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    Electron transport chain

    Electron transport chain

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    The electron transport chain in the mitochondrion is the site of oxidative phosphorylation in eukaryotes. The NADH and succinate generated in the citric acid cycle are oxidized, which releases the energy of oxygen to power ATP synthase.

    Photosynthetic electron transport chain of the thylakoid membrane.

    An electron transport chain (ETC[1]) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of protons (H+ ions) across a membrane. Many of the enzymes in the electron transport chain are membrane-bound.

    The flow of electrons through the electron transport chain is an exergonic process. The energy from the redox reactions creates an electrochemical proton gradient that drives the synthesis of adenosine triphosphate (ATP). In aerobic respiration, the flow of electrons terminates with molecular oxygen as the final electron acceptor that provides most of the energy.[2] In anaerobic respiration, other, lower-energy electron acceptors are used, such as sulfate.

    In an electron transport chain, the redox reactions are driven by the difference in the Gibbs free energy of reactants and products. The free energy released when a higher-energy electron donor and acceptor convert to lower-energy products, while electrons are transferred from a lower to a higher redox potential, is used by the complexes in the electron transport chain to create an electrochemical gradient of ions. It is this electrochemical gradient that drives the synthesis of ATP via coupling with oxidative phosphorylation with ATP synthase.[3]

    In eukaryotic organisms the electron transport chain, and site of oxidative phosphorylation, is found on the inner mitochondrial membrane. The energy of oxygen released in its reaction with reduced compounds such as cytochrome and (indirectly) NADH and FADH is used by the electron transport chain to pump protons into the intermembrane space, generating the electrochemical gradient over the inner mitochondrial membrane. In photosynthetic eukaryotes, the electron transport chain is found on the thylakoid membrane. Here, light energy drives electron transport through a proton pump and the resulting proton gradient causes subsequent synthesis of ATP. In bacteria, the electron transport chain can vary between species but it always constitutes a set of redox reactions that are coupled to the synthesis of ATP through the generation of an electrochemical gradient and oxidative phosphorylation through ATP synthase.[4]

    Contents

    1 Mitochondrial electron transport chains

    1.1 Mitochondrial redox carriers

    1.1.1 Complex I 1.1.2 Complex II 1.1.3 Complex III 1.1.4 Complex IV

    1.2 Coupling with oxidative phosphorylation

    1.3 Reverse electron flow

    2 Bacterial electron transport chains

    2.1 Electron donors

    2.2 Complexes I and II

    2.3 Quinone carriers

    2.4 Proton pumps

    2.5 Cytochrome electron carriers

    2.6 Terminal oxidases and reductases

    2.7 Electron acceptors

    3 Photosynthetic 4 See also 5 References 6 Further reading 7 External links

    Mitochondrial electron transport chains[edit]

    Most eukaryotic cells have mitochondria, which produce ATP from reactions of oxygen with products of the citric acid cycle, fatty acid metabolism, and amino acid metabolism. At the inner mitochondrial membrane, electrons from NADH and FADH2 pass through the electron transport chain to oxygen, which provides the energy driving the process as it is reduced to water.[5] The electron transport chain comprises an enzymatic series of electron donors and acceptors. Each electron donor will pass electrons to an acceptor of higher redox potential, which in turn donates these electrons to another acceptor, a process that continues down the series until electrons are passed to oxygen, the most energy-rich[2] and terminal electron acceptor in the chain. Each reaction releases energy because a higher-energy donor and acceptor convert to lower-energy products. Via the transferred electrons, this energy is used to generate a proton gradient across the mitochondrial membrane by "pumping" protons into the intermembrane space, producing a state of higher free energy that has the potential to do work. This entire process is called oxidative phosphorylation since ADP is phosphorylated to ATP by using the electrochemical gradient that the redox reactions of the electron transport chain have established driven by the energy of oxygen.

    Mitochondrial redox carriers[edit]

    Energy associated with the transfer of electrons down the electron transport chain is used to pump protons from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient (ΔpH) across the inner mitochondrial membrane. This proton gradient is largely but not exclusively responsible for the mitochondrial membrane potential (ΔΨM).[6] It allows ATP synthase to use the flow of H+ through the enzyme back into the matrix to generate ATP from adenosine diphosphate (ADP) and inorganic phosphate. Complex I (NADH coenzyme Q reductase; labeled I) accepts electrons from the Krebs cycle electron carrier nicotinamide adenine dinucleotide (NADH), and passes them to coenzyme Q (ubiquinone; labeled Q), which also receives electrons from Complex II (succinate dehydrogenase; labeled II). Q passes electrons to Complex III (cytochrome bc1 complex; labeled III), which passes them to cytochrome (cyt ). Cyt passes electrons to Complex IV (cytochrome oxidase; labeled IV), which uses the electrons and hydrogen ions to release the energy of molecular oxygen as it is reduced to water.

    Source : en.wikipedia.org

    Where in the mitochondrion does the electron transport chain take place?

    The inner mitochondrial membrane. The mitochondrion has an outer membrane and an inner membrane with folds (cisternae). The electron transport chain is a series of transmembrane proteins found in the inner membrane. The electrons are shuttled between these proteins which is used to pump protons (H^+) to the space between the inner and the outer membrane. This creates a gradient that is used to finally produce ATP = energy ready to go!

    Where in the mitochondrion does the electron transport chain take place?

    Biology Movement In and Out of Cells Transport Across the Cell Membranes

    1 Answer

    Joëlle Jul 27, 2016

    The inner mitochondrial membrane.

    Explanation:

    The mitochondrion has an outer membrane and an inner membrane with folds (cisternae). The electron transport chain is a series of transmembrane proteins found in the inner membrane.

    http://www.nature.com/scitable/topicpage/mitochondria-14053590 (part; rest home made)

    The electrons are shuttled between these proteins which is used to pump protons (

    H +

    ) to the space between the inner and the outer membrane. This creates a gradient that is used to finally produce ATP = energy ready to go!

    Answer link

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    Source : socratic.org

    Biochemistry, 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 photosynthesis. In the former, the electrons come from breaking down organic molecules, and energy is released. In the latter, the electrons enter the chain after being excited by light, and the energy released is used to build carbohydrates.

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    Biochemistry, Electron Transport Chain

    Maria Ahmad; Adam Wolberg; Chadi I. Kahwaji.

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    Last Update: September 8, 2021.

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    Introduction

    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. In the former, the electrons come from breaking down organic molecules, and energy is released. In the latter, the electrons enter the chain after being excited by light, and the energy released is used to build carbohydrates.

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    Fundamentals

    Aerobic cellular respiration is made up of three parts: glycolysis, the citric acid (Krebs) cycle, and oxidative phosphorylation. In glycolysis, glucose metabolizes into two molecules of pyruvate, with an output of ATP and nicotinamide adenine dinucleotide (NADH). Each pyruvate oxidizes into acetyl CoA and an additional molecule of NADH and carbon dioxide (CO2). The acetyl CoA is then used in the citric acid cycle, which is a chain of chemical reactions that produce CO2, NADH, flavin adenine dinucleotide (FADH2), and ATP. In the final step, the three NADH and one FADH2 amassed from the previous steps are used in oxidative phosphorylation, to make water and ATP.

    Oxidative phosphorylation has two parts: the electron transport chain (ETC) and chemiosmosis. The ETC is a collection of proteins bound to the inner mitochondrial membrane and organic molecules, which electrons pass through in a series of redox reactions, and release energy. The energy released forms a proton gradient, which is used in chemiosmosis to make a large amount of ATP by the protein ATP-synthase.

    Photosynthesis is a metabolic process that converts light energy into chemical energy to build sugars. In the light-dependent reactions, light energy and water are used to make ATP, NADPH, and oxygen (O2). The proton gradient used to make the ATP forms via an electron transport chain. In the light-independent reactions, sugar is made from the ATP and NADPH from the previous reactions.

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    Cellular

    In the electron transport chain (ETC), the electrons go through a chain of proteins that increases its reduction potential and causes a release in energy. Most of this energy is dissipated as heat or utilized to pump hydrogen ions (H+) from the mitochondrial matrix to the intermembrane space and create a proton gradient. This gradient increases the acidity in the intermembrane space and creates an electrical difference with a positive charge outside and a negative charge inside. The ETC proteins in a general order are complex I, complex II, coenzyme Q, complex III, cytochrome C, and complex IV.

    Complex I, also known as ubiquinone oxidoreductase, is made up of NADH dehydrogenase, flavin mononucleotide (FMN), and eight iron-sulfur (Fe-S) clusters. The NADH donated from glycolysis, and the citric acid cycle is oxidized here, transferring 2 electrons from NADH to FMN. Then they are transferred to the Fe-S clusters and finally from Fe-S to coenzyme Q. During this process, 4 hydrogen ions pass from the mitochondrial matrix to the intermembrane space, contributing to the electrochemical gradient. Complex I may also play an important role in causing apoptosis in programmed cell death.[1][2][3][4][1]

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

    Complex II, also known as succinate dehydrogenase, accepts electrons from succinate (an intermediate in the citric acid cycle) and acts as a second entry point to the ETC. When succinate oxidizes to fumarate, 2 electrons are accepted by FAD within complex II. FAD passes them to Fe-S clusters and then to coenzyme Q, similar to complex I. However; no protons are translocated across the membrane by complex II, therefore less ATP is produced with this pathway.[5][6]

    Succinate + FAD -> Fumarate + 2 H+(matrix) + FADH2

    FADH2 + CoQ -> FAD + CoQH2

    Glycerol-3-Phosphate dehydrogenase and Acyl-CoA dehydrogenase also accept electrons from glycerol-3-P and fatty acyl-CoA, respectively. Inclusion of these protein complexes allows for the donation to the ETC by cytosolic NADH (glycerol-3-P acts as a shuttle to regenerate cytosolic NAD from NADH) and fatty acids undergoing beta-oxidation within the mitochondria (acyl-CoA is oxidized to enoyl-CoA in the first step, producing FADH2).[7][8]

    Coenzyme Q, also known as ubiquinone (CoQ), is made up of quinone and a hydrophobic tail. Its purpose is to function as an electron carrier and transfer electrons to complex III. Coenzyme Q undergoes reduction to semiquinone (partially reduced, radical form CoQH-) and ubiquinol (fully reduced CoQH2) through the Q cycle. This process receives further elaboration under Complex III.

    Complex III, also known as cytochrome c reductase, is made up of cytochrome b, Rieske subunits (containing two Fe-S clusters), and cytochrome c proteins. A cytochrome is a protein involved in electron transfer that contains a heme group. The heme groups alternate between ferrous (Fe2+) and ferric (Fe3+) states during the electron transfer. Because cytochrome c can only accept a single electron at a time, this process occurs in two steps (the Q cycle), in contrast to the single-step complex I and II pathways. Complex III also releases 4 protons into the intermembrane space at the end of a full Q cycle, contributing to the gradient. Cytochrome c then transfers the electrons one at a time to complex IV.[9][10][11]

    Source : www.ncbi.nlm.nih.gov

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