A student claims that the electron transport chain is responsible for most of the ATP produced during aerobic respiration. Explain why this claim is correct, referring to the role of reduced NAD in this process.

OCR A-Level Biology A (H420) — 5.7 Respiration · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Aerobic respiration occurs in stages, each producing different amounts of ATP. The electron transport chain, located in the inner mitochondrial membrane, is the final stage and accounts for the majority of ATP yield per glucose molecule.

Model answer (4 marks)

Reduced NADH donates its electrons to the electron transport chain, where they move through a series of carriers and release energy.

The energy released drives the active pumping of protons from the mitochondrial matrix into the intermembrane space, creating a proton gradient.

This chemiosmotic (electrochemical) gradient stores potential energy across the inner membrane.

Protons flow back into the matrix via ATP synthase (chemiosmosis), and the energy of this flow is used to synthesise a large amount of ATP, making the ETC the main source of ATP in aerobic respiration.

Examiner tips

  • Use the term ‘reduced NADH’ and ‘electron transport chain’.
  • Explain the proton pumping and gradient clearly.
  • Mention ATP synthase and chemiosmosis.
  • Show the link between the gradient and the majority of ATP yield.

Common mistakes

  • Confusing NAD+ with NADH.
  • Saying the ETC only ‘generates’ ATP instead of ‘drives’ ATP synthase.

Mark scheme (4 marks)

  1. Reduced NAD donates electrons (and/or hydrogen) to the electron transport chain
  2. Energy released from electrons passing along the chain is used to actively pump protons (H⁺) across the inner mitochondrial membrane, from the matrix to the intermembrane space
  3. A proton gradient (chemiosmotic gradient / electrochemical gradient) is established across the inner mitochondrial membrane
  4. Protons flow back into the matrix through ATP synthase (chemiosmosis), driving the synthesis of a large quantity of ATP, accounting for the majority of ATP yield

Key terms in this question

electron transport chain · reduced NAD

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