Coupled redox gradient

Electron-cation symport has been realized in a double carrier process where the coupled, parallel transport of electrons and metal cations was mediated simultaneously by an electron carrier and by a selective cation carrier [6.47]. The transport of electrons by a nickel complex in a redox gradient was the electron pump for driving the selective transport of K+ ions by a macrocyclic polyether (Fig. 12). The pro-... 

Proton transfer is a particularly important transport process. Beyond acid-base reactions, proton transfer may be coupled to electron transfer in redox reactions and to excited-state chemistry. It is of enormous significance in biochemical processes where it is an essential step in hydrolytic enzyme processes and redox reactions spanning respiration, and photosynthesis where proton motion is responsible for sustaining redox gradients. In relatively recent times, proton transfer in the excited state has undergone significant study, primarily fueled by advances in ultrafast spectroscopy. 

Nitrification is ubiquitous in surface sediments and is often tightly coupled to denitrification in the sediment redox gradient. 

Ultimately all chemoautotrophs depend on a nonequilibrium redox gradient, without which there is no thermodynamic driver for carbon fixation. For example, the reaction involving the oxidation of H2S by microbes in deep-sea vents described above is ultimately coupled to oxygen... 

Chemical analysis for specific redox couples is especially useful in studies of groundwaters contaminated by waste disposal, where redox gradients are often steep, and the fate of redox-sensitive contaminants may be rate dependent or otherwise unpredictable. In such a study of sewage-contaminated groundwaters, Kent et al. (1994) found that reduction of O2, Cr(VI), and Se(VI) did not occur, although it was thermodynamically favored. They concluded that the disequilbrium resulted from unfavorable microbial conditions and heterogeneity of groundwater flow. 

Estuaries exhibit physical and chemical characteristics that are distinct from oceans or lakes. In estuaries, water renewal times are rapid (10 to 10 years compared to 1 to 10 years for lakes and 10 years for oceans), redox and salinity gradients are often transient, and diurnal variations in nutrient concentrations can be significant. The biological productivity of estuaries is high and this, coupled with accumulation of organic debris within estuary boundaries, often produces anoxic conditions at the sediment-water interface. Thus, in contrast to the relatively constant chemical composition of the... 

It is also important to understand how the potential gradient between an electrode and the bulk solution is established and controlled. Because the potential difference between the electrode and the bulk solution is not measurable, a second electrode must be employed. Although in general the potential difference between an electrode and solution cannot be determined, the potential difference between two electrodes in that solution can be determined. If the solution electrode potential difference of one of the electrodes is held constant by maintaining a rapid redox couple such as silver-silver chloride or mercury-mercurous chloride (calomel), then the potential... 

The term n in this relationship is simply the number of electrons involved in the redox reaction. Accordingly, a one-electron couple will change its electrode potential Em+.m by 59.1 mV per tenfold change in concentration. For this reason, an analyst will say the couple shows a slope of 59.1 mV per decade (if n = 1). The word slope alerts us to the fact that a gradient is involved. In fact, the slope in question is the gradient of a calibration curve such as that shown in Figure 3.6 when the x-axis is written as logioa(M" ). 

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

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