Sources of energy in a plant cell. Transformation of energy in the cell. Cellular respiration is the basis of life

ENERGY OF A LIVING CELL

The key processes that determine the difference between living and nonliving nature occur at the cellular level. The movement of electrons plays a decisive role in the transformation and transfer of energy within a living cell. But energy in no way originates within the cells themselves: it comes from outside. Special molecular mechanisms only slow down its movement tens of thousands of times, allowing other molecules to partially use this energy when performing work useful for the cell. Unspent energy escapes into the external environment in the form of heat. Tatyana Vasilyevna POTAPOVA, leading researcher at the Research Institute of Physics and Chemical Biology named after. A.N. Belozersky, Doctor of Biological Sciences.

Children of the Sun

The universe is filled with energy, but only a few types of it are suitable for living organisms. The main source of energy for the vast majority of biological processes on our planet is sunlight.

The cell is the basic unit of life; it continuously works to maintain its structure, and therefore requires a constant supply of free energy. Technologically, it is not easy to solve such a problem, since a living cell must use energy at a constant (and rather low) temperature in a dilute aqueous environment. In the course of evolution, over hundreds of millions of years, elegant and perfect molecular mechanisms have been formed that can act unusually effectively under very mild conditions. As a result, the efficiency of cellular energy turns out to be much higher than that of any engineering devices invented by man.

Cellular energy transformers are complexes of special proteins embedded in biological membranes. Regardless of whether free energy enters the cell from the outside directly with light quanta (in the process of photosynthesis) or as a result of the oxidation of food products with atmospheric oxygen (in the process of respiration), it triggers the movement of electrons. As a result, adenosine triphosphate (ATP) molecules are produced and the electrochemical potential difference across biological membranes increases.

ATP and membrane potential are two relatively stationary sources of energy for all types of intracellular work. Let us recall that the adenosine triphosphate molecule is a very valuable evolutionary acquisition. Energy extracted from an external source is stored in the form of "high-energy bonds" between phosphate groups. ATP very readily donates its phosphate groups either to water or to other molecules, so it is an indispensable intermediary for the transfer of chemical energy.

Electrical phenomena

in cellular energy

The mechanism by which ATP is created remained a mystery for many years until it was discovered that the process was essentially electrical. In both cases: for the respiratory chain (a set of proteins that carry out the oxidation of substrates with oxygen) and for a similar photosynthetic cascade, a proton current is generated through the membrane in which the proteins are immersed. Currents provide energy for ATP synthesis and also serve as a source of energy for some types of work. In modern bioenergy, it is common to consider ATP and proton current (more precisely, proton potential) as alternative and mutually convertible energy currencies. Some functions are paid for in one currency, some in another.

© T.V. Potapova

By the middle of the 20th century. biochemists knew for sure that in bacteria and mitochondria, electrons pass from reducing substrates to oxygen through a cascade of electron carriers called the respiratory chain. The mystery was how electron transfer and ATP synthesis were coupled. For more than 10 years, the hope of discovering the secret flared up and faded again. The decisive role was played not by overcoming technical difficulties, but by conceptual development. The coupling turned out to be, in principle, not chemical, but electrical. In 1961, the English scientist P. Mitchell published in the journal Nature a radical idea to solve the biochemical mystery of the century: the chemiosmotic hypothesis. Mitchell's idea was a truly revolutionary paradigm shift, a transformation of the conceptual framework, and at first caused heated debate.

In 1966, Mitchell wrote his first book, “Chemiosmotic coupling in oxidative and photosynthetic phosphorylation.” In the same year, Russian scientists, biophysicist E. Lieberman and biochemist V. Skulachev, figured out how to experimentally confirm Mitchell’s correctness. Using synthetic ions that penetrate a biological membrane, they showed that respiration and phosphorylation are indeed coupled through the proton potential. Another serious step in support of Mitchell was made by biophysicists from the Faculty of Biology of Moscow State University A. Bulychev, V. Andrianov, G. Kurella and F. Litvin. Using microelectrodes, they recorded the formation of a transmembrane electrical potential difference when large chloroplasts were illuminated.

A few more years of debate and meticulous testing in different laboratories around the world - and Mitchell's ideas were finally recognized. He was admitted to the Royal Society of Great Britain (and, accordingly, became Sir), received many prestigious international awards, and in 1978 was awarded the Nobel Prize, which, contrary to tradition, this time was awarded not for the discovery of a new phenomenon, but for guess about its existence.

The electron transfer chain turned out to be not just connected to the membrane, but woven into it in such a way that when an electron moves from the substrate to oxygen, it

We move from the inner surface to the outside. The membrane forms a closed bubble that does not allow protons to pass through, so as a result of the “pumping out” of protons, a potential difference is generated across the membrane: electrical negativity inside. At the same time, the pH increases: the environment inside the bubble becomes alkalized. The protons on the outside are at a much higher electrochemical potential than on the inside, as if under “pressure” from both the electrical potential and the pH gradient, which push the protons back through the membrane into the vesicle. A living cell uses the energy of such protons to perform different types of work.

Amazing advances in X-ray structural analysis of proteins have made it possible to see the complete spatial structures of individual protein complexes that make up the respiratory chain. Electron transport chain proteins, localized in mitochondrial membranes, are able to change their absorption spectrum, receiving and donating electrons. Microspectral methods make it possible to trace the sequence of electron transfer along a chain of proteins and find out exactly where part of the free energy of electrons is used for the synthesis of ATP.

According to Mitchell's idea, electrical energy is used to synthesize ATP from ADP and phosphate in mitochondrial membranes. Therefore, if the potential difference across the membrane is removed, it can be assumed that the synthesis will stop. It is precisely this effect that was demonstrated in experiments on artificial membranes using specially synthesized ions that sharply increase the conductivity of membranes for protons. 1

Some of the first experimental evidence of the validity of Mitchell's hypothesis was obtained in our country in | 1970 under the leadership of E.A. Lieberman * and V.P. Skulacheva. As indicators of changes in the electric field on the I membrane, synthetic ions were used, differing in their nature and charge sign, but similar in one thing: | they all easily penetrated the phospholipid film. After many attempts = the following elegant experimental model emerged.

A drop of phospholipids dissolved in an organic solvent is brought to a small hole in a Teflon plate, and it is instantly closed with a flat bimolecular film - an artificial membrane. A Teflon plate with an artificial membrane is immersed in a vessel with electrolyte, dividing it into two compartments with its own measuring electrode. All that remains is to embed a protein capable of generating electricity into the artificial membrane, and add penetrating ions to the electrolyte. Then the operation of the protein generator, which changes the potential difference on the membrane, will lead to the movement of penetrating ions through the phospholipid film, which will be recorded as a change in the potential difference between the compartments.

An even more convincing experimental model, allowing direct measurements of the electric current generated by cellular organelles and individual proteins, was developed and successfully used by L.A. Drachev, A.A. Kaulen and V.P. Skulachev. Particles generating electric current (mitochondria, bacterial chromatophores, or lipid vesicles with individual proteins embedded in them) were forced to adhere to a flat artificial membrane. The proton current generated by the generator molecules in response to a flash of light or the addition of appropriate chemical substrates was then detected directly by measuring electrodes on either side of the artificial membrane.

In 1973, U. Stockenius and D. Osterhelt

0 from the USA discovered an unusual light-sensing protein in the membranes of violet-j: bacteria living in salt lakes

1 rakh of the Californian deserts. This protein, like the visual pigment of the animal eye - rhodopsin, contained a derivative of vitamin A - retinal, for which it was named bacteriorhodopsin. American scientists Racker and Stokenius elegantly demonstrated the participation of bacteriorhodopsin in energy coupling. "By combining the newly discovered light-sensing protein of violet bacteria with ATP synthase in a model phospholipid membrane, they obtained a molecular ensemble capable of synthesizing ATP when the light is turned on.

At the end of 1973, Academician Yu.A. Ovchinnikov organized the Rhodopsin project for a comparative study of animal and bacterial light-sensitive pigments. As part of the project in the laboratory of V.P. Skulachev at Moscow State University, in model experiments on artificial membranes, it was proven that bacteriorhodopsin is a protein generator of electric current. Built-in

Cells incapable of photosynthesis (for example, humans) obtain energy from food, which is either plant biomass created as a result of photosynthesis, or the biomass of other living beings that feed on plants, or the remains of any living organisms.

Nutrients (proteins, fats and carbohydrates) are converted by the animal cell into a limited set of low molecular weight compounds - organic acids built from carbon atoms, which are oxidized to carbon dioxide and water using special molecular mechanisms. In this case, energy is released, it is accumulated in the form of an electrochemical potential difference on membranes and is used for the synthesis of ATP or directly to perform certain types of work.

The history of studying problems of energy conversion in an animal cell, like the history of photosynthesis, goes back more than two centuries.

In aerobic organisms, the oxidation of carbon atoms of organic acids to carbon dioxide and water occurs with the help of oxygen and is called intracellular respiration, which occurs in specialized particles - mitochondria. The transformation of oxidation energy is carried out by enzymes located in a strict order in the inner membranes of mitochondria. These enzymes make up the so-called respiratory chain and work as generators, creating an electrochemical potential difference across the membrane, through which ATP is synthesized, similar to what happens during photosynthesis.

The main task of both respiration and photosynthesis is to maintain the ATP/ADP ratio at a certain level, far from thermodynamic equilibrium, which allows ATP to serve as an energy donor, shifting the equilibrium of the reactions in which it participates.

The main energy stations of living cells are mitochondria - intracellular particles 0.1-10μ in size, covered with two membranes. In mitochondria, free energy from food oxidation is converted into free energy of ATP. When ATP combines with water, at normal concentrations of reactants, free energy of the order of 10 kcal/mol is released.

In inorganic nature, a mixture of hydrogen and oxygen is called “explosive”: a small spark is enough to cause an explosion - the instantaneous formation of water with a huge release of energy in the form of heat. The task performed by the enzymes of the respiratory chain is to produce an “explosion” so that the released energy is stored in a form suitable for the synthesis of ATP. What they do is transfer electrons in an orderly manner from one component to another (ultimately to oxygen), gradually lowering the hydrogen's potential and storing energy.

The following figures indicate the scale of this work. Mitochondria in an adult of average height and weight pump about 500 g of hydrogen ions per day across their membranes, forming a membrane potential. During this same time, H + -ATP synthase produces about 40 kg of ATP from ADP and phosphate, and ATP-using processes hydrolyze the entire mass of ATP back into ADP and phosphate.

Research has shown that the mitochondrial membrane acts as a voltage transformer. If electrons of the substrate are transferred from NADH directly to oxygen through the membrane, a potential difference of about 1 V will arise. But biological membranes - bilayer phospholipid films cannot withstand such a difference - a breakdown occurs. In addition, to produce ATP from ADP, phosphate and water, only 0.25 V is required, which means a voltage transformer is needed. And long before the advent of man, cells “invented” such a molecular device. It allows the current to be quadrupled and, due to the energy of each electron transferred from the substrate to oxygen, to transfer four protons through the membrane due to a strictly coordinated sequence of chemical reactions between the molecular components of the respiratory chain.

So, the two main pathways for the generation and regeneration of ATP in living cells: oxidative phosphorylation (respiration) and photophosphorylation (light absorption) - although supported by different external energy sources, both depend on the work of chains of catalytic enzymes immersed in membranes: the inner membranes of mitochondria , thylakoid membranes of chloroplasts or plasma membranes of some bacteria.

The vital activity of cells requires energy expenditure. Living systems (organisms) receive it from external sources, for example, from the Sun (phototrophs, which are plants, some types of protozoa and microorganisms), or produce it themselves (aerobic autotrophs) as a result of the oxidation of various substances (substrates).

In both cases, cells synthesize the universal high-energy molecule ATP (adenosine triphosphoric acid), the destruction of which releases energy. This energy is spent to perform all types of functions - active transport of substances, synthetic processes, mechanical work, etc.

The ATP molecule itself is quite simple and is a nucleotide consisting of adenine, ribose sugar and three phosphoric acid residues (Fig). The molecular weight of ATP is small and amounts to 500 daltons. ATP is a universal carrier and store of energy in the cell, which is contained in high-energy bonds between three phosphoric acid residues.

structural formula spatial formula

Figure 37. Adenosine triphosphoric acid (ATP)

Colors to represent molecules( spatial formula): white – hydrogen, red – oxygen, green – carbon, blue – nitrogen, dark red – phosphorus

The cleavage of just one phosphoric acid residue from an ATP molecule is accompanied by the release of a significant portion of energy - about 7.3 kcal.

How does the process of storing energy in the form of ATP occur? Let's consider this using the example of the oxidation (combustion) of glucose - a common source of energy for converting ATP chemical bonds into energy.

Figure 38. Structural formula

glucose (content in human blood - 100 mg%)

The oxidation of one mole of glucose (180 g) is accompanied by

is the release of about 690 kcal of free energy.

C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + E (about 690 kcal)

In a living cell, this enormous amount of energy is not released all at once, but gradually in a stepwise process and is regulated by a number of oxidative enzymes. At the same time, the released energy does not transform into thermal energy, as during combustion, but is stored in the form of chemical bonds in the ATP molecule (macroergic bonds) during the synthesis of ATP from ADP and inorganic phosphate. This process can be compared to the operation of a battery, which is charged from various generators and can provide energy to many machines and devices. In the cell, the role of a unified battery is performed by the system of adenosine-di- and tri-phosphoric acids. Charging the adenyl battery consists of combining ADP with inorganic phosphate (phosphorylation reaction) and forming ATP:

ADP + F inorg ATP + H 2 O

The formation of just 1 ATP molecule requires external energy expenditure of 7.3 kcal. Conversely, during ATP hydrolysis (battery discharge), the same amount of energy is released. Payment for this energy equivalent, called a “quantum of biological energy” in bioenergy, comes from external resources - that is, from nutrients. The role of ATP in cell life can be represented as follows:

Energy System System Functions

chemical reaccumulations using cells

energy resources

Fig. 39 General plan of cell energy

The synthesis of ATP molecules occurs not only due to the breakdown of carbohydrates (glucose), but also proteins (amino acids) and fats (fatty acids). The general scheme of cascades of biochemical reactions is as follows (Fig).

1. The initial stages of oxidation occur in the cytoplasm of cells and do not require the participation of oxygen. This form of oxidation is called anaerobic oxidation, or more simply - glycolysis. The main substrate for anaerobic oxidation is hexoses, mainly glucose. During the process of glycolysis, incomplete oxidation of the substrate occurs: glucose breaks down into trioses (two molecules of pyruvic acid). At the same time, to carry out the reaction in the cell, two ATP molecules are consumed, but 4 ATP molecules are synthesized. That is, by the method of glycolysis, the cell “earns” only two ATP molecules from the oxidation of 1 glucose molecule. From an energy efficiency point of view, this

an unprofitable process. During glycolysis, only 5% of the energy of the chemical bonds of the glucose molecule is released.

C 6 H 12 O 6 + 2P inorg + 2ADP 2 C 3 H 4 O 3 + 2ATP + 2H 2 O

Glucose pyruvate

2. Trioses formed during glycolysis (mainly pyruvic acid, pyruvate) are used

are oxidized for further more efficient oxidation, but in the cell organelles - mitochondria. In this case, the fission energy is released everyone chemical bonds, which leads to the synthesis of large amounts of ATP and oxygen consumption.

Fig. 40 Scheme of the Krebs cycle (tricarboxylic acids) and oxidative phosphorylation (respiratory chain)

These processes are associated with the oxidative cycle of tricarboxylic acids (synonyms: Krebs cycle, citric acid cycle) and with the chain of electron transfer from one enzyme to another (respiratory chain), when ATP is formed from ADP by adding one phosphoric acid residue (oxidative phosphorylation).

The concept “ oxidative phosphorylation“ determine the synthesis of ATP from ADP and phosphate due to the energy of oxidation of substrates (nutrients).

Under oxidation understand the removal of electrons from a substance, and, accordingly, the reduction and addition of electrons.

What is the role of oxidative phosphorylation in humans? The following rough calculation can give an idea of ​​this:

An adult with sedentary work consumes about 2800 kcal of energy per day from food. In order for this amount of energy to be obtained by ATP hydrolysis, 2800/7.3 = 384 moles of ATP, or 190 kg of ATP, will be required. Whereas it is known that the human body contains about 50 g of ATP. Therefore, it is clear that in order to meet the energy needs of the body, these 50 g of ATP must be broken down and synthesized thousands of times. In addition, the very rate of ATP renewal in the body changes depending on the physiological state - minimum during sleep and maximum during muscle work. This means that oxidative phosphorylation is not just a continuous process, but also widely regulated.

The essence of oxidative phosphorylation is the coupling of two processes, when an oxidative reaction involving external energy (exergic reaction) carries with it another, endergic reaction of phosphorylation of ADP with inorganic phosphate:

A in ADF + F n

oxidation phosphorylation

Here A b is the reduced form of a substance undergoing phosphorylating oxidation,

And o is the oxidized form of the substance.

In the Krebs cycle, pyruvate (CH 3 COCOOH) formed as a result of glycolysis is oxidized to acetate and combines with coenzyme A, forming acetyl-coA. After several stages of oxidation, the six-carbon compound citric acid (citrate) is formed, which is also oxidized to oxal acetate; then the cycle is repeated (Scheme of the tricarb acid cycle). During this oxidation, two CO 2 molecules and electrons are released, which are transferred to the acceptor (perceiving) molecules of coenzymes (NAD - nicotinamide dinucleotide) and then are involved in the chain of electron transfer from one substrate (enzyme) to another.

With the complete oxidation of one mole of glucose to CO 2 and H 2 O in the cycle of glycolysis and tricarboxylic acids, 38 ATP molecules are formed with a chemical bond energy of 324 kcal, and the total free energy yield of this transformation, as noted earlier, is 680 kcal. The efficiency of the release of stored energy into ATP is 48% (324/680 x 100% = 48%).

The overall equation for glucose oxidation in the Krebs cycle and the glycolytic cycle:

C 6 H 12 O 6 +6O 2 +36 ADP +P n 6CO 2 +36ATP + 42H 2 O

3. The electrons released as a result of oxidation in the Krebs cycle are combined with the coenzyme and transported into the electron transfer chain (respiratory chain) from one enzyme to another, where, during the transfer process, conjugation occurs (transformation of electron energy into the energy of chemical bonds) with the synthesis of molecules ATP.

There are three sections of the respiratory chain in which the energy of the oxidation-reduction process is transformed into the energy of the bonds of molecules in ATP. These sites are called phosphorylation points:

1. The site of electron transfer from NAD-H to flavoprotein, 10 ATP molecules are synthesized due to the oxidation energy of one glucose molecule,

2. Transfer of electrons in the area from cytochrome b to cytochrome c 1, 12 ATP molecules are phosphorylated per glucose molecule,

3. Electron transfer in the cytochrome c - molecular oxygen section, 12 ATP molecules are synthesized.

In total, at the stage of the respiratory chain, the synthesis (phosphorylation) of 34 ATP molecules occurs. And the total ATP yield in the process of aerobic oxidation of one glucose molecule is 40 units.

Table 1

Energy of glucose oxidation

For every pair of electrons transferred along the chain from NAD –H + to oxygen, three ATP molecules are synthesized

The respiratory chain is a series of protein complexes embedded in the inner membrane of mitochondria (Figure 41).

Fig. 41 Diagram of the location of respiratory chain enzymes in the inner membrane of mitochondria:

1-NAD-H-dehydrogenase complex, 1-complex, 3-cytochrome oxidase complex, 4-ubiquinone, 5-cyto-

chromium-c, 6-mitochondrion matrix, inner mitochondrial membrane, 8-intermembrane space.

So, complete oxidation of the initial substrate ends with the release of free energy, a significant part of which (up to 50%) is spent on the synthesis of ATP molecules, the formation of CO 2 and water. The other half of the free energy of substrate oxidation goes to the following needs of the cell:

1. For the biosynthesis of macromolecules (proteins, fats, carbohydrates),

2. For the processes of movement and contraction,

3. For active transport of substances across membranes,

4.To ensure the transfer of genetic information.

Fig. 42 General diagram of the process of oxidative phosphorylation in mitochondria.

1- outer membrane of the mitochondrion, 2- inner membrane, 3- ATP synthetase enzyme built into the inner membrane.

Synthesis of ATP molecules

ATP synthesis occurs in the inner membrane of mitochondria, looking into the matrix (Fig. 42 above). Specialized enzyme proteins are built into it, exclusively engaged in the synthesis of ATP from ADP and inorganic phosphate P n - ATP synthetase (ATP-S). In an electron microscope, these enzymes have a very characteristic appearance, for which they were called “mushroom bodies” (Fig). These structures completely line the inner surface of the mitochondrial membrane, directed into the matrix. Figuratively

in the words of the famous bioenergy researcher prof. Tikhonova A.N.,ATF-S is “the smallest and most perfect motor in nature.”

Fig.43 Localization

ATP synthetases in the mito membrane

chondria (animal cells) and chloroplasts (plant cells).

Blue areas are areas with high H + concentration (acid zone), orange areas are areas with low H + concentration.

Bottom: transfer of hydrogen ions H + across the membrane during the synthesis (a) and hydrolysis (b) of ATP

The efficiency of this enzyme is such that one molecule is capable of performing 200 cycles of enzymatic activation per second, while 600 ATP molecules are synthesized.

An interesting detail about the operation of this motor is that it contains rotating parts and consists of a rotor part and a stator, and the rotor rotates counterclockwise. (Fig. 44)

The membrane part of ATP-C, or the conjugation factor F0, is a hydrophobic protein complex. The second fragment of ATP-C - conjugation factor F 1 - protrudes from the membrane in the form of a mushroom-shaped formation. In the mitochondria of animal cells, ATP-C is embedded in the inner membrane, and the F 1 complex faces the matrix.

The formation of ATP from ADP and Fn occurs in the catalytic centers of the conjugation factor F 1. This protein can be easily isolated from the mitochondrial membrane, while it retains the ability to hydrolyze the ATP molecule, but loses the ability to synthesize ATP. The ability to synthesize ATP is a property of a single complex F 0 F 1 in the mitochondrial membrane (Figure 1 a). This is due to the fact that the synthesis of ATP with the help of ATP-C is associated with the transport of H + protons through it in the direction from F 0 rF 1 (Figure 1 a) . The driving force for ATP-C work is the proton potential created by the respiratory electron transport chain e - .

ATP-C is a reversible molecular machine that catalyzes both the synthesis and hydrolysis of ATP. In the ATP synthesis mode, the enzyme operates using the energy of H + protons transferred under the influence of the proton potential difference. At the same time, ATP-C also works as a proton pump - due to the energy of ATP hydrolysis, it pumps protons from an area with a low proton potential to an area with a high potential (Figure 1b). It is now known that the catalytic activity of ATP-C is directly related to the rotation of its rotor part. It was shown that the F 1 molecule rotates the rotor fragment in discrete jumps with a step of 120 0 . One revolution per 120 0 is accompanied by the hydrolysis of one ATP molecule.

A remarkable quality of the ATF-S rotating motor is its exceptionally high efficiency. It was shown that the work performed by the motor when turning the rotor part by 120 0 almost exactly coincides with the amount of energy stored in the ATP molecule, i.e. Motor efficiency is close to 100%.

The table shows comparative characteristics of several types of molecular motors operating in living cells. Among them, ATP-S stands out for its best properties. In terms of operating efficiency and the force it develops, it significantly surpasses all molecular motors known in nature and, of course, all those created by man.

Table 2 Comparative characteristics of molecular motors of cells (according to: Kinoshitaetal, 1998).

The F 1 molecule of the ATP-C complex is approximately 10 times stronger than the acto-myosin complex, a molecular machine specialized in performing mechanical work. Thus, many millions of years of evolution before man appeared who invented the wheel, the advantages of rotational motion were already realized by nature at the molecular level.

The amount of work that ATP-S does is amazing. The total mass of ATP molecules synthesized in the body of an adult per day is about 100 kg. This is not surprising, since the body undergoes numerous

biochemical processes using ATP. Therefore, in order for the body to live, its ATP-C must constantly spin, promptly replenishing ATP reserves.

A striking example of molecular electric motors is the work of bacterial flagella. Bacteria swim at an average speed of 25 µm/s, and some of them swim at a speed of more than 100 µm/s. This means that in one second the bacterium moves a distance 10 or more times greater than its own size. If a swimmer covered a distance ten times his own height in one second, then he would swim a 100-meter track in 5 seconds!

The rotation speed of bacterial electric motors ranges from 50-100 rpm to 1000 rpm, while they are very economical and consume no more than 1% of the cell’s energy resources.

Figure 44. Scheme of rotation of the rotor subunit of ATP synthetase.

Thus, both the enzymes of the respiratory chain and ATP synthesis are localized in the inner mitochondrial membrane.

In addition to ATP synthesis, the energy released during electron transport is also stored in the form of a proton gradient on the mitochondrial membrane. At the same time, an increased concentration of H + ions (protons) occurs between the outer and inner membranes. The resulting proton gradient from the matrix into the intermembrane space serves as the driving force for ATP synthesis (Fig. 42). Essentially, the inner membrane of mitochondria with built-in ATP synthetases is a perfect proton power plant, supplying energy for cell life with high efficiency.

When a certain potential difference (220 mV) is reached across the membrane, ATP synthetase begins to transport protons back into the matrix; in this case, the energy of protons is converted into the energy of synthesis of chemical bonds of ATP. This is how oxidative processes are coupled with synthetic

mi in the process of phosphorylation of ADP to ATP.

Energy of oxidative phosphorylation

fat

The synthesis of ATP during the oxidation of fatty acids and lipids is even more effective. With the complete oxidation of one fatty acid molecule, for example, palmitic acid, 130 ATP molecules are formed. The change in the free energy of acid oxidation is ∆G = -2340 kcal, and the energy accumulated in ATP is about 1170 kcal.

Energy of oxidative breakdown of amino acids

Most of the metabolic energy produced in tissues is provided by the oxidation of carbohydrates and especially fats; in an adult, up to 90% of all energy needs are covered from these two sources. The rest of the energy (depending on the diet from 10 to 15%) is supplied by the process of amino acid oxidation (Krebs cycle rice).

It has been estimated that a mammalian cell contains on average about 1 million (10 6 ) ATP molecules. In terms of all cells of the human body (10 16 –10 17 ) this amounts to 10 23 ATP molecules. The total energy contained in this mass of ATP can reach values ​​of 10 24 kcal! (1 J = 2.39x 10 -4 kcal). In a 70 kg person, the total amount of ATP is 50 g, most of which is consumed and re-synthesized daily.

One of the most difficult issues is the formation, accumulation and distribution of energy in the cell.

How does a cell produce energy? After all, it has neither a nuclear reactor, nor a power plant, nor a steam boiler, even the smallest one. The temperature inside the cell is constant and very low - no more than 40°. And despite this, cells process so many substances and so quickly that any modern plant would envy them.

How does this happen? Why does the resulting energy remain in the cell and not be released as heat? How does a cell store energy? Before answering these questions, it must be said that the energy entering the cell is not mechanical or electrical, but chemical energy contained in organic substances. At this stage, the laws of thermodynamics come into force. If energy is contained in chemical compounds, then it must be released through their combustion, and for the overall thermal balance it does not matter whether they burn immediately or gradually. The cell chooses the second path.

For simplicity, let’s liken a cell to a “power plant.” Especially for engineers, we will add that the “power plant” of the cell is thermal. Now let's challenge representatives of the energy sector to a competition: who will get more energy from fuel and use it more economically - a cell or any, the most economical, thermal power plant?

In the process of evolution, the cell created and improved its “power plant”. Nature took care of all its parts. The cell contains “fuel”, “motor-generator”, “its power regulators”, “transformer substations” and “high-voltage transmission lines”. Let's see what it all looks like.

The main “fuel” burned by the cell is carbohydrates. The simplest of them are glucose and fructose.

From everyday medical practice it is known that glucose is an essential nutrient. For severely malnourished patients, it is administered intravenously, directly into the blood.

More complex sugars are also used as energy sources. For example, regular sugar, scientifically called sucrose and consisting of 1 molecule of glucose and 1 molecule of fructose, can serve as such a material. In animals, the fuel is glycogen, a polymer consisting of glucose molecules linked in a chain. Plants contain a substance similar to glycogen - this is the well-known starch. Both glycogen and starch are storage substances. Both of them are put aside for a rainy day. Starch is usually found in underground parts of the plant, such as tubers like potatoes. There is also a lot of starch in the pulp cells of plant leaves (under a microscope, starch grains sparkle like small pieces of ice).

Glycogen accumulates in the liver of animals and is used from there as needed.

All sugars more complex than glucose must break down into their original “building blocks” - glucose molecules - before being consumed. There are special enzymes that cut, like scissors, long chains of starch and glycogen into individual monomers - glucose and fructose.

If there is a lack of carbohydrates, plants can use organic acids in their “firebox” - citric, malic, etc.

Sprouting oil seeds consume fat, which is first broken down and then converted into sugar. This is evident from the fact that as the fat in the seeds is consumed, the sugar content increases.

So, the types of fuel are listed. But it is not profitable for the cell to burn it right away.

Sugars are burned chemically in the cell. Conventional combustion is the combination of fuel with oxygen, its oxidation. But to oxidize, a substance does not have to combine with oxygen - it oxidizes when electrons in the form of hydrogen atoms are removed from it. This oxidation is called dehydrogenation(“hydros” - hydrogen). Sugars contain many hydrogen atoms, and they are not split off all at once, but one by one. Oxidation in the cell is carried out by a set of special enzymes that accelerate and direct the oxidation processes. This set of enzymes and the strict order of their work form the basis of the cellular energy generator.

The process of oxidation in living organisms is called respiration, so further we will use this more understandable expression. Intracellular respiration, so named by analogy with the physiological process of respiration, is very closely related to it. We will tell you more about the breathing processes further.

Let's continue comparing a cell with a power plant. Now we need to find in it those parts of the power plant without which it will run idle. It is clear that the energy obtained from burning carbohydrates and fats must be supplied to the consumer. This means that a cellular, “high-voltage transmission line” is needed. For a conventional power plant, this is relatively simple - high-voltage wires are stretched over the taiga, steppes, and rivers, and through them energy is supplied to plants and factories.

The cage also has its own universal “high voltage wire”. Only in it is energy transferred chemically, and the “wires”, naturally, are chemical compounds. To understand the principle of its operation, let’s introduce a small complication into the operation of a power plant. Let's assume that energy from a high-voltage line cannot be supplied to the consumer through wires. In this case, the easiest way would be to charge electric batteries from a high-voltage line, transport them to the consumer, transport used batteries back, etc. In the energy sector, this, of course, is unprofitable. And a similar method is very beneficial for the cell.

The cell uses a compound that is universal for almost all organisms - adenosine triphosphoric acid (we have already talked about it) as a battery in the cell.

In contrast to the energy of other phosphoester bonds (2-3 kilocalories), the binding energy of the terminal (especially the outermost) phosphate residues in ATP is very high (up to 16 kilocalories); therefore such a connection is called “ macroergic».

ATP is found in the body wherever energy is needed. The synthesis of various compounds, the work of muscles, the movement of flagella in protozoa - ATP carries energy everywhere.

“Charging” ATP in the cell occurs like this. Adenosine diphosphoric acid - ADP (ATP without 1 phosphorus atom) is suitable for the place where energy is released. When energy can be bound, ADP combines with phosphorus, which is found in large quantities in the cell, and “locks” energy into this bond. Now we need transport support. It consists of special enzymes - phosphoferases (“fera” - I carry), which, on demand, “grab” ATP and transfer it to the site of action. Next comes the turn of the last, final “power plant unit” - step-down transformers. They must lower the voltage and provide a safe current to the consumer. The same phosphoferases perform this role. The transfer of energy from ATP to another substance occurs in several stages. First, ATP combines with this substance, then an internal rearrangement of phosphorus atoms occurs and, finally, the complex disintegrates - ADP is separated, and energy-rich phosphorus remains “hanging” on the new substance. The new substance turns out to be much more unstable due to excess energy and is capable of various reactions.

ATP is the universal energy “currency” of the cell. One of the most amazing “inventions” of nature is the molecules of so-called “macroergic” substances, in the chemical structure of which there are one or more bonds that act as energy storage devices. Several similar molecules have been found in nature, but only one of them is found in the human body - adenosine triphosphoric acid (ATP). This is a rather complex organic molecule to which 3 negatively charged inorganic phosphoric acid residues PO are attached. It is these phosphorus residues that are connected to the organic part of the molecule by “macroergic” bonds, which are easily destroyed during various intracellular reactions. However, the energy of these bonds is not dissipated in space in the form of heat, but is used for the movement or chemical interaction of other molecules. It is thanks to this property that ATP performs in the cell the function of a universal energy storage device (accumulator), as well as a universal “currency”. After all, almost every chemical transformation that occurs in a cell either absorbs or releases energy. According to the law of conservation of energy, the total amount of energy generated as a result of oxidative reactions and stored in the form of ATP is equal to the amount of energy that the cell can use for its synthetic processes and the performance of any functions. As a “payment” for the opportunity to perform this or that action, the cell is forced to expend its supply of ATP. It should be especially emphasized: the ATP molecule is so large that it is not able to pass through the cell membrane. Therefore, ATP produced in one cell cannot be used by Another cell. Each cell of the body is forced to synthesize ATP for its needs independently in the quantities in which it is necessary to perform its functions.

Three sources of ATP resynthesis in human cells. Apparently, the distant ancestors of the cells of the human body existed many millions of years ago surrounded by plant cells, which supplied them with carbohydrates in abundance, while there was little or no oxygen. It is carbohydrates that are the most used component of nutrients for energy production in the body. And although most cells of the human body have acquired the ability to use proteins and fats as energy raw materials, some (for example, nerve, red blood, male reproductive) cells are capable of producing energy only through the oxidation of carbohydrates.

The processes of primary oxidation of carbohydrates - or rather, glucose, which is, in fact, the main substrate of oxidation in cells - occur directly in the cytoplasm: it is there that enzyme complexes are located, thanks to which the glucose molecule is partially destroyed, and the released energy is stored in the form of ATP. This process is called glycolysis, it can take place in all cells of the human body without exception. As a result of this reaction, two 3-carbon molecules of pyruvic acid and two molecules of ATP are formed from one 6-carbon molecule of glucose.

Glycolysis is a very fast, but relatively ineffective process. Pyruvic acid, formed in the cell after the completion of glycolysis reactions, almost immediately turns into lactic acid and sometimes (for example, during heavy muscular work) is released into the blood in very large quantities, since it is a small molecule that can freely pass through the cell membrane. Such a massive release of acidic metabolic products into the blood disrupts homeostasis, and the body has to turn on special homeostatic mechanisms to cope with the consequences of muscle work or other active action.

Pyruvic acid formed as a result of glycolysis still contains a lot of potential chemical energy and can serve as a substrate for further oxidation, but this requires special enzymes and oxygen. This process occurs in many cells that contain special organelles - mitochondria. The inner surface of mitochondrial membranes is composed of large lipid and protein molecules, including a large number of oxidative enzymes. Three-carbon molecules formed in the cytoplasm penetrate inside the mitochondria - usually acetic acid (acetate). There they are included in a continuously ongoing cycle of reactions, during which carbon and hydrogen atoms are alternately split off from these organic molecules, which, combining with oxygen, are converted into carbon dioxide and water. These reactions release a large amount of energy, which is stored in the form of ATP. Each molecule of pyruvic acid, having gone through a full cycle of oxidation in the mitochondria, allows the cell to obtain 17 molecules of ATP. Thus, the complete oxidation of 1 glucose molecule provides the cell with 2+17x2 = 36 ATP molecules. It is equally important that the process of mitochondrial oxidation can also include fatty acids and amino acids, i.e., components of fats and proteins. Thanks to this ability, mitochondria make the cell relatively independent of what foods the body eats: in any case, the required amount of energy will be produced.

Some of the energy is stored in the cell in the form of a smaller and more mobile molecule, creatine phosphate (CrP), than ATP. It is this small molecule that can quickly move from one end of the cell to the other - to where energy is most needed at the moment. KrF itself cannot give energy to the processes of synthesis, muscle contraction or conduction of a nerve impulse: this requires ATP. But on the other hand, KrP is easily and practically without losses capable of giving all the energy contained in it to the adenazine diphosphate (ADP) molecule, which immediately turns into ATP and is ready for further biochemical transformations.

Thus, the energy expended during the functioning of the cell, i.e. ATP can be renewed due to three main processes: anaerobic (oxygen-free) glycolysis, aerobic (with the participation of oxygen) mitochondrial oxidation, and also due to the transfer of the phosphate group from CrP to ADP.

Creatine phosphate source is the most powerful, since the reaction of Creatine Phosphate with ADP occurs very quickly. However, the reserve of CrF in the cell is usually small - for example, muscles can work with maximum effort due to CrF for no more than 6-7 s. This is usually enough to trigger the second most powerful - glycolytic - source of energy. In this case, the nutrient resource is many times greater, but as work progresses, homeostasis becomes increasingly stressed due to the formation of lactic acid, and if such work is performed by large muscles, it cannot last more than 1.5-2 minutes. But during this time, mitochondria are almost completely activated, which are capable of burning not only glucose, but also fatty acids, the supply of which in the body is almost inexhaustible. Therefore, an aerobic mitochondrial source can work for a very long time, although its power is relatively low - 2-3 times less than a glycolytic source, and 5 times less than the power of a creatine phosphate source.

Features of the organization of energy production in various tissues of the body. Different tissues have different levels of mitochondria. They are found least in bones and white fat, most in brown fat, liver and kidneys. There are quite a lot of mitochondria in nerve cells. Muscles do not have a high concentration of mitochondria, but due to the fact that skeletal muscles are the most massive tissue of the body (about 40% of an adult’s body weight), it is the needs of muscle cells that largely determine the intensity and direction of all energy metabolism processes. I.A. Arshavsky called this the “energy rule of skeletal muscles.”

With age, two important components of energy metabolism change at once: the ratio of the masses of tissues with different metabolic activities changes, as well as the content of the most important oxidative enzymes in these tissues. As a result, energy metabolism undergoes quite complex changes, but in general its intensity decreases with age, and quite significantly.