Electron Transport – Class 11 | Chapter – 13 | Biology Short Notes Series PDF

Electron Transport: The metabolic pathway of electron transport is called as electron transport system or ETS.

Glycolysis and Krebs cycle result in the formation of reduced coenzymes such as 10 molecules of NADH +H+ ions and 2 molecules of FADH2 and 4 molecules of ATP. These reduced coenzymes need to be oxidized to release energy stored in them. This is possible by the transport of electrons and protons from these coenzymes to oxygen through electron carriers present in the inner mitochondrial membrane. This metabolic pathway of electron transport is called as electron transport system or ETS.

ETS comprises of several energy carriers which include NADH dehydrogenase complex (Complex I), Ubiquinone (Complex Q), Succinate dehydrogenase complex (complex II), Cytochrome bc1 complex (Complex III), Cytochrome c, Cytochrome c oxidase (Complex IV)

In photosynthesis, water and oxygen are released as by-products. The four protein complexes, complex I to IV and the accessory mobile electron carriers form the electron transport chain. There are multiple electron transport chains inside a mitochondrial membrane. 

Process of Electron Transport System

Ubiquinone Oxidoreductase (Complex 1)

In the first step, a pair of electrons reach the first protein complex called NADH composed of 

NADH dehydrogenase,  iron-sulfur (Fe-S) and flavin mononucleotide (FMN). There is a cofactor attached to complex I which aids the process of creating the gradient so that four hydrogen ions can be released ahead into the intermembrane space. This cofactor is known as NADH dehydrogenase.

Succinate Dehydrogenase (Complex 2 and Ubiquinone)

Complex 2 directly receives electrons from succinate. It is a separate entry point to the electron transport chain. FADH2, which complex II accepts, does not first go through complex I. The enzyme cofactor Q or Ubiquinone is a mobile electron carrier. It plays an important role by collecting the electrons from complexes I and II and delivering them ahead in the electron transport chain. Coenzyme Q is a mobile factor; thus, it can move freely inside the mitochondrial membrane.

During this process, fewer ATPs are generated as the proton pump is not as activated as in complex I.Therefore this complex contributes less energy to the electron transport chain

Cytochrome C Reductase (Complex 3)

Cytochrome C receives only one electron from the coenzyme Q. Q carries a pair of enzymes, but complex III can accept only one. It carries electrons through a heme molecule. The heme molecule alternates between Ferrous (F++) and Ferric (F+++) oxides. The carrying capacity changes with its oxidation state. Protons are pumped through the membrane by complex III to facilitate the passing of the electrons to complex IV by cytochrome c. The electrons are transferred one at a time by the cytochrome to complex IV.

Cytochrome C Oxidase (Complex 4)

Complex IV protein consists of two heme molecules and three copper ions. These are cytochrome proteins. Their role is to bind with an oxygen molecule till it is reduced. Once reduced, it will combine with hydrogen ions to form water. Once hydrogen is removed from the surrounding, it will naturally form a gradient as all hydrogen ions will accumulate in intermembrane space. This will create a positive charge on one side.

Finally, Adenosine diphosphate is converted to Adenosine triphosphate using the potential energy of the hydrogen ion gradient.

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By Team Learning Mantras