Entering the link reaction

• The end product of glycolysis is?pyruvate?(3C)
• Pyruvate contains a substantial amount of chemical energy that can be further utilised in respiration to produce more ATP
• When?oxygen is available?pyruvate will?enter the mitochondrial?matrix?and?aerobic?respiration will continue
• Once in the matrix pyruvate takes part in the link reaction

The link reaction

• The link reaction takes place in the matrix of the mitochondria
• It is referred to as the link reaction because it?links?glycolysis?to the?Krebs cycle
• The steps are:
  • Oxidative carboxylation?reaction in which:
    • Carbon dioxide is removed?to produce a 2C molecule
    • This 2C molecule is then oxidised?(loss of hydrogen and 2 high energy electrons) to produce an?acetyl compound?and thereby reducing NAD to NADH
  • Combination of the acetyl compound with?coenzyme A?to form acetyl coenzyme A (acetyl CoA)
• It produces:
  • Acetyl CoA
  • Carbon dioxide (CO₂)
  • Reduced NAD (NADH)

pyruvate + NAD + CoA → acetyl CoA + carbon dioxide + reduced NAD

• Acetyl coenzyme A is supplied to the Krebs cycle where aerobic respiration continues

The link reaction occurs in the mitochondrial matrix. It dehydrogenates and decarboxylates the three-carbon pyruvate to produce the two-carbon acetyl CoA that can enter the Krebs Cycle.

Exam Tip

• The Krebs cycle (sometimes called the citric acid cycle) consists of a?series of enzyme-controlled reactions
• The?Krebs cycle?takes place in the matrix of the mitochondria
• Two carbon (2C) Acetyl CoA?enters the circular pathway from the?link reaction
• A four carbon (4C) accepts the 2C acetyl fragment from acetyl CoA to form a six carbon (6C)
  • Coenzyme A is released in this reaction to be reused in the link reaction
• The 6C?compound is then converted back to the 4C compound?through a series of?oxidation-reduction (redox) reactions

The Krebs Cycle uses acetyl CoA from the link reaction to produce reduced carbon dioxide, reduced NAD, reduced FAD and ATP

The reactions involved in the Krebs cycle

• The 4C compound is regenerated in the Krebs cycle through a series of?redox reactions
• Decarboxylation?of the 6C compound
  • Releasing?two?CO₂?as waste gas
• Oxidation?(dehydrogenation) of the 6C compound releases hydrogen atoms
• Reduction?of coenzymes NAD and FAD (by the released H atoms)
  • 3 NAD^+?and 1 FAD →?3NADH + H⁺?and 1 FADH₂
• Substrate-level phosphorylation
  • A phosphate is transferred from one of the intermediates to ADP, forming one?ATP

• As the link reaction produces two molecules of acetyl CoA (one per each pyruvate), the?Krebs cycle occurs twice
• Per glucose molecule, the Krebs cycle produces:
  • 4 CO₂
  • 2 ATP
  • 6 NADH + H⁺?(reduced NAD)
  • 2 FADH₂?(reduced FAD)