The electric potential
measured from within the cell
relative to the potential of the extracellular
fluid. The potential of the outside is zero by convention and the membrane potential is the potential difference between the outside and the inside.
The membrane potential is used for many different processes in cells. Here are a few examples:
- action potential: A nerve impulse, or action potential, is propagated along a nerve by depolarization of the membrane. The membrane potential drives ions across the nerve membrane, triggering channels in nearby regions to depolarize. This process moves down the nerve like a domino.
- energy: A proton gradient across a membrane (more protons on one side than the other) creates a potential which can be used to generate energy. This is how the photosynthetic machinery is coupled to ATP generation.
- transport: bacteria and other cells may use a membrane transporter protein to couple transport of ions with food. For example, if a cell has an excess store of sodium ion inside and it wishes to metabolize sugar for energy, it may use the membrane potential to release a sodium ion, but in the process, the energy gained is used to pull in a sugar.