respiration

• Respiration

  A catabolic, enzyme-controlled reaction inside cells to provide energy
• Respiration


  • All living organisms carry it out
  • 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 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 triose phosphates yields 2ATP by substrate level phosphorylation. Dehydrogenation releases 2 Hydrogen that are picked up by NAD. The resulting 2x 3C pyruvates 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 3C 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 deliver pairs of hydron 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. Reduced NAD utilises all 3 proton pumps and so 3xATP are released. Reduced FAD utilises only 2 proton pumps and only 2x ATP are released
• Respiratory substrates other than glucose

  • 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
  • Amino acids are deaminated in the liver into ammonia and keto acids, one of which is pyruvate that is used in the link reaction and others are fed into the Krebs cycle
• The method of ATP production can be found on the sheet "The importance of ATP"
• Anaerobic respiration

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

  1. Glycolysis occurs

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

  3. Reduced NAD reduces the pyruvate, forming lactate

  4. The oxidised NAD can again be reduced during glycolysis. The cycle continues
• Anaerobic respiration in plants

  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. The cycle continues
• Respiration energy budgets

  • Glycolysis: 2 ATP, 2 reduced NAD, 2 reduced FAD
  • Link reaction: 2 reduced NAD
  • Krebs cycle: 6 ATP, 6 reduced NAD, 2 reduced FAD
  • ATP from oxidative phosphorylation of reduced coenzymes NAD or FAD: 22
  • Total for aerobic respiration: 38
  • Total for anaerobic respiration: 2
• Without oxygen

  Biochemical reactions inside the mitochondria grind to a halt as any reduced NAD and FAD cannot be reoxidised to pick up more hydrogen
• Glycolysis

  Allows reduced NAD to transfer hydrogen to pyruvate, allowing NAD to again accept hydrogen from glucose in a reaction that produces a very small yield of 2 ATP