Final answer:
NADH donates electrons to Complex I, which are then transferred to ubiquinone (Q) before reaching Complex III. Meanwhile, FADH₂ donates electrons directly to Complex II, and these electrons are also passed to ubiquinone (Q) before moving to Complex III. Option a and b is correct.
Step-by-step explanation:
NADH and FADH₂ give up electrons at different points in the electron transport chain. NADH donates its electrons to Complex I, which is also known as NADH-coenzyme Q oxidoreductase or NADH dehydrogenase. The electrons are initially received by flavin mononucleotide (FMN) and then transferred through a series of iron-sulfur clusters to ubiquinone (Q), which then carries the electrons to Complex III, also known as cytochrome bc1 complex.
In contrast, FADH₂ donates electrons to Complex II, also known as succinate dehydrogenase. Complex II directly receives FADH₂ electrons, which are passed to ubiquinone (Q) without passing through Complex I. After ubiquinone is reduced to QH₂, it transports the electrons to Complex III. Since FADH₂ bypasses Complex I, fewer ATP molecules are produced from its electrons compared to NADH.
Therefore, the correct answers are: a. Complex I; Ubiquinone (CoQ) for NADH and b. Complex II; Ubiquinone (CoQ) for FADH₂.