Final answer:
Proton gradients are generated by complexes I, III, and IV through pumping protons from the mitochondrial matrix to the intermembrane space. Complex II does not pump protons, and neither does coenzyme Q, which serves as an electron shuttle.
Step-by-step explanation:
The generation of proton gradients across the inner mitochondrial membrane is a crucial step in the process of oxidative phosphorylation, and it is facilitated by specific complexes within the electron transport chain. Proton gradients are generated by complex I, complex III, and complex IV.
These complexes use the energy released from electron transport to pump protons (H+) from the mitochondrial matrix to the intermembrane space. Complex I is the first of these, using flavin mononucleotide (FMN) and iron-sulfur (Fe-S) clusters to transport electrons and establish the proton gradient.
Complex II, on the other hand, does not contribute to the proton gradient despite being part of the electron transport chain. It transports electrons but does not pump protons from the matrix to the intermembrane space. Coenzyme Q (ubiquinone) also does not directly pump protons; rather, it acts as a hydrophobic electron carrier, shuttling electrons between complexes within the inner membrane.
Therefore, the correct answer is that proton gradients are generated by all of the following complexes except complex II (B) and coenzyme Q (E).