Electrochemical gradient



In cellular biology, an electrochemical gradient refers to the electrical and chemical properties across a membrane. These are often due to ion gradients, particularly proton gradients, and can represent a type of potential energy available for work in a cell. This can be calculated as a membrane potential.

Overview

Electrochemical potential is important in conserved.

In biological processes the direction an ion will move 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.

Electrochemical gradients are analogous to hydroelectric dams and equivalent to the water pressure across the dam. Membrane sodium-potassium pump within the membrane are equivalent to turbines that convert the waters potential energy to other forms of physical or chemical energy, and the ions that pass through the membrane are equivalent to water that is now found 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.

Chemistry

The term is typically applied in contexts where a chemical reaction is to take place, such as one involving the transfer of an electron at a 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. More generally, however, it is used to characterize the inclined 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 compartments, the inclined 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 to which the solute's net flow across the membrane is also zero. This potential is predicted theoretically either by the reference electrodes.

Transmembrane ATPases or transmembrane proteins with ATPase domains are often used for making and utilizing ion gradients. The enzyme Na+/K+ ATPase use 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 are common to actively move ions across biological membranes.

Proton gradients

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

The electrochemical potential difference between the two sides of the membrane in pH.

Proton Motive Force: two protons are expelled at each coupling site, generating the Proton Motive Force. ATP is made indirectly using the PMF as a source of energy. Each pair of protons yields one ATP.

Some archaea, most notably halobacteria, make proton gradients by pumping in protons from the environment with the help of the solar driven 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; this is 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 - which they use 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 mitochondria where ATP synthase is located in the inner mitochondrial membrane, so that F1-part sticks into mitochondrial matrix, where ATP synthesis takes place.

References

  • Campbell, Reece (2005). Biology. Pearson Benjamin Cummings. ISBN 0-8053-7146-X. 
  • [1]

See also
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Electrochemical_gradient". A list of authors is available in Wikipedia.