External Effect Concentrations

Ecotoxicological effects of organic chemicals can be related to external and internal effect concentrations. Earlier studies already showed that a lot of information is available on external effect concentrations for different classes of compounds and different organisms. The main focus of this section therefore is on internal effect concentrations. 

Effect parameters in hazard or risk assessment of chemicals for the aquatic environment are usually based on external effect concentrations for a few types... 

The class of relatively unreactive chemicals which act, at least in acute toxicity tests, as narcotics [3] is the best known class of compounds for which several QSARs are established. Those chemicals exert the so-called base-line toxicity. Studies from KSnemann [3] and Veith et al. [91] have shown that external effect concentrations such as LC50s or NOECs for these chemicals depend on the octanol-water partition coefficient according to the following equation ... 

Table 4 shows that, for different organisms, the lethal body bnrdens for polar narcotics vary approximately by two orders of magnitnde, and thns again show a significant reduction in the variation of the ecotoxicological effect concentrations compared to the more than five orders of magnitnde differences that are foimd in external effect concentrations for this type of mechanism of action [e.g.l21]. 

Ecotoxicological effects, such as acute or sublethal responses, can be related to both external and internal concentrations. The former is still used in risk assessment procedures, while the latter has recently been investigated for its potential use in risk assessment. External concentrations may vary by many orders of magnitude for different chemicals, even when they exert the same mechanism of action. The variability in internal concentrations is much smaller. The assumptions which form the basis for a broad applicability of the internal concentration, namely that for a given mechanism of action, i) there would be no intraspecies variation, ii) there would be no interspecies variation, and iii) there would be no time or concentration dependency, have been studied. It was found that no assumption was completely valid. However, given the magnitude of variability found, these variations are much less than those which are found for external concentrations, while some of the reasons for the variations in the internal effect concentrations may be similar for the variation in external effect concentrations. 

Overviews of QSAR studies for aquatic toxicity of chemicals which show narcosis are extensively discussed in several publications [93,94]. At first sight, it is quite remarkable that QSAR equations for all kinds of different species are so similar. On the other hand, the explanation is rather simple. It is generally accepted that the mechanism of narcosis is not a very specific process and each compoimd has the same intrinsic activity. In other words the external concentration of a compound at a fixed effect (e.g. narcosis or death) is only a fimction of the probability of a compound to reach its site of action. For many chemicals for which bioaccumulation is not influenced by biotransformation reactions, this probability is correlated to the octanol-water partition coefficient (K ) and this explains directly the correlation between and the external effect concentrations. 

A well-known subacute effect is the growth reduction in algae. Hitherto, only external effect concentrations have been reported for this type of subacute effect, since experimental problems make it difficult to determine those internal effect concentrations, and existing bioaccumulation models for, e. g., fish, do not apply to algae, e.g. [78]. It must be noted that algae and other small organisms are prone to diffusive uptake for contaminants from the ambient environment for which the link between bioconcentration and the internal effect concentration concept would be very promising. 

Many structure-activity relationships can be used to deal with mixture toxicity. Bio accumulation models in combination with internal effect concentration may provide a good means to better predict when organisms are at risk. It must be noted, however, that in many cases there is significant variation in these internal effect concentrations, although even larger variation is found for external effect concentrations. The variation in the external effect concentrations is partly related to the variation in bioaccumulation and partly to interspecies and intraspecies variation. 

Na+/K -ATPase and for human skin fibroblast Na+/K+-ATPase ll52]. External effect concentrations were combined with tissue/water partition coefficients to estimate the internal effect concentrations. For these latter studies, external effect concentrations showed a much greater variation than the internal effect concentrations, as is found for in vivo external and internal effect concentrations. 

Fig. 3. A proposed signal transduction pathway regarding the external Ca effect on ginsenoside Rb synthesis by P. notoginseng cells. Ca signal changes are triggered by external Ca concentrations. The calcium signatures are decoded by calcium sensors, CaM and CDPK. UGRdGT is possibly modulated by the sensors in a direct or indirect (dashed lines) way. Changes of CDPK activity may result from increased synthesis or posttranslational modification of the enzyme (shown as CDPK ).

Alternative mechanisms are equally likely. One possibility arises from evidence that activation of a2-adrenoceptors reduces Ca + influx this will have obvious effects on impulse-evoked exocytosis. In fact, the inhibition of release effected by a2-adrenoceptor agonists can be overcome by raising external Ca + concentration. Finally, an increase in K+ conductance has also been implicated this would hyperpolarise the nerve terminals and render them less likely to release transmitter on the arrival of a nerve impulse. Any, or all, of these processes could contribute to the feedback inhibition of transmitter release. Similar processes could explain the effects of activation of other types of auto-or heteroceptors. 

The chemical composition of biological objects is extremely complex. They contain the macromolecules of proteins, lipids, and many other substances in addition to low-molecular-weight organic and inorganic compounds. Different external effects can produce both quantitative and qualitative composition changes some substances disappear and/or others appear. Some substances that are essential for the functioning of the cells or of the entire organism are present in very small concentrations, lO Mand less. 

Aluminium toxicity is a major stress factor in many acidic soils. At soil pH levels below 5.0, intense solubilization of mononuclear A1 species strongly limits root growth by multiple cytotoxic effects mainly on root meristems (240,241). There is increasing evidence that A1 complexation with carboxylates released in apical root zones in response to elevated external Al concentration is a widespread mechanism for Al exclusion in many plant species (Fig. 10). Formation of stable Al complexes occurs with citrate, oxalate, tartarate, and—to a lesser extent— also with malate (86,242,243). The Al carboxylate complexes are less toxic than free ionic Al species (244) and are not taken up by plant roots (240). This explains the well-documented alleviatory effects on root growth in many plant species by carboxylate applications (citric, oxalic, and tartaric acids) to the culture media in presence of toxic Al concentrations (8,244,245) Citrate, malate and oxalate are the carboxylate anions reported so far to be released from Al-stressed plant roots (Fig. 10), and Al resistance of species and cultivars seems to be related to the amount of exuded carboxylates (246,247) but also to the ability to maintain the release of carboxylates over extended periods (248). In contrast to P deficiency-induced carboxylate exudation, which usually increases after several days or weeks of the stress treatment (72,113), exudation of carboxylates in response to Al toxicity is a fast reaction occurring within minutes to several hours... 

Notice that in the region of fast chemical reaction, the effectiveness factor becomes inversely proportional to the modulus h2. Since h2 is proportional to the square root of the external surface concentration, these two fundamental relations require that for second-order kinetics, the fraction of the catalyst surface that is effective will increase as one moves downstream in an isothermal packed bed reactor. 

When a solid acts as a catalyst for a reaction, reactant molecules are converted into product molecules at the fluid-solid interface. To use the catalyst efficiently, we must ensure that fresh reactant molecules are supplied and product molecules removed continuously. Otherwise, chemical equilibrium would be established in the fluid adjacent to the surface, and the desired reaction would proceed no further. Ordinarily, supply and removal of the species in question depend on two physical rate processes in series. These processes involve mass transfer between the bulk fluid and the external surface of the catalyst and transport from the external surface to the internal surfaces of the solid. The concept of effectiveness factors developed in Section 12.3 permits one to average the reaction rate over the pore structure to obtain an expression for the rate in terms of the reactant concentrations and temperatures prevailing at the exterior surface of the catalyst. In some instances, the external surface concentrations do not differ appreciably from those prevailing in the bulk fluid. In other cases, a significant concentration difference arises as a consequence of physical limitations on the rate at which reactant molecules can be transported from the bulk fluid to the exterior surface of the catalyst particle. Here, we discuss... 

The rate at the surface is formally expressible in terms of the fluid phase concentration Cg and an external effectiveness 77 as... 

The external effectiveness is defined as the ratio of the actual rate with Cs at the surface to the hypothetical rate with concentration Cg at the surface, thus... 

Figure 4.78 Effect of an external inhibitor on the concentration profile of B in the extended basic system when operated as a packed bed reactor (n = 3). The external inhibitor concentration varies between 0 and X. The values of X are indicated above, the cycle time (r) is 1 min, and the values used for all other parameters are given in Table 4.12, set II.

Figure 11.19 Plots of the external effectiveness factor as a function of the substrate modulus Da for different values of the dimensionless bulk substrate concentration is the limiting first-order effectiveness factor attained at sufficiently low concentrations. Adapted from C.Horvath and J.M.Engasser. Biotechnol.Bioeng., 16, 909 (1974).

Greenaway, P. Calcium regulation in the freshwater mollusc Limnea stagnalis. 1. The effect of internal and external calcium concentration. J. exp. Biol. 54, 199 (1971)... 

The intermolecular general-base catalysis of the hydrolysis may also be measured. Comparing the rate constants for this with those of the intramolecular reaction shows that a 13-M solution of an external base is required to give the same first-order rate as the intramolecular reaction has.12 The effective concentration of the carboxylate ion in aspirin is therefore 13 M. This is a typical value for intramolecular general-acid-base catalysis. 

Since a reaction product catalyses the reaction, the initial concentration of product also has a strong effect on the TMRad. In the case illustrated in (Figure 12.6), an initial conversion of 10% leads to a reduction of the TMRad by a factor of 2. This also has direct implications for process safety the thermal history of the substance, that is, its exposure to temperature for a certain time increases initial product concentration, leading to effects comparable to those illustrated in Figure 12.5. Hence it becomes obvious that substances showing an autocatalytic decomposition are very sensitive to external effects, such as contaminations and previous thermal treatments. This is important for industrial applications as well as during the experimental characterization of such decompositions the sample chosen must be representative of the industrial situation, or several samples must be analysed. 

Lanir, Y., Seybold, J., Schneiderman, R. and Huyghe, J.M. (1998) Partition and diffusion of sodium and chloride ions in soft charged foam the effect of external salt concentration and mechanical deformation. Tissue Engineering 4, 365-378... 

Irradiation of externally added NDI rods 31 at 635 nm was detected as an increase in intervesicular pH in response to a reduction of internal quinone (Figure 11.17). The change in HPTS emission, which reflects internal proton consumption, is analyzed with Hill s equation (n = 3.9 1.2), which indicates that the active helix photosystem 28 is quadruple (four rods form the helix) with an effective concentration of 1.3 (iM. However, the shortened length of the rigid-rod p-barrel scaffold to the NDI dimer 30 is inactive (Figure 11.17) [43]. 

In solution culture experiments both strontium and caesium show hyperbolic absorption isotherms with respect to the external concentration of the element. Figure 7-15 (a) shows an example of a typical uptake isotherm for Sr while Shaw and Bell (1989) have demonstrated a similar isotherm for Cs. Baker (1981) has referred to such plant uptake responses as accumulator functions and has identified these as being typical of the absorption of elements over which plants can exert some degree of physiological control. Typically, the nutrient elements, including K and Ca, exhibit such isotherms and it can be postulated from the similarity in the uptake patterns of K and Cs on the one hand and Ca and Sr on the other that the radioions share, to some extent, the same uptake mechanisms as the nutrient ions. This has several important implications. Firstly, the direct competition for uptake sites between radioions and nutrient ions means that the external (soil) concentration of one is increased at the expense of the uptake of the other as the nutrient ions in question are vastly more abundant in soils than radioions it is K and Ca which will be effective in competitively excluding Cs and Sr, respectively. Secondly, the kinetics of this competition are concentration-dependent, so the assumption of first order kinetic movements of... 

Figure 13. Effect of external lysozyme concentration on the release of glucose oxidase from a glutaraldehyde-crosslinked, partially deacetylated chitin. Amount of glucose oxidase in gel 3 mg/ 18 mg gel.

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