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A2 · Practice questions · Oxidative phosphorylation and chemiosmosis

Electrons along, protons across.

Six original Cambridge-style questions on oxidative phosphorylation and chemiosmosis, including the effects of oxygen shortage and respiratory inhibitors.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Carry, split, pump, flow.

01
Recall
[2 marks]

Describe the role of NAD and FAD in aerobic respiration.

  • NAD and FAD are coenzymes that accept hydrogen removed by dehydrogenase enzymes in glycolysis, the link reaction and the Krebs cycle, becoming reduced. ✓
  • Reduced NAD and FAD transfer the hydrogen to carriers in the inner mitochondrial membrane, and are oxidised so they can be used again. ✓
02
Explain
[5 marks]

Explain how ATP is produced during oxidative phosphorylation.

  • Hydrogen from reduced NAD and FAD is passed to the electron transport chain on the inner mitochondrial membrane; each hydrogen atom splits into a proton and an energetic electron. ✓
  • Electrons pass along the chain of carriers, releasing energy. ✓
  • This energy is used to pump protons from the matrix into the intermembrane space, forming a proton gradient. ✓
  • Protons diffuse back into the matrix through ATP synthase (facilitated diffusion), which uses the energy to make ATP from ADP and phosphate: chemiosmosis. ✓
  • Oxygen is the final electron acceptor, combining with electrons and protons to form water. ✓
03
Explain
[3 marks]

Explain why the Krebs cycle stops when there is no oxygen, even though oxygen is not used in the Krebs cycle.

  • Oxygen is the final electron acceptor; without it electrons cannot leave the electron transport chain. ✓
  • So reduced NAD and reduced FAD cannot pass on their hydrogen and are not oxidised. ✓
  • No NAD or FAD is available to accept hydrogen in the dehydrogenation steps, so the Krebs cycle (and link reaction) stop. ✓
04
Explain
[3 marks]

Explain how the structure of the inner mitochondrial membrane is related to its function in oxidative phosphorylation.

  • It is folded into cristae, giving a large surface area for many electron transport chains and ATP synthase molecules. ✓
  • It is impermeable to protons, so a proton gradient can be maintained between the intermembrane space and the matrix. ✓
  • Protons can only return through ATP synthase, so their flow is coupled to ATP synthesis. ✓
05
Calculation
[2 marks]

In an active mitochondrion the pH of the intermembrane space is 7.0 and the pH of the matrix is 8.0. How many times greater is the proton concentration in the intermembrane space? Explain the difference.

  • pH is a logarithmic scale: one pH unit lower means a 10 times higher H+ concentration, so 10 times greater. ✓
  • Protons are pumped from the matrix into the intermembrane space using energy released by electrons in the electron transport chain. ✓
06
Data
[3 marks]

Isolated mitochondria were kept in a solution containing pyruvate and phosphate. Oxygen uptake was slow. When ADP was added, oxygen uptake rose sharply; when cyanide was then added, oxygen uptake stopped. Explain these results.

  • Without ADP, ATP synthase cannot work, so protons do not flow back; the gradient becomes steep and slows electron transport, so little oxygen is used. ✓
  • Adding ADP lets protons flow through ATP synthase, the gradient falls, electron transport speeds up and more oxygen is used as the final electron acceptor. ✓
  • Cyanide inhibits the last carrier in the chain, so electrons cannot be passed to oxygen and oxygen uptake stops. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.