Oxidation-reduction reactions concentration cells

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

The electrochemical approach uses a sterilizable stainless steel probe with a cell face constructed of a material which will enable oxygen to permeate across it and enter the electrochemical chamber which contains two electrodes of dissimilar reactants (forming the anode and cathode) immersed in a basic aqueous solution (Fig. 2). The entering oxygen initiates an oxidation reduction reaction which in turn produces an EMF which is amplified into a signal representing the concentration of oxygen in the solution. 

The differentiation between the coenzymes used in catabolism and anabolism is maintained because the enzymes that catalyze these oxidation-reduction reactions exhibit strong specificity for a particular coenzyme. For example, an enzyme that catalyzes an oxidation reaction can readily tell the difference between NAD and NADP if the enzyme is in a catabolic pathway, it will bind NAD, but not NADP. In addition, the relative concentrations of the coenzymes in the cell encourage binding of the appropriate coenzyme. For example, because NAD and NADH are catabolic coenzymes and catabolic reactions are most often oxidation reactions, the NAD concentration in the cell is much greater than the NADH concentration. (The cell maintains its [NAD" "]/[NADH] ratio near 1000.) Because NADP" and NADPH are anabolic coenzymes and anabolic pathways are predominantly reduction reactions, the concentration of NADPH in the cell is greater than the concentration of NADP" ". (The ratio of [NADP" "]/[NADPH] is maintained at about 0.01.)... 

The magnitude of the net cell potential AV° will signify the spontaneity of the oxidation-reduction reaction. However, it does not indicate the rate at which corrosion will occur. As noted before, we apply the superscript 0 to denote that we are considering the Standard Electrode Potentials. Engineers may be required to calculate the potential of a particular half-cell at concentrations and temperatures other than the standard conditions. For this purpose, we shall use the Nernst equation, which allows us to account for non-standard temperatures and solution concentrations. 

The biological importance of this enzyme has already been discussed (section II). Its role in producing glutamate, as the first organic amino compound, in bacteria and plants seems reasonably well established. In animals, which do not have the ability to produce all amino compounds from a simple nitrogen source, the enzyme seems to be concerned with the removal of excess amino compounds (equations 2-5) as well as with the production of glutamate for conversion to the acid amide (section V.B) or to take part in transamination reactions to form the non-essential amino compounds. Cellular control of the direction in which reaction occurs may well lie in the ratio of the concentrations of the oxidised and reduced forms of the cofactor. This ratio is not a fixed quantity but depends on the metabolic activity of the cell (NAD and NADP are cofactors for many oxidation-reduction reactions) as well as on the availability of molecular oxygen for the terminal step in respiration, by which the reduced cofactor is reoxidised by the cytochrome system (section IV.A.1). 

The electromotive force (emf), or cell potential, is the maximum voltage of a voltaic cell. It can be directly related to the maximum work that can be done by the cell. A standard electrode potential, or reduction potential, refers to the potential of an electrode in which molar concentrations and gas pressures (in atmospheres) have unit values. A table of standard electrode potentials is useful for establishing the direction of spontaneity of an oxidation-reduction reaction and for calculating the standard emf of a cell. 

The hydrogen ion is not special when it comes to electrochemical measurement of this type. Virtually every ionic species can take part in oxidation-reduction reactions, so the concentration of virtually any ion can be detected with a similar electrode. These ion-specific electrodes have some half-reaction inside and, across a porous glass shell, set up an electrochemical cell whose voltage can be measured and used to back-calculate the concentration of a particular ion. Figure 8.8 shows an ion-specific electrode. For the most part, they resemble pH electrodes, so care should be exercised to identify the exact ion an electrode detects. 

The relative concentrations of the coenzymes in a cell also encourage the appropriate coenzyme to be bound. For example, catabolic reactions are predominately oxidation reactions, and anabolic reactions are predominately reduction reactions. The cell maintains its [NAD ]/[NADH] ratio near 1000 and its [NADP ]/[NADPH] ratio at about 0.01. Thus, NAD is the oxidizing coenzyme and NADPH is the reducing coenzyme most available in a cell. 

Standard, reduction potentials are determined by measuring the voltages generated in reaction half-cells (Figure 21.2). A half-cell consists of a solution containing 1 M concentrations of both the oxidized and reduced forms of the substance whose reduction potential is being measured, and a simple electrode. 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

A concentration cell contains the same electroactive material in both half-cells, but in different concentration (strictly, with different activities). The emf forms in response to differences in chemical potential /r between the two half-cells. Note that such a concentration cell does not usually involve different electrode reactions (other than, of course, that shorting causes one half-cell to undergo reduction while the other undergoes oxidation). 

Electrochemical cells are made of two conducting electrodes, called the anode and the cathode. The oxidation reaction takes place at the anode, where electrons are released to flow through a wire to the cathode. At the cathode, reduction takes place. For the oxidation and reduction reactions to occur, the electrodes must be in a conducting solution called an electrolyte. The electrochemical cell voltage depends on the types of materials, usually conducting metals, used as electrodes, and the concentration of the electrolyte solution. (See Figure 6.5.)... 

The same enzymes that are found in die liver are present in small amounts in mucosal cells. They catalyze oxidations, reductions, hydrolysis, and conjugations. Specific activities of the enzymes are Iowct than in the liver, but the activity can increase by induction. For many drug molecules, die concentration is higher than in die liver, so there is a higher number of reaction cycles per tune unit and per mole of enzyme. Prollne eadopeptidasea cleave the Pro-NHz bond in proteins and nentides 1221. 

The oxidative power of peroxyl radicals, although sometimes increased by electron withdrawal, is smaller than that of RO radicals, but thanks to their longer lifetime [48], peroxyl radicals are ideal candidates for propagating oxidative chain reactions in biological membranes of aerobic cells, as discussed below. Traces of unprotected iron will therefore be sufficient to maintain a substantial free radical production from pre-formed hydroperoxides, even in the absence of important reductant concentrations [49]. 

---