In biological processes, the direction an ion moves by diffusion or active transport across a membrane is determined by the electrochemical gradient. In mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is also known as a proton motive force. This potential energy is used for the synthesis of ATP by oxidative phosphorylation.
With respect to a cell, organelle, or other subcellular compartment, the tendency of an electrically charged solute, such as a potassium ion, to move across the membrane is decided by the difference in its electrochemical potential on either side of the membrane, which arises from three factors:
- the difference in the concentration of the solute between the two sides of the membrane
- the charge or "valence" of the solute molecule
- the difference in voltage between the two sides of the membrane (i.e. the transmembrane potential).
A solute's electrochemical potential difference is zero at its "reversal potential", the transmembrane voltage at which the solute's net flow across the membrane is also zero. This potential is predicted, in theory, either by the Nernst equation (for systems of one permeant ion species) or by the Goldman-Hodgkin-Katz equation (for more than one permeant ion species). Electrochemical potential is measured in the laboratory and field using reference electrodes.
Transmembrane ATPases or transmembrane proteins with ATPase domains are often used for making and utilizing ion gradients. The enzyme Na+/K+ ATPase uses ATP to make a sodium ion gradient and a potassium ion gradient. The electrochemical potential is used as energy storage. Chemiosmotic coupling is one of several ways a thermodynamically unfavorable reaction can be driven by a thermodynamically favorable one. Cotransport of ions by symporters and antiporter carriers is commonly used to actively move ions across biological membranes.
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