CHEMICAL THREATS TO BIODIVERSITY

 CHEMICAL THREATS TO BIODIVERSITY

Chemistry has brought about a medical revolution which resulted in the average life expectancy rising from 47 in 1900 to 75 in the 1990s. The explosive expansion of world’s food supply has been because of development of chemicals that protect crops and enhance growth. In almost every area (food, clothing, shelter, transportation, communication, etc.) chemistry has brought about an improvement in the quality of life of people.

The Toxics Release Inventory (TRI), USA, established under the Emergency Planning and Community Right-to-Know Act, tracks the release of chemicals by different sectors of industry. Of the top ten industrial sectors tracked by the TRI, the chemical industry releases a greater amount of waste to the environment than the other nine industrial sectors combined.

 CHEMICAL TOXICOLOGY

There are various chemicals in the environment, some of which are toxic.These toxic chemicals are released by industries into air, water and soil. They upset the complex system of chemical reactions occurring in the human body. They can cause discomfort, illness, disability or even death. Chemical Toxicology is the science of the study of toxic chemicals and their modes of action.

Toxic Chemicals in Air Some of the extremely hazardous substances in the atmosphere are acrylonitrile, arsenic, asbestos, benzene, beryllium, cadmium, chlorinated solvents, chlorofluorocarbons, chromates, coke oven emissions, ethylene dibromide, ethylene oxide, lead, mercury, ozone, sulphur dioxide, vinyl chloride and toxic waste disposal emissions.  Toxic Chemicals in Water ,The toxic elements found in natural and waste waters are arsenic, cadmium, beryllium, boron, chromium, copper, fluorine, lead, manganese, mercury, molybdenum, selenium and zinc. The water bodies contain pesticides, mainly from the drainage of agricultural land.

Effect of Toxic Chemicals on Enzymes

Toxic chemicals attack enzymes, inhibiting their essential function. Heavy metal ions like Hg2+, Pb2+ and Cd2+ act as effective enzyme inhibitors.

Classification of Toxic Substances

1. Corrosive Poisons Corrosive poisons are toxic substances which destroy tissues, e.g., strong acids and alkalies, and many oxidants which can destroy tissues. Concentrated mineral acids such as sulphuric acid and bases like sodium hydroxide attack cellular structures. The cell dies because the proteins are hydrolysed by the acid or the base

2. Metabolic Poisons Metabolic poisons cause illness or death by interfering with a vital biochemical mechanism to such an extent that it ceases to function or does not function efficiently.

3. Neurotoxins Some metabolic poisons limit their action to the nervous system. These include poisons such as strychnine and curare. Neurotoxins act at the point where two nerve fibres come together (called a synapse).

Neurotoxins such as atropine and curare occupy the receptor sites on nerve endings of organs that are normally occupied by the impulse carrying acetylcholine. When atropine or curare occupies the receptor site, no stimulus is transmitted to the organ. Neurotoxins of this type are useful in medicines. For example, atropine is used to dilate the pupil of the eye to facilitate examination of its interior. When applied to the skin, atropine salts relieve pain by deactivating sensory nerve endings on the skin. Curare is used as a muscle relaxant. Nicotine is a natural compound that blocks receptor sites as is done by atropine and curare. This powerful poison causes stimulation and then depression of the central nervous system. Nicotine is present in tobacco and insecticides

4. Mutagens Mutagens are chemicals that can change the genes and chromosomes (hereditary pattern) to cause abnormalities in the offspring. They alter the structures of DNA and RNA, which compose the genes that transmit the traits of parent to offspring. Nitrous acid is a potent mutagen in bacteria, viruses, moulds and other organisms. It reacts with nitrogenous bases of DNA to produce new compounds. The result is an alteration in the genetic coding of DNA, so that a different protein is formed. Sodium nitrite is used as a preservative, colour enhancer or colour fixative in meat and fish products. Sodium nitrite is converted to nitrous acid by hydrochloric acid in the human stomach. Some examples of mutagens: Aflatoxin (from mould), Benzo (α) pyrene (from cigarette and coal smoke), Caffeine, Captan (a fungicide), Chloroprene, Dimethyl sulphate (used for methylation), LSD (lysergic acid diethylamide), Maleic hydrazide (plant growth inhibitor), Mustard gas (dichlorodiethyl sulphide), Ozone, solvents in glue (like toluene, acetone, cyclohexane, hexane and ethyl acetate), and TEM (triethylene melamine, an anticancer drug).

5. Teratogens The chemicals which affect reproduction are called teratogens. The study of birth defects produced by chemical agents is called Teratology. There are three known classes of teratogens: radiation, viral agents and chemical substances. Any chemical that can cross the placenta is a potential teratogen. Smoking a cigarette results in higher-than-normal blood levels of such substances as carbon monoxide, hydrogen cyanide, cadmium, nicotine and benzo (α) pyrene. Many of these substances are present in polluted air as well. Other examples of teratogens: Arsenic, Cobalt, Gallium, Lead, Lithium, Mercury,Thallium, Zinc, Caffeine, DES (diethyl stilbestrol) and PCBs (polychlorinated biphenyls).

6. Carcinogens Carcinogens are chemicals that cause cancer in animals and humans. An abnormal growth is classified as cancerous or malignant when examination shows it is invading neighbouring tissue. A growth is said to be benign if it is localised at its original site. Some compounds cause cancer at the point of contact. Other compounds cause cancer in an area remote from the point of contact. The liver, the site at which most toxic chemicals are removed from the blood, is particularly susceptible to such compounds.

7. Hallucinogens Hallucinogens can produce temporary changes in perception, thought and mood. They destroy one’s sense of judgement. LSD has been linked with birth defects.

8. Alcohols Methyl alcohol is highly poisonous and is a cumulative poison in human beings. It has a toxic effect on the optic nerve and large doses can cause blindness. In the body, it gets oxidised first to formaldehyde and then to formic acid, which is eliminated in the urine. The toxic effect on the optic nerve is caused by the oxidative products.