Saturable biological processes

Ahluwalia and Goyal94 have pointed out some disadvantages of biosorption such as the early saturation of biomass, which can be a problem since metal desorption is necessary prior to further use, irrespective of the metal value. Furthermore, the potential for biological process improvement is limited because cells are not performing an active metabolism. 

Products of this type seem to protect the humus from rapid incorporation into new biological processes. Additional factors that appear to be associated with the accumulation of organic matter in Mollisols are high exchange capacities, saturation with calcium, an abundance of mineral colloids and a high content of minerals of the smectite group (Fenton, 1983). 

The biological processes in biofilms are either described by 1-order or 0-order kinetics. However, the 0-order reaction is of specific importance for sewer biofilms as is also the case for treatment processes of wastewater in biofilters. The saturation constant, Ks, is normally insignificant compared with the substrate concentration, and the biofilm kinetics [cf. Equation (2.20)], is therefore 0-order. As shown in Figure 2.8, two different conditions exist the biofilm is either fully penetrated or partly penetrated, corresponding to either a fully effective or a partly effective biofilm. The distinction between these two situations can be expressed by means of a dimensionless constant, P, called the penetration ratio (Harremoes, 1978). For each of these two situations, the flux of substrate across the biofilm surface can neglect the stagnant liquid film being calculated [Equations (2.23) and (2.25)] ... 

Given that ET through proteins is fundamental to many biological processes and that H-bonds are prevalent in proteins, it is important to determine how efficiently ET can be mediated by H-bonds. Of the several studies which have been reported, that by Therien is of particular interest since it provides a comparative study of ET mediated by saturated bonds and by H-bonds.1341... 

A major contribution of this paper was pointing out the importance of bioturbation and bioirrigation on chemical processes associated with carbonate dissolution. In the movement of sulfidic sediment from depth to near the interface by biological processes, oxidation of the sediment produces sulfuric acid which ends up titrating alkalinity, lowering pH, and thus lowers saturation state (e.g., Berner and Westrich, 1985). Actually this process is very complex, involving many reactive intermediate compounds such as sulfite, thiosulfate, polythionates, etc. Aller and Rude (1988) demonstrated an additional complication to this process. Mn oxides may oxidize iron sulfides by a bacterial pathway that causes the saturation state of the solution to rise with respect to carbonate minerals, rather than decrease as is the case when oxidation takes place with oxygen. 

Many minerals have been shown to form as the result of direct or indirect biological processes (e.g., see Ehrlich 1999). Organismal production of a vast diversity of other phases, including phosphates (bones and teeth), actinide minerals (including uraninite and uranium phosphates), elemental sulfur, etc. are well known. In addition to processes described above, high concentrations of nanoparticles are generated in proximity to cells as the result of changes in solution saturation state due to uptake or release of protons or... 

There are biological processes, however, that involve saturable carrier or enzymatic systems, with a finite capacity for transport or catalysis. For instance, processes like active uptake at absorption sites, renal tubular secretion, or hepatic biotransformation of xenobiotics may become saturated at high exposure levels, yielding rates of disposition that are constant and independent of the concentration in blood. This is characteristic of zero-order kinetics. Biotransformations of ethanol in the liver and active tubular renal secretion of penicillin in urine are examples of biological processes that obey zero-order kinetics. Figure 7 illustrates the blood concentration... 

The behavior of a chemical and its metabolites in the body is described by the parameters of the pharmacokinetic model and is dependent, among other things, on the administered dose level. As a given dose level is repeatedly or continuously administered, the concentration in the body will increase until eventually the rate of input is equal to the rate of output, and the chemical and its metabolite(s) will then maintain a constant (steady state) concentration in the body until exposure ceases. When the dose levels are such that all the biological processes comprising the model are first order (or the concentrations are well below the values for saturable processes) the distribution and concentration of the chemical and its metabolites in the body will maintain values that are directly proportional to the dose level. This direct proportionality (which is a direct result of first order kinetic processes) confers linear pharmacokinetic behavior on the chemical within this range of dose levels. 

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