Uptake of the chemical

In summary, the two models of Figs. 27 and 28 provide new insights into the pH dependence of the surface topography of Si(l 11) in fluoride solutions (see Fig. 21). With increasing pH the uptake of the chemical reaction with water enhances the anisotropy of the dissolution since the chemical route depends critically on the atom coordination in contrast to the electrochemical one [122, 123 b]. 

The rate of uptake of the chemical per unit of water is proportional to both... 

The main routes of intake of a chemical are ingestion (dietary or nondietary), inhalation, dermal absorption, and parenteral (intravenous, intramuscular, intrathecal, intraperitoneal, and subcutaneous). The structure of a PBPK model is dependent on the intake routes, as the corresponding organs or tissues usually need to be explicitly modeled in order to describe the uptake of the chemical. It is therefore advisable to identify routes of uptake prior to developing the PBPK model. 

In selecting adjuvants that can be used to enhance biological efficacy one has to consider the specific interactions that may take place between the surfactant, agrochemical and target species. This is usually described in terms of an activation process for uptake of the chemical into the plant. This mechanism is particularly important for systemic agrochemicals. 

Now we can reexamine the lipid bilayer as a chemical barrier, looking at it from the perspective of I<. A chemical that is water-soluble is, by definition, not lipid-soluble and as such wiU not be able to dissolve into the membrane. As such, its rate of diffusion across the barrier will be minimal. Without the aid of carrier proteins, the uptake of the chemical wiU be minimal, as wiU its toxicity. In contrast, a compound that is soluble in oil or fat is easily absorbed across the lipid bilayer, and as such, wiU have a greater potential for eliciting a... 

Herbicides which are taken up by the root depend upon mass flow of soil water to the root and uptake of the chemical in the water removed from the soil by the plant. The concentration within the root has been shown by Briggs et to be related to the octanol/water partition coefficient. For compounds with a of less than zero, there is virtually no accumulation in the plant root as the rises, so the concentration in the root rises compared to the soil solution concentration. Briggs reported that the optimum for translocation to the leaves is 2. Herbicides with Xow values higher than 4 are accumulated in the roots but not translocated. Soil-acting compounds with systemic activity are, therefore, unlikely to have a Xow greater than 3.5. " Further details are discussed in Chapter 9. 

Sow tlie seeds evenly to ensure even uptake of the chemical. Do not plant seeds densely (as in Fig. 2c), although the number of seeds planted can be more or less than 10. 

The structure and mathematical expressions used in PBPK models significantly simplify the true complexities of biological systems. If the uptake and disposition of the chemical substance(s) is adequately described, however, this simplification is desirable because data are often unavailable for many biological processes. A simplified scheme reduces the magnitude of cumulative uncertainty. The adequacy of the model is, therefore, of great importance, and model validation is essential to the use of PBPK models in risk assessment. 

For convenience, the processes identified in Figure 2.1 can be separated into two distinct categories toxicokinetics and toxicodynamics. Toxicokinetics covers uptake, distribution, metabolism, and excretion processes that determine how much of the toxic form of the chemical (parent compound or active metabolite) will reach the site of action. Toxicodynamics is concerned with the interaction with the sites of action, leading to the expression of toxic effects. The interplay of the processes of toxicokinetics and toxicodynamics determines toxicity. The more the toxic form of the chemical that reaches the site of action, and the greater the sensitivity of the site of action to the chemical, the more toxic it will be. In the following text, toxicokinetics and toxicodynamics will be dealt with separately. 

From a toxicological point of view, the critical issue is how much of the toxic form of the chemical reaches the site of action. This will be determined by the interplay of the processes of uptake, distribution, metabolism, storage, and excretion. These processes will now be discussed in a little more detail. 

Further progress may derive from a more accurate definition of the chemical and physical properties of the humic substances present at the rhizosphere and how they interact with the root-cell apoplast and the plasma membrane. An interaction with the plasma membrane H -ATPase has already been observed however this master enzyme may not be the sole molecular target of humic compounds. Both lipids and proteins (e.g., carriers) could be involved in the regulation of ion uptake. It therefore seems necessary to investigate the action of humic compounds with molecular approaches in order to understand the regulatory aspects of the process and therefore estimate the importance of these molecules as modulators of the root-soil interaction. 

The major function of cutin is to serve as the structural component of the outer barrier of plants. As the major component of the cuticle it plays a major role in the interaction of the plant with its environment. Development of the cuticle is thought to be responsible for the ability of plants to move onto land where the cuticle limits diffusion of moisture and thus prevents desiccation [141]. The plant cuticle controls the exchange of matter between leaf and atmosphere. The transport properties of the cuticle strongly influences the loss of water and solutes from the leaf interior as well as uptake of nonvolatile chemicals from the atmosphere to the leaf surface. In the absence of stomata the cuticle controls gas exchange. The cuticle as a transport-limiting barrier is important in its physiological and ecological functions. The diffusion across plant cuticle follows basic laws of passive diffusion across lipophylic membranes [142]. Isolated cuticular membranes have been used to study this permeability and the results obtained appear to be valid... 

Chemically, all of these apparently involve skeletal rearrangements and uptake of the isocyano function at some stage during biosynthesis. 

The choice of the transporting reagent for a given material is made so that the reaction is as complete as possible in one direction, in the uptake, and the reverse reaction in the opposite direction at the deposition site. This requires that not only the choice of the reagent, but also the pressure and temperature ranges under which the reaction is most effectively, or quantitatively, performed, must be calculated (Alcock and Jeffes, 1967 1968). There will always be limitations placed on this choice by the demands of the chemical inertness and temperature stability of the containing materials in which the reaction is carried out. 

Environmental chemicals occur as pure liquid or solid compounds, dissolved in water or in nonaqueous liquids, volatilised in gases, dissolved in solids (absorbed) or bound to interfaces (adsorbed). Figure 5 gives a schematic view of the different physical states at which substrates are taken up by microbial cells. There is a consensus that water-dissolved chemicals are available to microbes. This is obvious for readily soluble chemicals, but there is also clear evidence for microbial uptake of the small dissolved fractions of poorly water soluble compounds. Rogoff already had shown in 1962 that bacteria take up phenanthrene from aqueous solution [55], In the intervening time many other researchers have made the same observation with various combinations of microorganisms and poorly soluble compounds [14,56,57]. 

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