12.2.7—NAD and FAD in respiration

Oxidative phosphorylation occurs at the inner mitochondrial membrane. Reduced NAD and FAD donate hydrogen/electrons to membrane electron carriers; the released energy builds a proton gradient, and proton flow through ATP synthase drives ATP formation. Oxygen accepts electrons at the end and water is formed.

1. Donate reducing power: Reduced NAD and reduced FAD deliver hydrogen/electrons to electron carriers in the inner mitochondrial membrane. The hydrogen separates into protons and energetic electrons.
2. Transfer electrons: Electrons pass through a series of membrane carriers. Their energy is released in steps rather than all at once.
3. Build the gradient: The released energy drives protons from the matrix into the intermembrane space. The inner membrane restricts proton movement, so an electrochemical gradient is established.
4. Couple proton flow to ATP: Protons return down the gradient through the channel protein ATP synthase. This facilitated movement supplies the energy for ADP and inorganic phosphate to form ATP.
5. Complete the chain: Oxygen accepts electrons and protons at the end of the chain, forming water. This final acceptance allows electron transfer and coenzyme reoxidation to continue under aerobic conditions.

Electron flow → gradient → ATP: reduced coenzymes provide electrons; electron-transfer energy pumps protons; the separated protons store potential energy; ATP synthase couples their return to ATP synthesis. Oxygen is the terminal electron acceptor, not the protein that directly makes ATP.

This card explains the oxidative-phosphorylation chain. It does not give a fixed ATP yield, inhibitor examples or detailed carrier names. Do not confuse electron movement with proton movement: electrons release the energy for pumping, while protons return through ATP synthase.