describe the role of NAD and FAD in transferring hydrogen to carriers in the inner mitochondrial membrane

Respiration – Role of NAD⁺ and FAD in Transferring Hydrogen to Carriers in the Inner Mitochondrial Membrane

Learning Objectives

• Explain the redox properties of NAD⁺ and FAD.
• Describe how NADH and FADH₂ donate electrons to the electron transport chain (ETC).
• Identify the specific protein complexes that accept electrons from NADH and FADH₂.
• Relate the flow of electrons to proton pumping and ATP synthesis.

Key Concepts

NAD⁺ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) are

essential co‑enzymes that act as electron carriers in cellular respiration. They accept

electrons and a proton (hydrogen) during catabolic reactions, becoming reduced to

NADH and FADH₂. The reduced forms then transport these high‑energy electrons to

the inner mitochondrial membrane where the electron transport chain resides.

Redox Reactions Involving NAD⁺ and FAD

The general reduction reactions are:

\$\text{NAD}^+ + 2e^- + H^+ \rightarrow \text{NADH}\$

\$\text{FAD} + 2e^- + 2H^+ \rightarrow \text{FADH}_2\$

In each case, two electrons and one (for NAD) or two (for FAD) protons are transferred,

storing the energy of the electrons in the reduced co‑enzyme.

Entry of Electrons into the Electron Transport Chain

The inner mitochondrial membrane contains four major protein complexes (Complex I–IV) and

mobile carriers (ubiquinone and cytochrome c). NADH and FADH₂ donate their

electrons at different points:

• Complex I (NADH‑ubiquinone oxidoreductase): Accepts electrons from NADH.
• Complex II (Succinate‑ubiquinone oxidoreductase): Accepts electrons from FADH₂ (produced by succinate dehydrogenase in the TCA cycle).

Summary Table of Electron Entry and Proton Pumping

Mechanism of Proton Translocation

As electrons move through Complex I, III, and IV, the energy released is used to pump

protons from the mitochondrial matrix into the intermembrane space. This creates an

electrochemical gradient (proton‑motive force) that drives ATP synthesis via

ATP synthase (Complex V):

\$\text{ADP} + Pi + 4H^+{out} \rightarrow \text{ATP} + H2O + 3H^+{in}\$

Overall Contribution to Cellular Respiration

The combined action of NADH and FADH₂ provides the majority of the ATP generated

during aerobic respiration. Roughly 90 % of the ATP yield comes from oxidative

phosphorylation driven by the electron transport chain.

Key Points to Remember

1. NAD⁺ and FAD act as reversible electron carriers, becoming NADH and FADH₂ after accepting electrons and protons.
2. NADH donates electrons to Complex I, which pumps the most protons per electron pair.
3. FADH₂ enters at Complex II and bypasses the first proton‑pumping step, yielding fewer ATP.
4. The proton gradient generated by electron flow powers ATP synthase to produce ATP.
5. Understanding the distinct entry points explains why NADH yields \overline{2}.5 ATP while FADH₂ yields \overline{1}.5 ATP per molecule.