Electron flow from reduced

V). The centers resemble PSII of chloroplasts and have a high midpoint electrode potential E° of 0.46 V. The initial electron acceptor is the Mg2+-free bacteriopheophytin (see Fig. 23-20) whose midpoint potential is -0.7 V. Electrons flow from reduced bacteriopheophytin to menaquinone or ubiquinone or both via a cytochrome bct complex, similar to that of mitochondria, then back to the reaction center P870. This is primarily a cyclic process coupled to ATP synthesis. Needed reducing equivalents can be formed by ATP-driven reverse electron transport involving electrons removed from succinate. Similarly, the purple sulfur bacteria can use electrons from H2S. 

Calculate AG0 for electron flow from reduced cytochrome c to oxygen. 

Figure 1. Possible pathways of electron flow from reduced photosystem I.

A FIGURE 8-20 Schematic depiction of the cytochrome c oxidase complex showing the pathway of electron flow from reduced cytochrome c to O2. Heme groups are denoted by red diamonds. Blue arrows indicate electron flow. Four electrons, sequentially released from four molecules of reduced cytochrome c, together with four protons from the matrix, combine with one O2 molecule to form two water molecules. Additionally, for each electron transferred from cytochrome c to oxygen, one proton is transported from the matrix to the intermembrane space, or a total of four for each O2 molecule reduced to two H2O molecules. 

As stated earlier, electrons flow from a reducing agent to an oxidizing agent. This flow of electrons is a kind of kinetic energy that can be used to produce electric current. 

However, in contrast to the cyclic flow of electrons in purple bacteria, some electrons flow from the reaction center to an iron-sulfur protein, ferredoxin, which then passes electrons via ferredoxin NAD reductase to NAD+, producing NADH. The electrons taken from the reaction center to reduce NAD+ are replaced by the oxidation of H2S to elemental S, then to SOf, in the reaction that defines the green sulfur bacteria. This oxidation of H2S by bacteria is chemically analogous to the oxidation of H20 by oxygenic plants. 

Because the voltage is positive, the net reaction is spontaneous in the forward direction. Cd(.v) is oxidized and Ag+ is reduced. Electrons flow from the left-hand electrode to the right-hand electrode. 

Iron atoms, Fe, for example, are better reducing agents than copper ions, Cu2+. So when a piece of iron metal and a solution containing copper ions are placed in contact with each other, electrons flow from the iron atoms to the copper ions, as Figure 11.6 illustrates. The result is the oxidation of iron atoms and the reduction of copper ions. 

We now see that mitochondria contain a variety of molecules—cytochromes, flavins, ubiquinone, and iron-sulfur proteins—all of which can act as electron carriers. To discuss how these carriers cooperate to transport electrons from reduced substrates to 02, it is useful to have a measure of each molecule s tendency to release or accept electrons. The standard redox potential, E°, provides such a measure. Redox potentials are thermodynamic properties that depend on the differences in free energy between the oxidized and reduced forms of a molecule. Like the electric potentials that govern electron flow from one pole of a battery to another, E° values are specified in volts. Because electron-transfer reactions frequently involve protons also, an additional symbol is used to indicate that an E° value applies to a particular pH thus, E° refers to an E° at pH 7. 

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