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| Introduction |
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| All living organisms require a continuous supply of energy for carrying out various functions. The main source of energy for all the functions, in all living organisms is cellular respiration. |
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| Cellular respiration is an energy releasing, enzymatically controlled catabolic process, which involves a step-wise oxidative breakdown of organic substances inside living cell. The organic substance i.e., glucose is oxidised inside the mitochondria to produce energy which is liberated in a controlled manner and which is partly stored in the high energy bonds of ATP molecules as biologically useful energy. |
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| Cellular respiration is similar to combustion in the following ways: |
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| a) It helps breakdown of complex organic substances |
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| b) It utilises oxygen. |
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| c) Carbon dioxide is produced. |
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| d) There is release of energy. |
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| Respiratory substrates |
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| The organic substances, which are catabolised in the living cells to release energy are called as respiratory substrates. Though any food stuff - carbohydrate, fat or protein may act as a respiratory substrate, the common respiratory substrate is glucose. |
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| Glucose is used as respiratory fuel because |
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| a) it can be oxidised easily |
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| b) it is present abundantly. |
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| Fats are used as respiratory substrates in some cases. Proteins are used as respiratory substrates only when carbohydrates and fat reserves have been used up. |
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| When proteins are used as respiratory fuels, it is called as protoplasmic respiration. |
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| When fats and carbohydrates are used as respiratory fuel, it is called as floating respiration. |
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| Protoplasmic respiration is not carried on for long as it liberates ammonia, which is toxic. |
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