Electron Transport Chain in Mitochondria? 7 Facts

Electron transport chain (ETC) is a multistep redox reaction process which occur inside the specific cell organelles. Let us discuss the electron transport chain in detail below.

The electron transport chain takes place inside mitochondria in eukaryotes. It involves a serial action of four protein complexes for the coupling of the redox reactions. Here, a chemical gradient is created by using electrons from electron carriers.

Schematic representation of Electron Transport Chain Image from Wikipedia

Catabolism of organic molecules release electrons which then enter into the chain and are excited by the light. As a result of this, energy is produced.

Proteins involved in Electron Transport Chain in Mitochondria

Electron transport chain involves the usage of a series of multiprotein complexes. Let us discuss more about the proteins involved in this process.

The proteins involved in electron transport chain in mitochondria are:

  • Complex I: It is also called ubiquinone oxidoreductase. It is generally formed of FMN (flavin mononucleotide), eight iron-sulfur clusters (Fe-S) and DADH dehydrogenase. It contributes four hydrogen ions in the ETC process which are moved from matrix to intermembrane space of mitochondria.

      (NADH + H+) + CoQ + 4H+ (matrix) -> NAD+ + CoQH2 + 4H+

  • Complex II: It is also called succinate dehydrogenase. It acts as a secondary entry point in ETC by accepting electrons from succinate. It is not involved in protons translocation and hence less ATP molecules are released by this pathway.

      Succinate + FAD -> Fumarate + 2H+ + FADH2

       FADH2 + CoQ -> FAD + CoQH2

  • Coenzyme Q: It is composed of quinone and a hydrophobic tail. It is also known as ubiquinone (CoQ). Coenzyme Q is an electron carrier and also helps in transferring electrons to the next complex in the series.
  • Complex III: This complex is formed of cytochrome b, Rieske subunits and cytochrome c subunits and is also called cytochrome c reductase. It is involved in electron transfer and can only accept single electrons at a time. It contributes in generating the electron gradient by contributing four protons at the end of full Q cycle into the intermembrane space.
  • Complex IV: It is also known as cytochrome c oxidase that oxidizes cytochrome c and then transfer the electrons to oxygen. It is the final electron carrier in the process of electron transport chain. It also contributes in the proton gradient by releasing four protons into intermembrane space.

     2 cytochrome c + ½ O2 + 4H+ -> 2cytochrome c + 1 H2O + 2H+

  • Complex V: It is also known as ATP synthase. It functions in the synthesis of ATP using the proton gradient which builds up in ETC across the inner membrane of mitochondria.

The f0 and f1 subunits of ATP synthase undergoes certain conformational changes that regulates the ATP synthesis. With every four H+ ions, one molecule of ATP is produced. This action of ATP synthase can also be reversed, consuming ATP to generate proton gradient. This reverse action has been seen in certain bacteria.

Where does electron transport chain occur in Mitochondria?

Mitochondria is the central organelle where all the energy generating processes are conducted. Let us see in detail where electron transport chain occurs in mitochondria.

Oxidative phosphorylation and electron transport chain happens in the inner mitochondrial membrane. A series of protein complexes which are embedded in the mitochondrial membrane facilitates the electron transport chain process.  

Eukaryotic mitochondria Image from Wikipedia

The inner membrane of mitochondria contains folds called cristae which helps in increasing the capacity of mitochondria for the synthesis of ATP molecules. The folds allow the packing of more amount of ATP synthase and other ETC enzymes into the mitochondria.

How does electron transport chain occur in mitochondria?

The process of electron transport chain is actively regulated and highly monitored. Let us discuss how the process takes place inside mitochondria.

The electron transport chain in mitochondria involves the collaborative action of four complexes which works together in coupling of redox reactions and generates electrochemical gradient ultimately leading to the synthesis of ATP.

The entire process is referred to as oxidative phosphorylation which involves two process of electron transport chain and chemiosmosis. It occurs in mitochondria and in chloroplasts as a part of cellular respiration and photosynthesis.

What are the steps of electron transport chain in Mitochondria?

Electron transport chain occurs in a series of steps which are highly regulated inside the mitochondria. Let us talk about these steps in detail.

Listed below are the steps included in the electron transport chain in mitochondria:

Step 1: Complex I or NADH dehydrogenase comes in contact with NADH molecule and receives two electrons from it after oxidizing it to NAD+. Two hydrogens per NADH molecules are yielded from complex I which are transported to the intermembrane space.

Step 2: Complex II oxidize FADH2 into FAD and receives two electrons.

Step 3: The electrons received from the complex I and complex II are transferred to Ubiquinone, which is an electron carrier.

Step 4: Ubiquinone carries the electrons to Complex III which in turn pumps one hydrogen per electron out of the matrix.

Step 5: Electrons are moved to Cytochrome c protein which caries the electrons to Complex IV.

Step 6: Complex IV is an electron acceptor which carries oxygen. This complex requires four electrons for its function. It creates two molecules of water and pump out rest of the protons into intermembrane space.

Step 7: This step is the final step of the process which involves the formation of ATP with the help of ATP synthase and the process is named as Chemiosmosis.

The final step of this aerobic cellular respiration process is the production of ATP molecules that occurs inside the mitochondria. High energy electrons are gathered by NAD+ and FAD which helps in the conversion of ADP into ATP.

Functions of electron transport chain in Mitochondria

Electron transport chain is a significant process in mitochondria. Let us discuss its importance in mitochondria in detail.

Functions of electron transport chain in mitochondria are listed below:

  • Inside mitochondria, ETC produces a transmembrane proton electrochemical gradient.
  • It actively participates in the production of adenosine triphosphate molecules in mitochondria.
  • Electron transport chain is the part of oxidative phosphorylation in eukaryotic mitochondrial membrane.  
  • Conservation of energy in the form of chemiosmotic gradient is the basic purpose of electron transport chain.

How many ATP are produced in electron transport chain?

The process of ETP give rise to ATP at the end. Let us see the number of ATP produced in this process.

Inside the inner membrane of mitochondria, the electron transport chain yields approximately 30-32 ATP molecules in the final step based on the latest studies.

At the end of this process, the electrons dropped off from NADH and FADH2 molecules which inturn results in more ATP generation. Oxygen is directly used in the process and is converted to water at the end.

Conclusion

To conclude the article, we can say that electron transport chain occurs in the inner membrane of mitochondria and is crucial for generating the proton gradient and hence the energy in the form of ATP molecules.

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