Respiration: (f) the importance of coenzymes in cellular respiration

The idea

Coenzymes are small organic molecules that assist enzymes by carrying chemical groups or electrons between reactions. In respiration they are essential because they accept and donate hydrogen and electrons during oxidation and reduction. NAD accepts hydrogen to become reduced NAD, and FAD similarly becomes reduced FAD during the Krebs cycle; these reduced coenzymes deliver electrons to the electron transport chain, where their oxidation drives ATP synthesis by oxidative phosphorylation. Coenzyme A carries acetyl groups into the Krebs cycle. Without coenzymes, dehydrogenation reactions could not proceed and the yield of ATP would fall, so they link the stages of respiration and enable controlled transfer of reducing power.

What earns the marks

• Coenzymes are non-protein organic molecules that assist enzymes by carrying groups or electrons between reactions.
• NAD accepts hydrogen and electrons to become reduced NAD during glycolysis and the Krebs cycle.
• FAD accepts hydrogen to become reduced FAD in the Krebs cycle.
• Reduced NAD and reduced FAD deliver electrons to the electron transport chain, where their oxidation supports ATP synthesis by oxidative phosphorylation.
• Coenzyme A carries acetyl groups into the Krebs cycle.
• Without coenzymes, dehydrogenation reactions and the transfer of reducing power would not occur, lowering ATP yield.

Common mistakes and how to put them right

• Describing coenzymes as enzymes.

  Put it right: Coenzymes are not proteins and are not catalysts themselves; they assist enzymes by carrying groups or electrons.

• Saying reduced NAD is oxidised in the Krebs cycle.

  Put it right: Reduced NAD is produced in glycolysis and the Krebs cycle and is oxidised at the electron transport chain.

• Ignoring coenzyme A: it carries acetyl groups into the Krebs cycle and is distinct from NAD and FAD.

Exam tips

• State the specific role of each coenzyme (NAD, FAD, coenzyme A) rather than a general statement.
• Link reduced coenzymes to the electron transport chain and ATP synthesis to show their importance.
• Use the terms oxidation and reduction correctly: NAD is reduced when it accepts hydrogen and oxidised when it donates electrons.