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Oxygen is essential in Aerobic cellular respiration, which is made of 3 parts i.e. glycolysis, Krebs cycles and oxidative phosphorylation (oxygen is required only in this part). Oxidative phosphorylation has two major components i.e. Electron Transport System or chain and Chemiosmosis.
Electron Transport System
During this process, a large amount of energy is released, which is stored in the phosphate bond of ATP produced from ADP and inorganic. This process takes place in the inner mitochondrial space which is connected with matrix and intra-membrane space. But in bacteria, it takes place in the plasma membrane. All Respiratory Chain enzymes (enzymes which participate in the respiratory chain) are found in F1 particles of mitochondria.
Compared to any other part of cellular respiration a large amount of ATP is produced in this phase. It is the only phase in glucose metabolism that makes use of atmospheric oxygen.
In this process, hydrogen is a donor of electrons and atmospheric oxygen is the acceptor of electrons.
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Mechanism
A Complex is a structure which comprises a weak protein molecule or atom that is weakly connected to a protein, involved in this process and plays an important role. Hydrogen and electrons pass through different carriers i.e. complex and ultimately react with oxygen and form water molecules. There are 4 types of complexes.
1. Complex I, 2. Complex II, 3. Complex III, 4. Complex IV.
Complex I:
is made up of NADH dehydrogenase, flavin mononucleotide (FMN), and eight iron-sulfur (Fe-S) clusters. The oxidation of NADH [ nicotinamide adenine dinucleotide (NAD) + hydrogen] occurs here, resulting in 2 electron transfers from NADH to CoQ. During this process, 4 hydrogen ions pass from the mitochondrial matrix to the intermembrane space.
(NADH + H+) + CoQ + 4 H+(matrix) -> NAD+ + CoQH2 + 4 H+(intermembrane)
Complex II:
Complex II is involved in the oxidation of succinate to fumarate, During the oxidation 2 electrons reach the coenzyme Q through the series of Fe-S clusters. similar to complex I. However; no protons are translocated across the membrane by complex II. thus it is not a part of creating the proton gradient in the ETC.
Succinate + FADH2 + CoQ → Fumarate + FAD+ + CoQH2
CoQ:
is coenzyme-Q which is also known as ubiquinone. Its function is as an electron carrier and transfer electrons to complex III.
Complex III:
It's comprised of cytochrome b, c, and a specific Fe-S cluster, known as cytochrome reductase. A cytochrome is a protein involved in electron transfer that contains a heme group. Complex III catalyzes the transfer of two electrons from CoQH2 to cytochrome c. Resulting in 4 protons released into the intermembrane space at the end. Thus the reduced CoQH2 has oxidized back CoQ while the iron centre (Fe3+) in the cytochrome c is reduced to Fe2+.
CoQH2 + 2 cyt c (Fe3+) → CoQ + 2 cyt c (Fe2+) + 4H+
Cytochrome c forms a connection with complex IV with the help of CoQ. Although CoQ carries pairs of electrons, cytochrome c can only accept one at a time.
Complex IV :
Its have two cytochromes i.e. a, and a3, which are made of two heme groups and three copper ions. After the Oxidation of Cytochrome c, the released electron transfer to the oxygen (the final electron acceptor) to the bound dioxygen, thus forming water (H2O) take place. The free energy from the electron transfer causes 4 protons to move into the intermembrane space helping in the forming of the proton gradient. Oxygen reduces via the following reaction.
2 cytochrome c(red) + ½O2 + 4 H+(matrix) –> 2 cytochrome c(ox) + 1 H2O + 2 H+(intermembrane)
ATP synthesis
It's also known as complex V. It uses the ETC-generated proton gradient across the inner mitochondrial membrane to form ATP. ATP-synthase contains F0 and F1 subunits, which act as a rotational motor. F0 down the flow of H+ ions from intermembrane space to matrix. And F1 subunits catalyze the formation of ATP from ADP and Pi. For every 4 H+ ions, 1 ATP is produced.
Net Gain in Electron Transport System
1. No. Of released NADH2
2. No. Of released FADH2
3. Total ATP from Electron Transport System – 34ATP
4. End product:
NAD+, FAD, ATP, H2O
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