Toxic material released in the environment may remain at the point of their discharge or’ be transported to other place. The distance up to which they are transported depends mainly on the mobility of the agent carrying them. Moving waters and air currents are effective media for transfer and transport of toxic substances. The mobility of a toxic material is however, not confined to their dispersion within a single medium.

Materials released into water may get distributed in soil and air as well. Toxic material released into atmosphere may contaminate soil as well as water. Wastes discarded on the soil may contaminate the water which seeps through it or the air may get contaminated when it carries the vapours or small particles of the toxic material.

Pollutants can enter a biological system from any of the three media, such as water, soil on which carry them. In lower plants and animals, plasma membrane is the main barrier between outside environment and the biological system inside the cell.

In higher plants and animals we possess much better protective devices such as coating of wax, cuticles, multilayered epidermis pollutants have to take other routes. These routes may be through the roots, root-hairs, rhizoid stomata etc. in plants and through nasal mucosa and mucosal lining of mouth, respiratory track and gastro-intestinal track in animals. In many cases, the epidermis is pervious to certain types of pollutants.

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Nature has provided higher animals including man with an efficient device which helps them to guard against pollutants which regularly enter through intestinal track. Materials absorbed from intestines are routed through hepatic portal system to the liver wherein harmful constituents are reacted upon by liver’s enzymatic machinery.

A number of harmful materials absorbed intestines are degraded or converted to harmless state before reaching the blood circulation However, this is not so in the case of pollutants entering the system through respiratory track. The air in alveoli of lungs is in direct contact with blood in capillaries across a thin membrane. The material absorbed enters straight into the blood circulation. This becomes more significant in to light of the fact that we breathe in by weight about 16 times more air than total foodstuff and water consumed orally.

The plasma membrane through which all substances entering a cell must pass is lipoprotein membrane being made up of a thin layer of lipid or lipid like substance covered on both sides by a layer of proteins. The basic structure of plasma membrane is strikingly uniform throughout the living world.

The rate of entry of the toxic material into a biological system is influenced by the size of its molecules, its concentration in the medium, the nature and solubility of the substance in water as well as in lipids and fats. Substances soluble in lipoid material (lipophilic ones) are carried across the plasma membrane simply by dissolution and diffusion through the lipid layer. Lipoid insoluble but water soluble toxic material (hydrophilic ones) may require an active transport mechanism. Smaller molecules such as those of water simply pass through pores in membrane.

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Inside a biological system the toxic material may be either stored or is reacted upon by the enzyme system and excreted. The phenomenon of accumulation of toxic agent inside cells and tissues of living organisms at concentrations above those of immediate environment is known is Bio-accumulation.

The toxic material soluble in lipoid substances or capable of forming complex with macromolecules within the cell may be stored for long duration of time as they are excreted very slowly. By virtue of complex formation and dissolution is lipoid materials which are storage products of cellular metabolism, these substances stay away from the enzymatic reaction which act on then the toxic material once inside a living organism may be concentrated several thousand times the process of Bio-magnification which causes enhancement in the concentration of toxic along the food chain.

Toxic agents who enter a living organism at the level of primary producers get concentrated many thousand times as they move up along the food chain to higher trophic levels. Thus, it is not only air, water or soil which may carry the toxic material of persistant nature.

The toxic agent may accumulate and be biomagnified through the food chain thus contaminating the entire biotic component of the locality. If the food we eat comes from organisms growing in contaminated environment, plants or animals, there is every possibility that it carries small amounts of toxic substance – the chances of getting higher concentration of toxic agent being higher and higher at higher trophic levels.