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Energy and Respiration: Question 8

Syllabus 12.2

Multiple choice A2 1 mark

Reduced NAD and reduced FAD both deliver hydrogen (as protons and electrons) to the electron transport chain during oxidative phosphorylation, but oxidation of one molecule of reduced FAD ultimately results in less ATP being synthesised than oxidation of one molecule of reduced NAD.

Which statement correctly explains this difference?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall how reduced NAD and reduced FAD feed into the electron transport chain

Reduced NAD and reduced FAD are both produced during earlier stages of respiration (reduced NAD from glycolysis, the link reaction and the Krebs cycle; reduced FAD from the Krebs cycle only) and both release their hydrogen, as protons and electrons, to the electron transport chain on the inner mitochondrial membrane.

The key difference is where each coenzyme hands over its electrons:

  • Electrons from reduced NAD enter the chain at an early carrier, so they pass through more of the proton-pumping sites along the chain.
  • Electrons from reduced FAD enter the chain at a later carrier, bypassing the first proton-pumping site used by reduced NAD.

Because the amount of ATP synthesised depends on how many protons are pumped into the intermembrane space (which in turn depends on how much of the chain the electrons pass through), electrons donated by reduced FAD result in fewer protons being pumped and, ultimately, less ATP being synthesised by chemiosmosis than electrons donated by reduced NAD.

Step 2: Evaluate each option

  • A: correct. Reduced FAD enters the electron transport chain later than reduced NAD, bypassing an earlier proton-pumping site, so fewer protons are pumped and less ATP is made.
  • B: incorrect, reduced FAD and reduced NAD both carry the same number of hydrogen atoms; the ATP yield difference is about where electrons enter the chain, not how many hydrogens are carried.
  • C: incorrect. Reduced FAD is not produced during glycolysis; it is produced in the Krebs cycle, in the mitochondrial matrix, and its electrons do pass through the electron transport chain.
  • D: incorrect. Reduced FAD’s electrons do pass through part of the electron transport chain (and do still cause some protons to be pumped); it does not deliver electrons directly to oxygen.

Final answer

  • Reduced FAD enters the electron transport chain at a later point than reduced NAD, so fewer protons are pumped and less ATP is synthesised, option A.