Edexcel International A Level Biology

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7.3 Link Reaction & Krebs Cycle

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Link Reaction & Krebs Cycle

The Link reaction

  • The end product of glycolysis is pyruvate
  • Pyruvate contains a substantial amount of chemical energy that can be further utilised in respiration to produce more ATP
  • The enzymes and coenzymes that are required for the link reaction are found in the mitochondrial matrix
  • When oxygen is available pyruvate will enter the mitochondrial matrix and aerobic respiration will continue
  • Pyruvate moves across the double membrane of the mitochondria via active transport
    • It requires a transport protein and a small amount of ATP
  • Once in the mitochondrial matrix pyruvate takes part in the link reaction

Entering Link Reaction

Pyruvate enters the mitochondrial matrix from the cytosol (cytoplasm) by active transport

  • 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:
    • Pyruvate is oxidised (hydrogen is removed) by enzymes to produce acetate, CH3CO(O) (also known as acetic acid)
    • Pyruvate is also decarboxylated (carbon is removed) in the form of carbon dioxide
    • Reduction of NAD to NADH or reduce NAD by collecting hydrogen from pyruvate
    • Acetate combines with coenzyme A to form acetyl coenzyme A (acetyl CoA)
  • No ATP is produced during the link reaction
  • It produces:
    • Acetyl coA
    • Carbon dioxide (CO2)
    • Reduced NAD (NADH)

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

Link Reaction

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

  • Every molecule of glucose produces two pyruvate molecules
  • The link reaction and the Krebs cycle will therefore occur twice for every molecule of glucose
  • Thus, each molecule of glucose will produce:
    • Two molecules of acetyl CoA
    • Two molecules of CO2
    • Two molecules of reduced NAD

The Krebs Cycle

  • The Krebs cycle (sometimes called the citric acid cycle) consists of a series of enzyme-controlled reactions
  • 2 carbon (2C) Acetyl CoA enters the circular pathway from the link reaction in glucose metabolism
    • Acetyl CoA formed from fatty acids (after the breakdown of lipids) and amino acids enters directly into the Krebs Cycle from other metabolic pathways
  • 4 carbon (4C) oxaloacetate accepts the 2C acetyl fragment from acetyl CoA to form the 6 carbon (6C) citrate
    • Coenzyme A is released in this reaction to be reused in the next link reaction
  • Citrate is then converted back to oxaloacetate through a series of oxidation-reduction (redox) reactions

The Krebs Cycle-1

The Krebs Cycle uses acetyl CoA from the link reaction and the regeneration of oxaloacetate to produce reduced NAD, reduced FAD and ATP

Regeneration of oxaloacetate

  • Oxaloacetate is regenerated in the Krebs cycle through a series of redox reactions
  • Decarboxylation of citrate
    • Releasing 2 CO2 as waste gas
  • Oxidation (dehydrogenation) of citrate
    • Releasing H atoms that reduce coenzymes NAD and FAD
    • These will be used during oxidative phosphorylation
    • 3 NAD and 1 FAD → 3NADH + H+ and 1 FADH2
  • Substrate linked phosphorylation
    • A phosphate is transferred from one of the intermediates to ADP, forming 1 ATP to supply energy
  • Because two acetyl-CoA molecules are produced from each glucose molecule, two cycles are required per glucose molecule
  • Therefore, at the end of two cycles, the products are:
    • Two ATP
    • Six NADH (reduced NAD)
    • Two FADH(reduced FAD)
    • Four CO2

Exam Tip

The Krebs cycle is often referred to as cyclical or circular. This is because the acceptor molecule oxaloacetate is regenerated throughout the reaction so that it can start all over again by adding another acetyl CoA.

You may be asked to name the important molecules in the Krebs cycle like oxaloacetate and citrate.

It is also worth noting how the number of carbon atoms in the substrate molecule changes as the cycle progresses.

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.