Coupled transport membranes theory

Bartsch, R.A. Charewicz, W.A. Kang, S.I. Walkowiak, W. Proton-coupled transport of alkali metal cations across liquid membranes by ionizable crown ethers. In Liquid Membranes Theory and Applications Noble, R.D., Way, J.D., Eds. ACS Symp. Ser. No. 347 American Chemical Society Washington, D.C., 1987 86-97. 

The authors of hundreds of articles, published in this field, in trying to show the uniqueness of their works, have given new names and features to techniques and technologies that are similar to each other. This confuses and disorients readers, especially students and young researchers. The same is true for theories hundreds of theories in this field need critical analysis and classification. In this chapter, recent aspects of carrier-facilitated, coupled transport through liquid membranes are reviewed with a classification and grouping of the theories. 

In the chemiosmotic theory for oxidative phosphorylation (Chap. 14), electron flow in the electron-transport chain is coupled to the generation of a proton concentration gradient across the inner mitochondrial membrane. Derive an expression for the difference in electrochemical potential for a proton across the membrane. 

The phenomenological coefficients are important in defining the coupled phenomena. For example, the coupled processes of heat and mass transport give rise to the Soret effect (which is the mass diffusion due to heat transfer), and the Dufour effect (which is the heat transport due to mass diffusion). We can identify the cross coefficients of the coupling between the mass diffusion (vectorial process) and chemical reaction (scalar process) in an anisotropic membrane wall. Therefore, the linear nonequilibrium thermodynamics theory provides a unifying approach to examining various processes usually studied under separate disciplines. 

Mitchell s theory holds that an electrochemical proton gradient across the membrane (which is only slightly permeable to many ionized species and particularly to H ") is formed by the vectorial transport of into the thylakoid lumen coupled to electron transport, as a consequence of the alternate disposition across the membrane of electron carriers which can bind protons and others which cannot be protonated. 

The experimental use of artificial electron acceptors and donors has demonstrated, in agreement with Mitchell s theory, that electron transport can be coupled to ATP synthesis only when the chemical structure and the lipophilicity of the electron carriers added is such as to allow vectorial proton transport across the membrane [72]. 

According to Mitchell s chemiosmotic theory, photophosphorylation is driven by energy derived from electron transfer coupled to proton translocation. The results of postillumination discussed in the previous section further supports the notion that a proton gradient is the driving force for phosphorylation. It is therefore possible in principle that a similar proton gradient produced by artificial means might also be able to drive phosphorylation in a chloroplast membrane, entirely in the dark, i. e., without the aid of photo-induced electron transport. Such a scheme was indeed realized by the so-called acid-bath ATP-forma-tion demonstrated by Jagendorf and Uribe " in 1966. 

In these equations, Pj and P2 are the two conformational states of the transport protein, and equilibrium constants (K) and rate constants (k) in an electric field are shown to be these constants in zero field multiplied by a nonlinear term that is the product of A Me and the electric field across the membrane, Em. The r in these equations is the apportionation constant and has a value between 0 and 1 (14). This property of a membrane protein has been explored, and a model called electroconformational coupling has been proposed to interpret data on the electric activation of membrane enzymes (13-17). A four-state membrane-facilitated transport model has been analyzed and shown to absorb energy from oscillating electric fields to actively pump a substrate up its concentration gradient (see the section entitled Theory of Electroconformational Coupling). 

A second aspect of the chemiosmotic coupling theory postulates that the proton-motive force (pmf) drives energy-consuming processes in the membrane by a reversed flow of protons [8] (Fig. 2). The energy of AjSji is thus either converted into ATP by a reversed action of the ATPase complex, or drives osmotic work such as the formation of solute gradients by secondary transport or drives mechanical work such as flagellar movements. 

The chemiosmotic theory is based on two fundamental assumptions. First, the enzymes of the electron transport chain and ATPase are vectorially distributed on a membrane. The position of the enzymes active sites determines on which side of the membrane the product of the reaction is delivered. Second, the membrane is impermeable to ions. Uncouplers are thought to abolish the permeability barrier to protons presented by the membrane. It is difficult to evaluate the validity of the hypothesis, because all arguments that have been marshalled in its support can also be used to support the other proposed mechanism of coupling. 

A principal difference between the chemiosmotic and the secretion theory is the involvement of metabolic energy in the process of the polar transport. According to the secretion theory, energy is directly coupled to carriers, in those cases, where auxin transport appears to be uphill whereas the new theory postulates that auxin transport is thermodynamically downhill, but with metabolic energy expended to maintain the pH and electrical gradients in addition to maintaining an ordered membrane with asymmetric permeability to lAA" (Goldsmith 1977, pp 446 and 491). 

This new theory has been extensively discussed in the review of Goldsmith in 1977 (see also Rubery 1980), and her introductory sentences (p 441) outline the basic concept. Cells, being more permeable to undissociated auxin molecules than to auxin anions, can accumulate auxin when the pH of the cytoplasm is above that of the walls... Carriers may be but are not necessarily involved in passage across the cell membrane... Unlike the conventional theory of polar secretion, which presumes active transport of auxin with direct coupling of energy to carriers in those cases when auxin transport appears to be uphill. 

If we speak specifically about polymer electrolyte fuel cells (PEFCs), to which this book is devoted, the theory of the polymer electrolyte membrane is the theory of proton transport in a complex water-containing porous environment it is also the story of water in the membrane, of its sorption and distribution coupled with the proton transport. 

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