Electron Transport System

Introduction

We, as living organisms, need the energy to sustain life. This energy is generated inside our cells with the help of the food , water we consume and the air we breathe. The energy production process in animals and plants occurs at a cellular level. In the case of animals, this process happens by breaking down organic material like carbohydrates, consumed as food, by combining it with oxygen, that is breathed in, and converting it into energy. This is called cellular respiration. 

In plants, energy is produced by combining light, carbon dioxide and water. Both these processes use the electron transport system to create a pathway inside the inner membrane of a cell’s mitochondria where cellular respiration or photosynthesis takes place to transport electrons that reduce the organic material or carbon dioxide to release energy.

Electron Transport System

The Electron Transport System is a mitochondrial pathway through which electrons move across four proteins, with the help of mobile coenzymes and undergo multiple redox reactions to release energy. In the electron transport chain, a gradient is created by four proteins placed alongside. The process occurs when each electron moves past these four proteins and undergo redox reactions at the end to finally release Adenosine triphosphate. This process is known as oxidative phosphorylation. 

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. The relay process that occurs inside the transport system can be better understood as follows:

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.

Conclusion

In animals, cellular respiration is the mechanism by which ATP or adenosine triphosphate is produced and energy is released. It occurs in the mitochondria, known as the cell’s powerhouse. The process of cellular respiration ends with a phase of oxidative phosphorylation and the Electron Transport Chain is a crucial part of the process. Understanding the four complexes will help you clarify how energy is produced within the cell. 

In plants, the process occurs during photosynthesis. Plants absorb light, water and carbon dioxide from the environment and through the process of photosynthesis, this is converted to energy in the form of ATP. Electron transport chain is present inside the mitochondrial membrane in both animal and plant cells. The electron transport chain is a common process for both.