Question
Explain the three pathways of breakdown in living organisms.
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Answer
Cellular respiration is the process of breakdown of food substances into simpler ones to release the stored energy. On the basis of presence or absence of oxygen, it can occur through aerobic or anaerobic pathway.
i. Aerobic respiration: Glucose in the presence of oxygen (air), breaks down to carbon dioxide and water with release of large amount of energy. It occurs in three steps, glycolysis, citric acid cycle and ETC.
Glycolysis is breakdown of glucose into pyruvate. It is oxygen independent pathway occurs in cytoplasm. Pyruvate is decarboxylated into acetyl coA in cytoplasm only.
Citric acid cycle is the second phase of cellular respiration in which acetyl Co are enzymatically oxidized into carbon dioxide and the released energy is stored in NADH and $FADH _2$.
Electron transport chain includes downhill flow of electrons to final electron acceptor (Oxygen, which is then reduced into water) through a chain of membranebound carriers to facilitate the uphill transport of protons across a proton-impermeable membrane and ATP synthesis.
Enzymes of citric acid cycle are present in mitochondrial matrix while the electron transport chain is present in inner mitochondrial membrane, hence, the last two phases of aerobic respiration takes place in mitochondria only.
ii. Anaerobic respiration: Glycolysis is breakdown of glucose into pyruvate. It is oxygen independent pathway and is common to both aerobic and anaerobic respiration. Fate of pyruvate depends on oxygen availability. Anaerobic respiration can occur via lactic acid fermentation or alcohol fermentation.
- Alcohol fermentation due to limited oxygen availability, pyruvate remains in cytoplasm where pyruvate decarboxylase and alcohol dehydrogenase enzymes carry out the second phase of anaerobic respiration and produce ethanol and carbon dioxide with limited amount of energy being released.
- Lactic acid fermentation due to limited oxygen availability, pyruvate remains in cytoplasm where it is broken down into lactic acid with limited amount of energy being released.
i. Aerobic respiration: Glucose in the presence of oxygen (air), breaks down to carbon dioxide and water with release of large amount of energy. It occurs in three steps, glycolysis, citric acid cycle and ETC.
Glycolysis is breakdown of glucose into pyruvate. It is oxygen independent pathway occurs in cytoplasm. Pyruvate is decarboxylated into acetyl coA in cytoplasm only.
Citric acid cycle is the second phase of cellular respiration in which acetyl Co are enzymatically oxidized into carbon dioxide and the released energy is stored in NADH and $FADH _2$.
Electron transport chain includes downhill flow of electrons to final electron acceptor (Oxygen, which is then reduced into water) through a chain of membranebound carriers to facilitate the uphill transport of protons across a proton-impermeable membrane and ATP synthesis.
Enzymes of citric acid cycle are present in mitochondrial matrix while the electron transport chain is present in inner mitochondrial membrane, hence, the last two phases of aerobic respiration takes place in mitochondria only.
ii. Anaerobic respiration: Glycolysis is breakdown of glucose into pyruvate. It is oxygen independent pathway and is common to both aerobic and anaerobic respiration. Fate of pyruvate depends on oxygen availability. Anaerobic respiration can occur via lactic acid fermentation or alcohol fermentation.
- Alcohol fermentation due to limited oxygen availability, pyruvate remains in cytoplasm where pyruvate decarboxylase and alcohol dehydrogenase enzymes carry out the second phase of anaerobic respiration and produce ethanol and carbon dioxide with limited amount of energy being released.
- Lactic acid fermentation due to limited oxygen availability, pyruvate remains in cytoplasm where it is broken down into lactic acid with limited amount of energy being released.
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