Passage of an electron pair down the electron transport chain releases free energy which is used to phosphorylate ADP molecules with inorganic phosphate (Pi) and thus forms ATP.

The formation of ATP molecules coupled to the transfer of electrons, derived from the oxidation oj organic compounds, through the mitochondrial electron transport chain is called as oxidative phosphorylation. The molecular mechanism of this ATP synthesis is best explained through the
chcmiosmotic hypothesis proposed by Peter Mitchell in 1961. For this revolutionary hypothesis, Peter Mitchcll was awarded the Xobel Prize in chemistry in 1978. The essential feature ofthis hypothesis is that during electron transport, a proton gradient is formed across the membran^e which results in the generation of a proton motive forcc.

It is the proton motive force of the gradient which drives the synthesis of ATP when protons forcc their way through an integral membrane protein complex known as the coupling factor.

Shows the Victoria transfer of protons from the matrix side of the inner mitochondrial membrane to the intermitochondrial space when electrons are transferred through complexes I, III and IV. A total of 10 protons are transferred when two electrons are transported from XADH to oxygen molecule via the components of the electron transport chain. For the transport of electrons coming from succinate via FADII2 which bypasses complex I, 6 protons are transported per two electrons transferred.

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This establishes a proton motive force across the inner mitochondrial membrane. This proton motive force drives the synthesis of ATP when the protons force their way into the matrix through” an integral membrane protein complex known as ATP synthase.

This complex is also described in many text books under the name F0-F1 particle or coupling factor. It is estimated that for 3 protons transported through ATP synthase, one ATP is synthesized. Because 9 protons are transported for each pair of electrons transferred trough the chain, a total of approximately 3 ATP molecules are synthesized for each molecule of XADH oxidized in the mitochondrial electron transport chain.

For the oxidation of each molecule of XADII formed in the cytoplasm during glycolysis and FADH2 formed inside mitochondria during citric acid cycle, 2 molecules of ATP are formed since complex I is bypassed. A balance sheet of total ATP formed for each molecule of glucose oxidized during cellular aerobic respiration is shown in Table 1.