on its chemical property. Excretion is the most permanent means whereby toxic
substances are removed from the body.
4.4 MECHANISM OF ACTION
The toxic action of pollutants involves either compounds with intrinsic toxicity
or activated metabolites. These interact with cellular components at specific
sites of action to cause toxic effects, which may occur anywhere in the body.
The consequences of such action may be reflected in changes in physiological
and biochemical processes within the exposed organism. These changes may be
manifested in different ways, including impaired central nervous system (CNS)
function and oxidative metabolism, injury to the reproductive system, or
altered DNA leading to carcinogenesis.
The duration of toxic action depends on the characteristics of the toxicant
and the physiological or biochemical functioning of the host organism.
Generally, the toxic action of a xenobiotic may be terminated by storage,
biotransformation, or excretion.
The mechanisms involved in xenobiotic-induced toxicity are complex and
much remains to be elucidated. The ways in which xenobiotics can induce
adverse effects in living organisms include:
disruption or destruction of cellular structure
direct chemical combination with a cell constituent
inhibition of enzymes
initiation of a secondary action
free-radical-mediated reactions
disruption of reproductive function
These mechanisms are examined in the following sections.
4.4.1 DISRUPTION OR DESTRUCTION OF CELLULAR STRUCTURE
A toxicant may induce an injurious effect on plant or animal tissues by
disrupting or destroying the cellular structure. As mentioned previously,
atmospheric pollutants, such as SO 2 , NO 2 , and O 3 , are phytotoxic – they can
cause plant injuries. Sensitive plants exposed to any of these pollutants at
sufficiently high concentrations may exhibit structural damage when their
tissue cells are destroyed. Studies show that low concentrations of SO 2 can
injure epidermal and guard cells, leading to enhanced stomatal conductance
and greater entry of the pollutant into leaves.
1 Similarly, after entry into the
substomatal cavity of the plant leaf, O 3 , or the free radicals produced from it,
may react with protein or lipid membrane components, disrupting the cellular
structure of the leaf.
3,5
In animals and humans, inhalation of sufficient quantities of NO 2 and
sulfuric acid mists can damage surface layers of the respiratory system.
52
Environmental Toxicology
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Ref: 4365 MING-HO YU Chap-004 Page: 52 45-64
substances are removed from the body.
4.4 MECHANISM OF ACTION
The toxic action of pollutants involves either compounds with intrinsic toxicity
or activated metabolites. These interact with cellular components at specific
sites of action to cause toxic effects, which may occur anywhere in the body.
The consequences of such action may be reflected in changes in physiological
and biochemical processes within the exposed organism. These changes may be
manifested in different ways, including impaired central nervous system (CNS)
function and oxidative metabolism, injury to the reproductive system, or
altered DNA leading to carcinogenesis.
The duration of toxic action depends on the characteristics of the toxicant
and the physiological or biochemical functioning of the host organism.
Generally, the toxic action of a xenobiotic may be terminated by storage,
biotransformation, or excretion.
The mechanisms involved in xenobiotic-induced toxicity are complex and
much remains to be elucidated. The ways in which xenobiotics can induce
adverse effects in living organisms include:
disruption or destruction of cellular structure
direct chemical combination with a cell constituent
inhibition of enzymes
initiation of a secondary action
free-radical-mediated reactions
disruption of reproductive function
These mechanisms are examined in the following sections.
4.4.1 DISRUPTION OR DESTRUCTION OF CELLULAR STRUCTURE
A toxicant may induce an injurious effect on plant or animal tissues by
disrupting or destroying the cellular structure. As mentioned previously,
atmospheric pollutants, such as SO 2 , NO 2 , and O 3 , are phytotoxic – they can
cause plant injuries. Sensitive plants exposed to any of these pollutants at
sufficiently high concentrations may exhibit structural damage when their
tissue cells are destroyed. Studies show that low concentrations of SO 2 can
injure epidermal and guard cells, leading to enhanced stomatal conductance
and greater entry of the pollutant into leaves.
1 Similarly, after entry into the
substomatal cavity of the plant leaf, O 3 , or the free radicals produced from it,
may react with protein or lipid membrane components, disrupting the cellular
structure of the leaf.
3,5
In animals and humans, inhalation of sufficient quantities of NO 2 and
sulfuric acid mists can damage surface layers of the respiratory system.
52
Environmental Toxicology
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-004.3d]
Ref: 4365 MING-HO YU Chap-004 Page: 52 45-64