The link reaction and Krebs cycleEdexcel A-Level Biology B: Subtopic test

1. 1
   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.

   (a)

   What type of reaction removes carbon dioxide from pyruvate in the link reaction?

   [1 mark]

   • ADecarboxylation
   • BPhosphorylation
   • CHydrolysis
   • DCondensation

   (b)

   How many carbon atoms are in the compound formed from each pyruvate in the link reaction?

   [1 mark]

   • A1
   • B3
   • C2
   • D4

   (c)

   Explain why radioactive carbon dioxide is released from the mitochondria.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   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.

   (a)

   What amount of carbon dioxide is released?

   [1 mark]

   • A3.0 mmol
   • B9.0 mmol
   • C6.0 mmol
   • D12.0 mmol

   (b)

   What amount of NADH is formed in the link reaction and Krebs cycle together?

   [1 mark]

   • A3.0 mmol
   • B9.0 mmol
   • C6.0 mmol
   • D12.0 mmol

   (c)

   Calculate the amount of ATP formed directly in the Krebs cycle. Show your working.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   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.

   (a)

   Explain why carbon dioxide is still released after the poison is added, but at a lower rate.

   [3 marks]

   (b)

   Explain why the poison reduces the total ATP made by the mitochondria by much more than the ATP made directly in the Krebs cycle.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student wants to account for every carbon atom in a molecule of glucose that is completely oxidised in aerobic respiration in a liver cell. She knows that glycolysis converts the glucose into two molecules of pyruvate and that no carbon dioxide is released in glycolysis.

   (a)

   Describe how carbon atoms from pyruvate are removed as carbon dioxide in the link reaction and the Krebs cycle, and state the other products formed.

   [6 marks]

   (b)

   For one molecule of glucose, calculate the number of carbon dioxide molecules and NADH molecules produced by the link reaction and the Krebs cycle together, and the ATP made in the Krebs cycle. Explain how your carbon dioxide total accounts for the carbon in glucose.

   [6 marks]

   Total for question 4: 12 marks

End of questions