Electrochemical potential gradient

If a diffusion potential occurs inside the membrane, the relation between mass transport and electrochemical potential gradient — as the driving force for the diffusion of ions — has to be examined in more detail. This can be done by three different approaches ... 

The proportionality constant between the current and the electrochemical potential gradient is controlled by the partial electrical conductivity [Pg.546]

In all these systems, the energy source is an electrochemical potential gradient and transport occurs in the direction —grad jU, (i.e. in the direction of decreasing electrochemical potential). It is often stated in the literature that this spontaneous type of transport occurs in the direction of the electrochemical potential gradient this is an imprecise formulation. 

Active transport. The definition of active transport has been a subject of discussion for a number of years. Here, active transport is defined as a membrane transport process with a source of energy other than the electrochemical potential gradient of the transported substance. This source of energy can be either a metabolic reaction (primary active transport) or an electrochemical potential gradient of a substance different from that which is actively transported (secondary active transport). 

The powerful biological machinery of energy conversion proceeds via redox reactions in aqueous media that involve electron and proton transfer between molecular entities. - Nature devised concerted sequences of these processes that generate electrochemical potential gradients across cell membranes and thereby enable the storage and the release of electrical energy. 

If the effect of external mechanical forces on the system is negligible (i.e. if convection does not occur), the only driving force for a transport process is the electrochemical potential gradient. For a change in a system occurring only along the x-coordinate,... 

Rottenberg, H. (1998) The generation of proton electrochemical potential gradient by cytochrome c oxidase. Biochim. Biophys. Acta 1364, 1-16. 

Solid state reactions occur mainly by diffusional transport. This transport and other kinetic processes in crystals are always regulated by crystal imperfections. Reaction partners in the crystal are its structure elements (SE) as defined in the list of symbols (see also [W. Schottky (1958)]). Structure elements do not exist outside the crystal lattice and are therefore not independent components of the crystal in a thermodynamic sense. In the framework of linear irreversible thermodynamics, the chemical (electrochemical) potential gradients of the independent components of a non-equilibrium (reacting) system are the driving forces for fluxes and reactions. However, the flux of one independent chemical component always consists of the fluxes of more than one SE in the crystal. In addition, local reactions between SE s may occur. 

In heterogeneous solid state reactions, the phase boundaries move under the action of chemical (electrochemical) potential gradients. If the Gibbs energy of reaction is dissipated mainly at the interface, the reaction is named an interface controlled chemical reaction. Sometimes a thermodynamic pressure (AG/AK) is invoked to formalize the movement of the phase boundaries during heterogeneous reactions. This force, however, is a virtual thermodynamic force and must not be confused with mechanical (electrical) forces. 

In chemically homogeneous ionic crystals, may be the only driving force. In inhomogeneous systems, the electrochemical potential gradient Vrij = Vnt+ZjF-Vtp acts upon the mobile charged species i. The additivity of Vp, and stems from the very small electric charge number needed to establish the internal electric field, which is on the order of 1 [V/cm]. These charges are too small to interfere with the concentrations that determine the chemical potentials p,. 

Phosphorylation Is Driven by Proton Movements Electron Transport Creates an Electrochemical Potential Gradient for Protons across the Inner Membrane... 

The chemiosmotic theory postulates that protons moving back into the matrix via an ATP-synthase drive the formation of ATP. Evidence for this is that an electrochemical potential gradient for protons can support the formation of ATP in the absence of electron-transfer reactions. A transient pH gradient that pulls protons into the matrix can be set up by first incubating mitochondria at pH 9, so that the inside becomes alkaline, and then quickly lowering the pH of the suspension medium to 7 (fig. 14.21). 

Uncouplers dissipate the electrochemical potential gradient by carrying protons across the membrane respi-... 

The electrochemical potential gradient also drives the uptake of P, and ADP into the mitochondrial matrix and the export of ATP to the cytosol. 

Flow of protons back into the bacterial cell, down the electrochemical potential gradient, is mediated by an ATP-synthase resembling the ATP-synthase of the mitochondrial inner membrane (see chapter 14). As in mitochondria, the movement of protons through the F0 base-piece of the enzyme drives the formation of ATP (see fig. 15.13). 

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

In the Z scheme, photosystem II, the cytochrome b6f complex and photosystem I operate in series to move electrons from H20 to NADP+ and to create an electrochemical potential gradient for protons across the thylakoid membrane. In addition to this linear pathway, chloroplasts in some plant species may use a cyclic electron-transfer scheme that includes photosystem I and the cytochrome b6f... 

Jagendorf, A. T., and E. Uribe, ATP formation caused by acid-base transition of spinach chloroplasts. Proc. Natl. Acad. Sci. USA 55 197, 1966. Chloroplasts can form ATP in the dark if an electrochemical potential gradient for protons is set up across the thylakoid membrane. 

Some Transporters Facilitate Diffusion of a Solute down an Electrochemical Potential Gradient... 

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