Molecular oxygen flux

The molecular oxygen flux through a perovskite OTM is related to the chemical potential gradient Vp(02) ... 

Reaction (12-9) shows the photochemical dissodation of NO2. Reaction (12-10) shows the formation of ozone from the combination of O and molecular O2 where M is any third-body molecule (principally N2 and O2 in the atmosphere). Reaction (12-11) shows the oxidation of NO by O3 to form NO2 and molecular oxygen. These three reactions represent a cyclic pathway (Fig. 12-4) driven by photons represented by hv. Throughout the daytime period, the flux of solar radiation changes with the movement of the sun. However, over short time periods (—10 min) the flux may be considered constant, in which case the rate of reaction (12-9) may be expressed as... 

Reactions (1) and (2) essentially convert solar radiant energy into thermal energy. The parameters which determine the rate of ozone formation (UV photon flux, atomic and molecular oxygen number density and the total gas number density) are not constant with altitude and so the ozone concentration and hence Tg varies with altitude. The net result is that Tg increases thoughout the stratosphere until a maximum is reached at the stratopause whence Tg begins to decrease again. 

Like the examples mentioned above, most examples of metabolic flux analysis by metabolite balancing have redox balances as a central constraint used in the determination of the flux distribution. However, the redox balance is, especially under aerobic conditions, subject to uncertainties which make it less suitable for estimation of the fluxes. Part of the reason for this is to be found in futile cycles, e. g., oxidation of sulfides to disulfides, where reductive power is needed to reduce the disulfides. The net result of this reaction is reduction of molecular oxygen to water, and oxidation of NADPH to NADP+. Since the consumption rate of oxygen of these specific reactions is impossible to measure, the result may be that the NADPH consumption is underestimated. This is in accordance with the finding that when the NADPH-producing reactions are estimated independently of the NADPH-consuming reactions, there is usually a large excess of NADPH that needs to be oxidized by reactions not included in the network, e. g., futile cycles [11-13]. 

Fig. 8. Vertical distribution of molecular oxygen (Oj) mixing ratio in prebiological paleo-atmosphere. Calculations for molecular hydrogen (Hj) = 17 ppmv, carbon dioxide (CO2) = 280 ppmv, and three different solar ultraviolet fluxes.

As yet, details of the fluxes involved in the processes that generate and consume molecular oxygen are too poorly constrained to estabhsh a balanced O2 budget. A summary of the processes believed to dominate controls on atmospheric O2, and reasonable best guesses for the magnitude of these fluxes, if available, are shown in Figure 7 (from Keeling et al., 1993). 

Since there are no standards for flux determinations, as there are for chemical concentration measurements in water or sediments, evaluating their accuracy is difficult. One is forced to rely on comparison of different anal5dical approaches and modeling of nutrient and oxygen distributions to achieve a consensus. Of the different methods used to determine the organic carbon export presented in the following section, the first two (sediment traps and thorium isotope disequilibria) are used to evaluate the POC flux only. They must be combined with estimates of the DOC flux to achieve the total. The second two methods, molecular oxygen and carbon isotope mass balance, determine the sum of DOC and POC export. 

Benthic oxygen fluxes as a function of longitude and depth on the northwest US continental margin. The fluxes were determined using either benthic lander measurements (boxes) or calculated from micro-electrode oxygen gradients in the top few centimeters of porewaters, assuming molecular diffusion (circles). Error bars are the standard deviation of replicate measurements. Redrawn from Archer and Devol (1992). 

If it were supposed, for example, that the first of these reactions is at equilibrium and the second is rate-determining, the net flux of molecular oxygen through the interface at the high pressure side can be described by,... 

Fig. 2 A schematic view of xenobiotic metabolism. Cytochrome P450 (CYP) and other hepatic enzymes such as epoxide hydrolase (EH), conjugating enzymes, UDP glucuronyl transferase (UGT), sulphotransferase (SULT), or glutathione S-transferase (GST) are important in metabolic handling of xenobiotics. The enzymes depend on molecular oxygen and NADPH (CYP), UDPG (UGT), PAPS (SULT), and reduced glutathione (GSH) (GST). The cellular membrane has many transporters, OATS, ABC transporters, and OCTS, which are important for the transmembrane flux of many xenobiotics including the products of the conjugating enzymes...

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