Question

The electrons of FADH₂ are transferred through three complexes of the electron transport chain, whereas those of NADH are transferred through all four complexes. Why does this difference occur?

a) FADH₂ has fewer electrons than NADH
b) FADH₂ enters the electron transport chain at a later stage
c) FADH₂ transfers electrons directly to the fourth complex
d) NADH transfers electrons to an additional complex not used by FADH₂

Answer 1

Electrons from FADH₂ enter the ETC at Complex II, missing the Complex I, which results in a lower ATP yield compared to NADH, which transfers electrons starting from Complex I through to Complex IV.

Step-by-step explanation:

The difference in how FADH₂ and NADH electrons are processed through the electron transport chain (ETC) is a consequence of their respective points of entry. Electrons delivered by NADH enter through Complex I, subsequently being transferred through Complex III and Complex IV, whereas FADH₂ electrons enter the ETC at Complex II, skipping Complex I and then moving to Complexes III and IV.

Complex II directly accepts electrons from the oxidation of FADH₂, using the enzyme succinate dehydrogenase from the citric acid cycle. This process does not involve Complex I and as a result, electrons from FADH₂ do not contribute to the proton gradient establishment at that stage. Hence, the energy released by FADH₂ through the ETC is less compared to NADH, resulting in the production of fewer ATP molecules.

To summarize, the entry of FADH₂ at Complex II means it bypasses the proton pump of Complex I, which directly affects its lower ATP yield in comparison to NADH.