Link reaction and Krebs cycleAQA A-Level Biology: Revision notes
Section 1
Entry into the mitochondrion
If respiration is aerobic, pyruvate from glycolysis enters the mitochondrial matrix by active transport. The matrix contains the enzymes for the link reaction and Krebs cycle, and the carriers on the inner membrane then use the reduced coenzymes made here.
Section 2
The link reaction
Pyruvate (3C) is oxidised to acetate (2C). Carbon dioxide is lost (decarboxylation) and hydrogen is removed (dehydrogenation) and accepted by NAD, forming reduced NAD. The acetate combines with coenzyme A to form acetylcoenzyme A. This happens twice per glucose, because glycolysis gives two pyruvate.
The link reaction does not make ATP. It makes reduced NAD and carbon dioxide.
Section 3
The Krebs cycle: reactions
Acetylcoenzyme A (2C) reacts with a four-carbon molecule, releasing coenzyme A and forming a six-carbon molecule. Through a series of oxidation-reduction reactions, the six-carbon molecule is converted back to the four-carbon molecule, so the cycle can continue. The reactions take place in the matrix.
Section 4
The Krebs cycle: products
In the cycle, carbon dioxide is lost (decarboxylation) and hydrogen is removed (dehydrogenation) and accepted by NAD and FAD, producing reduced NAD and reduced FAD. ATP is made by substrate-level phosphorylation. Per turn: 2 CO₂, 3 reduced NAD, 1 reduced FAD, 1 ATP; per glucose, double these (two turns).
Per glucose: link reaction gives 2 CO₂ and 2 reduced NAD; Krebs gives 4 CO₂, 6 reduced NAD, 2 reduced FAD and 2 ATP.
Section 5
Why oxygen still matters
Neither stage uses oxygen directly, but they depend on it indirectly. Reduced NAD and reduced FAD must be re-oxidised by the electron transfer chain, where oxygen is the final electron acceptor. Without oxygen, NAD and FAD are not regenerated and the link reaction and Krebs cycle stop.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Link reaction and Krebs cycle
- Pyruvate produced by glycolysis in the cytoplasm of a liver cell is taken up by the mitochondria when oxygen is available.Explain, with reference to carbon atoms and NAD, what happens to pyruvate when it is oxidised to acetate in the link reaction.2 marks
- The Krebs cycle was studied in isolated mitochondria supplied with acetylcoenzyme A and oxygen.Explain why the Krebs cycle would stop if the isolated mitochondria were deprived of oxygen.2 marks
- Each glucose molecule gives two pyruvate molecules in glycolysis. Each pyruvate goes through the link reaction, which releases one carbon dioxide molecule and forms one reduced NAD, and each acetylcoenzyme A then enters one turn of the Krebs cycle. One turn of the Krebs cycle releases two carbon dioxide molecules and produces three reduced NAD, one reduced FAD and one ATP.Calculate the number of carbon dioxide, reduced NAD and ATP molecules produced by the link reaction and Krebs cycle for each glucose molecule.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).