During glycolysis, what happens to the NADH produced in the cytosol if oxygen is available?

• A. It is converted to NAD+ locally with no transport.
• B. It is degraded to CO2 and water.
• C. It is used to directly synthesize ATP in glycolysis.
• D. It is shuttled into mitochondria and oxidized in the electron transport chain.

Answer: (D)

When oxygen is present, the NADH produced in glycolysis must be reoxidized to NAD+ so glycolysis can continue. NADH itself cannot cross the inner mitochondrial membrane directly, so reducing equivalents are moved into the mitochondria by shuttle systems such as the malate-aspartate or glycerol-3-phosphate shuttles. In the mitochondria, those electrons enter the electron transport chain and are ultimately donated to oxygen, fueling oxidative phosphorylation and ATP production. This means the NADH is effectively shuttled into mitochondria and oxidized in the electron transport chain, regenerating cytosolic NAD+ and linking glycolysis to respiration. The other ideas don’t fit because NADH isn’t recycled locally without transport, isn’t used directly to make ATP in glycolysis, and isn’t simply degraded to CO2 and water.