Respiration

• Aerobic respiration


  A catabolic, enzyme-controlled reaction that occurs inside cells to provide energy
• Respiration is carried out by all living organisms
• Respiration


  • Energy rich respiratory substrates, such as glucose or fatty acids, are broken down to release energy
  • High-energy bonds (C-C, C-H and C-OH) are broken and lower-energy bonds are formed
  • The excess energy released is used to phosphorylate ADP to form ATP or is released as heat energy
• Glycolysis


  1. Glucose is phosphorylated using 2ATP into hexose phosphate

  2. The hexose phosphate splits into two triose phosphate molecules

  3. The oxidation of these 2 triose phosphate molecules yield 2 ATP each by substrate level phosphorylation. Overall glycolysis has a net gain of 2ATP. Dehydrogenation releases 2 Hydrogen that are picked up by NAD. The 2 pyruvate diffuse into the mitochondria
• Link reaction


  1. Oxidative decarboxylation of pyruvate catalysed by decarboxylase releases carbon dioxide

  2. Dehydrogenation catalysed by dehydrogenase releases pairs of hydrogen atoms converting NAD to reduced NAD

  3. The addition of coenzyme A forms acetyl CoA (2C) which enters the Krebs cycle
• Krebs cycle


  1. The acetate from acetyl CoA combines with a 4C compound to form a 6C compound

  2. Decarboxylation forms a 5C compound and dehydrogenation occurs reducing NAD

  3. Decarboxylation forms a 4C compound and dehydrogenation to reduce NAD. There is also substrate level phosphorylation giving 1 ATP

  4. Dehydrogenation forming reduced FAD

  5. Dehydrogenation forming reduced NAD
• Electron transport chain


  1. Reduced NAD and reduced FAD, from glycolysis, link and Kreb's reactions, deliver pairs of hydrogen atoms to the ETC

  2. They are oxidised, delivering protons (H+) and high-energy electrons (e–) to proton pumps on the inner mitochondrial membrane

  3. Each reduced NAD utilises all 3 proton pumps, releasing 3ATP. Each reduced FAD utilises only 2 proton pumps, releasing 2 ATP

  4. Oxygen is the final acceptor in the electron transfer chain and is reduced to water
• Respiratory substrates other than glucose

  • Lipids: Glycerol is converted into triose phosphate for use in glycolysis. Fatty acids are split into 2C acetate fragments which feed into the Krebs cycle as acetyl CoA
  • Proteins: Amino acids are deaminated in the liver into ammonia and keto acids, one of which (pyruvate) is used in the link reaction while the others are fed into the Krebs cycle as intermediates
• For each molecule of glucose, the link reaction and Kreb's cycle happen twice. This is because each molecule of glucose leads to the formation of two triose phosphate in glycolysis
• For one molecule of glucose: The net yield of ATP from substrate level phosphorylation is 2 from glycolysis, 2 from Kreb's. 2 reduced NAD from glycolysis – releasing 6 ATP. 2 reduced NAD from the link reaction – releasing 6 ATP. 6 reduced NAD from Kreb's cycle – releasing 18 ATP. 2 reduced FAD from Kreb's cycle – releasing 4 ATP
• Anaerobic respiration


  Respiration that occurs in the absence of oxygen
• Anaerobic respiration in animals and bacteria

  1. Glycolysis occurs

  2. Reduced NAD reduces the pyruvate, forming lactate

  3. The oxidised NAD can again be reduced during glycolysis
• Anaerobic respiration in plants and fungi

  1. Glycolysis occurs

  2. Triose phosphate is converted to pyruvate with the release of ATP and reduction of NAD

  3. Pyruvate is decarboxylated, releasing carbon dioxide and forming ethanal

  4. Reduced NAD reduces the ethanal, forming ethanol

  5. The oxidised NAD can again be reduced during glycolysis

  
• Respiration energy budgets per molecule of glucose

  • Glycolysis: 2 ATP produced from substrate level phosphorylation, 2 reduced NAD produced
  • Link reaction: 2 reduced NAD produced
  • Krebs cycle: 2 ATP produced from substrate level phosphorylation, 6 reduced NAD produced, 2 reduced FAD produced
  • ATP from oxidative phosphorylation of reduced coenzymes NAD or FAD: 6 ATP
  • Total yield of ATP in aerobic conditions: 38
  • Total for anaerobic respiration: 2 (net yield from glycolysis only)

  
• The maximum yield for aerobic respiration of 38ATP/molecule is rarely achieved because of losses
• Some protons can leak through the inner mitochondrial membrane without passing through ATP synthetase
• Transport of reactants such as ADP, Pi and pyruvate into the mitochondria can have an energy cost
• Current estimates are 30 to 32 ATP per molecule of glucose

  
• Any energy that is not used to synthesise ATP is released as heat energy