Membrane potential couple reactions

The individual steps of the multistep chemical reduction of COj with the aid of NADPHj require an energy supply. This supply is secured by participation of ATP molecules in these steps. The chloroplasts of plants contain few mitochondria. Hence, the ATP molecules are formed in plants not by oxidative phosphorylation of ADP but by a phosphorylation reaction coupled with the individual steps of the photosynthesis reaction, particularly with the steps in the transition from PSII to PSI. The mechanism of ATP synthesis evidently is similar to the electrochemical mechanism involved in their formation by oxidative phosphorylation owing to concentration gradients of the hydrogen ions between the two sides of internal chloroplast membranes, a certain membrane potential develops on account of which the ATP can be synthesized from ADP. Three molecules of ATP are involved in the reaction per molecule of COj. 

The characteristics of VITTM in Figure 6.5 can easily be understood taking into account the relation of Equation (36) as follows. Since the membrane potential where the positive current wave appears ( wi-w2 indicated by A in curve 1) is the sum of wi/m indicated by B in curve 2 and Em/w2 indicated by C in curve 3, the positive current wave in curve 1 is considered to be caused by the coupled reactions of both the transfer of K" " from Wl to M facilitated by DB18C6 (the positive current wave in curve 2) and that of CV+ from M to W2 (the final rise in curve 3). Hence, the potential region for the positive wave in curve 1 differs from that in curve 2. 

In real cells, multiple transmembrane pumps and channels maintain and regulate the transmembrane potential. Furthermore, those processes are at best only in a quasi-steady state, not truly at equilibrium. Thus, electrophoresis of an ionic solute across a membrane may be a passive equilibrative diffusion process in itself, but is effectively an active and concentra-tive process when the cell is considered as a whole. Other factors that influence transport across membranes include pH gradients, differences in binding, and coupled reactions that convert the transported substrate into another chemical form. In each case, transport is governed by the concentration of free and permeable substrate available in each compartment. The effect of pH on transport will depend on whether the permeant species is the protonated form (e.g., acids) or the unprotonated form (e.g., bases), on the pfQ of the compound, and on the pH in each compartment. The effects can be predicted with reference to the Henderson-Hasselbach equation (Equation 14.2), which states that the ratio of acid and base forms changes by a factor of 10 for each unit change in either pH or pfCt ... 

Figure 18.39. Mechanism of Mitochondrial ATP-ADP Translocase. The translocase catalyzes the coupled entry of ADP and exit of ATP into and from the matrix. The reaction cycle is driven by membrane potential. The actual conformational change corresponding to eversion of the binding site could be quite small.

Facilitated or carrier-mediated transport is a coupled transport process that combines a (chemical) coupling reaction with a diffusion process. The solute has first to react with the carrier to fonn a solute-carrier complex, which then diffuses through the membrane to finally release the solute at the permeate side. The overall process can be considered as a passive transport since the solute molecule is transported from a high to a low chemical potential. In the case of polymeric membranes the carrier can be chemically or physically bound to the solid matrix (Jixed carrier system), whereby the solute hops from one site to the other. Mobile carrier molecules have been incorporated in liquid membranes, which consist of a solid polymer matrix (support) and a liquid phase containing the carrier [2, 8], see Fig. 7.1. The state of the art of supported liquid membranes for gas separations will be discussed in detail in this chapter. 

A SrTio.4 Mgo.603 x catalyst was used, which had been previously shown to be an effective catalyst for this reaction. The use of the membrane significantly increased the yield of C2 hydrocarbons. This remains an area with significant unexplored potential. Progress can be made here by developing CMR systems with enhanced catalytic activities towards the CH4 coupling reaction, and asymmetric-type proton-hole or proton-electron conducting membranes with significantly increased conductivities. 

As in nature, networks are relatively fault tolerant concerning, for example, changes in synaptic connections. All these effects can be measured by the change of membrane potential during an action potential (cf. Sect. 3.2). This potential has a direct influence on the gate of a field-effect transistor, or, in another device, it influences the capacity between a microelectrode and the axon, which can be measured with a.c.-coupled amplifiers with high input impedances. AU measurement conditions have to be chosen so that no electrochemical reaction takes place at the electrode surface in order to avoid the formation of poisoning chemicals. 

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