M5:S6 Respiration

• Aerobic respiration has four stages: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation
• Glycolysis

  1. Glucose is phosphorylated

  2. Hexose bisphosphate is split into 2 triose phosphate

  3. Triose phosphate is oxidised to form 2 pyruvate
• Glycolysis is the first stage of both aerobic and anaerobic respiration and doesn't need oxygen to take place so it's an anaerobic process
• Glycolysis

  Splits one molecule of glucose (6C) into two smaller molecules of pyruvate (3C)
• Glycolysis - Phosphorylation

  1. Glucose is phosphorylated by adding 2 phosphates from 2 ATP

  2. Creates 1 hexose bisphosphate and 2 ADP
• Glycolysis - Oxidation

  1. Triose phosphate is oxidised, forming 2 pyruvate

  2. NAD collects the hydrogen, forming 2 reduced NAD

  3. 4 ATP produced, but 2 used up in stage one, so net gain of 2 ATP
• The link reaction takes place in the mitochondrial matrix
• Link Reaction

  1. Pyruvate is decarboxylated, removing 1 carbon as CO2

  2. NAD is reduced to NADH, converting pyruvate to acetate

  3. Acetate is combined with coenzyme A to form acetyl coenzyme A
• The link reaction occurs twice for every glucose molecule that enters glycolysis
• Two molecules of acetyl coenzyme A go into the Krebs cycle for every glucose molecule
• Two CO2 molecules are released as a waste product of respiration for every glucose molecule
• Two molecules of reduced NAD are formed and go to the last stage (oxidative phosphorylation) for every glucose molecule
• Crista

  Folds in the inner mitochondrial membrane
• Glycolysis occurs in the cytoplasm of cells
• Glycolysis is an anaerobic process
• 4 ATP molecules are used up in glycolysis
• Acetyl coenzyme A

  A 2-carbon molecule that is the product of the link reaction and feeds into the Krebs cycle
• One glucose molecule is broken down into two molecules of pyruvate during glycolysis
• There is an overall gain of 2 ATP molecules in glycolysis
• The Krebs cycle involves a series of oxidation-reduction reactions, which take place in the matrix of the mitochondria
• The Krebs cycle happens once for every pyruvate molecule, so it goes round twice for every glucose molecule
• Krebs Cycle

  1. Acetyl group from acetyl CoA combines with oxaloacetate to form citrate

  2. Citrate is converted to a 5C molecule, with decarboxylation and dehydrogenation occurring

  3. 5C molecule is converted to a 4C molecule, with decarboxylation and dehydrogenation occurring, producing 1 reduced FAD and 2 reduced NAD

  4. ATP is produced by substrate-level phosphorylation
• Oxaloacetate is regenerated for use in the next Krebs cycle
• 1 ATP, 3 reduced NAD and 1 reduced FAD are produced in each turn of the Krebs cycle
• Oxidative phosphorylation takes place in the inner mitochondrial membrane
• Oxidative Phosphorylation

  1. Hydrogen atoms from reduced NAD and reduced FAD are released as they are oxidised

  2. Electrons move along the electron transport chain, losing energy at each carrier

  3. Energy is used to pump protons from the mitochondrial matrix into the intermembrane space

  4. Protons move down the electrochemical gradient back into the mitochondrial matrix via ATP synthase, driving ATP synthesis
• Oxygen is the final electron acceptor at the end of the electron transport chain
• 32 ATP can be made from one glucose molecule in aerobic respiration
• The Krebs cycle occurs in the mitochondrial matrix
• Decarboxylation happens twice during one turn of the Krebs cycle
• Electrons lose energy as they move along the electron transport chain in oxidative phosphorylation
• Carbon monoxide inhibits the final electron carrier in the electron transport chain

  This affects ATP production via the electron transport chain
• Carbon monoxide inhibits the final electron carrier in the electron transport chain

  This also affects ATP production via the Krebs cycle
• Anaerobic respiration doesn't use oxygen
• Anaerobic respiration doesn't involve the link reaction, the Krebs cycle or oxidative phosphorylation
• Types of anaerobic respiration

  • Alcoholic fermentation
  • Lactate fermentation
• Lactate Fermentation

  1. Reduced NAD transfers hydrogen to pyruvate to form lactate and NAD

  2. Production of lactate regenerates NAD so glycolysis can continue
• Lactate fermentation occurs in mammals
• Cells can tolerate high levels of lactate for short periods, e.g. during exercise
• Too much lactate is toxic and is removed from cells into the bloodstream, where the liver converts it back to glucose