Explain why NAD is essential for cell respiration to continue.

IB DP Biology Standard 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).

NAD is a coenzyme present in cells in limited amounts. During cell respiration, NAD accepts hydrogen (electrons and protons) from oxidation reactions, becoming reduced to NADH.

Model answer (4 marks)

NAD is required as the hydrogen/electron acceptor in the oxidation steps of glycolysis and the Krebs cycle.
Because only a small amount of NAD is present in the cell, it must be continually regenerated.
In aerobic respiration NADH is re‑oxidised to NAD at the inner mitochondrial membrane by the electron transport chain, allowing NAD to be reused.
In anaerobic respiration NAD is regenerated by reducing pyruvate to lactate (or ethanol and CO₂), so glycolysis can continue.

Examiner tips

Use the exact terms ‘hydrogen/electron acceptor’, ‘regenerated’, ‘electron transport chain’, and ‘anaerobic fermentation’.
Show the sequence: oxidation → NAD⁺ → NADH → re‑oxidation → NAD⁺ again.
Mention both aerobic and anaerobic pathways.

Common mistakes

Confusing NAD⁺ with NADH as the acceptor; they accept electrons, not donate.
Omitting the regeneration step or only mentioning one pathway (aerobic or anaerobic).

Mark scheme (4 marks)

NAD is needed as a hydrogen/electron acceptor in oxidation reactions during glycolysis and the Krebs cycle.
NAD is present in limited/small quantities in the cell, so it must be continuously regenerated/recycled.
In aerobic respiration, NADH is re-oxidised to NAD at the inner mitochondrial membrane/electron transport chain, allowing NAD to be reused.
In anaerobic respiration, NAD is regenerated by the reduction of pyruvate to lactate (or ethanol and carbon dioxide), allowing glycolysis to continue.

Key terms in this question

NAD