Question

NADH must be constantly oxidized in the cell (lose electrons). This results in the NADH becoming NAD. Explain why NADH must be “oxidized” and tell at what step this happens in fermentation and in respiration.

Answer 1

NADH must be oxidized for the glycolysis to continue, if oxygen is absent, Pyruvate formed by glycolysis is converted to lactate, or ethanol depending upon the organism.

pyruvate + NADH NAD+ + lactate

( this reaction is catalyzed by lactate hydrogenase)

pyruvate + NADH NAD+ + ethanol+ CO2

( this reaction is catalyzed by alcohol dehydrogenase).

so NAD+ can be used in glycolysis to produce ATP.

In aerobic respiration NADH formed during glycolysis, pyruvate oxidation and TCA cycle is oxidized during electron transport chain, NADH is oxidized by complex I and transfers the electrons to coenzyme Q which carries electrons to complex III, from there electrons are carried by cytochrome c to complex IV from where electrons are transferred to oxygen and it is reduced to water.