Oxidative phosphorylationEdexcel A-Level Biology B: Subtopic test

1. 1
   Isolated mitochondria are suspended in a buffered medium containing a pH electrode. The outer membranes of the mitochondria are permeable to small ions. When NADH and oxygen are added, the pH of the medium falls, and ATP is formed if ADP and phosphate are also present.

   (a)

   Where in a mitochondrion is the electron transport chain located?

   [1 mark]

   • AIn the outer mitochondrial membrane
   • BIn the inner mitochondrial membrane
   • CIn the fluid of the matrix
   • DIn the cytoplasm outside the mitochondrion

   (b)

   Which molecule supplies the electrons that enter the electron transport chain in this experiment?

   [1 mark]

   • AOxygen, which is reduced
   • BADP, which is phosphorylated
   • CWater, which is split
   • DNADH, which is oxidised to NAD

   (c)

   Explain why the pH of the medium falls after NADH and oxygen are added.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A researcher adds a chemical to isolated mitochondria that makes the inner membrane freely permeable to protons. Electrons continue to pass along the electron transport chain, and oxygen continues to be used at a high rate, but ATP production almost stops.

   (a)

   Which statement describes the role of ATP synthase?

   [1 mark]

   • AIt is a channel through which protons flow down their gradient, and the energy is used to join ADP and phosphate
   • BIt pumps protons from the matrix into the intermembrane space
   • CIt accepts electrons from NADH at the start of the chain
   • DIt accepts electrons at the end of the chain to form water

   (b)

   Why does oxygen continue to be used at a high rate after the chemical is added?

   [1 mark]

   • AOxygen is needed to make ATP synthase work
   • BOxygen is needed to make the inner membrane permeable to protons
   • CElectrons still pass along the chain, and oxygen is still the final electron acceptor, forming water
   • DOxygen reacts directly with glucose in the matrix

   (c)

   Explain why ATP production almost stops.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cyanide binds to the last protein carrier in the electron transport chain and prevents it from passing electrons to oxygen. Cells exposed to cyanide soon run short of ATP, even though plenty of glucose is available. The Krebs cycle in these cells also stops.

   (a)

   Explain why cells exposed to cyanide soon run short of ATP.

   [3 marks]

   (b)

   Explain why the Krebs cycle also stops in cells exposed to cyanide.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Heart muscle cells contract continuously and contain large numbers of mitochondria, with inner membranes folded into densely packed cristae. Epidermal skin cells have fewer mitochondria, with fewer cristae.

   (a)

   Describe how oxidative phosphorylation produces ATP, starting from reduced NAD.

   [6 marks]

   (b)

   Explain how the structure of the mitochondria in heart muscle cells allows ATP to be produced at a high rate.

   [6 marks]

   Total for question 4: 12 marks

End of questions