tailieunhanh - ECOTOXICOLOGY: A Comprehensive Treatment - Chapter 7

Khả dụng sinh học là một khái niệm được chấp nhận rộng rãi dựa trên những kiến thức tiềm ẩn trước khi một cơ thể có thể tích lũy hoặc hiển thị một phản ứng sinh học một chất hóa học, phần tử đó hoặc hợp chất phải được sẵn có cho cơ thể. Trong khi khái niệm về sinh khả dụng được chấp nhận rộng rãi, các quy trình kiểm soát nó được hiểu rõ. (Benson et al. 1994) | 7 Bioaccumulation Bioavailability is a widely accepted concept based on the implicit knowledge that before an organism may accumulate or show a biological response to a chemical that element or compound must be available to the organism. While the concept of bioavailability is widely accepted the processes that control it are poorly understood. Benson et al. 1994 OVERVIEW With the exception of radionuclides a toxicant must first come into contact with the initial site of action before an effect can manifest. This might in some cases involve a straightforward contact with a biological surface where a localized effect occurs. Even in this case the toxicant will penetrate to some extent into cells. In many other cases the toxicant moves from the surface of first contact into the organism where it interacts elsewhere with the site s of action. Once within the organism many processes modify redistribute or remove the toxicant. Bioaccumulation is the net result of these uptake transformation translocation and elimination mechanisms. Toxicant qualities influencing bioaccumulation-associated processes will be described here. UPTAKE Cellular Mechanisms Atoxicant present in an external medium or one already inside the organism can come in contact with and then be moved across a cell membrane. External media might be gaseous . inhaled air liquid . inhaled or imbibed water or solid . ingested food . A toxicant already in the organism perhaps moving within the circulatory system can be present in its original form complexed or transformed to some metabolite s or conjugate s . Regardless the same general mechanisms are involved in the toxicant transport into and out of cells. Before discussing cellular transport mechanisms it is important to mention that some substances might not pass into cells but instead enter by moving through the tight junctions between cells. Solvent drag of a substance into a fish through gill cell junctions see Evans et al. 1999 is