Electrochemical gradient

An electrochemical gradient is a spatial variation of both electrical potential and chemical concentration across a membrane; that is, a combination of the membrane potential (delta-psi) and the pH gradient (delta-pH). Both components are often due to ion gradients, particularly proton gradients (differences in the concentrations of hydronium ions), and the result can be a type of potential energy available for work in a cell. This can be calculated as a thermodynamic measure, termed electrochemical potential, that combines the concepts of energy stored in the form of chemical potential, which accounts for an ion's concentration gradient across a cellular membrane, and electrostatics, which accounts for an ion's tendency to move relative to the membrane potential.

Overview

Electrochemical potential is important in electroanalytical chemistry and industrial applications such as batteries and fuel cells. It represents one of the many interchangeable forms of potential energy through which energy may be conserved.

In biological processes, the direction of 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. Also, energy can be used to pump water up into the lake above the dam. In similar manner, chemical energy in cells can be used to create electrochemical gradients.

Chemistry

The term is typically applied in contexts wherein a chemical reaction is to take place, such as one involving the transfer of an electron at a battery electrode. In a battery, an electrochemical potential arising from the movement of ions balances the reaction energy of the electrodes. The maximum voltage that a battery reaction can produce is sometimes called the standard electrochemical potential of that reaction (see also Electrode potential and Table of standard electrode potentials). In instances pertaining specifically to the movement of electrically charged solutes, the potential is often expressed in units of volts. See: Concentration cell.

Biological context

In biology, the term is sometimes used in the context of a chemical reaction, in particular to describe the energy source for the chemical synthesis of ATP. In more general terms, however, it is used to characterize the tendency of solutes to simply diffuse across a membrane, a process involving no chemical transformation.

Ion gradients

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.

Proton gradients

The proton gradient can be used as intermediate energy storage for heat production and flagellar rotation. In addition, it is an interconvertible form of energy in active transport, electron potential generation, NADPH synthesis, and ATP synthesis/hydrolysis.

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.

Proton motive force

Two protons are expelled at each coupling site, generating the proton motive force (PMF). ATP is made indirectly using the PMF as a source of energy. Each pair of protons yields one ATP.

Some archaea, the most notable ones being 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 F₁F_O 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, in general, 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.

See also

• Concentration cell
• Transmembrane potential difference
• Action potential
• Cell potential
• Electrodiffusion
• Galvanic cell
• Electrochemical cell

References

• Campbell & Reece (2005). Biology. Pearson Benjamin Cummings. ISBN 0-8053-7146-X. 
• Stephen T. Abedon, "Important words and concepts from Chapter 8, Campbell & Reece, 2002 (1/14/2005)", for Biology 113 at the Ohio State University