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| Electron Transport System |
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| The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system. |
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| Electron Transport System (ETS) in Mitochondrion |
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| The electron transport system occurs in the inner membrane of the mitochondria. The electron transport system contains mainly six components arranged in the following sequence. |
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| 1) NAD |
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| 2) FAD |
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| 3) Cytochrome B |
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| 4) Cytochrome C |
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| 5) Cytochrome A |
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| 6) Cytochrome A3 |
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| Electrons from NADH produced during citric acid are oxidized by an NADH dehydrogenase (complex I) and electrons are then transferred to ubiquinone located within the inner member. Ubiquinone also receives reducing equivalents via FADH2 , through the activity of the enzyme, succinate dehydrogenase (complex II). The reduced ubquinone called ubquinol is then oxidised with the transfer of electrons to cytochrome c, (complex III) eytochrome c is a small protein attached to outer surface of the inner membrane and is the mobile carrier for transfer of electrons between complex between III and IV. Complex IV is cytochrome c oxidase complex containing cytochromes a and a 3 and 2 copper centers. |
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| When electrons pass from one carrier to another via complex I to IV, in the electron transport chain, they are coupled to ATP synthase (complex V) for the production of ATP. |
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| The number of ATP molecules synthesized depends on the nature of the electron donor. Oxidation of 1 molecule of NADH gives 3 molecules of ATP, while that of 1 molecule of FADH2 produces 2 molecule of ATP. |
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| Specific enzymes of the electron transport chain receive electrons from NADH2 or FADH2 produced by glycolysis and Krebs cycle. The electrons are then transported successively from one enzyme or cytochrome to the next with a loss of energy at each step. At each step of electron transport, the electron acceptor has a higher electron affinity than the electron donor. The energy from such electron transport is utilised in transporting protons (H+) from the matrix, across the inner membrane to its outer chamber. |
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| This creates a higher proton concentration outside the inner membrane than in the matrix. This difference in proton concentration on the outer and inner sides of the inner mito chondrial membrane is known as proton gradient. |
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| At the end of the chain the electrons and the accompanying protons are combined with oxygen to form water. Oxygen is thus, the terminal electron acceptor of the mito chondrial respiratory chain. |
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| Oxidative phosphorylation |
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| The whole process by which oxygen effectively allows the production of ATP by Phosphorylation of ADP, is called oxidative phosphorylation. |
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| Movement of Electrons Through Respiratory Chain and the Synthesis of ATP Molecules |
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| It is the synthesis of energy rich ATP molecules with the help of energy liberated by oxidation of reduced coenzymes, NADH2 and FADH2 produced during respiration. The enzyme required for their synthesis is called ATP synthatase. It is present in F1 or head piece of F0 - F1 particle in mito chondrial membrane. The enzyme ATP synthatase becomes active in ATP formation whenever there is a proton gradient. |
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| Chemiosmotic ATP synthesis in the F0 - F1 complex |
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| Transport of 2 electrons from NADH2 by the electron transport chain, simultaneously transfers 3 pairs of protons to the outer compartment. One high energy ATP bond is produced per pair of protons returning to the matrix through the inner membrane particles. |
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| When one molecule of NADH molecule is oxidised, 3 molecules of ATP are synthesized. When one molecule of FADH is oxidised, only 2 ATP molecules are synthesised, as the latter donates its electron further down the chain. |
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| Energetics of Glucose Metabolsim |
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| The metabolism of a glucose molecule releases 40 molecules of ATP. But glycolysis utilises 2 molecules of ATP. So the total number of ATP molecules in glucose metabolism is 40 - 2 = 38 ATP molecules. |
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| The synthesis of ATP is as follows: |
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| a) Glycolysis = 8 ATP |
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| b) Oxidative decarboxylation NADH = 3 ATP |
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| c) Kreb's cycle |
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| i) Oxidation of isocitric acid via NAD+ to oxalo succinic acid = 3 ATP |
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| ii) Oxidation of ketoglutaric acid via NAD+ to succinyl CoA = 3 ATP |
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| iii) Oxiation of succinic acid FAD to fumaric acid = 2 ATP |
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| iv) Oxidation to malic acid via NAD+ to oxaloacetic acid = 3 ATP |
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| v) Conversion of succinyl CoA to succinic acid = 1 ATP |
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| Total ATP molecules produced for each pyruvic acid = 15 ATP |
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| Thus the oxidation of one pyruvic acid molecule yields 15 ATP molecules. Since 2 molecules of pyruvic acid are produced from each glucose molecule the total number of ATP molecules of pyruvic acid are 15 x 2 = 30 ATP molecules. |
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| Glycolysis yields 8 molecules of ATP. |
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| Thus, the complete oxidation of one glucose molecule yields 30 + 8 = 38 ATP molecules. |
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| In most eukaryotic cells, 2 molecules of ATP are required for transporting the NADH produced in glycolysis into the mitochondrion for further oxidation. Hence the net gain of ATP is 36 molecules of ATP. |
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| Utility of step-wise oxidation |
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| 1) Energy is released in steps and in small amounts so it prevents sudden increase in body temperatue. |
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| 2) Wastage of energy is avoided, as more time is available to use and store the energy in ATP molecules. |
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| 3) Energy produced can be regulated according to body needs. |
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| 4) Intermediate products of cell respiration can be used in different metabolic pathways. |
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| Example: Acetyl CoA |
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| 5) Kreb's cycle is a common pathway of oxidative breakdown of carbohydrates, fatty acids and amino acids. |
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| 6) The activities of enzymes for different steps of respiration can be enhanced or inhibited by specific compounds. This helps in controlling the rate of respiration. |
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| Kreb's cycle is a catabolic pathway, it also provides a number of intermediate products for anabolic pathway. Therefore, Kreb's cycle is often called amphibolic pathway. |
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| Significance of citric acid cycle |
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| 1) During this pathway many intermediate compounds are formed, which are used in the synthesis of other biomolecules like amino acids, nucleotides, chlorophyll, cytochromes and fats. |
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| For example, succinyl COA is the starting molecule for synthesis of chlorophyll. |
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| 2) This is the major pathway for generation of ATP molecules, the energy currency of the cell. |
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| What is the efficiency of cellular respiration? |
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| The total yield from 38 ATP molecules is 1292 K J (one ATP yields 34 KJ of energy). Energy released by one molecule of glucose on complete oxidation is 2870 KJ. Thus the efficiency is 45%. This shows that only a part of this energy is used to make ATP and much of the energy generated during respiration is released in the form of heat. |
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