Cells need a safe way to move energy from food molecules to the machinery that makes ATP. NAD+ and FAD are electron carriers that act like small molecular shuttles during cellular respiration. They pick up high-energy electrons during glucose breakdown and deliver them to the mitochondrial electron transport chain.
This transfer of electrons is central to how cells capture usable energy instead of losing it as heat.
NAD+ is reduced to NADH when it gains electrons, while FAD is reduced to FADH2 when it gains electrons and hydrogen atoms. Later, NADH and FADH2 are oxidized when they donate electrons to the electron transport chain in the inner mitochondrial membrane. The movement of electrons through the chain powers proton pumping, which creates a proton gradient.
ATP synthase uses that gradient to make ATP from ADP and phosphate.
Key Facts
- NAD+ + 2 e- + H+ = NADH
- FAD + 2 e- + 2 H+ = FADH2
- Oxidation is loss of electrons, and reduction is gain of electrons.
- NADH donates electrons to Complex I of the electron transport chain.
- FADH2 donates electrons to Complex II of the electron transport chain.
- A glucose molecule can produce 10 NADH and 2 FADH2 during cellular respiration before oxidative phosphorylation.
Vocabulary
- NAD+
- NAD+ is an electron carrier that accepts high-energy electrons and becomes NADH during cellular respiration.
- FAD
- FAD is an electron carrier that accepts electrons and hydrogen atoms to become FADH2.
- Reduction
- Reduction is the gain of electrons by an atom, ion, or molecule.
- Oxidation
- Oxidation is the loss of electrons by an atom, ion, or molecule.
- Electron Transport Chain
- The electron transport chain is a series of protein complexes that pass electrons and use their energy to pump protons across the inner mitochondrial membrane.
Common Mistakes to Avoid
- Saying NAD+ stores ATP directly is wrong because NAD+ stores energy temporarily as high-energy electrons in NADH, not as phosphate bonds.
- Mixing up oxidation and reduction is wrong because NAD+ is reduced when it gains electrons, while NADH is oxidized when it loses electrons.
- Assuming NADH and FADH2 enter the electron transport chain at the same point is wrong because NADH donates to Complex I and FADH2 donates to Complex II.
- Forgetting that oxygen is the final electron acceptor is wrong because without oxygen, electrons cannot flow normally through the chain and ATP production drops sharply.
Practice Questions
- 1 If one glucose molecule produces 10 NADH before oxidative phosphorylation, how many electrons can those NADH molecules deliver in total if each NADH carries 2 electrons?
- 2 If 3 FAD molecules are reduced to FADH2, how many electrons and how many hydrogen ions are accepted in total?
- 3 Explain why NAD+ must be regenerated from NADH for glycolysis and the citric acid cycle to continue.