During which stage of aerobic glucose catabolism is FAD reduced to FADH2?

Introduction / Context:
NAD+ and FAD are key oxidized cofactors that capture electrons during catabolism. Knowing where each is reduced helps map energy flow into the electron transport chain.

Given Data / Assumptions:

• NADH is produced in multiple steps (glycolysis, PDH, Krebs).
• FADH2 is more restricted in its points of formation.
• Aerobic conditions with intact mitochondrial function.

Concept / Approach:
Within the Krebs cycle, succinate dehydrogenase catalyzes succinate → fumarate while reducing FAD to FADH2. This enzyme is also Complex II of the ETC, directly funneling electrons from FADH2 into the chain.

Step-by-Step Solution:
Identify the reaction: succinate + FAD → fumarate + FADH2.Locate it within metabolism: Krebs cycle in the mitochondrial matrix/inner membrane interface.Conclude that FADH2 formation occurs during the Krebs cycle.

Verification / Alternative check:
Biochemical maps consistently show FADH2 generated only at the succinate dehydrogenase step for glucose oxidation.

Why Other Options Are Wrong:

• Electron transport: consumes NADH/FADH2, does not form them.
• Fermentation/Glycolysis: employ NAD+/NADH, not FAD/FADH2.

Common Pitfalls:
Assuming all dehydrogenations use NAD+; FAD is used when the reaction chemistry requires a stronger oxidant or involves C–C to C=C conversions.

Final Answer:
Krebs cycle (citric acid cycle)