Electron Transport Chain

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The steps in the respiratory process are to generate and use NADH+H+ and FADH2 stored energy. This is done when they are oxidized by the electron transport system, and the electrons are delivered to O2 resulting in H2O creation. The biochemical path the electron is traveling from one carrier to another is called the electron transport network.

What is the Electron Transport Chain?

The electron transportation chain is the last aerobic respiration portion and is the only part of the glucose metabolism that uses atmospheric oxygen. Oxygen continuously passes through plants; it enters the body via the respiratory system of animals.

Electron transport is a sequence of redox reactions that mimic a relay race or bucket brigade in which electrons are easily transported from one part to the end point of the chain where the electrons decrease molecular oxygen and produce water.

There are four protein-composed electron transport chain complexes, labelled I through IV in the electron transport chain diagram below, and the assembly of these four complexes together with related active, accessory electron carriers is described named the electron transport chain. The electron transport chain is present in multiple copies in the eukaryote inner mitochondrial membrane and in the prokaryote plasma membrane. 

But note that the prokaryote electron transport chain may not require oxygen as some live-in anaerobic conditions. All electron transport chains are commonly characterized by the presence of a proton pump to create a proton gradient across a membrane.

Below electron transport system diagram illustrates the electron transport system in mitochondria.

(Image to be added soon)

Complex I

Aboard NADH, two electrons are transported to the first complex. Complex I consists of flavin mononucleotide (FMN) and the iron-sulfur (Fe-S) enzyme. FMN, originating from vitamin B2 (also known as riboflavin), is one of several prothetic classes or co - factors in the chain of electron transport. 

A prosthetic group is a molecule that is not protein required for a protein 's activity. Prosthetic groups may be organic or inorganic, and are non-peptide molecules bound to a protein that promotes their work.

Prosthetic groups include co-enzymes that are the enzyme prosthetic groups. NADH dehydrogenase is the enzyme in complex I, a very large protein containing 45 chains of amino acids. Complex I can pump four hydrogen ions into the intermembrane space across the membrane from the matrix; this is how the gradient of hydrogen ions is established and maintained between the two compartments separated by the inner mitochondrial membrane.

Q and Complex II

Complex II receives FADH2 directly, which does not traverse complex I. The compound which connects the first and second complexes to the third complex is ubiquinone (Q). The Q molecule is lipid soluble, and moves freely through the membrane's hydrophobic core. On reduction to QH2, ubiquinone transfers the electrons to the next complex in the electron transport chain.

Q derives the NADH derived electrons from complex I and the FADH2 derived electrons from complex II, like succinate dehydrogenase. This enzyme and FADH2 form a small complex that directly supplies electrons to the electron transmission chain, bypassing the first complex. 

Since these electrons circumvent the proton pump in the first complex and thus do not energize, less ATP molecules are made from the FADH2 electrons. Basically, the amount of ATP molecules produced is directly proportional to the number of protons pumped through the mitochondrial membrane inside.

Complex III

The third complex comprises of cytochrome b, another Fe-S protein, cytochrome c proteins, Rieske center (2Fe-2S center) and this complex is also known as cytochrome oxidoreductase.

Cytochrome proteins have a group of prosthetic hemes. The heme molecule of hemoglobin is similar to the heme because it includes electrons rather than oxygen. These lowers and oxidizes the iron ion at its center as it moves through the electrons, fluctuating between different oxidation states: Fe2 + (reduced) and Fe3 + (oxidized).

Because of the effects of the different proteins linking them, the heme molecules in the cytochromes have slightly different characteristics which makes each group. Complex III pushes protons through the membrane and transfers their electrons to cytochrome c for transportation to the fourth protein and enzyme complex. Cytochrome c is the accepter of Q electrons; while Q holds pairs of electrons, cytochrome c can accept only one at a time.

Complex IV

The fourth complex consists of the cytochrome c, a, and a3 proteins. This complex contains two classes of hemes (one in each cytochrome a and a3) and three ions of copper (a pair of CuA and one CuB in cytochrome a3).

The cytochromes hold a molecule of oxygen very tightly between the iron and copper ions until the oxygen is reduced altogether. The reduced oxygen then picks up two hydrogen ions to produce water (H2O) from the surrounding medium. Deleting the hydrogen ions from the system also contributes to the ion gradient used in the chemiosmosis process.

FAQ (Frequently Asked Questions)

1. Explain the 3 Main Steps in the Electron Transport Chain?

The three main Electron transport chain steps are as follows:

  • Pumps with protons generate an electrochemical gradient (proton motive force)

  • ATP synthase synthesizes ATP by using the resulting release of protons (chemiosmosis).

  • Oxygen allows water to form electrons and protons.

2. What is the Importance of Electron Transport Chain in Cellular Respiration?

The electron transport chain is a series of four protein complexes that couple redox reactions, creating an electrochemical gradient that leads to the creation of ATP in a complete system named oxidative phosphorylation. It occurs in mitochondria in both cellular respiration and photosynthesis  

3. Explain the Main Biochemical Function of the Electron Transport Chain?

The electron transport chain is a series of four protein complexes that couple redox reactions, creating an electrochemical gradient that leads to the creation of ATP in a complete system named oxidative phosphorylation. It occurs in mitochondria in both cellular respiration and photo synthesis.

4. Explain How is Water Produced in the Electron Transport Chain?

This begins with the movement of protons through the cell through NADH and FADH2, producing ATP through a series of reactions. The hydrogen from the coenzymes enters the oxygen consumed by the cell towards the end of the electron transport chain, and interacts with it to form water.