What Is a Proton Motive Force?
Cells in your body harvest energy from food molecules through a process called cellular respiration. Part of this process involves pumping protons across a membrane to create a proton-motive force that can drive synthesis of another type of molecule.
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The last stage of cellular respiration takes place inside small membrane-enclosed sacs called mitochondria. Proteins embedded in the inner membrane form part of an electron transport chain; molecules called NADH and FADH2 donate electrons to the chain, and each protein in the chain passes electrons to the next protein in the chain, using the energy released to pump a hydrogen ion across the membrane. Since atoms of hydrogen have only one proton, a hydrogen ion is essentially just a lone proton, although hydrogen ions in water actually attach to water molecules to form hydronium ions (H3O+) rather than remaining independent.
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Molecules and ions in solution diffuse or spread from areas of high concentration to areas of low concentration. To see this principle in action, try adding a drop of food coloring to a glass of water; the color will slowly spread throughout the glass. By pumping protons (hydrogen ions) across the inner membrane, the electron transport chain creates a concentration gradient, where positively charged hydrogen ions are more concentrated outside the membrane than they are inside. The ions "want" to diffuse back across but cannot cross the membrane. This difference in concentration is called the proton motive force.
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Function
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The mitochondria use the proton-motive force to synthesize a molecule called adenosine triphosphate or ATP that your cells can use to power other processes. A protein called ATP synthase is embedded in the inner membrane of the mitochondria. As hydrogen ions build up outside the inner membrane, they start to flow through a conduit provided by ATP synthase.
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References
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