Electron Transport System

The electron transport system is present in the inner mitochondrial membrane of mitochondria. It also refers as “Electron transport chain” and “ETS” in abbreviated form. ETS involves a transfer of electrons through a series of protein complexes from higher (NADH+) to lower energy state (O2), by releasing protons into the cytosol. The movement of a proton or H+ from a matrix to cytosol generates a proton motive force and creates an electrochemical gradient. The proton molecules then tend to diffuse down the electrochemical gradient again into the mitochondrial matrix and releases ATP via ATP synthase.

An oxygen atom is the last carrier, which accepts the electron and combines with the free hydrogen ions in the mitochondrial matrix to give water. Thus the oxygen carrier maintains the membrane potential by removing the de-energized from the inner mitochondrial membrane.

Content: Electron Transport System

  1. Definition of Electron Transport System
  2. Step by Step Explanation
  3. Components of ETS
  4. Location of ETS
  5. Summary
  6. Equation
  7. Mechanism of Electron Transport System

Definition of Electron Transport System

Electron transport system can define as a mechanism of cellular respiration that occurs in the inner membrane of mitochondria. It is the third and the last stage of cellular respiration and also refers as “Electron transport chain or Respiratory chain”. The electron transport system is an aerobic pathway. In ETS, the electrons flow from high to low energy state and finally removed by the oxygen carrier that combines with free protons to produce waste as water.

flow of electrons in Electron transport system

The electron transport system consists of hydrogen carrier complexes, electron carriers and an ATP synthase ion channel. An electron transport system creates a chemiosmotic gradient which allows the diffusion of a proton into the matrix by the production of ATP. The ATP is then used up by the cell to perform cellular and metabolic activities.

Step by Step Explanation

The process of electron transport system includes the following steps:

Step 1: Generation of proton motive force
generation of proton motive force in ETC

In the first step of the electron transport chain, the NADH+ and FADH2 molecule of glycolysis and Kreb’s cycle is oxidized into NAD+ and FAD, releases high energy electrons and protons. The electron diffuses into the inner mitochondrial membrane, which consists of a series of large protein complexes. The passage of an electron from one carrier protein to others loses some of the energy or ATP.

The ATP is then used up by the complexes to move proton from matrix to the intermembrane space. Thus, the diffusion of a proton across the inner mitochondrial membrane is the process refers to as “Chemiosmosis”, which creates a proton motive force across the electrochemical gradient.

Step 2: Synthesis of high energy molecule ATP
Synthesis of ATP in ETS

The H+ generates a proton motive force, which helps them to move downhill the concentration gradient of the inner mitochondrial membrane. H+ ion tends to diffuse back into the mitochondrial matrix through the channel protein by the help of transmembrane enzyme (ATP synthase), by producing ATP.

Step 3: Oxygen reduction
Oxygen reduction in ETS

For the continuation of the electron transport system, the de-energized electrons are released out by the help of electron acceptor O2 molecule. The oxygen accepts the electron from the fourth complex and then binds with the free protons. Finally, oxygen gets reduced to yield H2O.

Components of ETS

The electron transport system consists of the following elements:

components of electron transport system

Complex I

It is composed of Flavin mononucleotide and iron-sulphur protein. Complex I also refers as “NADH dehydrogenase” which oxidizes NADH+ into NAD+ and releases two electrons and four protons. NADH dehydrogenase pumps out four protons from the matrix to the cytosol and transfers two electrons in the inner mitochondrial membrane. Thus, NADH dehydrogenase creates high H+ ion concentration across the electrochemical gradient.


Coenzyme-Q also refers to as “Ubiquinone” that connects the complex I and II. Ubiquinone is a lipid-soluble complex, which can move freely in the hydrophobic core of the mitochondrial membrane. Q reduces into QH2 and delivers its electron to the third complex. Coenzyme-Q receives the electron released from the NADH and FADH2 molecules.

Complex II

It consists of an enzyme “Succinate dehydrogenase” and composed of iron and succinate. Complex II oxidizes FADH2 into FAD+. Succinate dehydrogenase plus FADH2 combines and directly transfers the electron to the ETC, bypassing complex I. It does not energize the complex I and produces few ATPs.

Complex III

It is composed of “Cytochrome-b” which consist of Fe-S protein with Rieske centre (2Fe-Fs). Complex III also refers to as “Oxidoreductase”. In cytochromes, the prosthetic group is heme, which carries an electron. As the electron passes, the iron is reduced to Fe2+ and oxidized to Fe3+. Complex III or cytochrome-b transfers an electron to the next complex cytochrome c.

Cytochrome c

Cytochrome-c also contains Fe-S protein and prosthetic heme group. It only accepts one electron at a time and further transports its electron to the fourth complex.

Complex IV

It is composed of Cytochrome a and a3, which contains two heme groups (one in each). Cytochrome-a3 consists of three copper ions (two CuA and one CuB). The function of complex IV is to hold the oxygen carrier firmly between the iron and copper ions until the reduction of oxygen into a water molecule. Oxygen combines with the two proton molecules and releases water by maintaining the membrane ion potential.

Complex V

It is the protein ion channel which consists of a transmembrane enzyme “ATP synthase” and thus also refers as ATP synthase complex. Complex V allows the passage of proton ion from the high to low concentration, against the potential gradient. The chemiosmotic passage of proton causes molecular rotation of the enzyme  ATP synthase and therefore release energy in the form of ATP.
Diagram of electron transport chain

Location of ETS

The electron transport system and its protein complexes along with ATP synthase channel protein are located in the inner mitochondrial membrane. In the diagram, we can see the site of the electron transport chain, which is present in between the cytosol and matrix.

location of electron transport system

There are four large protein complexes in the electron transport chain that mediates the transfer of an electron from one to the other. In addition to protein complexes, there are individual electron carriers present like Co-Q and Cyt-C. Both Coenzyme-Q and Cytochrome-c are the diffusible electron carriers and can travel within the membrane. Along with that, there is one ion channel protein (ATP-synthase) which mediates the transport of proton down the concentration gradient by producing ATP.


ETS can define as the system of producing energy in the form of ATP via a series of chemical reactions. The ETS is located in the inner membrane of mitochondria and contains electron carrier protein complexes, electron acceptor and channel protein.

Electrons pass from one complex to the other by redox reactions. Some energy produces during electron transfer, which captures as a proton gradient and used up by the ATP synthase to derive ATP.

schematic diagram of ETC

In ETC, the electrons formed by the reduction of FADH2 and NADH transfers to the electron carrier Co-Q. Coenzyme-Q or Q then reduces into QH2 and then passes its electron to the third protein complex (cyt-b). Complex III contains a heme group, where the Fe3+ accepts electron coming from Co-Q and reduces into Fe2+.

The third complex further transfers the electron to cyt-c where Fe3+ reduces into Fe2+ and transfers an electron to the fourth complex. Complex IV accepts, and Fe3+ reduces into Fe2+ and transfer an electron to the oxygen carrier. The oxygen carries the de-energized and combines with the free proton ions in the matrix and release waste in the form of water.


The overall reaction in the electron transport chain can be equated in a way given in the diagram. In the electron transport chain, per molecule of glucose can produce 34 molecules of ATP, given in the equation below:

net production in ETC
Thus, the net production of energy in the electron transport chain is 34 ATP molecules.

Mechanism of Electron Transport System

Electron transport chain also refers to as “Respiratory chain”, which is the third or final stage of cellular respiration. It requires the presence of oxygen to carry out the cellular respiration. In ETC, the energy produced during the transfer of an electron from one carrier to the other.

An electron loses some of the energy during the transport, that harnesses to pump proton into the cytosol, by creating a chemiosmotic gradient. A chemiosmotic gradient becomes charged, by the potential energy of the electrons. And finally, the potential energy converts into chemical energy (ATP) by the ATP synthase complex.

Thus, the electron transport system is an energy-producing mechanism, which obeys the principle of “Takes energy to make energy”. The electron transport system consists of a series of redox reactions where the electrons lose energy.  The membrane uses the energy lost by an electron, to diffuse proton back into the matrix and create a high energy molecule ATP.

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