Existence of Threshold Concentrations

The substrate concentrations may be too low for effective transport into the cells. 

There may be a limiting substrate concentration required for induction of the appropriate catabolic enzyme. At low substrate concentrations the necessary enzymes wonld simply not be synthesized, and this could be the determining factor in some circumstances (Janke 1987). 

All these observations emphasize that tests for biodegradability carried ont at high snbstrate concentrations may not adequately predict the rates of degradation occurring in natnral ecosystems where only low concentrations of xenobiotics are enconntered (Alexander 1985). This phenomenon is therefore of enormons environmental importance, since it would imply the possible extreme persistence of low concentrations in natnral ecosystems. The further exploration of this phenomenon is probably only limited in practice by the access to analytical methods for measnring snfficiently 

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There is substantial evidence from the aquatic environment for the existence of threshold concentrations below which rates of degradation of xenobiotics are slow or even negligible (Alexander 1985). A number of hypotheses have been put forward including the critical concentrations required for... 

Another reason for scepticism with regard to the existence of threshold concentrations is that for some substances, and some effects, different chemicals act together so that the total effect depends on the sum of their concentrations. This has been found to be the case for a number of oestrogenic substances (Silva et al, 2002, and see Note 3 for Chapter 5). The apparent no-effect level of such chemicals crucially depends on which other chemicals are present. In other words, laboratory tests in which animals are exposed to individual substances may have little relevance to the real world situation in which we are exposed to hundreds of different chemicals. 

Obtained results, apparently, indicate on existence of threshold concentration that organomodified layered silicate can distribute at nanometer level in such polymer, with forming of nanocomposite of exfoliate structure. 

The effect of substrate concentration and the important issue of the existence or otherwise of threshold concentrations below which degradation is not effectively accomplished. 

Existence of threshold substrate concentration below which rates of degradation are low or negligible... 

Odors are measured by their intensity. The threshold value of one odor to another, however, can vary greatly. Detection threshold is the minimum physical intensity necessary for detection by a subject where the person is not required to identify the stimulus, but just detect the existence of the stimulus. Accordingly, threshold deterrninations are used to evaluate the effectiveness of different treatments and to estabflsh the level of odor control necessary to make a product acceptable (8). Concentration can also produce different odors for the same matenal. For example, indole (qv) in low concentrations has the smell of jasmine and a low threshold of perception. In high concentrations, it has a strong odor of feces and CX-naphthyl amine as well as a considerably higher threshold of perception. 

The main conclusion of the percolation theory is that there exists a critical concentration of the conductive fraction (percolation threshold, c0), below which the ion (charge) transport is very difficult because of a lack of pathways between conductive islands. Above and near the threshold, the conductivity can be expressed as ... 

Investigations of pure cultures of bacteria clearly show the existence of a threshold concentration for the carbon source below which replication does not occur. This value is about 18 pg/1 for Escherichia coli and Pseudomonas sp. growing on glucose, 180 pg/1 for Aeromonas hydrophila growing on starch,... 

While large interindividual variability in the steady-state plasma concentrations among patients treated with similar doses of a given antipsychotic is well established, the existence of a critical range of plasma concentration for therapeutic response or significant adverse effects remains controversial. A body of data from a number of fixed-dose studies, however, indicates a possible threshold for response or a linear or curvilinear relationship between plasma levels and clinical response for agents such as ... 

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