Which molecules donate electrons to the electron transport chain?

Which molecules donate electrons to the electron transport chain?

• A. NADH and FADH2
• B. NADPH only
• C. FAD only
• D. ATP

Answer: (D)

Electrons are donated to the electron transport chain by reduced electron carriers that arise from fuel breakdown: NADH and FADH2. NADH transfers electrons to Complex I, while FADH2 donates electrons to Complex II; from there, electrons move through the chain via ubiquinone and cytochromes, pumping protons to create a gradient. That proton-motive force powers ATP synthase to make ATP from ADP and Pi. ATP itself isn’t a donor of electrons, it’s the product of the process. NADPH isn’t used to feed the mitochondrial ETC in typical respiration, and relying on ATP or FAD alone misses the essential NADH contributor (and, in the case of FADH2, its role as a separate donor).

Electrons are donated to the electron transport chain by reduced electron carriers that arise from fuel breakdown: NADH and FADH2. NADH transfers electrons to Complex I, while FADH2 donates electrons to Complex II; from there, electrons move through the chain via ubiquinone and cytochromes, pumping protons to create a gradient. That proton-motive force powers ATP synthase to make ATP from ADP and Pi. ATP itself isn’t a donor of electrons, it’s the product of the process. NADPH isn’t used to feed the mitochondrial ETC in typical respiration, and relying on ATP or FAD alone misses the essential NADH contributor (and, in the case of FADH2, its role as a separate donor).