Mechanism of Respiration – Electron transport chain

Mechanism of Respiration – Electron Transport Chain

Electron transport system (ETS) is a chain of electron carriers
consisting of NAD+, FAD+, CoQ and cytochromes (cyt. b, cyt. c, cyt. a
and cyt. a3).

The glucose molecule is completely oxidized by the end of
the citric acid cycle.

But, energy is not released, unless NADH2 and FADH2 are oxidized through electron transport system.

Transfer of electrons and protons from NADH2 and FADH2 to oxygen through a series of components like flavoprotein, cytochrome is called electron transport chain.

This process leads to coupling of electrons to form high-energy
phosphate bonds in the form of ATP from ADP is called oxidative
phosphorylation.

The electron transport components are arranged in the
inner membrane of mitochondria.

According to modern concept, the electron carriers in the electron
transport system are arranged in four complexes – complex I, complex II,
complex III and complex IV.

When NAD+ is a primary acceptor of electrons, the electrons are transported from complex I to II, II to III and then to complex IV. When electrons are transported from one complex to next complex, an ATP is produced.

Thus, one molecule of NADH2 generates three ATPs. When FAD+ is a primary acceptor of electrons, the electrons are transported from complex II to III and then to complex IV.

Thus, one molecule of FADH2 generates two ATPs.

The molecular oxygen forms the terminal constituent of the electron
transport system.

It is the ultimate recipient of electrons and picks up the protons from the substrate to form water.

Energy yield

Complete oxidation of one glucose molecule yields a net gain of 38ATP.

Out of 38ATP molecules, 4ATP are obtained by direct substrate level
phosphorylation, 30ATP through oxidation of NADH2 and 4ATP through
oxidation of FADH2.

Since, a large number of ATP molecules are produced in the mitochondria, they are called the ‘power houses of the cell’.