02 Respiration

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02 Respiration

  1. 1. Respiration as an energy transfer process ALBIO9700/2006JK
  2. 2. Respiration• A process in which organic molecules act as a fuel• Molecules broken down to release chemical potential energy which is used to synthesise ATP• Many cells can only use glucose as their respiratory substrate but others break down fatty acid, glycerol and amino acids in respiration• Glucose breakdown occur in 4 stages: – Glycolysis – Link reaction – Krebs cycle – Oxidative phosphorylation ALBIO9700/2006JK
  3. 3. Glycolysis• Splitting/lysis of glucose• 6C glucose molecule to 2 molecules of 3C pyruvate• ATP needed at first but energy released in later steps can be used to make ATP• Net gain of 2 ATP molecules per glucose molecule broken down• Takes place in cytoplasm• Pyruvate enters link reaction in mitochondria ALBIO9700/2006JK
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  5. 5. Link reaction• Pyruvate passed from cytoplasm into the mitochondrial matrix (active transport)• Decarboxylated (CO2 removed), dehydrogenated and combined with coenzyme A (CoA) to give acetyl coenzyme A• Coenzyme A: – adenine + ribose + pantothenic acid – acts as a carrier of acetyl groups to Krebs cycle• Pyruvate + CoA + NAD ↔ acetyl CoA + CO2 + reduced NAD• Fatty acids from fat metabolism also used to produce acetyl coenzyme A ALBIO9700/2006JK
  6. 6. Krebs cycle• aka citric acid cycle/tricarboxylic acid cycle• Closed pathway of enzyme-controlled reactions: – Acetyl CoA + oxaloacetate (4C) → citrate (6C) – Citrate decarboxylated and dehydrogenated to yield CO2 (waste) and hydrogens (accepted by NAD and FAD) – Oxaloacetate is regenerated to combine with another acetyl CoA• For each turn of the cycle, 2 CO2 produced, 1 FAD and 3 NAD reduced and 1 ATP generated• O2 not used• Most important contribution: release of hydrogen (used in oxidative phosphorylation to provide energy to make ATP) ALBIO9700/2006JK
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  9. 9. Oxidative phosphorylation• Energy for the phosphorylation of ADP to ATP comes from the activity of the electron transport chain (mitochondrial membranes)• NADH and FADH2 are passed to the electron transport chain (ETC)• Hydrogen H+ and e-• e- transferred to the first of a series of electron carriers; H+ remains in solution in mitochondrial matrix• e- transferred to O2 (in solution), H+ drawn from solution to reduce O2 to H2O• Transfer of e- along series of electron carriers makes energy available which is used to convert ADP + Pi to ATP ALBIO9700/2006JK
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  11. 11. • Potentially, 3 ATP from each NADH molecule/2 ATP from each FADH2 molecule• This yield cannot be realised unless ADP and Pi are available inside the mitochondrion• 25% of total energy yield is used to transport ADP into the mitochondrion and ATP into cytoplasm• Each NADH molecule entering the chain produces on average 2½ molecules of ATP and each FADH2 produces 1½ molecules of ATP ALBIO9700/2006JK
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  13. 13. Sites of events of respiration in a cell ALBIO9700/2006JK
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  15. 15. Anaerobic respiration• When free oxygen is not present, hydrogen cannot be disposed of by combination with oxygen• ‘dumping’ hydrogen stops ETC and affects glycolysis• 2 anaerobic pathways in cytoplasm which solve the problem: – Ethanol pathway (yeast and some plant tissues) – Lactate pathway (microorganisms and mammalian muscles) ALBIO9700/2006JK
  16. 16. • Ethanol pathway – Hydrogen from NADH is passed to ethanal (CH3CHO), releasing NAD and allows glycolysis to continue – Alcoholic fermentation: pyruvate decarboxylated to ethanal; ethanal reduced to ethanol (C2H5OH) by alcohol dehydrogenase• Lactate pathway – Pyruvate acts as the hydrogen acceptor and is converted to lactate by lactate dehydrogenase – NAD is released and allows glycolysis to continue• These reactions allow continued production of ATP even though oxygen is not available as the hydrogen acceptor ALBIO9700/2006JK
  17. 17. • However, ethanol and lactate are toxic, and so reactions cannot continue indefinitely• Pathway leading to ethanol cannot be reversed and the remaining chemical potential energy of ethanol is wasted• Lactate pathway can be reversed in mammals (carried by blood plasma to the liver and converted back into pyruvate; 20% oxidised to CO2 and H2O via aerobic respiration when O2 is available again; remainder converted to glycogen)• O2 needed to allow this removal of lactate is called oxygen debt ALBIO9700/2006JK
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