Electron Transport System: The electron transport system (ETS), or oxidative phosphorylation, transfers an electron from an electron donor to an electron acceptor by redox reactions through many protein complexes containing heme groups and copper ions. Additionally, electrons-protons are transferred in this process, making a chemical gradient. A large amount of ATP is created with this movement of electrons and protons in plants, which is then used for a variety of functions. Read this article to know more about electron transport systems.

Define Electron Transport System

The electron transport system (ETS), or oxidative phosphorylation, transfers an electron from an electron donor to an electron acceptor by redox reactions through many protein complexes containing heme groups and copper ions.

Types of Processes

The electron transport system is seen in two types of processes, i.e., during photosynthesis and during the breakdown of glucose in the aerobic environment to produce ATP.

1. Photosynthesis

Photosynthesis is when green parts of the plants use water, light, and carbon dioxide to form glucose and oxygen. It’s an energy transformation process in which light energy is converted into chemical energy.

Photosynthesis takes place in two stages:

1. Light Dependent Reaction (Light Reaction) is a reaction that happens within the chloroplast when light is absorbed by the chlorophyll molecule embedded in the thylakoid membrane. It performs several biochemical reactions in order to create ATP and NADPH (assimilatory power as Aron describes it) and release oxygen.
2. Light Independent Reaction (Dark Reaction), a series of chemical reactions not requiring light in photosynthesis and involving the reduction of carbon dioxide to form carbohydrates.

2. Photophosphorylation 

Photophosphorylation is a process that converts ADP into ATP in the presence of sunlight. It is of two types:

(i) Non- Cyclic Photophosphorylation

This photophosphorylation process involves non-cyclic electron movement in forming ATP molecules and NADPH molecules, and it involves PSI and PSII.

Electron Transport System in Non- Cyclic Photophosphorylation

Site: The photosystems (PSI and PSII) and electron transport chain complexes are all embedded in the thylakoid membrane present in the chloroplast.

Mechanism of Non- Cyclic Photophosphorylation

1. The absorption of light by the reaction centre of PSII initiates the electron transport chain.
2. Upon absorption of light, the photosystem II (P680) excites its electron, making it a strong oxidizing agent. Thus, the oxygen-evolving complex (OES) which is associated with PS II and present near the lumen side of the thylakoid, splits the water molecule to release oxygen and electrons.

                                       H₂O 2H⁺ +2e^– + ½ 0₂

3. When water is photolyzed, electrons are produced, which are then passed to oxidized P680, restoring the electron deficiency.                                     
4. Electron moves to a primary electron acceptor (pheophytin) present in Photosystem II.
5. From pheophytin, electrons get transported to a mobile electron carrier called plastoquinone.
6. Plastoquinone also receives protons from the stroma of the chloroplast.
7. These electrons and protons reduce plastoquinone to plastoquinol (PQH₂).
8. The electron is then transported to the cytochrome b6f protein complex generating 2H⁺ ions.
9. This hydrogen ion gets pumped from the lumen of the thylakoid to the stroma through ATP synthase, resulting in the formation of ATP.
10. The electron is then transported from the cytochrome b6f protein complex to plastocyanin.
11. Further, this electron is transported to Photosystem I, which transfers electrons to Ferredoxin reducing substance and then to ferredoxin, which is towards the stroma of thylakoid.
12. The final electron acceptor is NADP⁺ reductase, which reduces NADP⁺ to NADPH.
13. The proton accumulated into the stroma of thylakoid creates a proton gradient which is released when the proton is being diffused by CF₀-CF₁ complex present into the thylakoid membrane. This movement of protons converts ADP to ATP.

(ii) Cyclic Photophosphorylation

The photophosphorylation process in which the movement of electrons is in a cyclic manner involving only Photosystem I (P700) and forming ATP molecules is called cyclic photophosphorylation.

Electron Transport Flow of Cyclic Photophosphorylation

Site: Cyclic photophosphorylation occurs on the stroma lamellae or fret lamella of the chloroplast.

Mechanism of Cyclic Photophosphorylation

1. A photosystem I absorb light that excites electrons; these electrons are then picked up by a primary electron acceptor ferredoxin reducing substance (FRS) and then transferred to the ferredoxin.
2. The electrons are transferred from ferredoxin to cytb6-f protein complex, generating 2H⁺ ions.
3. This hydrogen ion gets pumped from the lumen of the thylakoid to the stroma through ATP synthase, resulting in the formation of ATP.
4. The electron is then transported from the cytochrome b6f protein complex to plastocyanin.
5. These electrons are then transferred from plastocyanin to the reaction centre of Photosystem I.
6. The excited electrons of cyclic photophosphorylation generate the proton gradient that they employ to synthesize ATP. No reduction of NADP⁺ occurs in cyclic photophosphorylation.

Cellular Respiration

It is a set of metabolic reactions which breaks the glucose molecule in the presence or absence of oxygen to release ATP molecules. If the glucose molecule is being utilized in the presence of oxygen, it releases ATP, NADH+H⁺ , and FADH₂.

Further, NADH+H⁺ and FADH₂ are oxidized to form NAD⁺ and FAD⁺ and release ATP. This whole process of breakdown and movement of an electron from one protein complex to another in the inner membrane of mitochondria is known as the electron transport system or oxidative phosphorylation.

Electron Transport System in Oxidative Phosphorylation

Site: Oxidative phosphorylation occurs in the inner mitochondrial membrane.

The inner mitochondrial membrane contains five huge protein complexes known as complexes I, II, III, IV, and V, which remove electrons from NADH and FADH₂, regenerating NAD⁺ and FAD⁺. While doing so, they generate a proton gradient across the membrane that may be used to drive ATP synthesis. In aerobic bacteria, ETS operates in the plasma membrane.

Complex	Enzyme Complex	Prosthetic Group(s)
Complex I	NADH Dehydrogenase	FMN, Fe-S
Complex II	Succinate Dehydrogenase	FAD, Fe-S
Complex III	Cytochrome c reductase (Cytochrome b- c1 complex)	Haeme, Fe-S
Complex IV	Cytochrome c oxidase (Cytochrome a – a3)	Hemes; CuA, CuB
Complex V	ATPase or F0 -F1 complex or ATP synthase

CoQ (coenzyme Q) or Ubiquinone (UQ) is a small, mobile carrier that transfers electrons between the primary dehydrogenases and cytochrome b.

Complex I (NADH dehydrogenase): It is composed of NADH dehydrogenase with FMN as a cofactor, along with non-heme-iron proteins. It removes two electrons from NADH and transfers them to UQ (ubiquinone) in the mitochondrial membrane. As the two electrons pass through, two protons are pumped across complex-I into the intermembrane space.

Complex II (succinate dehydrogenase): FADH₂ transfers electrons directly to complex II. Complex II is the only membrane-bound enzyme of the Krebs’ cycle. In this complex, electrons are generated, but no protons are pumped. Ubiquinone (UQ) transfers electrons from complexes I and II to complex III.

Complex III (cytochrome c reductase): Ubiquinone donates its electrons to Complex III, whose principal components are the heme proteins known as cytochromes b and c1 and a non-heme-iron protein. It also pumps 2H⁺ into the intermitochondrial space per 2 electron movement. The electrons are further transported to cytochrome c, from where it is transported complex-IV.

Complex IV (cytochrome c oxidase) – Contains the heme proteins known as cytochrome a and cytochrome a3 and copper-containing proteins. The cytochrome holds the oxygen molecule between copper and iron until the oxygen is reduced. Oxygen is the final electron acceptor, with water being the final product of oxygen reduction. Each NADH₂ and FADH₂ get oxidised to yield 2 electrons, and these are enough to reduce half  O₂ molecule to H₂O.

Complex V (ATP Synthase): Converts an H⁺ gradient into ATP, producing 1 ATP per 2H⁺. ATP synthase is a multiple subunit complex that binds ADP and inorganic phosphate at its catalytic site inside the mitochondrion and requires a proton gradient for activity. Protons accumulate in the inner mitochondrial space resulting in a lower concentration of protons in the mitochondrial matrix. So, a proton gradient is created, leading to the movement of protons from inner mitochondrial space to the matrix through the F₀-F₁ complex present in the inner mitochondrial membrane.

In the F₀-F₁  complex, F₀ is localized in the membrane (an integral membrane protein complex). F₁, the headpiece, is a peripheral membrane protein complex that protrudes from the inside of the inner membrane into the matrix; and is the site for the synthesis of ATP from ADP and inorganic phosphate.

Summary

The electron transport system includes a series of protein complexes containing heme groups and copper ions that transport the electron and proton. This flow creates a proton motive force making the transport of protons from higher proton concentration to lower proton concentration which moves from the F₀-F₁ complex present in the inner mitochondrial membrane and CF₀- CF₁  complex in the thylakoid membrane.

The membrane of the thylakoid and inner mitochondrial complex has an ATP synthase enzyme that helps in the synthesis of ATP. The movement of 2H⁺ leads to the formation of a 1 ATP molecule. So, NADH oxidizes to form 3 ATP, and FADH₂ oxidizes to form 2 ATP molecules. The final electron acceptor in mitochondria is half molecule of O₂ to form H₂O.

Frequently Asked Questions (FAQs)

Q.1. How many protons are required to form 1 ATP molecule?
Ans: When 2 protons get a release from the F0-F1 complex present in the inner mitochondrial membrane, 1 ATP is formed.

Q.2. How many complexes are there in an electron transport system in mitochondria?
Ans: There are five complexes in an Electron transport system embedded in the inner mitochondrial membrane.

Q.3. What is Light Reaction?
Ans: The light reaction is the first reaction in which light energy is absorbed by chlorophyll molecules present in the reaction centre of the photosystem and convert into chemical energy to produce O₂, ATP, and NADPH.

Q.4. In which layer of mitochondria, the Electron Transport System takes place?
Ans: The electron transport system takes place in the inner mitochondrial membrane.

Q.5. What is the final electron acceptor in the electron transport chain?
Ans: The final electron acceptor in the electron transport chain is oxygen.

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