Question

Compare the number of protons pumped when electrons enter through Complex I versus Complex II. In each case, which activated electron carrier transfers its electrons to the respective complexes?

a. Complex I pumps more protons, NADH to Complex I, FADH2 to Complex II
b. Complex II pumps more protons, NADH to Complex I, FADH2 to Complex II
c. Complex I pumps more protons, FADH2 to Complex I, NADH to Complex II
d. Complex II pumps more protons, FADH2 to Complex I, NADH to Complex II

Answer 1

The correct answer is that Complex I pumps more protons compared to Complex II, receiving electrons from NADH, while Complex II receives electrons from FADH2 and does not pump protons.

Step-by-step explanation:

Electron Transport Chain and Proton Pumping

The student inquires about the electron transport chain (ETC) and the different outcomes when electrons are transferred via NADH and FADH2. Complex I receives electrons from NADH and is capable of pumping more protons across the inner mitochondrial membrane, thus contributing to a larger proton gradient and, subsequently, more ATP generation. In contrast, Complex II, which receives electrons from FADH2, does not pump protons, resulting in less ATP generation as compared to the transfer of electrons through Complex I. Specifically, NADH is the activated electron carrier that transfers its electrons to Complex I, whereas FADH2 transfers its electrons to Complex II. Electrons from NADH and FADH2 then both pass to Coenzyme Q (ubiquinone) and subsequently to Complex III and IV where protons are pumped to generate the gradient utilized for ATP synthesis.

Therefore, the correct answer to the student's question is a. Complex I pumps more protons, NADH to Complex I, FADH2 to Complex II.