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.
An electrochemical gradient has two components. First, the electrical component is caused by a charge difference across the lipid membrane. Second, a chemical component is caused by a differential concentration of ions across the membrane. The combination of these two factors determines the thermodynamically favourable direction for an ion's movement across a membrane.
An electrochemical gradient is analogous to the water pressure across a hydroelectric dam. Membrane transport proteins such as the sodium-potassium pump within the membrane are equivalent to turbines that convert the water's potential energy to other forms of physical or chemical energy, and the ions that pass through the membrane are equivalent to water that ends up at the bottom of the dam. Alternatively, energy can be used to pump water up into the lake above the dam. Similarly, chemical energy in cells can be used to create electrochemical gradients.
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 theoretically either by the Nernst equation (for systems of one permeant ion species) or 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.
The electrochemical potential difference between the two sides of the membrane in mitochondria, chloroplasts, bacteria, and other membranous compartments that engage in active transport involving proton pumps, is at times called a chemiosmotic potential or proton motive force (see chemiosmosis). In this context, protons are often considered separately using units of either concentration or pH.
Some archaea, most notably halobacteria, make proton gradients by pumping in protons from the environment with the help of the solar-driven enzyme bacteriorhodopsin, which is used here for driving the molecular motor enzyme ATP synthase to make the necessary conformational changes required to synthesize ATP.
Proton gradients are also made by bacteria by running ATP synthase in reverse, and are used to drive flagella.
The F1FO ATP synthase is a reversible enzyme. Large enough quantities of ATP cause it to create a transmembrane proton gradient. This is used by fermenting bacteria - which do not have an electron transport chain, and hydrolyze ATP to make a proton gradient - for flagella and the transportation of nutrients into the cell.
In respiring bacteria under physiological conditions, ATP synthase generally runs in the opposite direction creating ATP while using the proton motive force created by the electron transport chain as a source of energy. The overall process of creating energy in this fashion is termed oxidative phosphorylation. The same process takes place in mitochondria where ATP synthase is located in the inner mitochondrial membrane, so that F1 part sticks into the mitochondrial matrix where ATP synthesis takes place.