Fill in the blanks about each of the following sequential steps of the electron transfer chain (ETC) of the inner mitochondrial membrane.

a. The initial electron donor of the ETC is _______, which hands its two electrons off to FMN, within Complex I.
b. Electrons from both Complex I and Complex II are ultimately donated to the same molecule, .
c. Within Complex III, electrons from ubiquinol are ultimately donated to , which then passes those electrons on to Complex IV.
d. is the final electron acceptor of the ETC.
e. The downhill movement of electrons through the ETC creates a proton gradient across the inner mitochondrial membrane. The _ subunit of the ATP synthase creates a channel that allows for the downhill movement of protons from the intermembrane space to the matrix.
f. The concomitant release of free energy drives the synthesis of ATP by the ____ subunit of that same protein.

Electrons enter the Electron Transport Chain (ETC) from Nicotinamide Adenine Dinucleotide (NADH), which is an electron donor generated during different catabolic reactions (e.g., glucose oxidation). Ubiquinone or 'Coenzyme Q' is an electron acceptor that accepts electrons from both Complex I and Complex II and donates electrons to Complex III. The oxygen (O) is the final electron acceptor that accepts the electrons after they have passed through the ETC and ATPase. Complex III (also known as Ubiquinol-cytochrome c oxidoreductase) catalyzes the electron transfer from ubiquinol to Cytochrome c, which is coupled to proton translocation across the membrane. The F0 subunit of the ATP synthase forms a channel (embedded in the mitochondrial membrane) through which protons can flow. Moreover, the F1 subunit of the ATP synthase uses the energy released by the proton electrochemical gradient to drive the synthesis of ATP.