What role do NADH and FADH2 play in cellular respiration?

• A. They serve as substrates for glycolysis
• B. They donate electrons to the electron transport chain
• C. They act as energy storage molecules
• D. They replace ATP during energy deficit

Answer: (B)

NADH and FADH2 are crucial players in cellular respiration, particularly in the process known as oxidative phosphorylation, which occurs in the electron transport chain (ETC). Their primary role is to serve as electron carriers. During earlier stages of cellular respiration, such as glycolysis and the citric acid cycle (Krebs cycle), NAD+ and FAD are reduced to NADH and FADH2, respectively, as they accept electrons and protons.

Once formed, NADH and FADH2 transport these high-energy electrons to the electron transport chain located in the inner mitochondrial membrane. Here, they donate the electrons, which initiate a series of redox reactions. As electrons pass through the complexes in the ETC, they lead to the pumping of protons across the mitochondrial membrane, creating a proton gradient. This gradient is fundamental for synthesizing ATP through chemiosmosis as protons flow back into the mitochondrial matrix via ATP synthase.

This electron donation is critical for generating a proton motive force that ultimately drives ATP production, emphasizing why the correct answer highlights their role in donating electrons to the electron transport chain.