Which statement about Complex II in the mitochondrial electron transport chain is true?

Which statement about Complex II in the mitochondrial electron transport chain is true?

• A. It catalyzes the oxidation of succinate to fumarate with transfer of electrons to FAD, forming FADH2.
• B. It reduces NAD+ to NADH.
• C. It pumps protons across the membrane.
• D. It donates electrons directly to cytochrome c.

Answer: (A)

The key idea is how Complex II (succinate dehydrogenase) handles electrons and what role it plays in the chain. It oxidizes succinate to fumarate and uses FAD as a bound cofactor, forming FADH2 in the process. The electrons from FADH2 are then transferred through iron-sulfur clusters in the enzyme to ubiquinone, reducing it to ubiquinol, which enters the electron transport chain. This is the only part of Complex II’s role: it accepts electrons from succinate and passes them to ubiquinone, without moving protons across the membrane. Because of this, it does not pump protons. Complex II also does not reduce NAD+—that reaction is carried out elsewhere (notably by Complex I). Finally, the electrons do not go directly to cytochrome c; they reach cytochrome c only after being handed off to ubiquinone and then to Complex III. So the statement that correctly describes Complex II is that it oxidizes succinate to fumarate with transfer of electrons to FAD, forming FADH2.

The key idea is how Complex II (succinate dehydrogenase) handles electrons and what role it plays in the chain. It oxidizes succinate to fumarate and uses FAD as a bound cofactor, forming FADH2 in the process. The electrons from FADH2 are then transferred through iron-sulfur clusters in the enzyme to ubiquinone, reducing it to ubiquinol, which enters the electron transport chain. This is the only part of Complex II’s role: it accepts electrons from succinate and passes them to ubiquinone, without moving protons across the membrane. Because of this, it does not pump protons. Complex II also does not reduce NAD+—that reaction is carried out elsewhere (notably by Complex I). Finally, the electrons do not go directly to cytochrome c; they reach cytochrome c only after being handed off to ubiquinone and then to Complex III. So the statement that correctly describes Complex II is that it oxidizes succinate to fumarate with transfer of electrons to FAD, forming FADH2.