∞ generated and posted on 2016.12.13 ∞
Hydrogen ionconcentration gradient across a membrane.
| Protons are hydrogen ions and a Proton Motive Force (or Proton-Motive Force) is a hydrogen ion electrochemical gradient across a lipid bilayer, particular as found across membranes of mitochondria and chloroplasts. |
A proton motive force is established via the action of either electron transport systems or instead membrane transport proteins that are known as proton pumps.
A proton motive force can be tapped by protein complexes known as ATP synthases, which are reverse-running proton pumps that allow protons to cross membranes but in a manner that the drive of these protons to move from regions of high proton concentration to regions of low proton concentration (i.e., down their concentration gradient) captured to phosphorylate ADP, producing ATP.
One sees the tapping of proton motive forces particularly in both cellular respiration and photosynthesis, where they are used to generate ATP.
Figure legend: The line in the middle represents a semipermeable membrane which does not permit movement of the protons (H+) across, i.e., as equivalent to a lipid bilayer. Proton concentrations are much greater to the right than they are to the left. This represents a concentration that can 'motivate' a net movement across the membrane – if that becomes possible – one going from the right to the left (large arrow). Since this is net movement, there is still movement that occurs going from left to the right (smaller, lighter arrow), just not as much. In addition to the concentration gradient, there is a proton-associated positive charge to the right of the membrane and a proton-dearth-associated negative charge to the left. The result is that protons quite strongly 'want' to cross the membrane from the right to the left, both to even out concentrations and to even out charges.
The following video discusses the above figure:
The overall process of ATP generation via the harnessing of a proton motive force is called chemiosmosis.