| Chemiosmotic Potential |
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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 an ion will move by Diffusion or Active Transport across membrane is determined by the electrochemical gradient. In Mitochondria and Chloroplast s, 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 . In generic terms, Electrochemical Potential is the Mechanical Work done in bringing 1 mole of an ion from a standard state to a specified Concentration and Electrical Potential . GENERAL OVERVIEW 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 Transport Protein s such as the 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 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 Volt s. BIOLOGICAL CONTEXT In biology too, 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:
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 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 Electrode s. 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 Symporter s 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, NADPH synthesis, and ATP synthesis/hydrolysis. The electrochemical potential difference between the two sides of the membrane in Mitochondria , Chloroplast s, Bacteria and other membranous compartments that engage in Active Transport involving Proton Pump s, is at times called a '''chemiosmotic potential''' or '''proton motive force''' (see Chemiosmotic Hypothesis ). In this context, Proton s are often considered separately using units either of 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 , here it is used 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; this is used to drive flagellas. 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 . The same process takes place in 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 SEE ALSO
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