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| Cellular Respiration |
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| Glucose is a carbohydrate, a compound of carbon and hydrogen. The bonds between the carbon and the hydrogen atoms are very strong. In the cells, the substrate, often glucose, is broken down into carbon dioxide and water in the presence of oxygen. This process breaks the bonds between carbon and hydrogen and thus releases energy. This is called respiration. It has three main steps- |
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| Glycolysis |
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| This takes place in the cytoplasm of the cells. The glucose is converted to 2 molecules of pyruvic acid. No oxygen is required for this step. |
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| Glycolysis |
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| Kreb's cycle |
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| It takes place in the mitochondria. The pyruvic acid formed during glycolysis is oxidized and forms carbon dioxide and water. This reaction also releases molecules called the coenzymes - NADH2 and FADH2. The hydrogen atoms associated with these are the hydrogen atoms released during the oxidation of pyruvic acid. |
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| Electron transport chain (ETC) |
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| It takes place in the mitochondrial membranes. During this step, the hydrogen ions (or a pair of electrons) are transported from one carrier to another and they are finally used to reduce oxygen to water. During this transfer of electrons, lot of energy is released which is in the form of ATP. ATP is thus an energy rich molecule and can be called the energy currency of the cell. |
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| Electron Transport Chain |
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| Thus the overall equation for respiration is |
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| If one observes the equation of respiration, the number of moles of oxygen needed is equal to the number of moles of carbon dioxide produced. This carbon dioxide to oxygen ratio is called the respiratory quotient. For carbohydrates, the respiratory quotient is 1 as can be seen below: |
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| Different substrates give different RQ values. The more the value of RQ more is the efficiency. Some of the substrates and their respiratory quotients are given below: |
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| Respiratory quotients of a variety of substrates |
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