5.7.4

• Pyruvate

  pyruvate produced in glycolysis is transported across the outer + inner mitochondrial membrane via a specific pyruvate-H+ symport, a transport protein that transports two ions or molecules in the same direction, + in the matrix

  then :

  1. pyruvate is converted to a two-carbon acetyl group during the link reaction
  2. the acetyl group is oxidised during the Krebs Cycle
• The link reaction (decarboxylation)

  occurs in the mitochondrial matrix , link between anaerobic and aerobic respiration

  pyruvate is decarboxylated + dehydrogenated, catalysed by a large multi-enzyme complex, pyruvate dehydrogenase, which catalyses the sequence of reactions that occur during the link reaction

  no ATP is produced
• The Krebs cycle

  occurs in the mitochondrial matrix

  • the Krebs cycle is a series of enzyme-catalysed reactions that oxidise the acetate from the link reaction to two molecules of carbon dioxide, while conserving energy by reducing the coenzyme NAD + FAD

  → these reduced coenzymes then carry the hydrogen atoms to the electron transport chain on the cristae, where they will be involved in the production of many more ATP molecules

  
• The Krebs cycle 2

  1. the acetyl group released from acetyl CoA combines with a four-carbon compound, oxaloacetate, to form a six-carbon compound, citrate
  2. citrate is decarboxylated + dehydrogenated, producing a five-carbon compound, one molecule of carbon dioxide + one molecule of reduced NAD
  3. this five-carbon compound is further decarboxylated + dehydrogenated, producing a four-carbon compound, one molecule of carbon dioxide + one molecule of reduce NAD
• The Krebs cycle 3

  5. this four-carbon compound combines temporarily with, and is then released from, coenzyme A; at this stage, substrate-level phosphorylation takes place, producing one molecule of ATP

  6. the four-carbon compound is dehydrogenated, producing a different four-carbon compound + a molecule of reduced FAD

  7. rearrangement of atoms in the four-carbon compound, catalysed by an isomerase enzyme, followed by further dehydrogenation, regenerate a molecule of oxaloacetate, so cycle can continue

  for every molecule of glucose there are two turns of the Krebs cycle
• the products of the link reaction + the Krebs cycle

  although oxygen is not directly used in the link reaction + Krebs cycle, these stages will not occur in the absence of oxygen, so they are aerobic

  by the end of the Krebs cycle, the production of carbon dioxide from glucose is produced

  
• the products of the link reaction and the Krebs cycle 2

  other substrates besides glucose:

  • fatty acids are broken down to many molecules of acetate that enter the Krebs cycle via acetyl CoA
  • glycerol may be converted to pyruvate + enter the Krebs cycle via the link reaction
  • amino acids may be deaminated + rest of the molecule can enter the Krebs cycle directly or be changed to pyruvate or acetyl CoA