muscle work. If the muscle works actively for more than two minutes, then aerobic respiration kicks in: with it, ATP production occurs slowly, but it provides enough energy to maintain physical activity for a long time (up to several hours).
Your comments:
They themselves make accusations about mistakes, even having no idea that what they are saying is correct.
ATP water. apparently people didn’t study much in school
Why is natural oxygen needed?
What is oxygen for?
Increased mental performance;
Increasing the body's resistance to stress and reducing nervous stress;
Maintaining a normal level of oxygen in the blood, thereby improving the nutrition of skin cells and organs;
The functioning of internal organs is normalized, metabolism is accelerated;
Weight loss - oxygen promotes active breakdown of fats;
Normalization of sleep - due to the saturation of cells with oxygen, the body relaxes, sleep becomes deeper and lasts longer;
Solving the problem of hypoxia (i.e. lack of oxygen).
Natural oxygen, according to scientists and doctors, is quite capable of coping with these tasks, but, unfortunately, in urban conditions, problems arise with a sufficient amount of oxygen.
Scientists have determined that 200 years ago a person received up to 40% of natural oxygen from the air, and today this figure has decreased by 2 times - to 21%.
Why do living organisms need oxygen?
Animals can survive without food for several weeks, without water for several days. But without oxygen they die within minutes.
Oxygen is a chemical element, and one of the most common on earth. It is found all around us, making up about one-fifth of the air (and almost the rest is nitrogen).
Oxygen combines with almost all other elements. In living organisms it combines with hydrogen, carbon and other substances, making up approximately two-thirds of the total weight in the human body.
At normal temperatures, oxygen reacts with other elements very slowly, forming new substances called oxides. This process is called an oxidation reaction.
Oxidation occurs constantly in living organisms. Food is the fuel of living cells. When food is oxidized, energy is released that the body uses to move and for its own growth. The slow oxidation that occurs in living beings is often called internal respiration.
A person inhales oxygen through the lungs. From the lungs it enters the circulatory system and is carried throughout the body. By breathing air, we supply the cells of our body with oxygen for their internal respiration. Thus, we need oxygen to obtain energy, thanks to which the body can function.
People with breathing problems are often placed in oxygen chambers, where the patient breathes air that is forty to sixty percent oxygen, and he does not have to expend much energy to obtain the amount of oxygen he needs.
Although oxygen is constantly taken from the air by living beings for breathing, its reserves, however, never run out. Plants release it during their nutrition, thereby replenishing our oxygen supplies.
Why does the body need oxygen?
Oxygen- one of the most common elements not only in nature, but also in the composition of the human body.
The special properties of oxygen as a chemical element have made it, during the evolution of living beings, a necessary partner in the fundamental processes of life. The electronic configuration of the oxygen molecule is such that it has unpaired electrons, which are highly reactive. Possessing therefore high oxidizing properties, the oxygen molecule is used in biological systems as a kind of trap for electrons, the energy of which is extinguished when they are associated with oxygen in a water molecule.
There is no doubt that oxygen is “at home” for biological processes as an electron acceptor. The solubility of oxygen in both the aqueous and lipid phases is also very useful for an organism whose cells (especially biological membranes) are built from physically and chemically diverse materials. This allows it to diffuse relatively easily to any structural formations of cells and participate in oxidative reactions. True, oxygen is several times more soluble in fats than in an aqueous environment, and this is taken into account when using oxygen as a therapeutic agent.
Each cell of our body requires uninterrupted supply of oxygen, where it is used in various metabolic reactions. In order to deliver and sort it into cells, you need a fairly powerful transport apparatus.
Under normal conditions, the cells of the body need to supply about 200-250 ml of oxygen every minute. It is easy to calculate that the need for it per day is considerable (about 300 liters). With hard work, this need increases tenfold.
The diffusion of oxygen from the pulmonary alveoli into the blood occurs due to the alveolar-capillary difference (gradient) of oxygen tension, which when breathing normal air is: 104 (pO 2 in the alveoli) - 45 (pO 2 in the pulmonary capillaries) = 59 mm Hg. Art.
Alveolar air (with an average lung capacity of 6 liters) contains no more than 850 ml of oxygen, and this alveolar reserve can supply the body with oxygen for only 4 minutes, given that the body's average oxygen requirement in normal conditions is approximately 200 ml per minute.
It has been calculated that if molecular oxygen were simply dissolved in blood plasma (and it dissolves poorly in it - 0.3 ml in 100 ml of blood), then in order to ensure the cells’ normal need for it, it is necessary to increase the speed of vascular blood flow to 180 l in a minute. In fact, blood moves at a speed of only 5 liters per minute. Oxygen delivery to tissues is carried out by a wonderful substance - hemoglobin.
Hemoglobin contains 96% protein (globin) and 4% non-protein component (heme). Hemoglobin, like an octopus, captures oxygen with its four tentacles. The role of “tentacles” that specifically grasp oxygen molecules in the arterial blood of the lungs is played by heme, or rather the divalent iron atom located in its center. Iron is “attached” inside the porphyrin ring using four bonds. This complex of iron with porphyrin is called protoheme or simply heme. The other two iron bonds are directed perpendicular to the plane of the porphyrin ring. One of them goes to the protein subunit (globin), and the other is free, it directly catches molecular oxygen.
The polypeptide chains of hemoglobin are arranged in space in such a way that their configuration approaches a spherical one. Each of the four globules has a “pocket” in which heme is placed. Each heme is capable of capturing one oxygen molecule. A hemoglobin molecule can bind a maximum of four oxygen molecules.
How does hemoglobin “work”?
Observations of the respiratory cycle of the “molecular lung” (as the famous English scientist M. Perutz called hemoglobin) reveal the amazing features of this pigment protein. It turns out that all four gems work in concert, rather than independently. Each of the gems is, as it were, informed about whether its partner has added oxygen or not. In deoxyhemoglobin, all the “tentacles” (iron atoms) protrude from the plane of the porphyrin ring and are ready to bind an oxygen molecule. Having caught an oxygen molecule, iron is drawn inside the porphyrin ring. The first oxygen molecule is the most difficult to attach, and each subsequent one gets better and easier. In other words, hemoglobin acts according to the proverb “appetite comes with eating.” The addition of oxygen even changes the properties of hemoglobin: it becomes a stronger acid. This fact is of great importance in the transfer of oxygen and carbon dioxide.
Having become saturated with oxygen in the lungs, hemoglobin in the red blood cells carries it through the bloodstream to the cells and tissues of the body. However, before saturating hemoglobin, oxygen must dissolve in the blood plasma and pass through the red blood cell membrane. In practice, especially when using oxygen therapy, it is important for a doctor to take into account the potential capabilities of erythrocyte hemoglobin to retain and deliver oxygen.
One gram of hemoglobin under normal conditions can bind 1.34 ml of oxygen. Reasoning further, we can calculate that with an average hemoglobin content in the blood of 14-16 ml%, 100 ml of blood binds 18-21 ml of oxygen. If we take into account the blood volume, which averages about 4.5 liters in men and 4 liters in women, then the maximum binding activity of erythrocyte hemoglobin is about 750-900 ml of oxygen. Of course, this is only possible if all the hemoglobin is saturated with oxygen.
When breathing atmospheric air, hemoglobin is incompletely saturated - 95-97%. You can saturate it by using pure oxygen for breathing. It is enough to increase its content in the inhaled air to 35% (instead of the usual 24%). In this case, the oxygen capacity will be maximum (equal to 21 ml O 2 per 100 ml of blood). Oxygen will no longer be able to bind due to the lack of free hemoglobin.
A small amount of oxygen remains dissolved in the blood (0.3 ml per 100 ml of blood) and is transferred in this form to the tissues. Under natural conditions, the needs of tissues are satisfied by oxygen bound to hemoglobin, because oxygen dissolved in plasma is an insignificant amount - only 0.3 ml in 100 ml of blood. This leads to the conclusion: if the body needs oxygen, then it cannot live without hemoglobin.
During its life (it is approximately 120 days), the red blood cell does a tremendous job, transferring about a billion oxygen molecules from the lungs to the tissues. However, hemoglobin has an interesting feature: it does not always absorb oxygen with the same greed, nor does it give it to surrounding cells with the same willingness. This behavior of hemoglobin is determined by its spatial structure and can be regulated by both internal and external factors.
The process of saturation of hemoglobin with oxygen in the lungs (or dissociation of hemoglobin in cells) is described by an S-shaped curve. Thanks to this dependence, a normal supply of oxygen to cells is possible even with small differences in the blood (from 98 to 40 mm Hg).
The position of the S-shaped curve is not constant, and its change indicates important changes in the biological properties of hemoglobin. If the curve shifts to the left and its bend decreases, then this indicates an increase in the affinity of hemoglobin for oxygen and a decrease in the reverse process - the dissociation of oxyhemoglobin. On the contrary, a shift of this curve to the right (and an increase in the bend) indicates the exact opposite picture - a decrease in the affinity of hemoglobin for oxygen and a better release of it to tissues. It is clear that shifting the curve to the left is advisable to capture oxygen in the lungs, and to the right to release it to the tissues.
The dissociation curve of oxyhemoglobin changes depending on the pH of the environment and temperature. The lower the pH (shift to the acidic side) and the higher the temperature, the worse oxygen is captured by hemoglobin, but the better it is given to tissues during the dissociation of oxyhemoglobin. Hence the conclusion: in a hot atmosphere, oxygen saturation of the blood occurs ineffectively, but with an increase in body temperature, the unloading of oxyhemoglobin from oxygen is very active.
Red blood cells also have their own regulatory devices. It is 2,3-diphosphoglyceric acid, formed during the breakdown of glucose. The “mood” of hemoglobin in relation to oxygen also depends on this substance. When 2,3-diphosphoglyceric acid accumulates in red blood cells, it reduces the affinity of hemoglobin for oxygen and promotes its release to tissues. If there is not enough of it, the picture is the opposite.
Interesting events also occur in capillaries. At the arterial end of the capillary, oxygen diffusion occurs perpendicular to the movement of blood (from the blood into the cell). The movement occurs in the direction of the difference in partial pressures of oxygen, i.e., into the cells.
Cells give preference to physically dissolved oxygen, and it is used first. At the same time, oxyhemoglobin is unloaded from its burden. The more intensely an organ works, the more oxygen it requires. When oxygen is released, the hemoglobin tentacles are released. Due to the absorption of oxygen by tissues, the content of oxyhemoglobin in venous blood drops from 97 to 65-75%.
The unloading of oxyhemoglobin simultaneously promotes the transport of carbon dioxide. The latter, formed in tissues as the final product of combustion of carbon-containing substances, enters the blood and can cause a significant decrease in the pH of the environment (acidification), which is incompatible with life. In fact, the pH of arterial and venous blood can fluctuate within an extremely narrow range (no more than 0.1), and for this it is necessary to neutralize carbon dioxide and remove it from the tissues to the lungs.
It is interesting that the accumulation of carbon dioxide in the capillaries and a slight decrease in the pH of the environment just contribute to the release of oxygen by oxyhemoglobin (the dissociation curve shifts to the right, and the S-shaped bend increases). Hemoglobin, which plays the role of the blood buffer system itself, neutralizes carbon dioxide. In this case, bicarbonates are formed. Some of the carbon dioxide is bound by hemoglobin itself (resulting in the formation of carbhemoglobin). It is estimated that hemoglobin is directly or indirectly involved in the transport of up to 90% of carbon dioxide from tissues to the lungs. In the lungs, reverse processes occur, because oxygenation of hemoglobin leads to an increase in its acidic properties and the release of hydrogen ions into the environment. The latter, combining with bicarbonates, form carbonic acid, which is broken down by the enzyme carbonic anhydrase into carbon dioxide and water. Carbon dioxide is released by the lungs, and oxyhemoglobin, binding cations (in exchange for split-off hydrogen ions), moves to the capillaries of peripheral tissues. Such a close connection between the acts of supplying tissues with oxygen and removing carbon dioxide from tissues to the lungs reminds us that when using oxygen for medicinal purposes, one should not forget about another function of hemoglobin - to free the body from excess carbon dioxide.
The arterial-venous difference or oxygen pressure difference along the capillary (from the arterial to the venous end) gives an idea of the oxygen demand of tissues. The length of the capillary travel of oxyhemoglobin varies in different organs (and their oxygen needs are not the same). Therefore, for example, oxygen tension in the brain drops less than in the myocardium.
Here, however, it is necessary to make a reservation and recall that the myocardium and other muscle tissues are in special conditions. Muscle cells have an active system for capturing oxygen from the flowing blood. This function is performed by myoglobin, which has the same structure and works on the same principle as hemoglobin. Only myoglobin has one protein chain (and not four, like hemoglobin) and, accordingly, one heme. Myoglobin is like a quarter of hemoglobin and captures only one molecule of oxygen.
The unique structure of myoglobin, which is limited only to the tertiary level of organization of its protein molecule, is associated with interaction with oxygen. Myoglobin binds oxygen five times faster than hemoglobin (has a high affinity for oxygen). The myoglobin saturation (or oxymyoglobin dissociation) curve with oxygen has the shape of a hyperbola rather than an S-shape. This makes great biological sense, since myoglobin, located deep in muscle tissue (where the partial pressure of oxygen is low), greedily grabs oxygen even under conditions of low tension. A kind of oxygen reserve is created, which is spent, if necessary, on the formation of energy in the mitochondria. For example, in the heart muscle, where there is a lot of myoglobin, during diastole a reserve of oxygen is formed in the cells in the form of oxymyoglobin, which during systole satisfies the needs of muscle tissue.
Apparently, the constant mechanical work of the muscular organs required additional devices for catching and reserving oxygen. Nature created it in the form of myoglobin. It is possible that non-muscle cells also have some as yet unknown mechanism for capturing oxygen from the blood.
In general, the usefulness of the work of red blood cell hemoglobin is determined by how much it was able to carry to the cell and transfer oxygen molecules to it and remove the carbon dioxide that accumulates in the tissue capillaries. Unfortunately, this worker sometimes does not work at full capacity and through no fault of his own: the release of oxygen from oxyhemoglobin in the capillary depends on the ability of biochemical reactions in cells to consume oxygen. If little oxygen is consumed, then it seems to “stagnate” and, due to its low solubility in a liquid medium, no longer comes from the arterial bed. Doctors observe a decrease in the arteriovenous oxygen difference. It turns out that hemoglobin uselessly carries some of the oxygen, and besides, it carries less carbon dioxide. The situation is not pleasant.
Knowledge of the operating patterns of the oxygen transport system in natural conditions allows the doctor to draw a number of useful conclusions for the correct use of oxygen therapy. It goes without saying that it is necessary to use, together with oxygen, agents that stimulate zytropoiesis, increase blood flow in the affected body and help the use of oxygen in the tissues of the body.
At the same time, it is necessary to clearly know for what purposes oxygen is spent in cells, ensuring their normal existence?
On its way to its place of participation in metabolic reactions inside cells, oxygen overcomes many structural formations. The most important of them are biological membranes.
Every cell has a plasma (or outer) membrane and a bizarre variety of other membrane structures that bound subcellular particles (organelles). Membranes are not just partitions, but formations that perform special functions (transport, breakdown and synthesis of substances, energy production, etc.), which are determined by their organization and the composition of the biomolecules included in them. Despite the variability in membrane shapes and sizes, they consist predominantly of proteins and lipids. Other substances also found in membranes (for example, carbohydrates) are connected through chemical bonds to either lipids or proteins.
We will not dwell on the details of the organization of protein-lipid molecules in membranes. It is important to note that all models of the structure of biomembranes (“sandwich”, “mosaic”, etc.) assume the presence in the membranes of a bimolecular lipid film held together by protein molecules.
The lipid layer of the membrane is a liquid film that is in constant motion. Oxygen, due to its good solubility in fats, passes through the double lipid layer of membranes and enters the cells. Some of the oxygen is transferred to the internal environment of cells through carriers such as myoglobin. Oxygen is believed to be in a soluble state in the cell. Probably, it dissolves more in lipid formations, and less in hydrophilic ones. Let us remember that the structure of oxygen perfectly meets the criteria of an oxidizing agent used as an electron trap. It is known that the main concentration of oxidative reactions occurs in special organelles, mitochondria. The figurative comparisons that biochemists gave to mitochondria speak about the purpose of these small (0.5 to 2 microns in size) particles. They are called both “energy stations” and “power stations” of the cell, thereby emphasizing their leading role in the formation of energy-rich compounds.
It’s probably worth making a small digression here. As you know, one of the fundamental characteristics of living things is the efficient extraction of energy. The human body uses external sources of energy - nutrients (carbohydrates, lipids and proteins), which are crushed into smaller pieces (monomers) with the help of hydrolytic enzymes of the gastrointestinal tract. The latter are absorbed and delivered to the cells. Only those substances that contain hydrogen, which has a large supply of free energy, have energy value. The main task of the cell, or rather the enzymes contained in it, is to process substrates in such a way as to remove hydrogen from them.
Almost all enzyme systems that perform a similar role are localized in mitochondria. Here, the glucose fragment (pyruvic acid), fatty acids and carbon skeletons of amino acids are oxidized. After final processing, the remaining hydrogen is “stripped off” from these substances.
Hydrogen, which is separated from combustible substances with the help of special enzymes (dehydrogenases), is not in free form, but in connection with special carriers - coenzymes. They are derivatives of nicotinamide (vitamin PP) - NAD (nicotinamide adenine dinucleotide), NADP (nicotinamide adenine dinucleotide phosphate) and derivatives of riboflavin (vitamin B 2) - FMN (flavin mononucleotide) and FAD (flavin adenine dinucleotide).
Hydrogen does not burn immediately, but gradually, in portions. Otherwise, the cell could not use its energy, because when hydrogen interacts with oxygen, an explosion would occur, which is easily demonstrated in laboratory experiments. In order for hydrogen to release the energy contained in it in parts, there is a chain of electron and proton carriers in the inner membrane of mitochondria, otherwise called the respiratory chain. At a certain section of this chain, the paths of electrons and protons diverge; electrons jump through cytochromes (which, like hemoglobin, consist of protein and heme), and protons escape into the environment. At the end point of the respiratory chain, where cytochrome oxidase is located, electrons “slip” onto oxygen. In this case, the energy of the electrons is completely extinguished, and oxygen, binding protons, is reduced to a water molecule. Water no longer has energy value for the body.
The energy given off by electrons jumping along the respiratory chain is converted into the energy of chemical bonds of adenosine triphosphate - ATP, which serves as the main energy accumulator in living organisms. Since two acts are combined here: oxidation and the formation of energy-rich phosphate bonds (present in ATP), the process of energy formation in the respiratory chain is called oxidative phosphorylation.
How does the combination of the movement of electrons along the respiratory chain and the capture of energy during this movement occur? It's not entirely clear yet. Meanwhile, the action of biological energy converters would make it possible to solve many issues related to the salvation of body cells affected by a pathological process, which, as a rule, experience energy starvation. According to experts, revealing the secrets of the mechanism of energy formation in living beings will lead to the creation of more technically promising energy generators.
These are perspectives. For now, it is known that the capture of electron energy occurs in three sections of the respiratory chain and, therefore, the combustion of two hydrogen atoms produces three ATP molecules. The efficiency of such an energy transformer is close to 50%. Considering that the share of energy supplied to the cell during the oxidation of hydrogen in the respiratory chain is at least 70-90%, the colorful comparisons that were awarded to mitochondria become clear.
ATP energy is used in a variety of processes: for the assembly of complex structures (for example, proteins, fats, carbohydrates, nucleic acids) from building proteins, mechanical activity (muscle contraction), electrical work (the emergence and propagation of nerve impulses), transport and accumulation of substances inside cells, etc. In short, life without energy is impossible, and as soon as there is a sharp shortage of it, living beings die.
Let us return to the question of the place of oxygen in energy generation. At first glance, the direct participation of oxygen in this vital process seems disguised. It would probably be appropriate to compare the combustion of hydrogen (and the resulting formation of energy) with a production line, although the respiratory chain is a line not for assembling, but for “disassembling” matter.
At the origin of the respiratory chain is hydrogen. From it, the flow of electrons rushes to the final destination - oxygen. In the absence of oxygen or its shortage, the production line either stops or does not work at full capacity, because there is no one to unload it, or the efficiency of unloading is limited. No flow of electrons - no energy. According to the apt definition of the outstanding biochemist A. Szent-Gyorgyi, life is controlled by the flow of electrons, the movement of which is set by an external source of energy - the Sun. It is tempting to continue this thought and add that since life is controlled by the flow of electrons, then oxygen maintains the continuity of this flow
Is it possible to replace oxygen with another electron acceptor, unload the respiratory chain and restore energy production? In principle it is possible. This is easily demonstrated in laboratory experiments. For the body, selecting an electron acceptor such as oxygen so that it is easily transported, penetrates all cells and participates in redox reactions is still an incomprehensible task.
So, oxygen, while maintaining the continuity of the flow of electrons in the respiratory chain, under normal conditions contributes to the constant formation of energy from substances entering the mitochondria.
Of course, the situation presented above is somewhat simplified, and we did this in order to more clearly show the role of oxygen in the regulation of energy processes. The effectiveness of such regulation is determined by the operation of the apparatus for transforming the energy of moving electrons (electric current) into the chemical energy of ATP bonds. If nutrients are present even in the presence of oxygen. burn in the mitochondria “in vain”, the thermal energy released in this case is useless for the body, and energy starvation may occur with all the ensuing consequences. However, such extreme cases of impaired phosphorylation during electron transfer in tissue mitochondria are hardly possible and have not been encountered in practice.
Much more frequent are cases of dysregulation of energy production associated with insufficient oxygen supply to the cells. Does this mean immediate death? It turns out not. Evolution decided wisely, leaving a certain reserve of energy strength for human tissues. It is provided by an oxygen-free (anaerobic) pathway for the formation of energy from carbohydrates. Its efficiency, however, is relatively low, since the oxidation of the same nutrients in the presence of oxygen provides 15-18 times more energy than without it. However, in critical situations, body tissues remain viable precisely due to anaerobic energy production (through glycolysis and glycogenolysis).
This is a small digression that talks about the potential for the formation of energy and the existence of an organism without oxygen, further evidence that oxygen is the most important regulator of life processes and that existence is impossible without it.
However, no less important is the participation of oxygen not only in energy, but also in plastic processes. This side of oxygen was pointed out back in 1897 by our outstanding compatriot A. N. Bach and the German scientist K. Engler, who developed the position “on the slow oxidation of substances with activated oxygen.” For a long time, these provisions remained in oblivion due to too much interest of researchers in the problem of the participation of oxygen in energy reactions. Only in the 60s of our century the question of the role of oxygen in the oxidation of many natural and foreign compounds was again raised. As it turned out, this process has nothing to do with the generation of energy.
The main organ that uses oxygen to introduce it into the molecule of the oxidized substance is the liver. In liver cells, many foreign compounds are neutralized in this way. And if the liver is rightly called a laboratory for the neutralization of drugs and poisons, then oxygen in this process is given a very honorable (if not dominant) place.
Briefly about the localization and design of the oxygen consumption apparatus for plastic purposes. In the membranes of the endoplasmic reticulum, which penetrates the cytoplasm of liver cells, there is a short electron transport chain. It differs from a long (with a large number of carriers) respiratory chain. The source of electrons and protons in this chain is reduced NADP, which is formed in the cytoplasm, for example, during the oxidation of glucose in the pentose phosphate cycle (hence glucose can be called a full partner in the detoxification of substances). Electrons and protons are transferred to a special protein containing flavin (FAD) and from it to the final link - a special cytochrome called cytochrome P-450. Like hemoglobin and mitochondrial cytochromes, it is a heme-containing protein. Its function is dual: it binds the oxidized substance and participates in the activation of oxygen. The end result of such a complex function of cytochrome P-450 is that one oxygen atom enters the molecule of the oxidized substance, and the second enters the water molecule. The differences between the final acts of oxygen consumption during the formation of energy in mitochondria and during the oxidation of substances in the endoplasmic reticulum are obvious. In the first case, oxygen is used to form water, and in the second - to form both water and an oxidized substrate. The proportion of oxygen consumed in the body for plastic purposes can be 10-30% (depending on the conditions for the favorable occurrence of these reactions).
Raising the question (even purely theoretically) about the possibility of replacing oxygen with other elements is pointless. Considering that this path of oxygen utilization is also necessary for the exchange of the most important natural compounds - cholesterol, bile acids, steroid hormones - it is easy to understand how far the functions of oxygen extend. It turns out that it regulates the formation of a number of important endogenous compounds and the detoxification of foreign substances (or, as they are now called, xenobiotics).
It should, however, be noted that the enzymatic system of the endoplasmic reticulum, which uses oxygen to oxidize xenobiotics, has some costs, which are as follows. Sometimes, when oxygen is introduced into a substance, a more toxic compound is formed than the original one. In such cases, oxygen acts as an accomplice in poisoning the body with harmless compounds. Such costs take a serious turn, for example, when carcinogens are formed from procarcinogens with the participation of oxygen. In particular, the well-known component of tobacco smoke, benzopyrene, which was considered a carcinogen, actually acquires these properties when oxidized in the body to form oxybenzpyrene.
The above facts force us to pay close attention to those enzymatic processes in which oxygen is used as a building material. In some cases, it is necessary to develop preventive measures against this method of oxygen consumption. This task is very difficult, but it is necessary to look for approaches to it in order to use various techniques to direct the regulating potencies of oxygen in the direction necessary for the body.
The latter is especially important in the case of the use of oxygen in such an “uncontrolled” process as peroxide (or free radical) oxidation of unsaturated fatty acids. Unsaturated fatty acids are part of various lipids in biological membranes. The architecture of membranes, their permeability and the functions of the enzymatic proteins included in the membranes are largely determined by the ratio of various lipids. Lipid peroxidation occurs either with the help of enzymes or without them. The second option is no different from free radical oxidation of lipids in conventional chemical systems and requires the presence of ascorbic acid. The participation of oxygen in lipid peroxidation is, of course, not the best way to utilize its valuable biological qualities. The free radical nature of this process, which can be initiated by divalent iron (the center of radical formation), allows it to quickly lead to the disintegration of the lipid backbone of membranes and, consequently, to cell death.
Such a catastrophe does not occur in natural conditions, however. Cells contain natural antioxidants (vitamin E, selenium, some hormones) that break the chain of lipid peroxidation, preventing the formation of free radicals. Nevertheless, the use of oxygen in lipid peroxidation, according to some researchers, also has positive aspects. Under biological conditions, lipid peroxidation is necessary for membrane self-renewal, since lipid peroxides are more water-soluble compounds and are more easily released from the membrane. They are replaced by new, hydrophobic lipid molecules. Only the excessiveness of this process leads to the collapse of membranes and pathological changes in the body.
It's time to take stock. So, oxygen is the most important regulator of vital processes, used by the cells of the body as a necessary component for the formation of energy in the respiratory chain of mitochondria. The oxygen requirements of these processes are met unequally and depend on many conditions (on the power of the enzymatic system, the abundance in the substrate and the availability of oxygen itself), but still the lion's share of oxygen is spent on energy processes. Hence, the “living wage” and the functions of individual tissues and organs during an acute lack of oxygen are determined by endogenous oxygen reserves and the power of the oxygen-free pathway of energy production.
However, it is no less important to supply oxygen to other plastic processes, although a smaller part of it is consumed for this. In addition to a number of necessary natural syntheses (cholesterol, bile acids, prostaglandins, steroid hormones, biologically active products of amino acid metabolism), the presence of oxygen is especially necessary for the neutralization of drugs and poisons. In case of poisoning by foreign substances, one can perhaps assume that oxygen is of greater vital importance for plastic than for energy purposes. In case of intoxication, this side of the action finds practical application. And only in one case does the doctor have to think about how to put a barrier to oxygen consumption in the cells. We are talking about inhibition of the use of oxygen in lipid peroxidation.
As we can see, knowledge of the characteristics of the delivery and routes of oxygen consumption in the body is the key to unraveling the disorders that arise during various types of hypoxic conditions, and to the correct tactics for the therapeutic use of oxygen in the clinic.
Zooengineering Faculty of Moscow Agricultural Academy. Unofficial site