What is the respiratory chain?


The respiratory chain is a process of energy production in our body cells. It is connected to the citrate cycle and is the last step in the breakdown of sugar, fats and proteins. The respiratory chain is located in the inner membrane of the mitochondria.

In the respiratory chain, reduction equivalents (NADH+H+ and FADH2) that have been formed in the meantime are oxidised again (electrons are given off), allowing a proton gradient to be established. This is finally used to form the universal energy carrier ATP (adenosine triphosphate). Oxygen is also needed to enable the respiratory chain to run completely.

Sequence 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 store energy in the meantime and are oxidized again in the respiratory chain.

This process takes place at the first two enzyme complexes of the respiratory chain. Complex 1: NADH+H+ reaches the first complex (NADH-ubiquinone oxidoreductase) and gives off two electrons. At the same time 4 protons are pumped from the matrix space into the intermembrane space.

Complex 2: FADH2 gives off its two electrons at the second enzyme complex (succinate-ubiquinone oxidoreductase), but no protons enter the intermembrane space. Complex 3: The released electrons are transferred 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-oxidoreductase).

Here the electrons are transferred to oxygen (O2), so that with two additional protons water (H2O) is formed. Thereby 2 protons again enter the intermembrane space. Complex 5: A total of eight protons have now been pumped from the matrix space into the intermembrane space.

The basic prerequisite for the electron transport chain is the increasing electronegativity of the enzyme complex. This means that the ability of the enzyme complexes to attract negative electrons becomes stronger and stronger. In addition to the first end product, water, a proton gradient was established in the intermembrane space by the respiratory chain.

In this space 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 tunnel, driven by the difference in concentration, the protons flow back into the matrix space. In this way, ADP (adenosine diphosphate) and inorganic phosphate are converted into ATP, which is available to the entire organism. The proton pump is the fifth and last enzyme complex of the respiratory chain.

Through it, the protons flow from the intermembrane space back into the matrix space. This is only made possible by the previously established difference in concentration between the two reaction spaces. The energy stored in the proton gradient is used to synthesize ATP (adenosine triphosphate) from phosphate and ADP. ATP is the universal energy carrier of our body and is essential for a large number of reactions. Since it is generated at the proton pump, this is also known as ATP synthase.