Māris kļaviņŠ, azamat azizov, JĀnis zaļoksnis environment, pollution, development: the case of uzbekistan

98 
ENVIRONMENT, POLLUTION, DEVELOPMENT:  THE CASE OF UZBEKISTAN
 6.2. 
 Effects of toxic substances on living 
organisms
The effect of substances on a living organism depends on their 
transformation once they have entered the organism. The behaviour 
of chemical substances in ecosystems is influenced by migration of the 
substances in the soil, surface water, air and transfer in food chains.
The toxic effect of substances can manifest itself after the sub-
stan ces have come into the contact with an organism. The most likely 
way to ingest the toxic substances is with water or food through the 
digestive tract. However, the effect of substances that can be taken 
by inhalation or through the skin is also important. When inhaled, 
depend ing on their properties, the toxic substances enter the lungs 
and desorb from their surface (human’s lung surface is 50-100 m
2
), 
directly entering the circulatory system. In toxicology, the process, 
where by a substance enters the organism is called absorption. For 
example, ingested or inhaled substances are considered to be outside 
the organism until they pass through cells in respiratory organs or 
gastrointestinal tract. Toxic impact on the living organism is exhibited 
only by absorbed substances, although a locally toxic effect, which 
can manifest itself, for example, as irritation, may take place before 
absorption of the substance. 
Concentration of the substance in organism and the intake de-
pends on the mode of consumption. In some cases, great amounts of 
substances cannot be absorbed, while in other cases even negligible 
amounts of a substance can be absorbed easily. For example, the 
intake of dichlorodiphenyltrichloroethane (DDT) through the skin 
is insignificant, however, even small amounts of DDT are effectively 
absorbed in gastrointestinal tract. When the substance enters an 
organism, it crosses various barriers – cell membranes, which must be 
penetrated both to enter the cell and to arrive at the next cell. Thereby 
only the substances which can effectively cross the cell membranes 
can be considered as toxic. The cell’s metabolism processes proceed 
through particular segments in its membrane, which consists from 
proteins (Figure 6.2). 
An important stage of toxic substances’ intake is their transport 
through the membranes of cells. A special transport mechanism of 
substances is endocytosis. Through this mechanism, macromolecular 
substances and large particles can enter a cell. Endocytosis means 
that the substance is encompassed by a fragment of cell’s membrane 
(Figure 6.3). The encircled substance and the part of membrane 
subsequently separate from the membrane and gets inside of the cell 
where, for example, it may be destroyed. Endocytosis is a substantial 
process in case of lung and liver cells. This way, for instance, 
asbestos particles are absorbed into the lung cells. 
The most important 
phases of substances’ 
circulation once they 
enter the organism are:
 
Œ absorption;
 
Œ distribution of 
substances;
 
Œ biotransformation 
(metabolism);
 
Œ excretion.
Absorption, distribution, 
biotransformation and 
excretion are intercon‑
nected processes. 
The probability of 
substance’s absorption is 
influenced by:
 
Œ the mode of intake of 
the substance;
 
Œ concentration of 
substance;
 
Œ physical and chemical 
properties of the 
substance. 

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6. Action of toxic substances in the environment – basic concepts of ecotoxicology 
 99
Cytoskeleton
Lipid biolayer
Proteins
Carbohydrates
Channel 
proteins
Cholesterol
Membrane
An important way of substance intake is through gastrointestinal 
tract. The environ mental conditions change along the length of 
gastrointestinal tract rather significantly, first of all, the pH reaction. 
In the oral cavity the environ mental reaction usually is close to 
neutral,  in  stomach  it  is  pH  ~2,  but  in  intestinal  canal  it  can  be 
pH ~6. The large surface of intestinal canal and its connectedness 
with circulatory system ensures assimilation of substances in this 
segment of gastrointestinal system. The intake of toxic substances 
can also take place through skin, trachea, lungs, or, in case of aquatic 
animals, through the gills. In undamaged skin the main barrier to 
substance’s entry into organism is epidermis. Since epidermis and 
other upper layers of skin are formed by lipophilic compounds, in