tailieunhanh - Introduction to ENVIRONMENTAL TOXICOLOGY Impacts of Chemicals Upon Ecological Systems - CHAPTER 5

Tuyến đường phơi sáng và chế độ hành động QUY TRÌNH THIỆT HẠI Với nồng độ đủ cao, chất độc môi trường có thể gây ra ảnh hưởng xấu đến quá trình sinh lý của sinh vật sống. Để cho một chất gây ô nhiễm gây độc tính của nó trên cơ thể sinh vật tiếp xúc với nó, chất gây ô nhiễm đầu tiên phải nhập máy chủ và tiếp cận trang web mục tiêu của nó. Mặc dù nó là khó khăn, nếu không phải không thể, khái quát cơ chế chính xác mà mỗi chất gây ô nhiễm. | CHAPTER 5 Routes of Exposure and Modes of Action THE DAMAGE PROCESS Given sufficiently high concentrations environmental toxicants can cause adverse effects on the physiological processes of a living organism. In order for a pollutant to exert its toxicity on an organism exposed to it the pollutant must first enter the host and reach its target site. Although it is difficult if not impossible to generalize the precise mechanism by which each specific pollutant affects living organisms a few commonalties that are shared by different pollutants are summarized here to provide a general background. The damage processes in plants and in animals are considered separately below. ATMOSPHERIC POLLUTANTS AND PLANTS An atmospheric pollutant-induced plant injury may follow a pathway that includes exposure uptake transport storage metabolism and excretion Figure . To cause injury to any vegetation an air pollutant must first be taken up by the plant in question. Although the atmospheric concentration of a pollutant is important the actual amount that gets into the plant is of more concern. The conductance through the stoma which regulates the passage of ambient air into the cells is especially critical. Uptake is dependent upon the physical and chemical properties along the gas-to-liquid diffusion pathway. Pollutant flow may be restricted by the physical structures of the leaf or scavenging by competing chemical reactions. The leaf orientation and morphology including epidermal characteristics and air movement across the leaf are important determinants affecting the initial flux of gases to the leaf surface. More pollutant would enter a leaf when there is some air movement. Stomatal resistance is a critical factor affecting pollutant uptake. The resistance is determined by stomatal number size anatomical characteristics and the size of the stomatal aperture. Little or no uptake occurs when the stoma is closed. Stomatal opening is regulated by internal CO2 content .