All biochemical reactions in the cells of any organism proceed with the expenditure of energy. The respiratory chain is a sequence of specific structures that are located on the inner membrane of mitochondria and serve to form ATP. Adenosine triphosphate is a universal source of energy and is able to accumulate in itself from 80 to 120 kJ.
The electron respiratory chain - what is it?
Electrons and protons play an important role in the formation of energy. They create a potential difference on opposite sides of the mitochondrial membrane, which generates a directed movement of particles - a current. The respiratory chain (aka ETC, electron transport chain) mediates the transfer of positively charged particles into the intermembrane space and negatively charged particles into the thickness of the inner mitochondrial membrane.
The main role in the formation of energy belongs to ATP synthase. This complex complex transforms the energy of the directed motion of protons into the energy of biochemical bonds. By the way, an almost identical complex is found in plant chloroplasts.
Complexes and enzymes of the respiratory chain
The transfer of electrons is accompanied by biochemical reactions in the presence of an enzymatic apparatus. These biologically active substances, numerous copies of which form large complex structures, serve as mediators in the transfer of electrons.
Complexes of the respiratory chain are the central components of the transport of charged particles. In total, there are 4 such formations in the inner membrane of mitochondria, as well as ATP synthase. All these structures are united by a common goal - the transfer of electrons along the ETC, the transfer of hydrogen protons into the intermembrane space and, as a result, the synthesis of ATP.
The complex is an accumulation of protein molecules, among which there are enzymes, structural and signal proteins. Each of the 4 complexes performs its own function, only peculiar to it. Let's see for what tasks these structures are present in the ETC.
I complex
The respiratory chain plays the main role in the transfer of electrons in the thickness of the mitochondrial membrane. The reactions of abstraction of hydrogen protons and their accompanying electrons are one of the central ETC reactions. The first complex of the transport chain takes over molecules of NADH+ (in animals) or NADPH+ (in plants) followed by elimination of four hydrogen protons. Actually, because of this biochemical reaction, complex I is also called NADH - dehydrogenase (after the name of the central enzyme).
The composition of the dehydrogenase complex includes 3 types of iron-sulfur proteins, as well asflavin mononucleotides (FMN).
II complex
The operation of this complex is not associated with the transfer of hydrogen protons into the intermembrane space. The main function of this structure is to supply additional electrons to the electron transport chain through the oxidation of succinate. The central enzyme of the complex is succinate-ubiquinone oxidoreductase, which catalyzes the elimination of electrons from succinic acid and transfer to lipophilic ubiquinone.
The supplier of hydrogen protons and electrons to the second complex is also FADH2. However, the efficiency of flavin adenine dinucleotide is less than that of its analogues - NADH or NADPH.
Complex II includes three types of iron-sulfur proteins and the central enzyme succinate oxidoreductase.
III complex
The next component, ETC, consists of cytochromes b556, b560 and c 1, as well as iron-sulfur protein Riske. The work of the third complex is associated with the transfer of two hydrogen protons into the intermembrane space, and electrons from lipophilic ubiquinone to cytochrome C.
The peculiarity of Riske protein is that it dissolves in fat. Other proteins of this group, which were found in the respiratory chain complexes, are water-soluble. This feature affects the position of protein molecules in the thickness of the inner membrane of mitochondria.
The third complex functions as ubiquinone-cytochrome c-oxidoreductase.
IV complex
He is also a cytochrome-oxidant complex, is the end point in the ETC. His work is toelectron transfer from cytochrome c to oxygen atoms. Subsequently, negatively charged O atoms will react with hydrogen protons to form water. The main enzyme is cytochrome c-oxygen oxidoreductase.
The fourth complex includes cytochromes a, a3 and two copper atoms. Cytochrome a3 played a central role in electron transfer to oxygen. The interaction of these structures is suppressed by nitrogen cyanide and carbon monoxide, which in a global sense leads to the cessation of ATP synthesis and death.
Ubiquinone
Ubiquinone is a vitamin-like substance, a lipophilic compound that moves freely in the thickness of the membrane. The mitochondrial respiratory chain cannot do without this structure, since it is responsible for the transport of electrons from complexes I and II to complex III.
Ubiquinone is a benzoquinone derivative. This structure in the diagrams can be denoted by the letter Q or abbreviated as LU (lipophilic ubiquinone). Oxidation of the molecule leads to the formation of semiquinone, a strong oxidizing agent that is potentially dangerous for the cell.
ATP synthase
The main role in the formation of energy belongs to ATP synthase. This mushroom-like structure uses the energy of the directional movement of particles (protons) to convert it into the energy of chemical bonds.
The main process that occurs throughout the ETC is oxidation. The respiratory chain is responsible for the transfer of electrons in the thickness of the mitochondrial membrane and their accumulation in the matrix. Simultaneouslycomplexes I, III and IV pump hydrogen protons into the intermembrane space. The difference in charges on the sides of the membrane leads to the directed movement of protons through ATP synthase. So H + enter the matrix, meet electrons (which are associated with oxygen) and form a substance that is neutral for the cell - water.
ATP synthase consists of F0 and F1 subunits, which together form a router molecule. F1 is made up of three alpha and three beta subunits, which together form a channel. This channel has exactly the same diameter as hydrogen protons. When positively charged particles pass through ATP synthase, the head of the F0 molecule rotates 360 degrees around its axis. During this time, phosphorus residues are attached to AMP or ADP (adenosine mono- and diphosphate) using high-energy bonds, which contain a large amount of energy.
ATP synthases are found in the body not only in mitochondria. In plants, these complexes are also located on the vacuole membrane (tonoplast), as well as on the thylakoids of the chloroplast.
Also, ATPases are present in animal and plant cells. They have a similar structure to ATP synthases, but their action is aimed at the elimination of phosphorus residues with the expenditure of energy.
Biological meaning of the respiratory chain
Firstly, the end product of ETC reactions is the so-called metabolic water (300-400 ml per day). Secondly, ATP is synthesized and energy is stored in the biochemical bonds of this molecule. 40-60 are synthesized per daykg of adenosine triphosphate and the same amount is used in the enzymatic reactions of the cell. The lifetime of one ATP molecule is 1 minute, so the respiratory chain must work smoothly, clearly and without errors. Otherwise, the cell will die.
Mitochondria are considered to be the energy stations of any cell. Their number depends on the energy consumption that is necessary for certain functions. For example, up to 1000 mitochondria can be counted in neurons, which often form a cluster in the so-called synaptic plaque.
Differences in the respiratory chain in plants and animals
In plants, the chloroplast is an additional "energy station" of the cell. ATP synthases are also found on the inner membrane of these organelles, and this is an advantage over animal cells.
Plants can also survive high concentrations of carbon monoxide, nitrogen and cyanide through a cyanide-resistant pathway in the ETC. The respiratory chain thus terminates at ubiquinone, the electrons from which are immediately transferred to oxygen atoms. As a result, less ATP is synthesized, but the plant can survive adverse conditions. Animals in such cases die with prolonged exposure.
You can compare the efficiency of NAD, FAD and the cyanide-resistant pathway by using the rate of ATP production per electron transfer.
- with NAD or NADP, 3 ATP molecules are formed;
- FAD produces 2 ATP molecules;
- cyanide-resistant pathway produces 1 ATP molecule.
Evolutionary value of ETC
For all eukaryotic organisms, one of the main sources of energy is the respiratory chain. The biochemistry of ATP synthesis in the cell is divided into two types: substrate phosphorylation and oxidative phosphorylation. ETC is used in the synthesis of energy of the second type, i.e. due to redox reactions.
In prokaryotic organisms, ATP is formed only in the process of substrate phosphorylation at the stage of glycolysis. Six-carbon sugars (mainly glucose) are involved in the cycle of reactions, and at the output the cell receives 2 ATP molecules. This type of energy synthesis is considered the most primitive, since in eukaryotes 36 ATP molecules are formed in the process of oxidative phosphorylation.
However, this does not mean that modern plants and animals have lost the ability to substrate phosphorylation. It's just that this type of ATP synthesis has become only one of the three stages of obtaining energy in the cell.
Glycolysis in eukaryotes takes place in the cytoplasm of the cell. There are all the necessary enzymes that can break down glucose into two molecules of pyruvic acid with the formation of 2 molecules of ATP. All subsequent stages take place in the mitochondrial matrix. The Krebs cycle, or tricarboxylic acid cycle, also takes place in the mitochondria. This is a closed chain of reactions, as a result of which NADH and FADH2 are synthesized. These molecules will go as consumables to the ETC.
