The link reaction and Krebs cycleEdexcel A-Level Biology B: Revision notes
Section 1
Where and why
After glycolysis, pyruvate is transported into the mitochondrial matrix. There the link reaction and the Krebs cycle complete the oxidation of pyruvate. Both take place in the matrix, and both need the products of glycolysis, but they do not make much ATP themselves. Their main outputs are carbon dioxide, reduced coenzymes (NADH) and a little ATP.
Detailed knowledge of the intermediate compounds of the Krebs cycle is not required.
Section 2
The link reaction
Each pyruvate (3C) is decarboxylated (one carbon removed as CO₂) and dehydrogenated (hydrogen removed, reducing NAD to NADH). This forms a 2-carbon compound (acetyl, carried on coenzyme A) that enters the Krebs cycle.
Per pyruvate: 1 CO₂ and 1 NADH. Per glucose, with two pyruvate: 2 CO₂ and 2 NADH.
Section 3
The Krebs cycle
The 2-carbon compound combines with a 4-carbon compound to form a 6-carbon compound. This is decarboxylated twice and dehydrogenated several times, regenerating the 4-carbon compound so that the cycle can continue.
Per turn (per pyruvate): 2 CO₂, 3 NADH, 1 other reduced coenzyme (FADH₂) and 1 ATP. The ATP is made directly, not using a membrane.
Per glucose, the cycle turns twice: 4 CO₂, 6 NADH, 2 FADH₂ and 2 ATP.
Count per glucose, not per pyruvate: the link reaction and the cycle each happen twice.
Section 4
Totals and worked example
Per glucose, link reaction + Krebs cycle:
- CO₂: 2 + 4 = 6
- NADH: 2 + 6 = 8
- ATP: 2
The 6 CO₂ account for all 6 carbon atoms in glucose (glycolysis releases none).
Worked example. 3.0 mmol of pyruvate is oxidised. CO₂ = 3 × 3.0 = 9.0 mmol. NADH = 4 × 3.0 = 12.0 mmol (1 from the link reaction and 3 from the cycle for each pyruvate). ATP = 1 × 3.0 = 3.0 mmol.
Section 5
What happens next
The NADH and other reduced coenzymes carry hydrogen to the inner mitochondrial membrane, where oxidative phosphorylation uses them to make most of the ATP. If the link reaction or Krebs cycle are blocked, less reduced coenzyme is supplied, so much less ATP is made overall.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The link reaction and Krebs cycle
- A researcher supplies isolated liver mitochondria with pyruvate in which all three carbon atoms are labelled with radioactive carbon-14. The mitochondria are in an aerated medium that contains NAD, ADP and phosphate. She collects the gas released from the medium and finds that it is radioactive.Explain why radioactive carbon dioxide is released from the mitochondria.2 marks
- A student incubates isolated mitochondria with 3.0 mmol of pyruvate and excess oxygen, NAD, ADP and phosphate. All of the pyruvate is completely oxidised by the link reaction and the Krebs cycle.Calculate the amount of ATP formed directly in the Krebs cycle. Show your working.2 marks
- Mitochondria are isolated and supplied with pyruvate, NAD, ADP, phosphate and oxygen. A researcher measures the rate of carbon dioxide release and the amount of NADH formed. She then repeats the experiment after adding a poison that blocks the Krebs cycle but has no effect on the link reaction. She records measurements during the first few minutes after the poison is added.Explain why carbon dioxide is still released after the poison is added, but at a lower rate.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).