Explain the role of the electron transport chain in linking the oxidation of NADH to the net synthesis of ATP during aerobic respiration.

IB DP Biology Higher Level (2023 syllabus) — C1.2 Cell respiration · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

NADH donates electrons and protons to Complex I (NADH dehydrogenase), becoming oxidised to NAD⁺. The electrons are transferred through a series of carriers (FMN, iron–sulfur clusters, ubiquinone, cytochromes), releasing energy at each step. This energy is used by Complexes I, III and IV to pump protons from the mitochondrial matrix into the intermembrane space, creating a proton‑motive gradient. Protons then flow back into the matrix through ATP synthase (Complex V), driving the phosphorylation of ADP to ATP by chemiosmosis.

Examiner tips

Use the exact terms: Complex I, II, III, IV, V; NADH, NAD⁺; chemiosmosis; proton‑motive gradient.
Show the flow of electrons and the role of proton pumping to link NADH oxidation to ATP synthesis.
Mention that the energy released is used for proton pumping, not directly for ATP.
Keep the answer concise – 4 marks, so one sentence per point.

Common mistakes

Confusing Complex II (succinate dehydrogenase) with Complex I; forgetting that Complex II does not pump protons.
Losing the detail that NADH is oxidised to NAD⁺ at Complex I.
Using vague language such as ‘energy is used’ without specifying proton pumping and chemiosmosis.

Mark scheme (4 marks)

NADH donates electrons (and protons/H⁺) to Complex I (NADH dehydrogenase) at the start of the electron transport chain, becoming oxidised to NAD⁺.
Electrons are passed along a series of carriers (e.g. FMN, iron–sulfur clusters, ubiquinone, cytochromes), losing energy at each transfer.
The energy released at Complexes I, III, and IV is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, establishing an electrochemical / proton-motive gradient.
Protons flow back into the matrix down their electrochemical gradient through ATP synthase (Complex V), driving the phosphorylation of ADP to ATP (chemiosmosis).

Key terms in this question

electron transport chain · NADH