Schematic representation of the respiratory chain with the complexes and the ATP synthase in the inner membrane of the mitochondria. The electrons are transported via coenzyme Q to complex III and then via cytochrome c to complex IV, where they reduce oxygen to water in the atomic chain.
Electrons supplied by NADH, FMNH2 and FADH2 are transferred to an oxidizing agent in a series of redox processes in the respiratory chain. The exergonic connection of hydrogen and oxygen to water in the respiratory chain – especially in eukaryotes – is divided into individual steps. The electrons bound to the hydrogen and electron carriers NADH and FADH2 and the hydrogen bound to them originate from the oxidation of external electron donors, for example – by means of the citrate cycle – the degradation of fatty acids and glycolysis. In eukaryotes the respiratory chain is located in the inner membrane of the mitochondria, in prokaryotes in the cell membrane.
Electron donors other than fats and sugars and electron acceptors other than oxygen are also found there.
Respiratory chain as electron transport chain
The respiratory chain is the common path by which all electrons originating from the most diverse nutrients of the cell are transferred to oxygen. From the electron transport via the respiratory chain, the cell achieves its greatest energy gain, which it receives from various oxidations – because the electrons have a relatively high energy content when they flow into the respiratory chain.
2 reaction sequences
The electrons of NADH and FADred are transferred in the respiratory chain first to the common acceptor coenzyme Q and then further to a sequence of cytochromes. They are stained red and resemble the oxygen-transmitting hemoglobin in the erythrocytes in their structure. In any cytochrome molecule iron can exist in its divalent form or in its trivalent form. Thus, the cytochrome molecule in its FeIII form can be converted to the reduced state FeII by taking up an electron.
Electron transport via the respiratory chain can be described in a sequence of successive reactions linked by common intermediates. For each pair of hydrogen atoms obtained by each of the four dehydrogenation steps in the citrate cycle, which becomes a pair of H+ ions, one pair of electrons flows into the respiratory chain and ultimately reduces one atom of oxygen to water.