Electron transport system and Oxidative phosphorylation – Cellular respiration ( Part – V)

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By using electron transport system and oxidative phosphorylation steps, the cell utilizes energy stored in NADH + H⁺ and FADH formed during glycolysis and Krebs cycle.

The Electron Transport System also called the Electron Transport Chain is a metabolic pathway present in the inner mitochondrial membrane, where electrons available from oxidation of NADH and FADH₂ are passed along from one carrier to another in energy-releasing reactions.

The free energy released is stored in the form of an electrochemical gradient of protons across the membrane, which is used to synthesize ATP by a process called as oxidative phosphorylation.

Electron Transport System:

The electron transport chain can be described as a series of oxidation-reduction (redox) reactions leading to the release of energy. A summary of the reactions in the electron transport chain is:

NADH + 1/2O₂ + H⁺ + ADP + Pi → NAD⁺ + ATP + H₂O

Electrons from NADH formed during glycolysis and Krebs cycle are transferred to complex I, which is made up of NADH dehydrogenase and the Fe-S centers. In this step NADH is oxidized to NAD⁺ and also transports 4 H⁺ ions from matrix through the membrane to the inner membrane space, creating a proton gradient. Later, electrons are transferred to ubiquinone located in the inner membrane.
Complex II consists of succinic dehydrogenase, FAD, and Fe-S centers. Electron transfer through complex II is not accompanied by proton translocation.
Electrons from FADH2 ( part of complex II) are directly received by ubiquinone bypassing complex I. The reduced ubiquinone is then oxidized with the transfer of electrons to cytochrome c via cytochrome bc 1 complex (complex III).
Cytochrome c is a small protein which acts as a mobile carrier for transfer of electrons between complex III and IV.
Ubiquinone and cytochrome c are the two electron carriers which are not part of any electron carrier complex. They in fact forms a connecting link between complexes.
4 protons are translocated across the membrane for every pair of electrons transferred through complex III.
Complex IV is a cytochrome c oxidase complex containing cytochrome a and a₃, along with two copper centers.
As the electrons pass from one carrier by means of complex I to IV, they are coupled to ATP synthase for production of ATP from ADP + inorganic phosphate (addition of phosphate is called phosphorylation reaction).
Even though aerobic respiration take place in presence of oxygen, the role of oxygen confined to the last stage of the process.
The final step of electron transport in a mitochondrion is the successive transfer of electrons from reduced cytochrome c to oxygen to form water by picking up two hydrogen ions from the surrounding medium (after picking up extra electrons, oxygen tends to attract H⁺ or protons) .
oxygen is the final election acceptor in the electron transport chain. Even though oxygen comes into picture at a very stage of aerobic respiration, its presence is very vital as it runs the process of removing hydrogen from system by acting as final hydrogen acceptor ( as molecules pick up electrons, they also gain one or more hydrogen atoms).

The removal of the hydrogen ions from the system contributes to the ion gradient that forms the foundation for the process of chemiosmosis.

The reduction of O₂ is catalyzed by complex IV (cytochrome oxidase). In this step 2 protons are translocated from matrix to inner membrane space, across the membrane helping to form the proton gradient.

The proton gradient formed in above steps will be used to synthesize ATP by chemiosmosis hypothesis proposed by Peter Mitchell in 1961. (for more details on chemiosmosis hypothesis which occurs during photosynthesis and structure of ATP synthase please click here)

The difference in formation of ATP during photosynthesis is that during photophosphorylation light is used to drive the formation of proton gradient essential for phosphorylation or ATP synthesis.

In respiration, the energy released from oxidation-reduction process is used for ATP synthesis. hence it is called oxidative phosphorylation.

To sum up these reactions is that it leads into production of ATP from the energy of the electrons removed from hydrogen atoms and these atoms were once part of a glucose molecule which entered glycolysis.

Synthesis of ATP – Chemiosmosis

The flow of electrons through the electron carriers of electron transport system leads to accumulation or high concentration of H⁺ ions in inner membrane space when compared to matrix of mitochondria.

This unequal distribution of H⁺ ions across the membrane establishes both concentration and electrical gradients.

Many ions cannot just cross the cell membrane unless helped by ion channels.

An ATP synthase complex – made up of two subunits F₀and F₁ . The F1 subunits harbors the active site for phosphorylation of ADP plus inorganic phosphate to form ATP.

F₀ subnuit integrates in membrane and forms the channel to allow protons to pass through it along the active sit in F₁ subunit.

This flow of protons back into matrix from inner membrane space leads to formation of ATP.