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Applications of Chemical Kinetics in Environmental Systems
Published in Kalliat T. Valsaraj, Elizabeth M. Melvin, Principles of Environmental Thermodynamics and Kinetics, 2018
Kalliat T. Valsaraj, Elizabeth M. Melvin
The complete oxidation of glucose should liberate 38 molecules of ATP, equivalent to the free energy available in glucose. If that much has to be accomplished the electrons generated during the process should be stored in other compounds that then undergo reduction. “Electron acceptors” generally used by microbes in our natural environment include oxygen, nitrate, Fe(III), SO42−, and CO2. It is to mediate the transfer of electrons from substrate to electron acceptors that the microorganisms need intermediate electron transport agents. These agents can also store some of the energy released during ATP synthesis. Some examples of these intermediates are cytochromes and iron-sulfur proteins. The same function can also be performed by compounds that act as H+-carriers (e.g., flavoproteins). The redox potentials of some of the electron transport agents commonly encountered in nature are given in Table 4.17. Chappelle (1993) cites the example of E. coli that uses the NADH/NAD cycle to initiate redox reactions that eventually releases H+ ions out of its cell. The NADH oxidation to NAD in the cytoplasm is accompanied by a reduction of the flavoprotein that releases H+ from the cell to give an FeS protein, which further converts to flavoprotein via acquisition of 2H+ from the cytoplasm and in concert with coenzyme Q releases 2H+ out of the cell. The resulting cytochrome b transfers the electron to molecular oxygen forming water. The net result is the use of the energy from redox reactions to transport hydrogen ions out of the cell. The energy accumulated in the process is utilized to convert ADP to ATP. The entire sequence of events is pictorially summarized in Figure 4.52 and is sometimes called “chemiosmosis,” the process of harnessing energy from electron transport. More details of these schemes are given in advanced textbooks such as Schlegel (1992). The main point of this discussion is the part played by electron transport intermediates and ATP synthesis in the metabolic activities of a living cell. Thus microorganisms provide an efficient route by which complex molecules can be broken down. The entire process is driven by the energy storage and release capabilities of microorganisms that are integral parts of their metabolism.
Green hydrogen production by Rhodobacter sphaeroides
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2023
Dahbia Akroum-Amrouche, Hamza Akroum, Hakim Lounici
During respiration, cytochromes b and c (common components of photosynthesis and respiration) are preserved, while bacteriochlorophylls and carotenoids are inhibited. In the presence of oxygen, cytochrome c2 transfers the electrons from ubiquinone-cytochrome c2 oxidoreductase (the cytochrome b-c1 complex) to terminal cytochrome α-a3 oxidase. Under photosynthetic conditions, cytochrome c2 functions to complete the cyclic photophosphorylation chain by the transfer of electrons from the cytochrome b-c1 complex to reduce the photo-oxidized reaction center (Brandner et al. 1989; Lancaster and Michel 1996), (Figure 3).