site. Alternatively, SO 2 may oxidize and break apart the sulfur bonds in critical
enzymes of the membrane, disrupting cellular function.
As noted previously, soil acidification increases release of toxic metal ions,
such as Pb
2þ and Cd
2þ ions. These metal ions may directly damage roots by
disrupting water and nutrient uptake, resulting in water deficit or nutrient
deficiency. Soil acidification can also cause leaching of nutrients, leading to
nutrient deficiency and growth disturbance (Figure 4.1). A plant becomes
unhealthy as a result of one or more of the disturbances mentioned above.
Even before visible symptoms are discernable, an exposed plant may be
weakened and its growth impaired. In time, visible symptoms, such as chlorosis
or necrosis, may appear, followed by death.
4.3 MAMMALIAN ORGANISMS
4.3.1 EXPOSURE
An environmental pollutant may enter an animal or human through a variety
of pathways. Figure 4.3 shows the general pathways that pollutants follow in
mammalian organisms. As mentioned earlier, exposure of a host organism to a
pollutant constitutes the initial step in the manifestation of toxicity. A
mammalian organism may be exposed to pollutants through inhalation,
dermal contact, eye contact, or ingestion.
4.3.2 UPTAKE
The immediate and long-term effects of a pollutant are directly related to its
mode of entry. The portals of entry for an atmospheric pollutant are the skin,
eyes, lungs, and gastrointestinal tract. The hair follicles, sweat glands, and open
wounds are the possible entry sites where uptake from the skin may occur.
Both gaseous and particulate forms of air pollutants can be taken up through
the lungs. Uptake of toxicants by gastrointestinal tract may occur when
consumed foods or beverages are contaminated by air pollutants, such as Pb,
Cd, or sprayed pesticides.
For a pollutant to be taken up into the body and finally carried to a cell,
it must pass through several layers of biological membranes. These include
not only the peripheral tissue membranes, but also the capillary and cell
membranes. Therefore, the nature of the membranes and the chemical and
physical properties (e.g., lipophilicity) of the toxicant in question are important
factors affecting uptake. The mechanisms by which chemical agents pass
through the membrane include:
filtration through spaces or pores in membranes
passive diffusion through the spaces or pores, or by dissolving in the lipid
material of the membrane
Toxic Action of Pollutants
49
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-004.3d]
Ref: 4365 MING-HO YU Chap-004 Page: 49 45-64
enzymes of the membrane, disrupting cellular function.
As noted previously, soil acidification increases release of toxic metal ions,
such as Pb
2þ and Cd
2þ ions. These metal ions may directly damage roots by
disrupting water and nutrient uptake, resulting in water deficit or nutrient
deficiency. Soil acidification can also cause leaching of nutrients, leading to
nutrient deficiency and growth disturbance (Figure 4.1). A plant becomes
unhealthy as a result of one or more of the disturbances mentioned above.
Even before visible symptoms are discernable, an exposed plant may be
weakened and its growth impaired. In time, visible symptoms, such as chlorosis
or necrosis, may appear, followed by death.
4.3 MAMMALIAN ORGANISMS
4.3.1 EXPOSURE
An environmental pollutant may enter an animal or human through a variety
of pathways. Figure 4.3 shows the general pathways that pollutants follow in
mammalian organisms. As mentioned earlier, exposure of a host organism to a
pollutant constitutes the initial step in the manifestation of toxicity. A
mammalian organism may be exposed to pollutants through inhalation,
dermal contact, eye contact, or ingestion.
4.3.2 UPTAKE
The immediate and long-term effects of a pollutant are directly related to its
mode of entry. The portals of entry for an atmospheric pollutant are the skin,
eyes, lungs, and gastrointestinal tract. The hair follicles, sweat glands, and open
wounds are the possible entry sites where uptake from the skin may occur.
Both gaseous and particulate forms of air pollutants can be taken up through
the lungs. Uptake of toxicants by gastrointestinal tract may occur when
consumed foods or beverages are contaminated by air pollutants, such as Pb,
Cd, or sprayed pesticides.
For a pollutant to be taken up into the body and finally carried to a cell,
it must pass through several layers of biological membranes. These include
not only the peripheral tissue membranes, but also the capillary and cell
membranes. Therefore, the nature of the membranes and the chemical and
physical properties (e.g., lipophilicity) of the toxicant in question are important
factors affecting uptake. The mechanisms by which chemical agents pass
through the membrane include:
filtration through spaces or pores in membranes
passive diffusion through the spaces or pores, or by dissolving in the lipid
material of the membrane
Toxic Action of Pollutants
49
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-004.3d]
Ref: 4365 MING-HO YU Chap-004 Page: 49 45-64