Respiration

• Glycolysis takes place in the cytosol.
• Link reaction and Krebs cycle takes place in the mitochondrial matrix.
• Oxidative phosphorylation takes place in the inner mitochondrial membrane.
• Raw materials of glycolysis are glucose and oxygen.
• substrate level phosphorylation is when an enzyme transfers a phosphate group directly from a substrate to ADP to form ATP.
• Substrate level phosphorylation occurs in glycolysis and Krebs cycle.
• Products of glycolysis: 2 pyruvate molecules, 2 ATP molecules and 2 NADH molecules per glucose.
• Oxidative decarboxylation in Link reaction:

  • pyruvate decarboxylated (3C to 2C molecule)
  • oxidation by dehydrogenation yields NADH
• co-enzyme A + 2C molecule —> acetyl CoA
• End products of link reaction: 2 acetyl-CoA, 2 CO2, 2 NADH per glucose
• Acetyl CoA (2C) + oxaloacetate (4C) —> citrate (6C) during Krebs cycle
• Oxidative decarboxylation in Krebs cycle: citrate (6C) —> a-ketoglutarate (5C)

  CO2 and NADH formed
• A-ketoglutarate (5C) —> oxaloacetate (4C)

  • 3 dehydrogenation reactions form FADH2 and 2NADH
  • decarboxylation
  • substrate level phosphorylation (ADP to ATP)
• Products of Krebs cycle: 4 CO2, 6 NADH, 2 FADH2, 2 ATP
• Oxidative phosphorylation only takes place in the presence of oxygen.
• during oxidative phosphorylation, hydrogen atoms dissociate from NADH and FADH2, split to form H+ and electrons.
• H+ ions pumped across the inner mitochondrial membrane via active transport from the mitochondrial matrix into the inter membrane space.
• Electrons are transported down the electron transport chain containing electron carriers with progressively lower energy levels, releases energy that pumps H+ across inner mitochondrial membrane.
• H+ ion concentration is high in the intermembrane space, establishing a proton concentration gradient.
• H+ ions diffuse down the concentration across the inner mitochondrial membrane through ATP synthase back into the mitochondrial matrix. Chemiosmosis takes places, proton motive force is used to drive ATP synthesis (ATP synthase catalyses ADP to ATP).
• oxygen acts as the final electron acceptor to accept electrons from the electron transport chain and H+ ions to form water.
• 3 ATP is formed per NADH
• 2 ATP is formed from FADH2
• total ATP formed from oxidative phosphorylation: 34 ATP per glucose
• Anaerobic respiration consists of glycolysis followed by fermentation.
• Alcohol fermentation occurs in yeast and some bacteria
• In alcohol fermentation:

  • pyruvate is decarboxylated into ethanal/acetaldehyde (by pyruvate decarboxylase)
  • ethanal/acetaldehyde is reduced to ethanol (by alcohol dehydrogenase), NADH loses H+ (oxidised) in the process
  • NAD+ is reformed
  • 2 ATP is formed from glycolysis
• Lactic acid fermentation occurs in bacteria, fungi, muscle cells.
• In lactic acid fermentation:

  • pyruvate is reduced to lactic acid (by lactate dehydrogenase), NADH is oxidised back to NAD+
  • 2 lactate formed per glucose
  • 2 ATP formed during glycolysis
• Importance of anaerobic respiration:

  • synthesise small amount of ATP
  • regenerate NAD+
• Role of NAD+ and FAD+

  • accepts high energy electrons and protons to form NADH and FADH2
  • carries electrons from glycolysis, link reaction and Krebs cycle to the electron transport chain
  • acts as a coenzyme during oxidation by dehydrogenation
• role of oxygen:

  • final electron acceptor, combines with electrons and H+ ions to form water
  • allows oxidative phosphorylation to continue to generate ATP
  • NAD+ and FAD regenerated, can pick up more electrons and protons from glycolysis, link reaction and Krebs cycle
  • removes H+ from matrix, contributes to generation of proton gradient across inner mitochondrial membrane
• less glucose needed for aerobic respiration than anaerobic respiration:

  • aerobic respiration produces 38 net atp per glucose whereas anaerobic 2 net atp per glucose (substrate level phosphorylation during glycolysis)
  • oxidative phosphorylation synthesises 34 net atp in presence of oxygen, no oxidative phosphorylation during anaerobic conditions