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| Compensation Point |
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| At a given low concentration of CO2 and non-limiting light intensity, the photosynthetic rate of a given plant will be equal to the total amount of respiration. The atmospheric concentration of CO2 at which photosynthesis just compensates for respiration is referred to as the CO2 compensation point. The CO2 compensation point is reached when the amount of CO2 uptake is equal to that generated through respiration at a non-limiting light intensity. Net photosynthesis under these conditions is zero. In C3 plants, the CO2 compensation point is usually much higher than in C4 plants. |
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| Anaerobic oxidation of pyruvate |
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| Anaerobic respiration is an enzyme controlled partial breakdown of organic compounds without using oxygen and releasing only a fraction of energy. In micro organisms, anaerobic respiration is called fermentation. Fermentation is named after its products, like alcoholic fermentation, lactic acid fermentation. |
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| In anaerobic respiration, electrons are removed from the substrate during oxidation but are not finally transferred to molecular oxygen. The final electron acceptors are compounds such as pyruvic acid or acetaldehyde. The end products are lactic acid or ethyl alcohol. |
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| The mechanism of anaerobic respiration resembles that of respiration upto glycolysis. Pyruvate formed at the end of glycolysis is anaerobically broken down to yield various products depending upon the organism and the type of tissue. The 2 common products are ethyl alcohol and lactic acid. |
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| Alcoholic fermentation |
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| Anaerobic respiration occurs in sugary solution causing fermentaion, if the fungus (yeast) is not in contact with atmosphere. |
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| Pyruvate is first decarboxylated with the help of the enzyme pyruvate decarboxylase, Mg2+ and Thiamine pyrophosphate (TPP). This produces acetaldehyde and CO2 from pyruvic acid. |
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| Acetaldehyde then accepts hydrogen from NADH2 and is reduced to ethyl alcohol producing oxidised NAD+. This process is catalysed by the enzyme alcohol dehydrogenase. |
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| Accumulation of ethanol by fermentation in a culture of yeast may stop further multiplication and lead to the death of cells. |
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| Pathway of Anaerobic Respiration in Yeast (Fermentation) |
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| Lactic acid fermentation |
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| This is carried out by lactic acid bacteria Lacto bacillus some fungi and vertebrate muscle cells. |
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| Pyruvic acid from glycolysis receives hydrogen from NADH2 to form lactic acid and NAD+. The enzyme lactic dehydrogenase catalyses the reaction in the presence of Zn2+ and FMN (Flavin mononucleotide) CO2 is not produced. NAD+ is reused in glycolysis. |
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| Anaerobic respiration yields much less energy than aerobic respiration because. |
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| a) The respiratory substrate is not fully degraded. |
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| b) The organic end product of anaerobic respiration still contain energy which remains untapped and is wasted. |
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| c) Regeneration of NAD+ from NADH2 does not produce energy by transfer of hydrogen electrons over the ETS. |
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| d) ETS does not function and is absent. |
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| e) Oxygen is not used for accepting electrons and protons. |
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| Utility of Anaerobic Respiration |
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| a) It supplements the energy provided by aerobic respiration during intense muscular activity. |
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| b) Brewing industry produces beers and wines by fermentation of sugar solution with yeast. |
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| c) Baking industry uses CO2 released by yeast cells in alcoholic fermentation, in raising the dough and making bread spongy. |
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| d) Dairy industry produces yogurt, cheese and butter by fermenting milk sugar lactose to lactic acid with lactic acid bacteria. |
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| e) Tea and tobacco leaves are cured by fermentation with certain bacteria. |
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| f) Vinegar is produced by fermenting molasses with yeast to ethyl alcohol which is then oxidised to acetic acid. |
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| g) Bacterial fermentation is also used for tanning hides. |
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| h) Retting of hemp fibres is achieved by fermentation with pseudomonas fluorescence. |
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| i) Ensilage, a nutritive fodder for cattle, is prepared by fermentation with bacteria in air tight chambers called silos. |
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| j) Bodies of dead organisms are decomposed by micro organism through fermentation. |
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