The respiratory chain is a process for generating energy in our body cells. It joins the citrate cycle and is the final step in the removal of sugar, fats and proteins. The respiratory chain is located in the inner membrane of the mitochondria. In the respiratory chain meanwhile formed reduction equivalents (NADH + H + and FADH2) are again oxidized (electrons are released), whereby a proton gradient can be built up. This is finally used to form the universal energy source ATP (adenosine triphosphate). In order for the respiratory chain to run completely, oxygen is also needed.

Course of the respiratory chain

The respiratory chain is integrated into the inner mitochondrial membrane and consists of a total of five enzyme complexes. It follows the citrate cycle, in which the reduction equivalents NADH + H + and FADH2 are formed. These reduction equivalents temporarily store energy and are oxidized again in the respiratory chain. This process takes place on the first two enzyme complexes of the respiratory chain.
Complex 1: NADH + H + enters the first complex (NADH-ubiquinone oxidoreductase) and releases two electrons. Simultaneously, 4 protons are pumped from the matrix space into the intermembrane space.

Complex 2: FADH2 releases its two electrons from the second enzyme complex (succinate-ubiquinone oxidoreductase), but no protons enter the intermembrane space.

Complex 3: The released electrons are forwarded to the third enzyme complex (ubiquinone cytochrome c oxidoreductase), where another 2 protons are pumped from the matrix space into the intermembrane space.

Complex 4: Finally, the electrons reach the fourth complex (cytochrome c oxidase). Here, the electrons are transferred to oxygen (O2), so that with two additional protons water (H2O) is formed. Here again 2 protons enter the intermembrane space.

Complex 5: In total, eight protons were pumped from the matrix space into the intermembrane space. The basic requirement for the electron transport chain is the increasing electronegativity of the enzyme complex. This means that the ability of enzyme complexes to attract negative electrons becomes ever stronger.
In addition to the first end product water, a proton gradient in the intermembrane space was established through the respiratory chain. In this energy is stored, which is used to build up ATP (adenosine triphosphate). This is the task of the fifth and last enzyme complex (ATP synthase). The fifth complex spans the mitochondrial membrane like a tunnel. Through this, driven by the difference in concentration, the protons flow back into the matrix space. The result is ADP (adenosine diphosphate) and inorganic phosphate ATP, which is available to the entire organism.

What does the proton pump do?

The proton pump is the fifth and last enzyme complex in the respiratory chain. Through this, the protons flow back from the intermembrane space into the matrix space. This is made possible only by the previously established concentration difference between the two reaction spaces. The energy stored in the proton gradient is used to ultimately synthesize ATP (adenosine triphosphate) from phosphate and ADP.
ATP is the universal energy source of our body and is essential for a variety of reactions. Since it is generated at the proton pump, it is also called ATP synthase.

Balance of the respiratory chain

The ultimate end product of the respiratory chain is ATP (adenine triphosphate), which is a universal energy source of the body. ATP is synthesized by means of a proton gradient, which arises during the respiratory chain. NADH + H + and FADH2 have different efficiency. NADH + H + is oxidized back to NAD + at the respiratory chain on the first enzyme complex, pumping a total of 10 protons into the intermembrane space. In the oxidation of FADH2, the yield is lower because only 6 protons are transported into the intermembrane space. This is because FADH2 is introduced into the respiratory chain at the second enzyme complex, bypassing the first complex. To synthesize an ATP, 4 protons must flow through the fifth complex.
Thus, for each NADH + H + 2.5 ATP (10/4 = 2.5) and for each FADH2 1.5 ATP (6/4 = 1.5) are prepared.
During the degradation of a sugar molecule via the glycolysis, citrate cycle and respiratory chain, a maximum of 32 ATPs can be generated, which are available to the organism.

What role do the mitochondria play?

Mitochondria are cell organelles found in animal and plant organisms. In the mitochondria find various energy processes, including the respiratory chain. Since the respiratory chain is the key process for generating energy, mitochondria are also called the "power plants of the cell". They have a double membrane, resulting in a total of two separate reaction spaces. Inside is the matrix space and between the two membranes of the intermembrane space. These two spaces are fundamental to the flow of the respiratory chain. Only in this way can a proton gradient be built up, which is important for ATP synthesis.

What does cyanide do in the respiratory chain?

Cyanides are dangerous toxins that include, among others, compounds of hydrocyanic acid. They are able to bring the respiratory chain to a halt.
Specifically, the cyanide binds to the iron of the fourth complex of the respiratory chain. Consequently, the electrons can no longer be transferred to molecular oxygen. The entire respiratory chain can not run off.
The result is a lack of the energy source ATP (adenosine triphosphate) and it comes to a so-called "inner suffocation". Symptoms such as vomiting, unconsciousness and convulsions occur very quickly after cyanide poisoning and, if left untreated, lead to rapid death.

What is a respiratory chain defect?

A respiratory chain defect is a rare metabolic disease that often manifests in childhood. The causes are changes in genetic information (DNA). The mitochondria are limited in their function and the respiratory chain does not work properly. This is particularly noticeable in organs that consume a lot of energy in the form of ATP (adenosine triphosphate).
A typical symptom are, for example, muscle pain or muscle weakness.
Therapy of this disease is difficult because it is a hereditary disease. It should be paid attention to a sufficient energy supply (eg by dextrose sugar). Otherwise, a purely symptomatic treatment is appropriate.

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