What is the respiratory chain?

The respiratory chain is the last part of the degradation pathway of glucose. After the sugar has been metabolized in glycolysis and in the citrate cycle, the respiratory chain has the function of regenerating the reduction equivalents (NADH+H+ and FADH2) produced in the process.This produces the universal energy source ATP (adenosine triphosphate). Like the citrate cycle, the respiratory chain is localized in the mitochondria, which are therefore also called the “power stations of the cell”.

The respiratory chain consists of five enzyme complexes that are embedded in the inner mitochondrial membrane. The first two enzyme complexes each regenerate NADH+H+ (or FADH2) to NAD+ (or FAD). During oxidation of NADH+H+, four protons are transported from the matrix space to the intermembrane space.

Also in the following three enzyme complexes 2 protons each are pumped into the intermembrane space. This creates a concentration gradient which is used for ATP production. For this purpose, protons flow from the intermembrane space through an ATP synthase back into the matrix space.

The released energy is used to finally produce ATP from ADP (adenosine diphosphate) and phosphate. A further task of the respiratory chain is to capture the electrons produced by the oxidation of the reduction equivalents. This is achieved by transferring the electrons to oxygen. By combining electrons, protons and oxygen, normal water is thus produced at the fourth enzyme complex (cytochrome c oxidase). This also explains why the respiratory chain can only proceed when there is sufficient oxygen.

What tasks do the mitochondria have in cellular respiration?

Mitochondria are organelles that only occur in eukaryotic cells. They are also known as the “power plants of the cell”, since cell respiration takes place in them. The end product of cell respiration is the ATP (adenosine triphosphate).

This is a universal energy carrier, which is needed in the whole human organism. A prerequisite for cell respiration is the compartmentalization of the mitochondria. This means that there are separate reaction chambers in the mitochondrium.

This is achieved by an inner and outer membrane, so that there is an intermembrane space and an inner matrix space. In the course of the respiratory chain, protons (hydrogen ions, H+) are transported into the intermembrane space, resulting in a difference in the concentration of protons. These protons come from different reduction equivalents, such as NADH+H+ and FADH2, which are thereby regenerated to NAD+ and FAD.

ATP synthase is the last enzyme in the respiratory chain, where ATP is ultimately produced. Driven by the difference in concentration, the protons flow from the intermembrane space through the ATP synthase into the matrix space. This flow of positive charge releases energy which is used to produce ATP from ADP (adenosine diphosphate) and phosphate.

The mitochondria are particularly well suited for the respiratory chain because they have two reaction spaces due to the double membrane. In addition, many metabolic pathways (glycolysis, citrate cycle) that provide the starting materials (NADH+H+, FADH2) for the respiratory chain take place in the mitochondrion. This spatial proximity represents a further advantage and makes the mitochondria the optimal place for cell respiration. Here you can learn everything about the respiratory chain