Link reaction and Krebs cycleAQA A-Level Biology: Subtopic test

1. 1
   Pyruvate produced by glycolysis in the cytoplasm of a liver cell is taken up by the mitochondria when oxygen is available.

   (a)

   By which process does pyruvate move into the mitochondrial matrix?

   [1 mark]

   • ASimple diffusion
   • BActive transport
   • COsmosis
   • DEndocytosis

   (b)

   Which compound combines with the four-carbon molecule at the start of the Krebs cycle?

   [1 mark]

   • APyruvate
   • BReduced NAD
   • CCarbon dioxide
   • DAcetylcoenzyme A

   (c)

   Explain, with reference to carbon atoms and NAD, what happens to pyruvate when it is oxidised to acetate in the link reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The Krebs cycle was studied in isolated mitochondria supplied with acetylcoenzyme A and oxygen.

   (a)

   How many carbon atoms are in the molecule formed when acetylcoenzyme A reacts with the four-carbon molecule?

   [1 mark]

   • A2
   • B4
   • C5
   • D6

   (b)

   By which process is ATP made directly in the Krebs cycle?

   [1 mark]

   • AOxidative phosphorylation
   • BSubstrate-level phosphorylation
   • CPhotophosphorylation
   • DChemiosmosis

   (c)

   Explain why the Krebs cycle would stop if the isolated mitochondria were deprived of oxygen.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   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.

   (a)

   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]

   (b)

   Explain why the four-carbon molecule is not used up in the Krebs cycle, and what the answer to the calculation above shows about the fate of the carbon atoms in glucose.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Heart muscle cells contain many mitochondria and respire aerobically. A student states that because the link reaction and the Krebs cycle do not use oxygen directly, they will carry on in mitochondria even when no oxygen is available.

   (a)

   Describe the link reaction and the Krebs cycle.

   [6 marks]

   (b)

   Evaluate the student's statement.

   [6 marks]

   Total for question 4: 12 marks

End of questions