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Separation of oppositely charged ions by a membrane particularly as generated by actively transporting one or more such ions.
Such gradients are generated via the action of sodium-potassium pumps, proton pumps, and, also pumping protons, electron transport chains. In each of these cases an excess of positive charge accumulates on one side of the cell membrane, creating what is known as a membrane potential or, when generated by proton pumping, a proton motive force.
Electrochemical gradients are essentially batteries, with a voltage created across the membrane. The ions involved are held back because they are unable to pass through a lipid bilayer without facilitation. These ions, however, "want" very strongly to move not only down their concentration gradient but also towards the opposite (negative) charge found on the other side of the membrane. The potential energy associated with this "desire" can be tapped by proteins that allow ion movement across the membrane, converting their potential energy to kinetic energy and then tapping that kinetic energy to do work.
A crucial means by which the energy in electrochemical gradients is tapped is either by ATP synthases, which produces ATP based on proton electrochemical gradients (proton motive force), or by transport proteins that pump other substances across membranes. The latter process, called cotransport, can use any excess of ions to drive what is also termed secondary active transport since ATP is not directly driving the active transport of these other substances.
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