Chemical stimulation potential difference

The huge advances made in molecular biology since the late 1980s have provided the possibility of approaches to evaluating chemicals and potential drugs for carcinogenic potential in approaches which are different, less expensive and which take a shorter period of time than traditional long-term bioassays. This work has also been stimulated by dissatisfaction with the performance of traditional test systems. 

The selection or avoidance of potential host plants by phytophagous Insects Is guided by a complex combination of physical and chemical stimuli. Color, shape and olfactory cues may play a role In the Initial orientation, whereas acceptance or rejection of a plant depends on texture as well as chemical stimulants or deterrents. Initiation of feeding Is stimulated or deterred by the presence or absence of specific chemicals or groups of chemicals, many of which have been Identified. The selection of a suitable plant for ovlposltlon Is also crucial for survival of the progeny of most herbivorous Insects, but the chemical factors Involved are known In relatively few cases. Ovlposltlon stimulants and deterrents often appear to be quite different from the chemicals that elicit or Inhibit feeding responses of larvae. 

Ca2+ cycling into and out of the mitochondria leads to NAD depletion and a fall in ATP. The entry of Ca2+ into the mitochondria dissipates the potential difference across the mitochondrial membranes and so inhibits the function of ATP synthase, which relies on the charge difference across the membrane (Fig. 6.13 and 7.60). Export of Ca2+ from the mitochondrial matrix may occur and be stimulated by some chemicals. However, this will lead to repeated cycling, which damages the membrane and further compromises ATP synthesis. The export of Ca2+ also uses up ATP as a result of the Ca2+ ATPases involved. Hence ATP levels fall. 

In addition, ion concentration gradients existing between two sides of a membrane produce an electrical potential difference, ranging between 50 and 100 millivolts or mV (10 volt), the outside being positive with respect to the interior. This is the direct consequence of the distribution of cations, especially potassium and sodium ions. Any stimulation by electrical, mechanical, or chemical means at one point of the membrane will create a change in the potential membrane at that point. The altered potential, also called the active potential, will move as a wave over the membrane surface. This provides a means of rapid communication between different regions of a cell. In the case of an elongated nerve cell, this constitutes a nerve impulse. 

Siace the discovery of quantum mechanics,more than fifty years ago,the theory of chemical reactivity has taken the first steps of its development. The knowledge of the electronic structure and the properties of atoms and molecules is the basis for an understanding of their interactions in the elementary act of any chemical process. The increasing information in this field during the last decades has stimulated the elaboration of the methods for evaluating the potential energy of the reacting systems as well as the creation of new methods for calculation of reaction probabilities (or cross sections) and rate constants. An exact solution to these fundamental problems of theoretical chemistry based on quan-tvm. mechanics and statistical physics, however, is still impossible even for the simplest chemical reactions. Therefore,different approximations have to be used in order to sii lify one or the other side of the problem. 

It is not clear how generally useful the technique of adding test chemicals to intact plants may be. First, the chemicals to be added must be soluble in relatively polar (ideally, aqueous) solvents. Surfactants (i.e.. Tween or Triton compounds) may be needed on leaf surfaces are very waxy or otherwise hydrophobic. Nonpolar solvents rapidly dissolve the protective waxes off of the leaf surface and quickly lead to desiccation of the leaf. Second, the use of known nonhost plants raises the potential problem of overriding differences between hosts and nonhosts that cannot be overcome by an stimulant. Such differences may involve leaf shape, color, or physical and chemical characteristics of the leaf surface. Thus, care needs to be taken to demonstrate the attractiveness of nonhosts after treatment with known stimulants before results using potential stimulants can be evaluated. 

Hazard and Operability Analysis (Hazop) (Kletz, 1992) is one of the most used safety analysis methods in the process industry. It is one of the simplest approaches to hazard identification. Hazop involves a vessel to vessel and a pipe to pipe review of a plant. For each vessel and pipe the possible disturbances and their potential consequences are identified. Hazop is based on guide words such as no, more, less, reverse, other than, which should be asked for every pipe and vessel (Table 1). The intention of the quide words is to stimulate the imagination, and the method relies very much on the expertise of the persons performing the analysis. The idea behind the questions is that any disturbance in a chemical plant can be described in terms of physical state variables. Hazop can be used in different stages of process design but in restricted mode. A complete Hazop study requires final process plannings with flow sheets and PID s. 

In terms of improving our ability to predict soil C turnover, we identify five priorities for research (1) The interactive effects of temperature and moisture on microbial decomposition rates, because soils will experience novel and transient conditions (2) the mechanisms governing protection of OM through interactions with mineral surfaces and due to spatial structure (3) the mechanisms leading to slower OM turnover times with depth (4) the potential for nonlinear responses of decomposition to C availability—for example, the role of labile C inputs in stimulating decomposition of less labile OM (i.e., priming) and density-dependent microbial behavior and (5) how the chemical characteristics of organic compounds, as inputs from different plant species, charred (black) carbon, or microbial cell walls and by-products, influence mechanisms of stabilization and turnover. 

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